Patent Publication Number: US-2022226699-A1

Title: Golf club head and golf club

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2021-006050 filed on Jan. 18, 2021. The entire contents of this Japanese Patent Application are hereby incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a golf club head and a golf club. 
     Description of the Related Art 
     As designs advantageous for flight distance, there has been known an upsized head, a lengthened club, and a more flexible shaft for attaining an increased degree of bending, for example. A club that is easy to swing and has a high rebound performance can be obtained by reducing the weight of its shaft while not reducing the weight of its head. 
     On the other hand, a so-called toe-down phenomenon (hereinafter also simply referred to as “toe down”) is one of factors in reduction of flight distance. JPH11-267251 A and JPH10-43332 A have descriptions about the toe-down phenomenon. 
     SUMMARY 
     The inventors of the present disclosure have found that the designs advantageous for flight distance can increase the degree of the toe down. It is difficult for conventional golf clubs to attain both suppression of the toe down and increase of flight distance because of a tradeoff relationship between the two. 
     One of the objects of the present disclosure is to provide a golf club head that can suppress the toe down and is excellent in flight distance performance. 
     In one aspect, the present disclosure provides a golf club head including a face portion that forms a striking face, a crown portion that forms a crown outer surface, a sole portion that forms a sole outer surface, and a hosel portion that is configured to receive a shaft and that defines a shaft axis line. The crown portion includes a protruding portion on the crown outer surface. In a front view of the head as viewed from a face side, the protruding portion does not form any part of an outer contour line of the head. In a heel projection figure in which the head that is placed on a ground plane such that the shaft axis line is perpendicular to the ground plane and a face angle is set at 0° is viewed from a heel side along the ground plane, the protruding portion forms a part of the outer contour line of the head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a golf club according to a first embodiment; 
         FIG. 2A  is a front view of a head of the first embodiment as viewed from a face side, this head being in a reference state, and  FIG. 2B  shows the head of the first embodiment as viewed from the face side, this head being in a heel projection posture; 
         FIG. 3  is a plan view of the head of the first embodiment as viewed from a crown side; 
         FIG. 4  is a side view of the head of the first embodiment as viewed from a heel side; 
         FIG. 5  shows the head of the first embodiment as viewed from an inclined heel side, and  FIG. 5  is a heel projection figure; 
         FIG. 6  shows a part of an outer contour line of the head of the first embodiment as viewed from a toe-back side; 
         FIG. 7  shows a cross-sectional contour line of the outer surface of the head in a cross-sectional view taken along line A-A in  FIG. 3 ; 
         FIG. 8  shows a cross-sectional contour line of the outer surface of the head in a cross-sectional view taken along line B-B in  FIG. 3 ; 
         FIG. 9  shows a cross-sectional contour line of the outer surface of the head in a cross-sectional view taken along line C-C in  FIG. 3 ; 
         FIG. 10  shows a cross-sectional contour line of the outer surface of the head in a cross-sectional view taken along line D-D in  FIG. 3 ; 
         FIG. 11  is an enlarged view of a portion surrounded by a tetragon Q 1  in  FIG. 7 , and a virtually extended line of a crown base surface is additionally drawn in  FIG. 11 ; 
         FIG. 12  is an enlarged view of a portion surrounded by a tetragon Q 2  in  FIG. 9 , and a virtually extended line of the crown base surface is additionally drawn in  FIG. 12 ; 
         FIG. 13A  shows a silhouette of the heel projection figure in  FIG. 5 , and  FIG. 13B  shows a part of the contour line of the silhouette,  FIG. 13B  showing a part of the outer contour line of the heel projection figure of the head of the first embodiment; 
         FIG. 14  is a plan view of a head according to a second embodiment as viewed from the crown side; 
         FIG. 15  is a plan view of a head according to a third embodiment as viewed from the crown side; 
         FIG. 16  is a plan view of a head according to a fourth embodiment as viewed from the crown side; 
         FIG. 17  is a plan view of a head according to a fifth embodiment as viewed from the crown side; 
         FIG. 18  is a plan view of a head according to a sixth embodiment as viewed from the crown side; 
         FIG. 19A  shows a part of an outer contour line of a head according to a seventh embodiment as viewed from the toe-back side, and  FIG. 19B  shows a part of an outer contour line of a head according to an eighth embodiment as viewed from the toe-back side; 
         FIG. 20A  is a perspective view of a head according to a ninth embodiment, and  FIG. 20B  is a cross-sectional view taken along line b-b in  FIG. 20A , however, the depiction of a cross section of a head body is omitted in  FIG. 20B ; 
         FIG. 21A  is a perspective view of the head body of the head of the ninth embodiment, and  FIG. 21B  is a cross-sectional view taken along line b-b in  FIG. 21A , however, the depiction of the cross section of the head body is omitted in  FIG. 21B ; 
         FIG. 22A  is a perspective view of a head according to a tenth embodiment,  FIG. 22B  is a cross-sectional view taken along line b-b in  FIG. 22A , and  FIG. 22C  is a cross-sectional view taken along line c-c in  FIG. 22A , however, the depiction of the cross section of a head body is omitted in  FIG. 22B  and  FIG. 22C ; 
         FIG. 23A  is a perspective view of the head body of the head of the tenth embodiment,  FIG. 23B  is a cross-sectional view taken along line b-b in  FIG. 23A , and  FIG. 23C  is a cross-sectional view taken along line c-c in  FIG. 23A , however, the depiction of the cross section of the head body is omitted in  FIG. 23B  and  FIG. 23C ; 
         FIG. 24  shows the motion of a golf club during downswing; 
         FIG. 25A  and  FIG. 25B  are conceptual diagrams illustrating forces that act on a head having no protruding portion when the head is at a position  9 , and  FIG. 25C  is a conceptual diagram showing the posture of this head at impact; 
         FIG. 26A  and  FIG. 26B  are conceptual diagrams illustrating forces that act on a head having a protruding portion when the head is at the position  9 , and  FIG. 26C  is a conceptual diagram showing the posture of this head at impact; 
         FIG. 27A  shows average values of head speeds (H/S) of respective testers 1 to 9, a left column of each tester showing the result of a club A (having no protruding portion), and a right column of each tester showing the result of a club B (having a protruding portion), and  FIG. 27B  shows average values of distances between hit points and a face center for the respective testers 1 to 9, a left column of each tester showing the result of the club A (having no protruding portion), and a right column of each tester showing the result of the club B (having a protruding portion); 
         FIG. 28A  shows average values of face angles of the respective testers 1 to 9, a left column of each tester showing the result of the club A, and a right column of each tester showing the result of the club B, and  FIG. 28B  shows average values of smash factors of the respective testers 1 to 9, the smash factor being calculated by dividing an initial velocity of a hit ball (B/S) by a head speed (H/S), a left column of each tester showing the result of the club A, and a right column of each tester showing the result of the club B; 
         FIG. 29A  shows standard deviations of head speeds (H/S) of the respective testers 1 to 9, a left column of each tester showing the result of the club A, and a right column of each tester showing the result of the club B, and  FIG. 29B  shows standard deviations of the distances between the hit points and the face center for the respective testers 1 to 9, a left column of each tester showing the result of the club A, and a right column of each tester showing the result of the club B; 
         FIG. 30  shows standard deviations of the face angles of the respective testers 1 to 9, a left column of each tester showing the result of the club A, and a right column of each tester showing the result of the club B; and 
         FIG. 31  is a conceptual diagram for illustrating a reference state. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (Findings as Basis for the Present Disclosure) 
     The toe-down phenomenon occurs because the center of gravity of a head is positioned apart from a shaft axis line. A centrifugal force acts on the center of gravity of a head during a swing. As shown in  FIG. 1 , the center of gravity CG of a head is positioned on a toe side with respect to its shaft axis line Z. For this reason, the centrifugal force bends the shaft such that a toe-side portion of the head moves downward. In addition, as shown in  FIG. 4 , the center of gravity CG of a head is positioned on a back side with respect to its shaft axis line Z. For this reason, the centrifugal force bends the shaft such that the a back-side portion of the head moves downward. Accordingly, the centrifugal force bends and twists the shaft such that the toe-side portion and the back-side portion of the head move downward. The shaft is bent such that the toe-side portion of the head moves downward and is twisted in a direction in which the face of the head is opened. This is known as the toe-down phenomenon. The greater the centrifugal force is, the greater the degree of the toe down is. The greater the distance between the center of gravity of a head and its shaft axis line is, the greater the degree of the toe down is. 
     As explained above, the centrifugal force moves the toe-side portion of the head downward and also the back-side portion of the head downward. These phenomena can be separately explained as toe down and back down. In the present disclosure, however, these are collectively referred to as toe down (phenomenon). 
     For suppressing the toe down, a club length may be reduced. This, however, reduces the kinetic energy of the head, thereby reducing flight distance. For suppressing the toe down, a head weight may be reduced. This, however, also reduces the kinetic energy of the head, thereby reducing flight distance. 
     For suppressing the toe down, the distance of the center of gravity of a head may be reduced and/or the depth of the center of gravity of the head may be shallowed. This, however, narrows an area having a high rebound performance, thereby reducing an average flight distance. In addition, the orientation of the face cannot be stabilized in such a head, whereby an average flight distance is reduced. 
     For suppressing the toe down, the flexural rigidity of the tip end portion of the shaft may be increased. This, however, lowers the trajectory of a hit ball, thereby reducing flight distance. 
     For reducing the effect of the toe down, a lie angle may be set to an upright lie angle and/or a face angle may set to a hook face angle. However, a golf club having an upright lie angle and a hook face angle is difficult to address. 
     A golf club that is easy to swing increases head speed. The increased head speed, however, increases the centrifugal force acting on the center of gravity of the head, which increases the degree of the toe down. 
     As described above, the factors that increase flight distance can also increase the degree of the toe down. An excessively great degree of the toe down results in inappropriate hit point or inappropriate impact angle of the head. In addition, such an excessively great degree of the toe down causes variation in degree of the toe down, whereby consistent hit points or consistent impact angles of the head cannot be obtained. Accordingly, the toe down tends to cause energy loss at impact. 
     The inventors of the present disclosure have found that, as explained above, even when a golf club has a feature(s) that can increase flight distance, a great toe down can reduce the flight distance. The inventors of the present disclosure have also found that both flight distance performance and suppression of the toe down can be achieved by suppressing the toe down with novel means. 
     Hereinafter, the present disclosure will be described in detail according to the preferred embodiments with appropriate references to the accompanying drawings. 
     In the present disclosure, a reference state, a reference perpendicular plane, a toe-heel direction, a face-back direction, an up-down direction, a face center, a heel projection posture, an inclined toe-heel direction and a heel projection figure are defined as follows. 
     The reference state is a state where a head is placed at a predetermined lie angle on a ground plane HP. As shown in  FIG. 31 , in the reference state, a shaft axis line Z is contained in a plane VP that is perpendicular to the ground plane HP. The shaft axis line Z is the center line of a shaft. The plane VP is defined as the reference perpendicular plane. The predetermined lie angle is shown in a product catalog, for example. 
     In the reference state, a face angle is 0°. That is, in a plan view of the head as viewed from above, a tangent line to the head at its face center on a striking face is set to be parallel to the toe-heel direction. The definitions of the face center and the toe-heel direction are as explained below. 
     In the present disclosure, the toe-heel direction is the direction of an intersection line NL between the reference perpendicular plane VP and the ground plane HP (see  FIG. 31 ). 
     In the present disclosure, the face-back direction is a direction that is perpendicular to the toe-heel direction and is parallel to the ground plane HP. A face side in the face-back direction is also simply referred to as “face side”. A back side in the face-back direction is also simply referred to as “back side”. 
     In the present disclosure, the up-down direction is a direction that is perpendicular to the toe-heel direction and is perpendicular to the face-back direction. In other words, the up-down direction in the present disclosure is a direction perpendicular to the ground plane HP. 
     In the present disclosure, the face center is determined in the following manner. First, a point Pr is selected roughly at the center of a striking face in the up-down direction and the toe-heel direction. Next, a plane that passes through the point Pr, extends in the direction of a line normal to the striking face at the point Pr, and is parallel to the toe-heel direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Px of this intersection line is determined. Next, a plane that passes through the midpoint Px, extends in the direction of a line normal to the striking face at the midpoint Px, and is parallel to the up-down direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Py of this intersection line is determined. Next, a plane that passes through the midpoint Py, extends in the direction of a line normal to the striking face at the midpoint Py, and is parallel to the toe-heel direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Px of this intersection line is newly determined. Next, a plane that passes through this newly-determined midpoint Px, extends in the direction of a line normal to the striking face at this midpoint Px, and is parallel to the up-down direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Py of this intersection line is newly determined. By repeating the above-described steps, points Px and Py are sequentially determined. In the course of repeating these steps, when the distance between a newly-determined midpoint Py and a midpoint Py determined in the immediately preceding step first becomes less than or equal to 0.5 mm, the newly-determined midpoint Py (the midpoint Py determined last) is defined as the face center. 
     The heel projection posture means a posture of a head placed such that the shaft axis line Z is perpendicular to the ground plane HP and the face angle is 0°. The heel projection posture is shown in  FIG. 2B  and  FIG. 5 . In the heel projection posture, a heel-side portion of a sole is positioned considerably apart from the ground plane HP, and a toe-side portion of the sole or a toe-side portion of a side portion (skirt portion) is in contact with the ground plane HP. The heel projection posture is obtained by rotating a head which is in the reference state until the shaft axis line Z becomes perpendicular to the ground plane HP. By this rotation, the toe-heel direction of the head is inclined with respect to the ground plane HP (See  FIG. 2A ). However, in a planar view in which the head is viewed from above, the toe-heel direction of the head is not changed by this rotation. That is, in the heel projection posture, the face angle remains 0°. 
     A vector that extends in the toe-heel direction of a head which is in the heel projection posture can be decomposed into two vectors (components): a vector V 1  that is parallel to the ground plane HP, and a vector V 2  that is perpendicular to the ground plane HP (see  FIG. 2B ). The direction of the vector V 1  parallel to the ground plane HP is defined as an inclined toe-heel direction. The inclined toe-heel direction is perpendicular to the shaft axis line Z. A heel side in the inclined toe-heel direction is also referred to as an inclined heel side. A toe side in the inclined toe-heel direction is also referred to as an inclined toe side. In  FIG. 2B , an inclined heel direction is indicated by S-heel, and an inclined toe direction is indicated by S-toe. 
     The heel projection figure is a projected figure in which a head which is in the heel projection posture is viewed from the heel side along the ground plane HP. In other words, the heel projection figure is a figure obtained by projecting a head which is in the heel projection posture to the heel side along the inclined toe-heel direction.  FIG. 5  is a heel projection figure. 
       FIG. 1  is an overall view of a golf club  2  that includes a head  4  according to one embodiment of the present disclosure.  FIG. 2A  is a front view of the head  4 .  FIG. 2A  shows the head  4  which is in the reference state as viewed from the face side.  FIG. 2B  shows the head  4  which is in the heel projection posture as viewed from the face side.  FIG. 3  is a plan view of the head  4  as viewed from the crown side.  FIG. 4  is a side view of the head  4  as viewed from the heel side.  FIG. 5  shows the head  4  as viewed from the inclined heel side.  FIG. 5  is the heel projection figure of the head  4 . 
     As shown in  FIG. 1 , the golf club  2  includes the golf club head  4 , a shaft  6 , and a grip  8 . The shaft  6  has a tip end Tp and a butt end Bt. The head  4  is attached to a tip end portion of the shaft  6 . The grip  8  is attached to a butt end portion of the shaft  6 . 
     The golf club  2  is a driver (No. 1 wood). Typically, the club as a driver has a length of greater than or equal to 43 inches. Preferably, the golf club  2  is a wood-type golf club. 
     The shaft  6  is in a tubular form. The shaft  6  is hollow. The material of the shaft  6  is a carbon fiber reinforced resin. From the viewpoint of reducing the weight, a carbon fiber reinforced resin is preferable as a material for the shaft  6 . The shaft  6  is a so-called carbon shaft. Preferably, the shaft  6  is formed with a cured prepreg sheet. In the prepreg sheet, fibers are substantially oriented in one direction. Such a prepreg in which fibers are substantially oriented in one direction is also referred to as UD prepreg. “UD” is an abbreviation of “unidirectional”. A prepreg other than the UD prepreg may be used. For example, fibers contained in the prepreg sheet may be woven. The shaft  6  may include a metal wire. The material of the shaft  6  is not limited, and may be a metal, for example. 
     The grip  8  is a part that a golfer grips during a swing. Examples of the material of the grip  8  include rubber compositions and resin compositions. The rubber composition for the grip  8  may contain air bubbles. 
     Although not shown in the drawings, the head  4  is hollow. In the present embodiment, the head  4  is a wood type head. The head  4  may be a hybrid type (utility type) head. The head  4  may be an iron type head. The head  4  may be a putter type head. Examples of a preferable material for the head  4  include metals and fiber reinforced plastics. Examples of the metals include titanium alloys, pure titanium, stainless steel, maraging steel, and soft iron. Examples of the fiber reinforced plastics include carbon fiber reinforced plastics. The head  4  may be a composite head including a portion made of a metal and a portion made of a fiber reinforced plastic. 
     As shown in  FIG. 2  to  FIG. 5 , the head  4  includes a face portion  10 , a crown portion  12 , a sole portion  14  and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The striking face  10   a  is also simply referred to as a face. The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a.    
     As shown in  FIG. 1  and  FIG. 4 , the head  4  has a center of gravity CG (hereinafter also referred to as head center of gravity CG). In the present embodiment, the head center of gravity CG is positioned inside (in a hollow portion of) the head  4 . 
     A double-pointed arrow B in  FIG. 1  shows a distance of the center of gravity (hereinafter also referred to as gravity center distance) of the head  4 . The gravity center distance B means a distance between the shaft axis line Z and the head center of gravity CG. The gravity center distance B is a distance measured in the front view of the head  4 , not a distance measured three-dimensionally. The shaft axis line Z and the head center of gravity CG of the head which is in the reference state is projected to the reference perpendicular plane VP. The gravity center distance B is measured in this projected figure. 
     A double-pointed arrow C in  FIG. 4  shows a depth of the center of gravity (hereinafter also referred to as gravity center depth) of the head  4 . The gravity center depth C is a distance between the shaft axis line Z and the head center of gravity CG. The gravity center depth C is measured in the face-back direction. 
     The striking face  10   a  has a face center Fc as defined above. 
     The head center of gravity CG of the head  4  is not positioned on the shaft axis line Z. The head center of gravity CG is positioned apart of the shaft axis line Z. The head  4  has the gravity center distance B and the gravity center depth C. These gravity center distance B and gravity center depth C are causes of the toe down phenomenon. 
     The crown portion  12  includes a protruding portion  20  on the crown outer surface  12   a . The protruding portion  20  is hollow. The protruding portion  20  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. 
     In the front view of the head as viewed from the face side (see  FIG. 2A ), the protruding portion  20  is not viewable. In the front view of the head as viewed from the face side (see  FIG. 2A ), the protruding portion  20  does not form any part of an outer contour line CL 1  of the head  4 . 
     In the present embodiment, the entirety of the protruding portion  20  is formed on the crown outer surface  12   a . As shown in  FIG. 3 , the head  4  has an outer contour line CL 2  in the plan view of the head  4 . As shown in  FIG. 3 , the protruding portion  20  does not reach the outer contour line CL 2 . The protruding portion  20  does not extend to other portions than the crown outer surface  12   a.    
     The plan view of the head  4  is a projected figure obtained by projecting the head which is in the reference state onto a plane parallel to the ground plane HP. This plan view ( FIG. 3 ) is also referred to as a planar view. 
     In the plan view ( FIG. 3 ) of the head  4 , the protruding portion  20  may reach the outer contour line CL 2 . In other words, the protruding portion  20  may form a part of the outer contour line CL 2 . The protruding portion  20  may extend into other portions than the crown outer surface  12   a . For example, the protruding portion  20  may extend from the crown outer surface  12   a  onto the sole outer surface  14   a . For example, the protruding portion  20  may extend from the crown outer surface  12   a  onto the outer surface of a side portion (skirt portion). 
     In the side view ( FIG. 4 ) of the head  4  which is in the reference state as viewed from the heel side in the toe-heel direction, the entirety of the protruding portion  20  can be seen. This side view shows an outer contour line CL 3  of the crown outer surface  12   a . In this side view, the protruding portion  20  does not reach the outer contour line CL 3 . The entirety of the protruding portion  20  is positioned on the heel side with respect to the face center Fc. A part of the protruding portion  20  may reach the toe side with respect to the face center Fc. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  20  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. The convex curved surface is a curved surface that is convex toward the outside of the head  4 . As shown in  FIG. 3 , the crown base surface  12   b  includes a geometric center CR of the head  4  in the plan view. The geometric center CR is the geometric center of a figure indicated by the outer contour line CL 2 . 
       FIG. 6  shows a part of an outer contour line of the head  4  as viewed from the toe side.  FIG. 7  shows a cross-sectional contour line of the outer surface of the head  4  in a cross-sectional view taken along line A-A in  FIG. 3 .  FIG. 8  shows a cross-sectional contour line of the outer surface of the head  4  in a cross-sectional view taken along line B-B in  FIG. 3 .  FIG. 9  shows a cross-sectional contour line of the outer surface of the head  4  in a cross-sectional view taken along line C-C in  FIG. 3 .  FIG. 10  shows a cross-sectional contour line of the outer surface of the head  4  in a cross-sectional view taken along line D-D in  FIG. 3 . Each of  FIG. 7  to  FIG. 10  includes a cross-sectional contour line of the crown outer surface  12   a.    
     The protruding portion  20  includes a contour line CL 20 , an upper surface  22 , and a sidewall surface  24 . The contour line CL 20  is a boundary line between the crown base surface  12   b  and the protruding portion  20 . In the plan view of the head  4  ( FIG. 3 ), the contour line CL 20  of the protruding portion  20  has a substantially quadrilateral shape (substantially trapezoidal shape). In the present disclosure, the word “substantially” means that a shape in question may have a curved side(s) (not straight side(s)) and/or a rounded angle(s). In the contour line CL 20  in the plan view of the head ( FIG. 3 ), the radius of curvature of the curved side(s) is preferably greater than or equal to 25 mm, more preferably greater than or equal to 40 mm, and still more preferably greater than or equal to 50 mm. In the contour line CL 20  in the plan view of the head ( FIG. 3 ), the radius of curvature of the rounded angle(s) is preferably less than or equal to 10 mm, more preferably less than or equal to 7 mm, and still more preferably less than or equal to 5 mm. The contour line CL 20  forms the substantially quadrilateral shape. 
     The boundary between the upper surface  22  and the sidewall surface  24  can be defined by a ridgeline. In a cross-sectional contour line of the outer surface of the protruding portion  20 , the ridgeline can be specified as a vertex of an angle or a point having a radius of curvature of less than or equal to 5 mm. Although the radius of curvature of the cross-sectional contour line of the outer surface of the protruding portion  20  can vary depending on the direction of the cross section, a cross section that has the smallest radius of curvature is selected for determining the radius of curvature to specify the ridgeline. 
     In the plan view (planar view) of the head  4 , the protruding portion  20  can have a substantially polygonal shape. When this substantially polygonal shape is defined as a substantially N-sided polygonal shape, N can be an integer of greater than or equal to 3. N may be an integer that is greater than or equal to 3 and less than or equal to 20. 
     The contour line CL 20  has a first side CL 21 , a second side CL 22 , a third side CL 23  and a fourth side CL 24 . The first side CL 21  constitutes a side on the toe-face side of the protruding portion  20 . The first side CL 21  extends toward the back side as it goes to the toe side. The first side CL 21  connects the second side CL 22  and the fourth side CL 24 . 
     The second side CL 22  constitutes a side on the heel-face side of the protruding portion  20 . The second side CL 22  extends toward the back side as it goes to the heel side. The second side CL 22  connects the first side CL 21  and the third side CL 23 . 
     The third side CL 23  constitutes a side on the heel-back side of the protruding portion  20 . The third side CL 23  extends toward the back side as it goes to the toe side. The third side CL 23  connects the second side CL 22  and the fourth side CL 24 . The third side CL 23  constitutes a curved line that projects toward the outside of the head  4 . 
     The fourth side CL 24  constitutes a side on the toe-back side of the protruding portion  20 . The fourth side CL 24  extends toward the back side as it goes to the heel side. The fourth side CL 24  connects the third side CL 23  and the first side CL 21 . 
     The second side CL 22 , the third side CL 23 , and the fourth side CL 24  constitute a starting line of the sidewall surface  24 . That is, the second side CL 22 , the third side CL 23 , and the fourth side CL 24  constitute the boundary line between the sidewall surface  24  and the crown base surface  12   b . On the other hand, the first side CL 21  does not constitute a starting line of the sidewall surface  24 . The first side CL 21  constitutes the boundary line between the crown base surface  12   b  and the upper surface  22 . 
     In the present disclosure, a cross-sectional contour line in a cross section taken along the toe-heel direction is also simply referred to as a t-h cross-sectional contour line.  FIG. 7  shows one example of the t-h cross-sectional contour line. The t-h cross-sectional contour line of the crown outer surface  12   a  is also referred to as a crown t-h cross-sectional contour line.  FIG. 7  includes the crown t-h cross-sectional contour line. In the present disclosure, a cross-sectional contour line in a cross section taken along the face-back direction is also simply referred to as a f-b cross-sectional contour line.  FIG. 9  shows one example of the f-b cross-sectional contour line. The f-b cross-sectional contour line of the crown outer surface  12   a  is also referred to as a crown f-b cross-sectional contour line.  FIG. 9  includes the crown f-b cross-sectional contour line. 
     An inflection point of the crown t-h cross-sectional contour line can be a point that forms the contour line CL 20 . In other words, this inflection point can be a starting point of the protruding portion  20 . The t-h cross-sectional contour line of the crown base surface  12   b  is a curved line that projects toward the outside of the head  4 . The inflection point is a point at which the curved line that projects toward the outside of the head  4  changes into a curved line that projects toward the inside of the head  4 . 
     A vertex of an angle of the crown t-h cross-sectional contour line can be a point that forms the contour line CL 20 . In other words, this vertex can be a starting point of the protruding portion  20 . The t-h cross-sectional contour line of the crown base surface  12   b  is a curved line that projects toward the outside of the head  4 . A line that is connected to this curved line, bends, and extends toward the outside of the head  4  forms a vertex. This vertex points toward the inside of the head  4 . This vertex can be a starting point of the protruding portion  20 . 
     An inflection point of the crown f-b cross-sectional contour line can be a point that forms the contour line CL 20 . In other words, this inflection point can be a starting point of the protruding portion  20 . The f-b cross-sectional contour line of the crown base surface  12   b  is a curved line that projects toward the outside of the head  4 . The inflection point is a point at which the curved line that projects toward the outside of the head  4  changes into a curved line that projects toward the inside of the head  4 . 
     A vertex of an angle of the crown f-b cross-sectional contour line can be a point that forms the contour line CL 20 . In other words, this vertex can be a starting point of the protruding portion  20 . The f-b cross-sectional contour line of the crown base surface  12   b  is a curved line that projects toward the outside of the head  4 . A line that is connected to this curved line, bends, and extends toward the outside of the head  4  forms a vertex. This vertex points toward the inside of the head  4 . This vertex can be a starting point of the protruding portion  20 . 
     Typically, the contour line CL 20  can be determined by the inflection points or the vertices. For determining the contour line CL 20 , the crown t-h cross-sectional contour line may be selected in preference to the crown f-b cross-sectional contour line. In this case, the crown t-h cross-sectional contour line is used for specifying the inflection point or the vertex. When it is difficult to specify the inflection point or the vertex by using the crown t-h cross-sectional contour line, the crown f-b cross-sectional contour line can be used. When the contour line of the protruding portion  20  can be visually and clearly recognized, the contour line can be determined as the contour line CL 20 . 
     The protruding portion  20  is a portion that protrudes from the crown base surface  12   b . A virtually extended surface  12   c  that is obtained by extending the crown base surface  12   b  can be specified on the lower side of the protruding portion  20 . The protruding portion  20  is a portion that protrudes relative to the virtually extended surface  12   c . The virtually extended surface  12   c  can be considered as a part of the crown base surface  12   b  formed in a region in which the protruding portion  20  is formed if the protruding portion  20  is not present. The virtually extended surface  12   c  is formed so as to be continuous with the crown base surface  12   b . The virtually extended surface  12   c  is a curved surface that is convex toward the outside of the head  4 . The virtually extended surface  12   c  is smoothly continuous with the crown base surface  12   b.    
       FIG. 11  is an enlarged view of a portion that is surrounded by a tetragon Q 1  in  FIG. 7 .  FIG. 12  is an enlarged view of a portion that is surrounded by a tetragon Q 2  in  FIG. 9 . 
       FIG. 11  shows the crown t-h cross-sectional contour line with a virtually extended line  12   d  that can form the virtually extended surface  12   c . The virtually extended line  12   d  is a curved surface that is convex toward the outside of the head  4 . The virtually extended line  12   d  is smoothly continuous with the t-h cross-sectional contour line of the crown base surface  12   b . The virtually extended surface  12   c  can be formed by a set of virtually extended lines  12   d.    
     The virtually extended line  12   d  smoothly connects one side end of the t-h cross-sectional contour line of the protruding portion  20  and the other side end of the t-h cross-sectional contour line of the protruding portion  20 . The virtually extended line  12   d  can be drawn as a Bezier curve. A quadratic Bezier curve and a cubic Bezier curve are known as the Bezier curve. In the quadratic Bezier curve, the number of control points is one (excluding a starting point and an end point). In the cubic Bezier curve, the number of control points is two (excluding a starting point and an end point). The cubic Bezier curve is preferably used. Bezier curves drawn in  FIG. 11  and  FIG. 12  are cubic Bezier curves. 
     As shown in  FIG. 11 , the crown t-h cross-sectional contour line has a first starting point P 1  and a second starting point P 2 . The first starting point P 1  and the second starting point P 2  are located on the contour line CL 20 . 
     Points P 11  and P 12  that are located on the opposite side of the first starting point P 1  from the protruding portion  20  are plotted in order to define an effective tangent line to the crown t-h cross-sectional contour line at the first starting point P 1 . The point P 11  is a point located 0.5 mm apart from the first starting point P 1 . The point P 12  is a point located 0.5 mm apart from the point P 11 . “0.5 mm” for these points is a route length measured along the crown t-h cross-sectional contour line. The points P 11  and P 12  are located on the crown t-h cross-sectional contour line. A tangent line L 1  to a circle that passes through these three points P 1 , P 11  and P 12  at the point P 1  is determined. When the points P 1 , P 11  and P 12  are positioned on a single straight line, this straight line can be determined as the tangent line L 1 . 
     Similarly, points P 21  and P 22  that are located on the opposite side of the second starting point P 2  from the protruding portion  20  are plotted in order to define an effective tangent line to the crown t-h cross-sectional contour line at the second starting point P 2 . The point P 21  is a point located 0.5 mm apart from the second starting point P 2 . The point P 22  is a point located 0.5 mm apart from the point P 21 . “0.5 mm” for these points is a route length measured along the crown t-h cross-sectional contour line. The points P 21  and P 22  are located on the crown t-h cross-sectional contour line. A tangent line L 2  to a circle that passes through these three points P 2 , P 21  and P 22  at the point P 2  is determined. When the points P 2 , P 21  and P 22  are positioned on a single straight line, this straight line can be determined as the tangent line L 2 . 
     When the tangent line L 1  and the tangent line L 2  are determined, then an intersection point Px between the tangent line L 1  and the tangent line L 2  is specified. Furthermore, a middle point M 1  between the point P 1  and the point Px is specified, and a middle point M 2  between the point P 2  and the point Px is specified. 
     A Bezier curve can be drawn by using the point P 1  as the starting point, the middle point M 1  as the first control point, the middle point M 2  as the second control point, and the point P 2  as the end point. In  FIG. 11 , a Bezier curve drawn in this manner is the virtually extended line  12   d . Because of having two control points, this Bezier curve is a cubic Bezier curve. 
     The virtually extended line  12   d  can be defined at any position in the face-back direction. The virtually extended surface  12   c  can be defined as the set of these virtually extended lines  12   d.    
     A similar Bezier curve can be defined in the crown f-b cross-sectional contour line. As shown in  FIG. 12 , the crown f-b cross-sectional contour line has a first starting point P 1  and a second starting point P 2 . The first starting point P 1  and the second starting point P 2  are located on the contour line CL 20 . 
     Points P 11  and P 12  that are located on the opposite side of the first starting point P 1  from the protruding portion  20  are plotted in order to define an effective tangent line to the crown f-b cross-sectional contour line at the first starting point P 1 . The point P 11  is a point located 0.5 mm apart from the first starting point P 1 . The point P 12  is a point located 0.5 mm apart from the point P 11 . “0.5 mm” for these points is a route length measured along the crown f-b cross-sectional contour line. The points P 11  and P 12  are located on the crown f-b cross-sectional contour line. A tangent line L 1  to a circle that passes through these three points P 1 , P 11  and P 12  at the point P 1  is determined. When the points P 1 , P 11  and P 12  are positioned on a single straight line, this straight line can be determined as the tangent line L 1 . 
     Similarly, points P 21  and P 22  that are located on the opposite side of the second starting point P 2  from the protruding portion  20  are plotted in order to define an effective tangent line to the crown f-b cross-sectional contour line at the second starting point P 2 . The point P 21  is a point located 0.5 mm apart from the second starting point P 2 . The point P 22  is a point located 0.5 mm apart from the point P 21 . “0.5 mm” for these points is a route length measured along the crown f-b cross-sectional contour line. The points P 21  and P 22  are located on the crown f-b cross-sectional contour line. A tangent line L 2  to a circle that passes through these three points P 2 , P 21  and P 22  at the point P 2  is determined. When the points P 2 , P 21  and P 22  are positioned on a single straight line, this straight line can be determined as the tangent line L 2 . 
     When the tangent line L 1  and the tangent line L 2  are determined, then an intersection point Px between the tangent line L 1  and the tangent line L 2  is specified. Furthermore, a middle point M 1  between the point P 1  and the point Px is specified, and a middle point M 2  between the point P 2  and the point Px is specified. 
     A Bezier curve can be drawn by using the point P 1  as the starting point, the middle point M 1  as the first control point, the middle point M 2  as the second control point, and the point P 2  as the end point. In  FIG. 12 , a Bezier curve drawn in this manner is a virtually extended line  12   e.    
     The virtually extended line  12   e  can be defined at any position in the toe-heel direction. The virtually extended surface  12   c  can be defined as the set of these virtually extended lines  12   e.    
     In another embodiment, the protruding portion may reach an outer peripheral edge (outer contour line CL 4 ) of the crown portion (see  FIG. 19B  explained below). In this case, the number of the starting point(s) of the protruding portion which is/are formed on the boundary between the protruding portion and the crown base surface  12   b  can be only one in the crown t-h cross-sectional contour line and/or the crown f-b cross-sectional contour line. When only one starting point is present as in this case, a circular arc that is drawn so as to path through the starting point and have a radius of curvature at the starting point can be the virtually extended line  12   d . That is, in this case, the virtually extended line  12   d  can be a circle that passes through the following three points: a first point that is the starting point; a second point located 0.5 mm apart from the first point; and a third point located 0.5 mm apart from the second point. 
     For determining the virtually extended surface  12   c , the crown t-h cross-sectional contour line may be used in preference to the crown f-b cross-sectional contour line. The virtually extended surface  12   c  can be determined as a set of the virtually extended lines  12   d  obtained from the crown t-h cross-sectional contour lines. When the virtually extended surface  12   c  is not clearly determined by the set of the virtually extended lines  12   d , the virtually extended surface  12   c  may be determined as a set of the virtually extended lines  12   e  obtained from the crown f-b cross-sectional contour lines. 
     A height H 1  of the protruding portion  20  can be defined as a height from the virtually extended surface  12   c . As shown in  FIG. 11 , a normal line LN that is normal to the virtually extended surface  12   c  at a certain point f 1  has an intersection point f 2  at which the normal line LN intersects the outer surface of the protruding portion  20 . A distance between the point f 1  and the intersection point f 2  can be defined as the height H 1  of the protruding portion  20  at the intersection point f 2 . If the protruding portion does not intersect the normal line LN of the virtually extended surface  12   c  and has a point at which the protruding portion intersects a normal line that is normal to the crown base surface  12   b , the height H 1  of the protruding portion at the point is defined as a height from the crown base surface  12   b . Also in this case, the length of the normal line is the height H 1 . 
       FIG. 13A  shows a silhouette of the heel projection figure in  FIG. 5 .  FIG. 13B  shows a part of the contour line of the silhouette. The contour line of the silhouette is an outer contour line CL 6  of the heel projection figure of the head  4 .  FIG. 13B  is a part of the outer contour line CL 6  of the heel projection figure of the head  4 . 
     In the heel projection figure of the head  4 , the outer contour line CL 6  of the crown outer surface includes a protuberance  30 . The protuberance  30  is also referred to as a silhouette protuberance. As described above, the protruding portion  20  can be seen in the heel projection figure ( FIG. 5 ). The silhouette protuberance  30  is formed by the protruding portion  20 . A silhouette area S 1  is enlarged by the presence of the silhouette protuberance  30 . That is, the protruding portion  20  enlarges the silhouette area S 1  of the heel projection figure. The silhouette area S 1  means the area of the figure formed by the outer contour line CL 6  of the heel projection figure, which is also the area of the silhouette shown in  FIG. 13A . 
     An inflection point of the outer contour line CL 6  of the heel projection figure can be a starting point of the silhouette protuberance. A vertex of an angle of the outer contour line CL 6  of the heel projection figure can be a starting point of the silhouette protuberance. In the present embodiment, vertices, not inflection points, located on both sides of the silhouette protuberance  30  are the starting points of the silhouette protuberance  30 . As shown in  FIG. 13B , in the silhouette protuberance  30  of the present embodiment, vertices P 31  and P 32  of angles are the starting points of the silhouette protuberance  30 . 
     A cubic Bezier curve can be drawn also for the silhouette protuberance  30  in the same manner as discussed above. A two-dot chain line in  FIG. 13B  shows the Bezier curve. This Bezier curve is a curved line that smoothly connects curved lines adjacent to both ends of the silhouette protuberance  30 . This Bezier curve can be a virtual contour line  30   a  of the heel projection figure when the protruding portion  20  is not present. An area of a portion surrounded by the contour line of the silhouette protuberance  30  and the virtual contour line  30   a  is defined as an additional area S 2  added by the protruding portion  20 . In the present embodiment, the additional area S 2  is the area of a portion indicated with hatching in  FIG. 13B . Of the silhouette area S 1 , an area increased by the protruding portion  20  is the additional area S 2 . 
     Although the protruding portion forms a part of the outer contour line CL 6  of the head in the heel projection figure in this case, there is a case where the silhouette protuberance is not formed. For example, when the protruding portion reaches the outer peripheral edge (outer contour line CL 4 ) of the crown portion and extends along the outer peripheral edge, the silhouette protuberance might not be formed. However, also in such a case, the protruding portion is viewable in the heel projection figure, and increases the silhouette area S 1 . That is, also in this case, the additional area S 2  added by the protruding portion is present. For example, a silhouette area S 11  of a head in which the protruding portion is replaced by the virtually extended surface  12   c  and the protruding portion is removed, and a silhouette area S 12  of a head having the protruding portion can be considered. An area (S 12 -S 11 ) can be considered as the additional area S 2 . 
     From the viewpoint of suppression of the toe down and stabilization, the additional area S 2  is preferably greater than or equal to 30 mm 2 , more preferably greater than or equal to 50 mm 2 , and still more preferably greater than or equal to 100 mm 2 . From the viewpoint of lowering air resistance at a position  6 , there is a limit on the height and the volume of the protruding portion. From this viewpoint, the additional area S 2  is preferably less than or equal to 500 mm 2 , more preferably less than or equal to 400 mm 2 , and still more preferably less than or equal to 300 mm 2 . 
     From the viewpoint of suppression of the toe down and stabilization, a ratio (S 2 /S 1 ) is preferably greater than or equal to 0.005, more preferably greater than or equal to 0.008, and still more preferably greater than or equal to 0.015. From the viewpoint of lowering air resistance at the position  6 , there is a limit on the height and the volume of the protruding portion. From this point of view, the ratio (S 2 /S 1 ) is preferably less than or equal to 0.10, more preferably less than or equal to 0.08, and still more preferably less than or equal to 0.06. S 2 /S 1  is the ratio of the additional area S 2  to the silhouette area S 1 . 
     With reference to  FIG. 6 , an intersection line PL between the crown outer surface  12   a  and a plane forms a closed figure. This plane is referred to as a crown cut plane. In  FIG. 6 , this crown cut plane CP 1  is indicated by a two-dot chain line. Although not shown in the side view of  FIG. 6 , in the planar view, the intersection line PL between the crown outer surface  12   a  and the crown cut plane CP 1  forms a closed figure on the crown cut plane CP 1 . The intersection line PL is a line having an endless annular shape. Since  FIG. 6  is a side view, the intersection line PL is shown as points. 
     The crown outer surface  12   a  is cut off by the crown cut plane CP 1 . An object whose outer surface is constituted by the cutoff crown outer surface  12   a  and the crown cut plane CP 1  is referred to as a cutoff object. The volume of the cutoff object is referred to as a cutoff volume. The length of the intersection line PL is defined as L (mm), and the cutoff volume is defined as V (mm 3 ). The length L is the length of the intersection line PL measured as it is. In other words, the length L is the route length of the intersection line PL. For example, when the protruding portion has a conical shape, and the crown cut plane CP 1  cuts the conical shape so that the intersection line PL forms a circle, then the length L is the length of the circumference of the circle. In the embodiment of  FIG. 3 , the intersection line PL can have an endless substantially quadrilateral shape. In this case, the length L is the route length of the intersection line PL having this substantially quadrilateral shape. 
     A ratio (V/L) can be an index indicating the degree of protrusion of the crown outer surface  12   a . The greater the ratio (V/L) is, the greater the degree of protrusion is. The crown outer surface  12   a  preferably includes a portion having a ratio (V/L) of greater than a threshold X. In other words, it is preferable that a crown cut plane CP 1  can be set in the crown outer surface  12   a  such that the ratio (V/L) is greater than the threshold X. 
     A portion having a ration (V/L) of greater than the threshold X can forms at least a part of the protruding portion  20 . Preferably, the entirety of the intersection line PL in which the ratio (V/L) is greater than the threshold X constitutes the intersection line between the protruding portion  20  and the crown cut plane CP 1 . In other words, it is preferable that a crown cut plane CP 1  can be set in the protruding portion  20  such that the ratio (V/L) is greater than the threshold X. The crown cut plane CP 1  shown in  FIG. 6  is also set at a position where the entirety of the intersection line PL is the intersection line between the protruding portion  20  and the crown cut plane CP 1 . The ratio of the maximum value of the cutoff volume V to the volume of the protruding portion  20  when the entirety of the intersection line PL is the intersection line between the protruding portion  20  and the crown cut plane CP 1  is preferably greater than or equal to 50%, more preferably greater than or equal to 60%, and still more preferably greater than or equal to 70%. Note that the volume of the protruding portion  20  can be considered as the volume of a portion cut off by the virtually extended surface  12   c . When the entirety of the intersection line PL is the intersection line between the protruding portion  20  and the crown cut plane CP 1 , the crown cut plane CP 1  may also intersect the virtually extended surface  12   c.    
     From the viewpoint of increasing the degree of protrusion of the protruding portion  20  and increasing the additional area S 2  of the heel projection figure, the threshold X is preferably greater than or equal to 20, more preferably greater than or equal to 30, and still more preferably greater than or equal to 40. An excessively great degree of protrusion may cause a golfer to feel a sense of incongruity in the shape of the head. From this viewpoint, the threshold X is preferably less than or equal to 500, more preferably less than or equal to 450, and still more preferably less than or equal to 400. 
       FIG. 14  is a plan view of a head  40  according to a second embodiment. The difference between the head  40  and the above-described head  4  is only the shape of the protruding portion. 
     The head  40  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes a protruding portion  50  on the crown outer surface  12   a . The protruding portion  50  is hollow. The protruding portion  50  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. 
     As with the head  4 , in the head  40 , the protruding portion  50  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  50  is provided on the crown outer surface  12   a . The head  40  has an outer contour line CL 2  in the plan view (planar view) of the head  40 . The protruding portion  50  does not reach the outer contour line CL 2 . The protruding portion  50  does not extend to other portions than the crown outer surface  12   a . The entirety of the protruding portion  50  is positioned on the heel side with respect to the face center. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  50  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. The convex curved surface is a curved surface that is convex toward the outside of the head  40 . 
     The protruding portion  50  includes a contour line CL 50 , an upper surface  52 , and a sidewall surface  54 . The contour line CL 50  is a boundary line between the crown base surface  12   b  and the protruding portion  50 . In the plan view of the head  40 , the protruding portion  50  has a substantially quadrilateral shape (substantially trapezoidal shape). The contour line CL 50  forms the substantially quadrilateral shape. The contour line CL 50  has a first side CL 51 , a second side CL 52 , a third side CL 53 , and a fourth side CL 54 . 
     The first side CL 51  constitutes a side on the face side of the protruding portion  50 . The first side CL 51  extends toward the back side as it goes to the toe side. The first side CL 51  connects the second side CL 52  and the fourth side CL 54 . 
     The second side CL 52  constitutes a side on the heel side of the protruding portion  50 . The second side CL 52  extends toward the back side as it goes to the toe side. The second side CL 52  connects the first side CL 51  and the third side CL 53 . The second side CL 52  constitutes a curved line that projects toward the outside of the head  40 . 
     The third side CL 53  constitutes a side on the back side of the protruding portion  50 . The third side CL 53  extends toward the back side as it goes to the toe side. The third side CL 53  connects the second side CL 52  and the fourth side CL 54 . 
     The fourth side CL 54  constitutes a side on the toe side of the protruding portion  50 . The fourth side CL 54  extends toward the back side as it goes to the toe side. The fourth side CL 54  connects the third side CL 53  and the first side CL 51 . 
     The first side CL 51 , the second side CL 52 , and the third side CL 53  constitute a starting line of the sidewall surface  54 . That is, the first side CL 51 , the second side CL 52 , and the third side CL 53  constitute the boundary line between the sidewall surface  54  and the crown base surface  12   b . On the other hand, the fourth side CL 54  does not constitute a starting line of the sidewall surface  54 . The fourth side CL 54  constitutes the boundary line between the crown base surface  12   b  and the upper surface  52 . 
       FIG. 15  is a plan view of a head  60  according to a third embodiment. The difference between the head  60  and the above-described head  4  is only the shape of the protruding portion. 
     The head  60  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes a protruding portion  70  on the crown outer surface  12   a . The protruding portion  70  is hollow. The protruding portion  70  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. 
     As with the head  4 , in the head  60 , the protruding portion  70  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  70  is provided on the crown outer surface  12   a . The head  60  has an outer contour line CL 2  in the plan view (planar view) of the head  60 . The protruding portion  70  does not reach the outer contour line CL 2 . The entirety of the protruding portion  70  is located on the heel side with respect to the face center. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  70  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The protruding portion  70  includes a contour line CL 70 , an upper surface  72 , and a sidewall surface  74 . The contour line CL 70  is a boundary line between the crown base surface  12   b  and the protruding portion  70 . In the plan view (planar view) of the head  60 , the protruding portion  70  has a substantially pentagonal shape. The contour line CL 70  forms the substantially pentagonal shape. The sidewall surface  74  is formed along all sides constituting this substantially pentagonal shape. Although not visually recognized from the viewing angle in  FIG. 15 , the sidewall surface  74  is formed also along a side closest to the outer contour line CL 2 . 
       FIG. 16  is a plan view of a head  80  according to a fourth embodiment. The difference between the head  80  and the above-described head  4  is only the shape of the protruding portion. 
     The head  80  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes a protruding portion  90  on the crown outer surface  12   a . The protruding portion  90  is hollow. The protruding portion  90  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. 
     As with the head  4 , in the head  80 , the protruding portion  90  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  90  is provided on the crown outer surface  12   a . The head  80  has an outer contour line CL 2  in the plan view (planar view) of the head  80 . The protruding portion  90  does not reach the outer contour line CL 2 . The entirety of the protruding portion  90  is positioned on the heel side with respect to the face center. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  90  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The protruding portion  90  includes a contour line CL 90 , an upper surface  92 , and a sidewall surface  94 . The contour line CL 90  is a boundary line between the crown base surface  12   b  and the protruding portion  90 . In the plan view of the head  80 , the protruding portion  90  has a substantially quadrilateral shape. The contour line CL 90  forms the substantially quadrilateral shape. The sidewall surface  94  is formed along all sides constituting this substantially quadrilateral shape. Although not visually recognized from the viewing angle in  FIG. 16 , the sidewall surface  94  is formed also along a side closest to the outer contour line CL 2 . 
       FIG. 17  is a plan view (planar view) of a head  100  according to a fifth embodiment. The difference between the head  100  and the above-described head  4  is only the shape of the protruding portion. 
     The head  100  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes a protruding portion  110  on the crown outer surface  12   a . The protruding portion  110  is hollow. The protruding portion  110  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. 
     As with the head  4 , in the head  100 , the protruding portion  110  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  110  is provided on the crown outer surface  12   a . The head  100  has an outer contour line CL 2  in the plan view (planar view) of the head  100 . The protruding portion  110  does not reach the outer contour line CL 2 . The entirety of the protruding portion  110  is positioned on the heel side with respect to the face center. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  110  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The protruding portion  110  includes a contour line CL 110 , a ridgeline  112  formed by vertices, and sidewall surfaces  114 . The ridgeline  112  is formed by the sidewall surfaces  114  intersecting with each other. The protruding portion  110  does not have an upper surface. The contour line CL 110  is a boundary line between the crown base surface  12   b  and the protruding portion  110 . The protruding portion  110  is constituted by one ridgeline  112  and two sidewall surfaces  114 . The protruding portion  110  forms a projection having a ridgeline. 
       FIG. 18  is a plan view of a head  120  according to a sixth embodiment. The difference between the head  120  and the above-described head  4  is only the shape of the protruding portion. 
     The head  120  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes a protruding portion  130  on the crown outer surface  12   a . The protruding portion  130  is hollow. The protruding portion  130  forms projections on the crown outer surface  12   a  and forms recesses on a crown inner surface. 
     As with the head  4 , in the head  120 , the protruding portion  130  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  130  is provided on the crown outer surface  12   a . The head  120  has an outer contour line CL 2  in the plan view (planar view) of the head  120 . The protruding portion  130  does not reach the outer contour line CL 2 . The entirety of the protruding portion  130  is positioned on the heel side with respect to the face center. 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  130  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The protruding portion  130  is divided into a plurality of (two) parts. The protruding portion  130  has a first part  132  and a second part  134 . The first part  132  and the second part  134  are positioned apart from each other. A parting groove  136  is formed between the first part  132  and the second part  134 . This parting groove  136  extends in a bending manner. 
       FIG. 19A  shows a part of an outer contour line of a head  140  according to a seventh embodiment as viewed from the toe-back side. The head  140  includes a protruding portion  150 . The protruding portion  150  is the same as the protruding portion  20  of the first embodiment except that a part of the sidewall surface  24  formed along the third side CL 23  of the contour line CL 20  is recessed. In the head  140 , a space SP is formed between a highest portion  152  of the protruding portion  150  and the crown outer surface  12   a . The highest portion  152  is a portion whose height H 1  is the maximum. The definition of the height H 1  is as described above. From the viewpoint of increasing aerodynamic drag at a position  9 , the space SP is preferably provided on a contour proximate wall surface CW (detailed later). 
       FIG. 19B  shows a part of an outer contour line of a head  154  according to an eighth embodiment as viewed from the toe-back side. The head  154  includes a protruding portion  156 . The protruding portion  156  reaches the outer peripheral edge (outer contour line CL 4 ) of the crown portion. In a planar view of the head  154 , a part of the contour line of the protruding portion  156  coincides with the outer contour line CL 4  of the crown portion. 
       FIG. 20A  is a perspective view of a head  160  according to a ninth embodiment.  FIG. 20B  is a cross-sectional view taken along line b-b in  FIG. 20A .  FIG. 21A  is a perspective view of a head body  160   h  of the head  160 .  FIG. 21B  is a cross-sectional view taken along line b-b in  FIG. 21A . With regard to the head body in  FIG. 20B  and  FIG. 21B , the depiction of its cross section is omitted, and only the cross-sectional contour line of its outer surface is shown. 
     The head  160  includes a head body  160   h , a protruding portion  170 , and a fixing jig  172 . The protruding portion  170  is attachable to and detachable from the head body  160   h . The protruding portion  170  is formed by a protruding member  174  that is a member different from the head body. The protruding member  174  is detachably fixed to the head body  160   h  with the fixing jig  172 . 
     The head body  160   h  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The striking face  10   a  has a face center Fc as defined above. The crown portion  12  includes the protruding portion  170  on the crown outer surface  12   a . The protruding portion  170  is formed by the protruding member  174 . The protruding member  174  is detachably fixed to the crown outer surface  12   a.    
     As with the head  4 , in the head  160 , the protruding portion  170  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  170  is provided on the crown outer surface  12   a . The head  160  has an outer contour line CL 2  in a plan view (planar view) of the head  160 . 
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  170  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The head body  160   h  includes a port  162 . In the present embodiment, the port  162  constitutes a screw hole that forms a female screw. In the present embodiment, the fixing jig  172  is a male screw. The fixing jig  172  can be screw-connected to the port  162 . In  FIG. 20B , the depiction of screw threads in the screw portions of the port  162  and the fixing jig  172  is omitted. 
     The protruding member  174  includes a base portion  174   a  and a standing wall portion  174   b  that extends upward from the base portion  174   a . The standing wall portion  174   b  is formed at an edge of the base portion  174   a . In the planar view, the protruding member  174  has a substantially polygonal shape (substantially quadrilateral shape). In the planar view, the protruding member  174  has a plurality of (four) sides. The standing wall portion  174   b  is formed on one side of the plurality of (four) sides. The base portion  174   a  has a through hole  174   c  through which the fixing jig  172  is inserted. 
     As described above, in the present embodiment, the protruding member  174  is fixed by screwing with the fixing jig  172 . The structure (means) for fixing the protruding member  174  is not limited to screwing. 
     This structure having the standing wall portion  174   b  can increase the additional area S 2  while suppressing air resistance at the position  6 . The standing wall portion  174   b  is formed only on a contour proximate side CS (detailed later). The standing wall portion  174   b  constitutes the contour proximate wall surface CW (detailed later). The standing wall portion  174   b  effectively increases the additional area S 2 . 
       FIG. 22A  is a perspective view of a head  180  according to a tenth embodiment.  FIG. 22B  is a cross-sectional view taken along line b-b in  FIG. 22A .  FIG. 22C  is a cross-sectional view taken along line c-c in  FIG. 22A .  FIG. 23A  is a perspective view of a head body  180   h  of the head  180 .  FIG. 23B  is a cross-sectional view taken along line b-b in  FIG. 23A .  FIG. 23C  is a cross-sectional view taken along line c-c in  FIG. 23A . With regard to the head body in  FIG. 22B ,  FIG. 22C ,  FIG. 23B , and  FIG. 23C , the depiction of its cross section is omitted, and only the cross-sectional contour line of its outer surface is shown. 
     The head  180  includes the head body  180   h , a protruding portion  190 , and a fixing jig  192 . The fixing jig  192  includes a screw member  194  and a screw hole member  196 . The protruding portion  190  is attachable to and detachable from the head body  180   h . The protruding portion  190  is formed by a protruding member  198  that is a member different from the head body  180   h . The protruding member  198  is detachably fixed to the head body  180   h  with the fixing jig  192 . 
     The head body  180   h  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The hosel portion  16  has a shaft hole  16   a . The crown portion  12  includes the protruding portion  190  on the crown outer surface  12   a . The protruding portion  190  is formed by the protruding member  198 . The protruding member  198  is detachably fixed to the crown outer surface  12   a.    
     As with the head  4 , in the head  180 , the protruding portion  190  is not viewable in the front view of the head as viewed from the face side. The entirety of the protruding portion  190  is provided on the crown outer surface  12   a.    
     The crown outer surface  12   a  includes a crown base surface  12   b . Of the crown outer surface  12   a , a portion in which the protruding portion  190  is not present is formed by the crown base surface  12   b . The crown base surface  12   b  is a convex curved surface that is smooth and continuous. 
     The head body  180   h  includes a port  182 . In the present embodiment, the port  182  is a recess. The screw hole member  196  is fixed to the port  182 . This fixing can be effected by, for example, bonding or welding. The screw hole member  196  has a screw hole  196   a . The screw member  194  is screw-connected to the screw hole  196   a . Note that, in  FIG. 22B ,  FIG. 22C ,  FIG. 23B  and  FIG. 23C , the depiction of screw threads in screw portions of the screw member  194  and the screw hole  196   a  is omitted. 
     The protruding member  198  includes a base portion  198   a  and a standing wall portion  198   b  that extends upward from the base portion  198   a . The standing wall portion  198   b  is formed at an edge of the base portion  198   a . In a planar view, the protruding member  198  has a substantially polygonal shape (substantially quadrilateral shape). In the planar view, the protruding member  198  has a plurality of (four) sides. The standing wall portion  198   b  is formed on one side of the plurality of (four) sides. The base portion  198   a  has a through hole  198   c  through which the screw member  194  is inserted. 
     As described above, also in the present embodiment, the protruding member  198  is fixed by screwing with the fixing jig  192 . In the present embodiment, the screw hole is formed by the screw hole member  196 . The present embodiment is different from the ninth embodiment in that there is no need to form a screw hole in the head body  180   h.    
     It should be noted that the screw hole member  196  may be detachably attached to the head body  180   h . For example, the screw hole member  196  may be attached to the head body  180   h  by screw connection. In this case, the screw hole member  196  can be replaced. This replacement allows users to adjust the weight of the screw hole member  196 . For example, when the protruding member  198  is attached, the screw hole member  196  can be made relatively light, and, when the protruding member  198  is not attached, the screw hole member  196  can be made relatively heavy. In this case, the difference between a head weight when the protruding member  198  is attached and a head weight when the protruding member  198  is not attached can be reduced. Furthermore, the head weight when the protruding member  198  is attached can be made equal to the head weight when the protruding member  198  is not attached. 
       FIG. 24  shows the motion of the golf club  2  during downswing. A swinging motion starts from backswing, then transitions from the top of swing to downswing, and reaches impact. With the progress of the downswing, the head speed is accelerated. In addition, with the progress of the downswing, the posture of the head changes. 
     At a certain point of time during downswing, the shaft  6  of the golf club  2  is made parallel to the ground surface. The position of the golf club  2  at this point of time is also referred to as position  9 . A position of the golf club at impact is also referred to as position  6 . An intermediate position between the position  9  and the position  6  is also referred to as position  7 . 5 . These positions are named by considering the golf club  2  during swing as an hour hand of a clock. That is, for example, the position  9  coincides with the position of an hour hand at nine o&#39;clock in a clock with hands (analog clock). 
     Postures of a head during downswing are as follows. Wrists of a golfer turn during downswing (hereinafter, this is also referred to as wrist turn), and the face of the head turns by the time of impact. Accordingly, the head moves with its face-side portion preceding other portions at impact. That is, at impact, the head moves toward the face side in the face-back direction. Until the wrist turn occurs, the head moves with its heel-side portion preceding other portions. It was considered so far that, until the wrist turn occurs, the head moves toward the heel side in the toe-heel direction. 
     However, the inventors of the present disclosure have found that the traveling direction of a head at the position  9  is actually the heel side in the inclined toe-heel direction, not the heel side in the toe-heel direction. That is, the inventors of the present disclosure have found that, at the position  9 , the head moves in a state where its front in the traveling direction is as shown in the heel projection figure ( FIG. 5 ). The centrifugal force acting on the head during a period of time from the top of swing to the position  9  causes the toe down at the position  9 . In addition, wrist cock is released at the position  9 . When the wrist cock is released, the club rotates about the grip, and the posture of the head changes in the same manner as in the toe down. The inventors of the present disclosure have found that, due to these factors, the traveling direction of the head at the position  9  is actually the inclined toe-heel direction. 
     The protruding portion  20  is provided on the crown portion  12 , and the silhouette area of the heel projection figure is increased, thereby increasing aerodynamic drag (force of air resistance) received by the head  4  at the position  9 . This aerodynamic drag cancels a part of the centrifugal force acting on the head center of gravity CG. Accordingly, the increase of the aerodynamic drag reduces the force causing the toe down, and thus the toe down is suppressed. 
     Furthermore, the protruding portion  20  provided on the crown portion  12  can cause lifting force. At the position  9 , air flows in the inclined toe-heel direction. This air provides lifting force to the head  4  in the same manner as the principle of lifting force acting on wings of an airplane. The presence of the protruding portion  20  increases the lifting force at the position  9 . 
     The protruding portion  20  is not viewable in the front view of the head as viewed from the face side. The protruding portion  20  does not form any part of the outer contour line of the head in the front view of the head as viewed from the face side. Accordingly, the protruding portion  20  substantially does not affect aerodynamic drag (force of air resistance) at the position  6 . Substantially, the protruding portion  20  does not reduce the head speed. 
       FIG. 25A  and  FIG. 25B  are conceptual diagrams illustrating forces acting on a head  200  at the position  9 .  FIG. 25C  shows the head  200  at impact. The head  200  does not include a protruding portion on the crown portion.  FIG. 26A  and  FIG. 26B  are conceptual diagrams illustrating forces acting on the head  4  at the position  9 .  FIG. 26C  shows the head  4  at impact. The head  4  is the first embodiment described above. 
     A centrifugal force acts on the head  200  at the position  9 . The centrifugal force acts along a straight line that connects the center of rotation of the golf club  2  and the head center of gravity CG. This centrifugal force is decomposed into a force component F 1  parallel to the shaft axis line Z and a force component F 2  perpendicular to the shaft axis line Z. On the other hand, an aerodynamic drag (force of air resistance) and a lifting force act on the head  200  at the position  9 . The aerodynamic drag and the lifting force act in a direction in which these cancel the centrifugal force. The resultant force obtained from the aerodynamic drag and the lifting force is decomposed into a force component F 3  parallel to the shaft axis line Z and a force component F 4  perpendicular to the shaft axis line Z. These forces F 1  to F 4  are schematically shown using arrows. The centrifugal force is greater than the aerodynamic drag and the lifting force, and thus the toe down occurs. As a result, as shown in  FIG. 25C , the toe-side portion of the head  200  is lowered, the back-side portion of the head  200  is lowered, and the striking face  10   a  is opened. 
     By providing the protruding portion  20 , the aerodynamic drag and the lifting force acting on the head at the position  9  are increased. The presence of the protruding portion  20  enlarges the additional area S 2  in the heel projection figure, and increases the aerodynamic drag. Furthermore, the protruding portion  20  increases the velocity of airflow on the upper side of the head  4  at the position  9 , and increases the lifting force. Since the aerodynamic drag and the lifting force increase, the force F 3  and the force F 4  increase (see the solid black arrows in  FIG. 26A  and  FIG. 26B ). As a result, the force of canceling the centrifugal force is increased, and the toe down is suppressed. That is, lowering of the toe-side portion of the head  4  and lowering of the back-side portion of the head  4  are suppressed, and the opening of the striking face  10   a  is suppressed (see  FIG. 26C ). 
     Note that, in  FIG. 25A ,  FIG. 25B ,  FIG. 26A  and  FIG. 26B , the lengths of the arrows indicating respective forces and the relationship in length between the arrows are not accurate. Similarly, in  FIG. 25C  and  FIG. 26C , the postures of the heads and the relationship of those are not accurate. These drawings are shown for qualitatively understanding the advantageous effects of the present embodiment. 
     In each of the above-described embodiments except the embodiment of  FIG. 17 , the protruding portion includes an upper surface, and a sidewall surface that extends from the upper surface to the outer edge of the protruding portion. Because of the presence of the sidewall surface, the height H 1  of the protruding portion can be increased, and thus the additional area S 2  can be effectively increased. 
     In the head  4  of the first embodiment, the height H 1  of the upper surface  22  decreases toward a head center side. The term “head center” in the head center side can mean the geometric center CR in the plan view of the head  4  (see  FIG. 3 ). In the head which is in the reference state, a plurality of planes that are perpendicular to the ground plane HP, intersect the upper surface  22 , and pass through the geometric center CR can be set. In cross sections taken along these planes, the height H 1  of the upper surface  22  decreases toward the head center side (geometric center CR side). With this structure, the volume of the protruding portion  20  can be reduced while enlarging the additional area S 2 . In addition, with regard to the airflow at the position  9 , this structure suppresses the disturbance of the airflow, and helps the airflow to run along the crown outer surface  12   a . This airflow contributes to increase in lifting force. 
     In the head  4  of the first embodiment, the height H 1  of the upper surface  22  decreases toward the face side. That is, the height H 1  of the upper surface  22  decreases toward the face side in the cross section taken along the face-back direction (i.e., the crown f-b cross-sectional contour line described above). For this reason, the protruding portion  20  that enlarges the additional area S 2  and is unviewable when viewed from the face side can be easily formed. In addition, this structure reduces the influence of the protruding portion  20  on the airflow (airflow in the face-back direction) at impact, and can also reduce the influence on the head speed. 
     In the head  80  of the fourth embodiment, the height H 1  of the upper surface  92  decreases toward the back side. That is, the height H 1  of the upper surface  92  decreases toward the back side in the cross section taken along the face-back direction (i.e., the crown f-b cross-sectional contour line described above). Accordingly, regarding the airflow at impact (airflow in the face-back direction), the occurrence of turbulent airflow is suppressed, and the decrease of the head speed is suppressed. 
     In the head  140  of the seventh embodiment, the space SP is formed between the highest portion  152  of the protruding portion  150  and the crown outer surface  12   a . This structure having the space SP tends to receive airflow. This structure contributes to increase in aerodynamic drag at the position  9 . 
     In the head  100  of the fifth embodiment, the protruding portion  110  includes the ridgeline  112  formed by vertices, and the sidewall surfaces  114  extending from the ridgeline  112  to the outer edge CL 110  of the protruding portion  110 . Because of this structure, the volume of the protruding portion  110  can be reduced and the additional area S 2  can be enlarged. This protruding portion  110  can reduce the influence of the protruding portion  110  on airflow at impact (position  6 ) while increasing aerodynamic drag at the position  9 . 
     In the head  160  of the ninth embodiment, the head  160  includes the head body  160   h  forming the crown portion  12 , and the protruding member  174  that is detachably fixed to the head body  160   h  and constitutes the protruding portion  170 . In this structure, since the protruding portion  170  can be made by a material (a resin, for example) different from that of the head body  160   h , the weight of the protruding portion  170  can be reduced and/or the degree of freedom in forming of the protruding portion  170  can be increased. In addition, the performance of the head can be changed by attaching or detaching the protruding member  174 . The protruding member  174  is detachably fixed to the head body  160   h  with the fixing jig  172 . Accordingly, the protruding member  174  can be easily attached and detached. Furthermore, the protruding member  174  may be configured such that it is attached and detached using a dedicated tool, and thus this facilitates conformity to golf rules. 
     By increasing the height H 1 , the additional area S 2  can be increased. From this viewpoint, the maximum value of the height H 1  of the protruding portion is preferably greater than or equal to 1 mm, more preferably greater than or equal to 2 mm, and still more preferably greater than or equal to 3 mm. From the viewpoint of the degree of freedom in designing the position of the head center of gravity, the maximum value of the height H 1  of the protruding portion is preferably less than or equal to 20 mm, more preferably less than or equal to 17 mm, and still more preferably less than or equal to 15 mm. 
     Of the crown outer surface  12   a , an area of a portion located on the heel side with respect to the face center Fc is defined as Sh (mm 2 ). An area of the protruding portion is defined as St (mm 2 ). The area Sh and the area St are measured in the plan view of the head ( FIG. 3 , for example). In  FIG. 3 , the area Sh is the area of a portion located on the heel side with respect to a straight line LC that passes through the face center Fc and extends in the face-back direction. From the viewpoint of increasing the aerodynamic drag and lifting force at the position  9 , a ratio of the area St to the area Sh is preferably greater than or equal to 5%, more preferably greater than or equal to 15%, and still more preferably greater than or equal to 20%. From the viewpoint of the degree of freedom in designing the position of the head center of gravity, the ratio of the area St to the area Sh is preferably less than or equal to 70%, more preferably less than or equal to 60%, and still more preferably less than or equal to 50%. 
     As explained using  FIG. 3 , the protruding portion  20  has the contour line CL 20 . A distance D 1  between each point on the contour line CL 20  and the outer contour line CL 2  is defined. As shown in  FIG. 7 , the distance D 1  is defined as a distance (shortest distance) in the t-h cross-sectional contour line. 
     The sides constituting the contour line CL 20  include a side closest to the outer contour line CL 2 . In the embodiment of  FIG. 3 , the side closest to the outer contour line CL 2  is the third side CL 23 . This side is defined as a contour proximate side CS that means a side closest to the outer contour line CL 2  in the plurality of sides. In the embodiment of  FIG. 14 , the second side CL 52  is the contour proximate side CS. The minimum value of the distance D 1  of the contour proximate side CS is smaller than the minimum value of the distance D 1  of the other sides. The maximum value of the distance D 1  of the contour proximate side CS is smaller than the maximum value of the distance D 1  of the other sides. The maximum value of the distance D 1  of the contour proximate side CS is smaller than the minimum value of the distance D 1  of the other sides. 
     As shown in  FIG. 3 , the contour proximate side CS extends substantially along the outer contour line CL 2 . The outer contour line CL 2  includes the outer contour line CL 4  of the crown portion  12  in the plan view of the head. The contour proximate side CS extends substantially along the outer contour line CL 4 . 
     By providing the contour proximate side CS, the position of the protruding portion  20  is made closer to the outer contour line CL 4  on the heel side. For this reason, the additional area S 2  in the heel projection figure can be effectively increased. From this viewpoint, the maximum value of the distance D 1  in the entirety of the contour proximate side CS is preferably less than or equal to 25 mm, more preferably less than or equal to 20 mm, and still more preferably less than or equal to 15 mm. The maximum value may be 0 mm. When the protruding portion  20  extends to reach the outer contour line CL 4  of the crown portion  12 , the maximum value of the distance D 1  is 0 mm. 
     The contour proximate side CS preferably extends along the outer contour line CL 4 . In this case, the additional area S 2  can be effectively increased. From this viewpoint, a maximum value D 1 max and a minimum value D 1 min of the distance D 1  of the contour proximate side CS are taken into consideration. When the contour proximate side CS extends along the outer contour line CL 4 , a difference (D 1 max−D 1 min) is made smaller. From this viewpoint, the difference (D 1 max−D 1 min) is preferably less than or equal to 15 mm, more preferably less than or equal to 13 mm, and still more preferably less than or equal to 10 mm. The difference (D 1 max−D 1 min) is yet more preferably 0 mm. 
     From the viewpoint of efficiently increasing the additional area S 2 , the length of the contour proximate side CS is preferably greater than or equal to 20 mm, more preferably greater than or equal to 30 mm, and still more preferably greater than or equal to 40 mm. From the viewpoint of reducing air resistance at the position  6  by suppressing an excessive extension of the protruding portion toward the face side, the length of the contour proximate side CS is preferably less than or equal to 90 mm, more preferably less than or equal to 80 mm, and still more preferably less than or equal to 70 mm. This length of the contour proximate side CS is the actual length (route length which is three-dimensionally measured) of the contour proximate side CS. 
     It is preferable that the contour proximate side CS is provided with a sidewall surface. That is, the protruding portion preferably includes a sidewall surface having the contour proximate side CS as its lower edge. In the embodiment of  FIG. 3 , the contour proximate side CS is provided with the sidewall surface  24 . Such a part of the sidewall surface  24  which has the contour proximate side CS as its lower edge is also referred to as a contour proximate wall surface. The contour proximate wall surface CW can efficiently increase the additional area S 2  in the heel projection figure. 
     By increasing the height of the contour proximate wall surface CW, the additional area S 2  can be efficiently increased. From this viewpoint, the height H 1  of the upper edge of the contour proximate wall surface CW is preferably greater than or equal to 1 mm, more preferably greater than or equal to 2 mm, and still more preferably greater than or equal to 3 mm. From the viewpoint of the degree of freedom in designing the position of the head center of gravity, the height H 1  of the upper edge of the contour proximate wall surface CW is preferably less than or equal to 20 mm, more preferably less than or equal to 18 mm, and still more preferably less than or equal to 15 mm. 
     From the viewpoint of reducing air resistance at the position  6  while increasing the additional area S 2  in the heel projection figure, the upper edge of the contour proximate wall surface CW may include a point where the height H 1  of the protruding portion  20  is at the maximum. 
     In the plan view of  FIG. 3 , the first side CL 21  of the contour line CL 20  of the protruding portion  20  is a side (hereinafter, referred to as opposing side PS) that is opposed to the contour proximate side CS. The length of the opposing side PS is preferably shorter than the length of the contour proximate side CS. By shortening the opposing side PS, the influence of the protruding portion on airflow at the position  6  can be reduced while the additional area S 2  is enlarged. The length of the opposing side PS is the actual length (route length which is three-dimensionally measured) of the opposing side PS. A ratio of the length of the opposing side PS to the length of the contour proximate side CS is preferably less than or equal to 90%, more preferably less than or equal to 80%, and still more preferably less than or equal to 70%. The ratio of the length of the opposing side PS to the length of the contour proximate side CS may be 0%. 
     By lowering the height of the opposing side PS, air resistance at the position  6  can be reduced. From this viewpoint, the opposing side PS is not provided with a sidewall surface, preferably. 
     In the plan view of the head  4  ( FIG. 3 ), the protruding portion  20  includes a taper-shaped portion in which a width W 1  decreases from the contour proximate wall surface CW toward the opposing side PS. This taper-shaped portion contributes to reducing the influence of the protruding portion on air flow at the position  6  while enlarging the additional area S 2 . The width W 1  can be measured along a direction of a straight line that connects both ends of the opposing side PS. 
     Examples 
     Although advantageous effects of the present disclosure are demonstrated by the following examples, the present disclosure should not be construed restrictively on the basis of the descriptions of the examples. 
     [Test 1: Evaluation Performed by Actually Hitting Balls] 
     Testers actually hit balls with a club A having no protruding portion and a club B having a protruding portion to evaluate the effect of the protruding portion. 
     The testers were nine golfers who swing a driver at a head speed of 34 m/s to 39 m/s. As the club A having no protruding portion, a driver of XXIO Eleven (shaft flex: R, loft angle: 10.5°) was used. As the club B having a protruding portion, a club obtained by bonding a protruding portion that is formed by a sponge mockup to the crown portion of the head of the club A was used. The sponge mockup was made of a sponge material (EVA foam) and had a light weight. The club A and the club B were adjusted to have a same head weight. The position and the shape of the protruding portion were the same as those of the head  4  of the first embodiment described above. 
     Each of the above-mentioned nine testers hit eight golf balls with each club. For the respective hittings, head speeds, positions of hitting points, face angles at impact, and initial velocities of hit balls were measured. As to each club of each tester, the average value of the head speeds, the variation (standard deviation σ) of the head speeds, the average value of distances between the hitting points and the face center, the variation (standard deviation σ) of the distances between the hitting points and the face center, the average value of the face angles, the variation (standard deviation σ) of the face angles, and the average value of smash factors were calculated from measured data. 
       FIG. 27A  shows the average values of head speeds (H/S) for the respective testers 1 to 9. A left column for each tester shows the result of the club A (with no protruding portion), and a right column for each tester shows the result of the club B (with protruding portion). Each arrow indicates whether the head speed was increased or decreased by providing the protruding portion. The head speed remained almost unchanged whether the protruding portion was present or not. It was confirmed that head speed was not reduced by providing the protruding portion. 
       FIG. 27B  shows the average values of distances between the hit points and the face center for the respective testers 1 to 9. A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the distance was increased or decreased by providing the protruding portion. The distance was decreased in the results of eight testers out of the nine testers. It was confirmed that the presence of the protruding portion optimized (suppressed) the toe down, and made the hit point closer to the face center. 
       FIG. 28A  shows the average values of the face angles for the respective testers 1 to 9. A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the face angle was increased or decreased by providing the protruding portion. The face angle is most preferably 0°. Of the nine testers, five testers had a result in which the face angle became closer to 0° by the presence of the protruding portion. Of these five testers, three testers had a face angle of almost 0° in the club B. It was confirmed that the presence of the protruding portion optimized (suppressed) the toe down, and made the face angle closer to 0°, that is, a square face angle. 
       FIG. 28B  shows the average values of smash factors for the respective testers 1 to 9. The smash factor was calculated by dividing the initial velocity of a hit ball (B/S) by the head speed (H/S). A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the smash factor was increased or decreased by providing the protruding portion. The smash factor was increased in the results of eight testers out of the nine testers. It was confirmed that the presence of the protruding portion optimized (suppressed) the toe down, and improved the hit point and/or impact angle, whereby the smash factor was increased. 
       FIG. 29A  shows the standard deviations of the head speeds (H/S) for the respective testers 1 to 9. A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the variation of head speeds was increased or decreased by providing the protruding portion. The variation of head speeds remained almost unchanged whether the protruding portion was present or not. 
       FIG. 29B  shows the standard deviations of the distances between the hit points and the face center for the respective testers 1 to 9. A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the variation was increased or decreased by providing the protruding portion. The variation was decreased in the results of six testers out of the nine testers. It was confirmed that the presence of the protruding portion stabilized the toe down, and decreased the variation of hit points. 
       FIG. 30  shows the standard deviations of the face angles for the respective testers 1 to 9. A left column for each tester shows the result of the club A, and a right column for each tester shows the result of the club B. Each arrow indicates whether the variation was increased or decreased by providing the protruding portion. The variation was decreased in the results of six testers out of the nine testers. It was confirmed that the presence of the protruding portion stabilized the toe down, and decreased the variation of face angles. 
     [Test 2: Evaluation Using a Swing Machine] 
     Each of the above-described club A and club B was swung by a swing machine to observe toe down. This observation was performed by using Computer Controlled Hitting Machine manufactured by Golf Laboratories, Inc. as the swing machine and setting a head speed at impact at 40 m/s. The amount of bending of the shaft at impact for each club was also observed by attaching a strain gauge to the shaft. As a result, the amount of bending at impact toward the toe-down side of the club B (with the protruding portion) was reduced by 4% (about 3 mm) as compared with that of the club A (with no protruding portion). In addition, the amount of change in hitting points was measured using a pressure-sensitive paper. The hitting point of the club B was shifted toward the heel side and the sole side as compared with that of the club A. The distance between the hitting points of the club A and club B was about 6 mm. Thus, it was confirmed in the observation using the swing machine that the protruding portion suppressed the toe down. 
     [Test 3: Aerodynamics Simulation] 
     Simulations were performed to observe changes of aerodynamic drag and lifting force acting on heads. The simulations were performed by using “STAR-CCM” manufactured by Siemens Digital Industries Software as a software, and utilizing polyhedral rear fine meshes. The shape and the position of the protruding portion were the same as those of the head  4  of the first embodiment. The maximum value of the height H 1  of the protruding portion was 3 mm. The head speed during downswing was set to 20 m/s at the position  9 , 30 m/s at the position  7 . 5 , and 40 m/s at the position  6 . As a result, the aerodynamic drag at the position  9  was increased by 13% in the head advancing direction (i.e., the inclined toe-heel direction) at the position  9 . On the other hand, the aerodynamic drag at the position  6  (impact) was reduced by 2% in the head advancing direction (i.e., the face-back direction) at the position  6 . The lifting force at the position  9  was increased by 28% in the direction perpendicular to the head advancing direction (i.e., the inclined toe-heel direction) at the position  9 . The increased aerodynamic drag and the increased lifting force cancelled centrifugal force and reduced the magnitude of a force acting in the direction perpendicular to the shaft axis line. The increased aerodynamic drag and the increased lifting force reduced the magnitude of the force acting in the direction perpendicular to the shaft axis line by about 1% at the position  9 . As described above, it was confirmed that the protruding portion increased aerodynamic drag and lifting force at the position  9 , whereby the toe down was suppressed. 
     As shown by these evaluation results, the superiority of the present disclosure is clear. 
     Regarding the above-described embodiment, the following clauses are disclosed. 
     [Clause 1] 
     A golf club head including: 
     a face portion that forms a striking face; 
     a crown portion that forms a crown outer surface; 
     a sole portion that forms a sole outer surface; and 
     a hosel portion that is configured to receive a shaft and that defines a shaft axis line, wherein 
     the crown portion includes a protruding portion on the crown outer surface, 
     in a front view of the golf club head as viewed from a face side, the protruding portion does not form any part of an outer contour line of the golf club head, and 
     in a heel projection figure in which the golf club head that is placed on a ground plane such that the shaft axis line is perpendicular to the ground plane and a face angle is set at 0° is viewed from a heel side along the ground plane, the protruding portion forms a part of an outer contour line of the golf club head. 
     [Clause 2] 
     The golf club head according to clause 1, wherein the protruding portion includes an upper surface and a sidewall surface that extends from the upper surface to an outer edge of the protruding portion. 
     [Clause 3] 
     The golf club head according to clause 2, wherein a height of the upper surface decreases toward a head center side. 
     [Clause 4] 
     The golf club head according to clause 2, wherein a height of the upper surface of the protruding portion decreases toward the face side. 
     [Clause 5] 
     The golf club head according to clause 2, wherein a height of the upper surface of the protruding portion decreases toward a back side. 
     [Clause 6] 
     The golf club head according to any one of clauses 1 to 5, wherein a space is formed between a highest portion of the protruding portion and the crown outer surface. 
     [Clause 7] 
     The golf club head according to clause 1, wherein the protruding portion includes a ridgeline formed by vertices, and a sidewall surface that extends from the ridgeline to an outer edge of the protruding portion. 
     [Clause 8] 
     The golf club head according to any one of clauses 1 to 7, wherein the golf club head includes a head body that forms the crown portion, and a protruding member that forms the protruding portion and is detachably fixed to the head body. 
     [Clause 9] 
     The golf club head according to clause 8, wherein the protruding member is detachably fixed to the head body with a fixing jig. 
     [Clause 10] 
     The golf club head according to any one of clauses 1 to 9, wherein 
     in a planar view of the golf club head, when an area of the protruding portion is denoted by St, and, of the crown outer surface, an area of a portion located on the heel side with respect to a face center is denoted by Sh, then a ratio of the area St to the area Sh is greater than or equal to 5% and less than or equal to 70%. 
     [Clause 11] 
     A golf club including any one of the golf club heads according to clauses 1 to 10, a grip, and a shaft, wherein the golf club head is attached to a tip end portion of the shaft, and the grip is attached to a butt end portion of the shaft. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               2  Golf club 
               4 ,  60 ,  80 ,  100 ,  120 ,  140 ,  160 ,  180  Head 
               6  Shaft 
               10  Face portion 
               10   a  Striking face 
               12  Crown portion 
               12   a  Crown outer surface 
               12   b  Crown base surface 
               12   c  Virtually extended surface 
               12   d  Virtually extended line 
               12   e  Virtually extended line 
               14  Sole portion 
               16  Hosel portion 
               20 ,  70 ,  90 ,  110 ,  130 ,  150 ,  170 ,  190  Protruding portion 
               22  Upper surface 
               24  Sidewall surface 
               174 ,  198  Protruding member 
             CL 1  Outer contour line of a head in the front view as viewed from the face side 
             CL 6  Outer contour line of the heel projection figure 
             Z Shaft axis line 
             CG Head center of gravity 
           
         
       
    
     The above descriptions are merely illustrative and various modifications can be made without departing from the principles of the present disclosure. 
     The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. The use of the terms “a”, “an”, “the”, and similar referents in the context of throughout this disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. As used throughout this disclosure, the word “may” is used in a permissive sense (i.e., meaning “having the potential to”), rather than the mandatory sense (i.e., meaning “must”). Similarly, as used throughout this disclosure, the terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted.