Abstract:
A variable pitch propeller comprises a plurality of blades pivotally connected to a hub. Each of the plurality of blades has a base having a leading end and a trailing end. The trailing edge has a notch therein forming a discontinuity in a profile of the trailing edge. The notch is sized and shaped so as to accommodate passage therethrough of a portion of the leading edge of an adjacent blade when the blade and the adjacent blade are pivoted. A blade for a variable pitch propeller is also presented.

Description:
CROSS-REFERENCE 
     The present application claims priority to U.S. Provisional Patent Application No. 61/299,657, filed Jan. 29, 2010, and entitled “Blade for a Variable Pitch Propeller”, the entirety of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to blades for variable pitch propellers for marine applications. 
     BACKGROUND 
     Many boats and other watercraft are propelled by one or more inboard or outboard engines or a stern drive system, which drives one or more propellers. Each propeller typically has three or four blades, but may have as few as two or as many as five or six. The base of each blade is mounted at an angle, or pitch angle, relative to a radial plane transverse to the axis of rotation of the propeller shaft. Propellers may be constructed with blades having a fixed pitch. The fixed pitch is typically when the propeller blades are fixed at a pitch angle that provides maximum efficiency at a designated operating condition. Fixed pitch propellers typically have a reduced efficiency at operating conditions other than those designed for. Alternatively, the pitch of a propeller can be fixed to provide better acceleration or pulling capacity at lower speeds, which typically results in a reduced top speed. As a result, fixed pitch propellers typically are a compromise between acceleration, a boat velocity and fuel consumption. 
     One way to improve the efficiency of propellers at most speeds is to provide a propeller with blades having a variable pitch angle. This type of propeller has therefore a variable pitch and is called variable pitch propeller. One example of a variable pitch propeller is described in U.S. Pat. No. 6,896,564, which is incorporated herein by reference in its entirety.  FIG. 8  shows typical blades  300 ′ of a variable pitch propeller  54 ′. The blades  300 ′ are pivotally connected to a hub  94 ′ for rotating about their pitch axis  102 ′. An actuator (not shown in that figure) is linked to the blades  300 ′ to pivot them between a first pitch angle and a second pitch angle at which actuation of the propeller assembly  54 ′ produces forward thrusts of different strength (positive pitch). In a few cases, the first pitch angle produces a forward thrust, and the second pitch angle produces a rearward thrust (negative pitch) to the watercraft. 
     To avoid interferences between adjacent blades  300 ′ during pivoting, the blades  300 ′ usually have a smaller chord  347 ′ at the hub and a smaller tip area  301 ′ than the blades mounted on typical conventional outboard engine fixed propellers. However, a longer chord at the hub and a larger tip area are desirable as they maximize the available surface area of the blade and thus create a greater thrust, and favour flow control. Controlling the flow of water at the tip of the blade, and especially at the trailing edge, is critical for minimizing cavitation and losses when operating at high rpm. 
     In addition, the blades of conventional variable pitch propellers are secured to the hub of the propeller by one or more screws inserted into threaded apertures located at the base of the blade. The threaded ends of the screws are engaged in the blade, while the heads of the screws are in the hub. That way the screws are not apparent on the blade. While this arrangement does secure the blades to the hub, it makes it difficult for the user to replace a damaged blade from the propeller. To detach the blade from the hub, the user has to first disassemble the hub in order to access the screws, before being able to unscrew the blade from the hub. 
     Therefore, while conventional variable pitch propellers are adequate for their purposes, there is nonetheless room for improvement in the art. 
     SUMMARY 
     It is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art. 
     In one aspect, a variable pitch propeller suitable for use in association with a marine internal combustion engine is provided. The propeller comprises a hub and a plurality of blades. Each of the plurality of blades is pivotally connected to the hub so as to pivot about a pitch axis between a positive pitch angle and a negative pitch angle. Each of the plurality of blades has a base having a leading end and a trailing end. A leading edge has a distal end and a proximal end. The proximal end of the leading edge connects the leading end of the base. A trailing edge has a distal end and a proximal end. The distal end of the trailing edge connects the distal end of the leading edge. The proximal end of the trailing edge connects the trailing end of the base. The trailing edge has a notch therein forming a discontinuity in a profile of the trailing edge. The notch is sized and shaped so as to accommodate passage therethrough of a portion of the leading edge of an adjacent blade when the blade and the adjacent blade are pivoted between the positive pitch angle and the negative pitch angle. The propeller has a diameter of less than 40 inches. 
     Thus aspects of the invention provide a variable pitch propeller having blades with a notch (i.e. a small indentation) in their trailing edges. The notch forms a discontinuity in the profile of both the blade and the trailing edge. The notch is shaped and sized to accommodate the proximal end of the leading edge of an adjacent blade, thereby preventing interference between adjacent blades during pass-by. Were the notch not present, the adjacent blades would collide during rotation about their pitch axis. Various propeller blades of the prior art do not incorporate such notch or such discontinuity. Rather, the whole trailing edge is designed to prevent interference between blades, thus leaving little room for optimization of other factors, for example those effecting propeller performance, when designing the blade. One of those other factors relates to diminishing a load at the tip of the blade. In a rounded ear profile (large tip area), water pressure is more uniformly distributed on the blade&#39;s tip area, and the flow of water is more controlled at the tip of the blade. Therefore blades with rounded tips are usually of a preferred choice. Such blades are commonly seen in conventional outboard engines, but are too bulky to be used in variable pitch propeller engines. If such rounded ear propeller blades were allowed to pivot, they would (obviously undesirable) collide with each other. As a consequence, blades traditionally used in variable pitch propellers have straight trailing edges and ‘pointy’ tips (also known as ‘cleaver’ profile). Because only a small part of the present blade (i.e. the notch) is designed for insuring non-interference between adjacent blades during pass-by, the rest of trailing edge can be shaped so as to optimize these other factors. Hence, in one embodiment, the blade of the present invention is shaped to have a large rounded ear design. 
     In a further aspect, a portion of the trailing edge not comprising the notch is rounded and convex. 
     In a further aspect, the propeller rotates about a propeller axis. A longest distance between the trailing edge and the leading edge when projected onto the propeller axis defines a width of the blade. Half of a difference between the diameter of the propeller and a diameter of the hub defines a height of the blade. In one embodiment, the blade of the present invention has a width longer than the height, similar to what is seen in conventional non variable pitch propellers. Having a blade with a high height, (and as a consequence a high propeller diameter) forces the gear case to be located lower under the water line. When the gear case is low, it generates a higher drag. Reducing drag is a critical factor in designing propellers. Moreover, at high rpm, such high-height-low-width blade type reduces the tip area necessary to control the water flow path, which generates cavitation. Furthermore, such design increases the tip area, which, at high rpm, helps to better control the water flow path. 
     Traditionally, blades for variable pitch propellers have the entirety of their base connected to the hub. If the base were to extend past the connecting portion to the hub, the base of that blade would interfere with an adjacent blade when the blades are pivoted. Blades of the present invention have a notch to prevent this interference. Thus in some embodiments, blades of the present invention do have portions extending past their connection to the hub. The presence of these portions between the connecting portion and the leading and trailing edges provides a more balanced blade design. In these embodiments, because the blade&#39;s design is more balanced one each side of its pitch axis, forces necessary to pivot the blade about its pitch axis are decreased in highly tip loaded conditions. 
     In a further aspect, the propeller rotates about a propeller axis. The propeller axis defines horizontal. The base has a portion connecting to the hub. The portion has a first end and a second end. A horizontal distance between the first end and the second end of the portion of the base is shorter than a horizontal distance between the leading end and the trailing end of the base. 
     In one embodiment, the blade profile is off-centered toward the leading edge. As a consequence, a portion of the base between the pivot point and the leading edge is longer than half of the width of the blade. 
     In an additional aspect, the propeller rotates about a propeller axis. The propeller axis defines horizontal. A longest horizontal distance between the trailing edge and the leading edge defines a width of the blade. The pitch axis intersects the base at a pivot point. A horizontal distance between the pivot point and the leading end of the base is longer than half of the width of the blade. 
     Water infiltration decreases the thrust produced by a blade. In one embodiment, the base is designed to minimize water infiltration between the hub and the blade. A portion of the base located between the leading end and the connecting portion is shaped to follow a curvature of the hub (as seen when the blades are at a maximum pitch angle to produce a forward thrust). 
     In an additional aspect, the base has a portion for connecting to the hub (connecting portion). The base has a leading portion extending between the connecting portion and the leading end. When the positive pitch angle is at a maximum angle for producing a forward thrust a curvature of the leading portion of the base follows at least partially a curvature of an external surface of the hub. 
     The shape and size of the notch goes in hand with the design of the leading edge. In some embodiments, the notch is located at the proximal end of the trailing edge to accommodate the proximal end of the leading edge. 
     In an additional aspect, the notch is located toward the proximal end of the trailing edge. 
     The notch represents only a small fraction of the blade&#39;s surface and of the trailing edge. One way to assess the size of the notch with respect to the blade or to the leading edge is by comparing a chord length between a most distal point of the notch and a most distal point of the trailing edge from the pivot point. 
     In an additional aspect, the distal end of the trailing edge connects the distal end of the leading edge at a juncture point. The notch has a distal end and a proximal end. The proximal end of the notch is the proximal end of the trailing edge. The pitch axis intersects the base at a pivot point. A length of a chord between the pivot point and the distal end of the notch is at most 80% of a length between the juncture point and the pivot point. 
     Another way to assess the size of the notch with respect to the blade or to the leading edge is by comparing the height of the trailing edge with the height of the notch. 
     In an additional aspect, the propeller rotates about a propeller axis. The propeller axis defines horizontal. The pitch axis defines vertical perpendicular to the horizontal. The pitch axis intersects the base at a pivot point. The distal end of the trailing edge connects the distal end of the leading edge at a juncture point. The notch has a distal end and a proximal end. The proximal end of the notch is the proximal end of the trailing edge. A vertical distance between the distal end of the notch and the pivot point is at most 60% of a vertical distance between the juncture point and the pivot point. 
     In some embodiments, the notch does not have a shape corresponding to a shape of a portion of the leading edge received in the notch during pass-by. Because the conventional blades do not have the notch, their trailing edge is shaped so as to correspond to the shape of the leading edge for preventing interference, and hence do not feature such a notch. 
     In an additional aspect, the notch of each of the plurality of blades has a shape dissimilar to a shape of a portion of the leading edge of the adjacent blade received in the notch during pivoting about their pitch axis. 
     In one embodiment, the notch is V-shaped. 
     In a further aspect, the notch of each of the plurality of blades has two generally straight portions forming a V. 
     In an additional aspect, the two generally straight portions form an angle of at least 65 degrees. 
     In a further aspect, the notch has rounded corners. 
     In a further aspect, each of the plurality of blades is skewed. 
     In an additional aspect, each of the plurality of blades has a rake angle. 
     In a further aspect, the trailing edge of each of the plurality of blades has a cup. 
     In an additional aspect, the variable pitch propeller has a progressive pitch. 
     In a further aspect, the variable pitch propeller has a pitch of about 24 inches per revolution when the positive pitch angle is at a maximum angle for producing a positive thrust. 
     In an additional aspect, the plurality of blades comprises between 3 and 5 blades. 
     In a further aspect, adjacent blades of the plurality of blades are separated by a clearance gap. The clearance gap prevents contact between at least a portion of the leading edges and the trailing edges of the adjacent blades. 
     In a further aspect, the variable pitch propeller further comprises an integral propeller shaft for operative connection with a drive shaft. 
     In an additional aspect, the variable pitch propeller further comprises a propeller shaft. At least a portion of the drive shaft is perpendicular to the propeller shaft. 
     In a further aspect, the hub comprises an exhaust outlet for fluid connection with the exhaust manifold of the engine for exhausting spent combustion components through the hub. 
     In one aspect, a propeller blade suitable for use on a variable pitch propeller on a marine internal combustion engine is provided. The propeller blade comprises a base having a leading end and a trailing end. A leading edge has a distal end and a proximal end. The proximal end of the leading edge connects the leading end of the base. A trailing edge has a distal end and a proximal end. The distal end of the trailing edge connects the distal end of the leading edge. The proximal end of the trailing edge connects the trailing end of the base. The trailing edge has a notch therein forming a discontinuity in a profile of the trailing edge. The notch is sized and shaped so as to accommodate passage therethrough of a portion of the leading edge of an adjacent blade of the propeller when the blade is attached to the propeller and when the blade and the adjacent blade are pivoted between a positive pitch angle and a negative pitch angle. 
     In an additional aspect, when in operation, a longest distance between the trailing edge and the leading edge when projected onto a propeller axis defines a width of the blade. Half of a difference between a diameter of the propeller and a diameter of the hub defines a height of the blade. The height of the blade is shorter than the width of the blade. 
     In a further aspect, when in operation the blade is mounted on the propeller. The propeller is adapted to rotate about a propeller axis. The propeller axis defines horizontal. The base has a portion adapted to connect to the hub. The portion has a first end and a second end. A horizontal distance between the first end and the second end of the portion of the base is shorter than a horizontal distance between the leading end and the trailing end of the base. 
     In an additional aspect, when in operation the blade is mounted on the propeller and pivots about a pitch axis. The propeller is adapted to rotate about a propeller axis. The propeller axis defines horizontal. A longest horizontal distance between the trailing edge and the leading edge defines a width of the blade. The pitch axis intersects the base at a pivot point. A horizontal distance between the pivot point and the leading end of the base is longer than half of the width of the blade. 
     In an additional aspect, the base is adapted to be mounted to the hub at a connecting portion. The connecting portion is disposed between the leading end and the trailing end of the base. A leading portion of the base between the connecting portion and the leading end has a curvature. When the blade is mounted on a hub of the propeller and when the blade is at a maximum pitch angle for producing a positive thrust, the curvature of the leading portion follows at least partially a curvature of an external surface of the hub. 
     In a further aspect, a portion of the trailing edge not comprising the notch is rounded and convex. 
     In an additional aspect, the notch is located toward the proximal end of the trailing edge. 
     In a further aspect, when in operation the blade is adapted to pivot about a pitch axis. The distal end of the trailing edge connects the distal end of the leading edge at a juncture point. The notch has a distal end and a proximal end. The proximal end of the notch is the proximal end of the trailing edge. The pitch axis intersects the base at a pivot point. A length of a chord between the pivot point and the distal end of the notch is at most 80% of a length between the juncture point and the pivot point. 
     In an additional aspect, when in operation the blade is mounted on the propeller and pivots about a pitch axis. The propeller is adapted to rotate about a propeller axis. The propeller axis defines horizontal. The pitch axis defines vertical perpendicular to the horizontal. The pitch axis intersects the base at a pivot point. The distal end of the trailing edge connects the distal end of the leading edge at a juncture point. The notch has a distal end and a proximal end. The proximal end of the notch is the proximal end of the trailing edge. A vertical distance between the distal end of the notch and the pivot point is at most 60% of a vertical distance between the juncture point and the pivot point. 
     In an additional aspect, the notch has a shape dissimilar to a shape of a portion of the leading edge of the adjacent blade received in the notch during pivoting about their pitch axis. 
     In a further aspect, the notch has two generally straight portions forming a V. 
     In an additional aspect, the two generally straight portions form an angle of at least 65 degrees. 
     In a further aspect, the notch has rounded corners. 
     In yet another aspect, a propeller blade suitable for use on a variable pitch propeller on a marine internal combustion engine is provided. The propeller blade comprises a face, a back opposite to the face, and a base. The base is adapted to connect at least partially to the variable pitch propeller. Three apertures for connecting the blade to the propeller are located near the base. The three apertures are positioned on only one of the face and the back. 
     In an additional aspect, the three apertures are adapted to each receive a fastener for securing the blade to the variable pitch propeller. 
     In a further aspect, each of the three apertures is adapted to receive therein a head of a corresponding fastener. 
     In an additional aspect, the three apertures are disposed in a triangular pattern. 
     In a further aspect, wherein one of the three apertures is disposed nearer to the base than the other two of the three apertures. 
     In an additional aspect, the three apertures are disposed on the back of the blade. 
     In another aspect, a variable pitch propeller suitable for use in association with a marine internal combustion engine is provided. The propeller comprises a hub, and a plurality of blades. Each of the plurality of blades is pivotally connected to the hub so as to pivot about a pitch axis at a pitch angle. Each of the plurality of blades has a face and a back. The propeller also comprises a plurality of cam follower assemblies. Each of the plurality of blades has an associated unique cam follower assembly. Each cam follower assembly is pivoting its associated blade about its pitch axis to vary the pitch angle of the blade. Each cam follower assembly including a cam follower. Each blade and its associated cam follower assembly is arranged such that at least one plane containing the pitch axis passes between the cam follower and the face of the blade. 
     In the present application, terms related to spatial orientation such as forwardly, rearwardly, left, and right, should be interpreted are as they would normally be understood by a driver of a watercraft sitting thereon in a normal driving position, when the engine is mounted to the stern of the watercraft. When these terms are used in relation to a propeller alone, they should be interpreted as they would be understood if the propeller were installed on a watercraft. 
     For the purposes of this application, the term ‘blade face’ refers to the positive pressure side of the blade. When the blade is mounted on the variable pitch propeller and the variable pitch propeller is mounted on the boat, the blade face is the side of the blade that faces away from the boat. The term ‘blade back’ refers to the negative pressure (suction) side of the blade. When the blade is mounted on the variable pitch propeller and the variable pitch propeller is mounted on the boat, the blade back is the side of the blade that faces the boat. The term ‘propeller’ in the present specification includes all types of water-contacting rotors used to propeller a boat, such as the impeller of water jet drive engine. The term ‘diameter’ refers to the distance across the circle made by the blade tips mounted on the hub, as the propeller rotates. The term ‘blade tip’ refers to the part of the blade most remote from the center of the hub. The term ‘pitch’ refers to the distance that the propeller would move in one revolution if it were moving though a soft solid. The term ‘positive pitch’ refers to a distance in the forward direction. The term ‘negative pitch’ refers to a distance in the rearward direction. The term ‘pitch angle’ refers to an angle at which the blade is pivoted such as to produce a specific thrust. The term ‘positive pitch angle’ refers to an angle at which the blade is pivoted such as to produce a forward thrust (positive pitch). The term ‘negative pitch angle’ refers to an angle at which the blade is pivoted such as to produce a rearward thrust (negative thrust). The term ‘chord’ refers to a straight line joining the two extremities of an arc of circle or a curve. The term ‘profile’ refers to the shape (or contour) of the an element (blade, trailing edge etc.) as viewed directly in front of the blade face or back. The term ‘width’ refers to a distance between two points projected on the rotation axis of the propeller. The term ‘height’ refers to a distance between two points projected on a vertical axis perpendicular to the rotation axis of the propeller (pivot axis). The term ‘skew’ refers to a blade having a sweep back, i.e. a blade that does not have a radially symmetrical contour or profile. The term ‘rake’ refers to the angle between the face of the blade and the propeller axis as seen from a cut extending directly through the center of the hub. The term ‘cup’ refers to a small curved lip at the blade tip or at the trailing edge of the blade. The term ‘notch’ refers to a small indentation, nick or cut in an edge, a contour, a profile or a surface. The term ‘discontinuity’ refers to a lack of logical continuation or cohesion of two connected elements. The term ‘pass-by’ refers to two adjacent blades being at pitch angles such that a portion of the leading edge of one of the blade is contained in a zone left by the trailing edge of the other blade for passing by during rotation of the blades about their pitch axis. The term ‘chord at the hub’ refers to a chord between extremities of the base. 
     The present invention has at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attempting to attain the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
     Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
         FIG. 1  is a side elevation view of a marine outboard engine having a variable pitch propeller being an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the gear case of the outboard engine of  FIG. 1 ; 
         FIG. 3  is a close-up view of the gear case of  FIG. 2 , showing the piston in a different position; 
         FIG. 4  is a plan view of a cam of a variable pitch propeller assembly being the embodiment of the present invention; 
         FIG. 5A  is a right, rear perspective view of the variable pitch propeller assembly of  FIG. 4  showing the cam in a first position; 
         FIG. 5B  is a right, rear perspective view of the variable pitch propeller assembly of  FIG. 4  showing the cam in a second position; 
         FIG. 5C  is a right, rear perspective view of the variable pitch propeller assembly of  FIG. 4  showing the cam in a third position; 
         FIG. 6A  is a rear, right perspective view of a blade according to an embodiment of the present invention; 
         FIG. 6B  is a front, left perspective view of the blade of  FIG. 6A ; 
         FIG. 6C  is a longitudinal cross-sectional view of the propeller assembly with elements removed for clarity; 
         FIG. 6D  is a side elevation view of the blade of  FIG. 6A  shown with a portion of the variable pitch propeller assembly of  FIG. 4 ; 
         FIG. 6E  is a bottom view of the blade shown with a portion of the variable pitch propeller assembly of  FIG. 6D ; 
         FIG. 7A  is a front, left perspective view of the variable pitch propeller assembly of  FIG. 4  showing the blades of  FIG. 6A  oriented when the cam in the first position; 
         FIG. 7B  is a front, left perspective view of the variable pitch propeller assembly of  FIG. 4  showing the blades of  FIG. 6A  oriented when the cam in the second position; 
         FIG. 7C  is a front, left perspective view of the variable pitch propeller assembly of  FIG. 4  showing the blades of  FIG. 6A  oriented when the cam in the third position; and 
         FIG. 8  is a variable pitch propeller assembly showing blades of the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a marine outboard engine  40  having a variable pitch propeller assembly  54  with blades  300  being an embodiment of the present invention will be described. It should be understood that embodiments of the present invention may also be applied to other marine applications having internal combustion engines and propellers, such as inboard engines, surface drive engines, water jet drive engines, and stern drive engines. 
       FIG. 1  is a side view of a marine outboard engine  40  having a cowling  42 . The cowling  42  surrounds and protects an engine  44 , shown schematically. The engine  44  may be any suitable engine known in the art, such a 2-stroke or a 4-stroke internal combustion engine. An exhaust system  46 , shown schematically, is connected to the engine  44  and is also surrounded by the cowling  42 . 
     The engine  44  is coupled to a vertically oriented driveshaft  48 . The driveshaft  48  is coupled to a drive mechanism  50 , which includes a transmission  52  and the variable pitch propeller assembly  54  (shown schematically) mounted on a propeller shaft  56 . The propeller shaft  56  is generally perpendicular to the driveshaft  48 . It is contemplated that the propeller shaft  56  could not be perpendicular to the driveshaft  48 . The propeller shaft  56  defines a rotation axis  206  (shown in  FIG. 2 ). The drive mechanism  50 , the variable pitch propeller assembly  54  and the propeller shaft  56  will be described below in further detail. Other known components of an engine assembly are included within the cowling  42 , such as a starter motor and an alternator. As it is believed and understood that these components would be readily recognized by one of ordinary skill in the art, and thus further explanation and description of these components will not be provided herein. 
     A stern bracket  58  is connected to the cowling  42  via the swivel bracket  59  for mounting the outboard engine  40  on a watercraft. The stern bracket  58  can take various forms, the details of which are conventionally known. 
     A linkage  60  is operatively connected to the cowling  42 , to allow steering of the outboard engine  40  when coupled to a steering mechanism of a boat, such as a steering wheel. 
     The cowling  42  includes several primary components, including an upper motor cover  62  with a top cap  64 , and a lower motor cover  66 . A lowermost portion, commonly called the gear case  68 , is attached to the exhaust system  46 . The upper motor cover  62  preferably encloses the top portion of the engine  44 . The lower motor cover  66  surrounds the remainder of the engine  44  and the exhaust system  46 . The gear case  68  encloses the transmission  52  and supports the drive mechanism  50 , which will be described below in further detail. 
     The upper motor cover  62  and the lower motor cover  66  are made of sheet material, preferably plastic, but could also be metal, composite or the like. The lower motor cover  66  and/or other components of the cowling  42  can be formed as a single piece or as several pieces. For example, the lower motor cover  66  can be formed as two lateral pieces that mate along a vertical joint. The lower motor cover  66 , which is also made of sheet material, is preferably made of composite, but could also be plastic or metal. One suitable composite is fiberglass. 
     A lower edge  70  of the upper motor cover  62  mates in a sealing relationship with an upper edge  72  of the lower motor cover  66 . A seal  74  is disposed between the lower edge  70  of the upper motor cover  62  and the upper edge  72  of the lower motor cover  66  to form a watertight connection. 
     A locking mechanism  76  is provided on at least one of the sides of the cowling  42 . Preferably, locking mechanisms  76  are provided on each side of the cowling  42 . 
     The upper motor cover  62  is formed with two parts, but could also be a single cover. As seen in  FIG. 1 , the upper motor cover  62  includes an air intake portion  78  formed as a recessed portion on the rear of the cowling  42 . The air intake portion  78  is configured to prevent water from entering the interior of the cowling  42  and reaching the engine  44 . Such a configuration can include a tortuous path. The top cap  64  fits over the upper motor cover  62  in a sealing relationship and preferably defines a portion of the air intake portion  78 . Alternatively, the air intake portion  78  can be wholly formed in the upper motor cover  62  or even the lower motor cover  66 . 
     Referring to  FIG. 2 , the drive mechanism  50  will now be described. 
     A bevel gear  80  is mounted on one end of the driveshaft  48 . The bevel gear  80  meshes with the bevel gear  82  that is mounted to the propeller shaft  56 . The driveshaft  48  drives the propeller shaft  56  to propel a watercraft (not shown) in either the forward direction or the reverse direction, depending on the pitch angle of the blades  300 , as will be described below in further detail. An optional transmission assembly (not shown) is capable of disengaging the engine  44  from the bevel gear  82 , resulting in a neutral state wherein the propeller shaft  56  is not driven and the watercraft is no longer propelled in a body of water. A neutral state may alternatively be achieved by varying the pitch angle of the blades  300 , as will be discussed below in further detail. 
     Referring to  FIG. 2 , the variable pitch propeller assembly  54  being an embodiment of the present invention will now be described. 
     The variable pitch propeller assembly  54  includes four propeller blades  300  disposed at 90 degrees from each other. The blades  300  are received in recesses  126  formed in the hub  94 . It is contemplated that in other embodiments the variable pitch propeller assembly  54  could comprise a different number of blades  300 . In this embodiment, the blades  300  are made of stainless steel. It is contemplated that in other embodiments the blades  300  could be made of another material, such as another metal or an alloy. Sealing rings  138  provide a water-tight seal between the hub  94  and the blades  300 . 
     Each blade  300  has a corresponding cam follower assembly  100 . A D-shaped or otherwise non-circular end  104  of the cam follower assembly  100  is received in a complementarily-shaped aperture  106  in the corresponding blade  300 , such that rotating the cam follower assembly  100  causes the blade  300  to pivot about a pitch axis  102  to vary the pitch angle of the blade  300 . In this embodiment, each pair of blades  300  on opposite sides of the hub  94  has coaxial pitch axis  102 . It is contemplated that the blade  300  may be connected to the cam follower assembly  100  by any other suitable connection, such as a spline connection. It is further contemplated that in other embodiments the cam follower assembly  100  may be formed integrally with the blade  300  in a one piece construction. Each cam follower assembly  100  has a cam follower  108  that is received in a corresponding recess  110  in the cam  168 . The cam follower assemblies  100  are rotated by the cam  168  in a manner that will be described in further detail below. It should be understood that in other embodiments more or fewer than four propeller blades  300  may be used, in which case the cam  168  would have a recess  110  corresponding to each propeller blade  300 . The propeller blades  300  will be described in greater details below. 
     A spacer  112  is bolted to the rear portion of the hub  94 , to allow an increased range of motion for the cam  168 , as will be described in further detail below. A cap  134  is received in the rear portion of the spacer  112 . The cap  134  improves the aesthetic and hydrodynamic properties of the variable pitch propeller assembly  54 , and provides a path for exhaust from the exhaust chamber  130  to exit (in a known manner) via channels (not shown) in the hub  94 , and spacer  112 . 
     Referring now to  FIG. 3 , a variable pitch system for varying the pitch angle of the blades  300  of the variable pitch propeller assembly  54  will now be described. 
     The variable pitch system is operated by an actuator  140  in the form of a linear hydraulic actuator. The actuator  140  includes a housing  142  formed inside the propeller shaft  56 , a piston  144  that can reciprocate within the housing  142 , a shaft  146  fixed to the piston  144 , and a cam  168  ( FIG. 2 ) fixed to the rearward end of the shaft  146 . The actuator  140  is coupled to a set of hydraulic valves  114  and  116 , and is supported by tapered bearings  118  and  120 . The hydraulic valves  114 ,  116  are preferably controlled by an electronic control unit (ECU, not shown) to ensure precise operation of the actuator  140 . The hydraulic valve  114  permits hydraulic fluid to enter the chamber  122  via the annular channel  124  and the apertures  148 , to urge the piston  144  rearwardly toward the position shown in  FIG. 3 . The hydraulic valve  116  permits hydraulic fluid to enter the chamber  150  via the hydraulic line  152  and the apertures  154 , to urge the piston  144  forwardly toward the position shown in  FIG. 2 . It should be understood that the valves  114 ,  116  could be placed at any suitable location. It is contemplated that in other embodiments a valveless hydraulic actuator could alternatively be used, for example by replacing the valves  114 ,  116  with connections to a reversible hydraulic motor. Operation of the reversible hydraulic motor in one direction would cause fluid to enter the chamber  122 , and operation of the reversible hydraulic motor in the reverse direction would cause fluid to enter the chamber  150 . Sealing rings  132  provide a seal between the piston  144 , the chamber  122  and the hydraulic line  152 . The reciprocating movement of the piston  144  drives the shaft  146 , which extends through an end of the housing  142  to the hub  94  to drive the cam  168 . The hub  94  and spacer  112  together form a channel  156  ( FIG. 2 ) in which the cam  168  reciprocates. It is contemplated that any other suitable type of actuator  140  may alternatively be used. 
     Referring now to  FIG. 4 , the cam  168  has four recesses  110  formed therein, corresponding to the four blades  300  (only one recess  110  is actually shown in  FIG. 4 ). A first segment  202  of the recess  110  is oriented at a first angle  204  relative to the rotation axis  206  of the propeller shaft  56 . A second segment  208  of the recess  110 , disposed rearwardly of the first segment  202 , is oriented at a second angle  210  relative to the rotation axis  206  of the propeller shaft  56 . A third segment  212  of the recess  110 , disposed rearwardly of the second segment  208 , is oriented at a third angle  214  relative to the rotation axis  206  of the propeller shaft  56 . The second angle  210  is greater than each of the first angle  204  and the third angle  214 . Each cam follower  108  is received in, and engages, a corresponding recess  110 . The cam follower  108  has a round pin (not shown) to fit the recess  110  shown in  FIG. 4 . The cam follower  108  shown in  FIG. 6E  appears to have an oval profile due to the fact that a separate piece has been fitted to the round pin. It is contemplated that the cam follower  108  could not have a rounded profile, and could not have a separate piece fitted to the round pin to form a generally oval cam follower. It should be understood that in other embodiments a variable pitch propeller assembly  54  having more or fewer than four blades  300  would have a cam  168  with a corresponding number of recesses  110 . For example, in some embodiments, the variable pitch propeller assembly  54  may have three blades  300  evenly spaced around the hub  94 , and a three-sided cam  168  having three corresponding recesses  110 . 
     Referring to  FIGS. 5A ,  5 B and  5 C, as the cam  168  reciprocates within the channel  156  (shown in  FIG. 2 ), the portion of the recess  110  in contact with the corresponding cam follower  108  causes the cam follower  108  to rotate and thereby vary the pitch angle of the corresponding blade  300 . When the cam  168  is forwardly of a first position at the first angle  204  (shown in  FIG. 4 , a corresponding position of the blade  300  being shown in  FIG. 5A ) and rearwardly of a second position at the second angle  210  (shown in  FIG. 4 , a corresponding position of the blade  300  being shown in  FIG. 5B ), the cam follower  108  engages the first segment  202  of the cam  168 . The relatively shallow first angle  204  (of the recess  110 ) causes the pitch angle of the blade  300  to vary relatively slowly as the cam  168  moves between the first position and the second position. This permits fine adjustments of the pitch angle of the blades  300 , to achieve the desired performance characteristics while propelling the watercraft in the forward direction. It is contemplated that in other embodiments the first angle  204  would be chosen such as to accelerate the cam  168  between the first and the second positions. 
     When the cam  168  is forwardly of the second position and rearwardly of a third position the third angle  214  (shown in  FIG. 4 , a corresponding position of the blade  300  being shown in  FIG. 5C ), the cam follower  108  engages the second segment  208  of the cam  168 . The relatively steep second angle  210  causes the pitch angle of the blade  300  to vary relatively quickly as the cam  168  moves between the second position and the third position. The pitch angle of the blade  300  varies at a second rate that is preferably greater than the first rate. This permits rapid shifting of the pitch angle of the blades  300  between the second position, in which the watercraft is propelled in the forward direction, and the third position, in which the watercraft is propelled in the reverse direction. In this manner, the watercraft can quickly and conveniently be propelled in the reverse direction by varying the pitch angle of the blades  300  without changing the direction of rotation of the variable pitch propeller assembly  54 . The increased rate of pitch angle change between the second and third positions reduces the degree of travel of the cam  168 , allowing for a more compact arrangement. It is contemplated that in other embodiments the watercraft would also be propelled in the forward direction when the cam  168  is in the third position. It is further contemplated that the rate of pitch angle change could be constant or decreased between the second and the third positions. 
     It should be understood that there exists a pitch angle of the blades  300  corresponding to a zero thrust point, at which the rotation of the variable pitch propeller assembly  54  provides no thrust in either of the forward or reverse directions. The zero thrust point occurs between the pitch angle range in which the watercraft is propelled in the forward direction and the pitch angle range in which the watercraft is propelled in the reverse direction, when the cam  168  is between the second position and the third position. A neutral state may therefore be achieved by setting the pitch angle of the blades  300  to the zero thrust pitch angle, without turning off the engine  44  or disengaging the engine  44  from the variable pitch propeller assembly  54 . 
     When the cam  168  is positioned forwardly of the third position, the cam follower  108  engages the third segment  212  of the cam  168 . In this position, the rotation of the variable pitch propeller assembly  54  propels the watercraft in the reverse direction, and the pitch angle of the blades  300  varies at a third rate that is less than the second rate. The third segment  212  extends to the end  216  of the cam  168 , to allow the variable pitch propeller assembly  54  to be assembled by sliding the cam followers  108  through the end  216  of the cam  168  into their respective recesses  110 . The third angle  214  is preferably parallel to the propeller shaft  56 , to allow for simple assembly, in which case the third rate is zero. 
     Referring now to  FIGS. 6A-6E , the blade  300  will be described in greater details. Throughout the figures, the blade  300  will have dimension described using the word ‘width’ for designating horizontal distances, and the word ‘height’ for designating vertical distances. Horizontal is defined by the rotation axis  206  of the propeller shaft  56 . Vertical is defined by the pivot axis  102 , which is perpendicular to the rotation axis  206 . 
     The blades  300  each have a face  380  (shown in  FIG. 6A ) and a back  390  (shown in  FIG. 6B ). As best seen in  FIG. 6B , the blades  300  each have three apertures  360  which receive bolts  361  (shown in  FIG. 6C ) for connecting to the hub  94  via a connector  95  (shown in  FIG. 6B ). It is contemplated that the three apertures  360  could receive fasteners other than the bolts  361 . The three apertures  300  are disposed in a triangular pattern. One of the three apertures  360  is proximate to the hub  94 , while two of the three apertures  360  are more distal from the hub  94 . The three apertures  360  are disposed on the back  390  of the blade  300 . The bolts  361  are easily accessible from the apertures  360 . A user desiring to replace a blade  300  unscrews the bolts  361  to detach the blade  300  from the hub  94  without having to remove the whole variable pitch propeller assembly  54 . It is contemplated that the blades  300  could be attached to the hub  94  by means other than with bolts. It is also contemplated that the blades  300  could have less or more than three apertures  360  to connect to the hub  94 . It is also contemplated that the apertures  360  could be disposed in a fashion other than in a triangle, and that two apertures  361  could be disposed proximate to the hub  94 . It is contemplated that the apertures  361  could be disposed on the face  380  of the blade  300 , and that the apertures  361  could not all be disposed on a same side (face  380  or back  390 ) of the blade  300 . 
     As best seen in  FIGS. 6D and 6E , a plane  103  containing the pitch axis  102  passes between the face  380  of the blade  300  and the cam follower  108 . As the cam follower  108  moves, the plane  103  rotates as the blade  300  rotates, and the face  380  of the blade  300  stays on one side of the plane  103  and the cam follower  108  stays on the other side of the plane  103 . It is contemplated that, in certain embodiments, the plane  103  could be disposed between the cam follower  108  and only a portion of the face  380  of the blade  300 . It is also contemplated that, in other embodiments, there may not be such plane  103 . 
     The blade  300  has a base  302  proximate to the hub  94 , a leading edge  304 , and a trailing edge  306 . The base  302  has a leading end  310  connected to the leading edge  304  and a trailing end  312  connected to the trailing edge  306 . The base  302  has a curved profile that follows a curvature of the hub  94  (as viewed when the blade  300  is mounted on the hub  94  and is at a maximum pitch angle for producing a forward thrust). It is contemplated that in other embodiments the base  302  would not have a curved profile that follows the curvature of the hub  94 . 
     The base  302  has a portion  308 , located between the leading end  310  and the trailing end  312 , for connecting the blade  300  to the connector  95  to the hub  94 . The portion  308  has a midpoint  309 , which is a pivot point  309  of the blade  300 . The pivot point  309  is at the intersection between the pitch axis  102  and the base  302 . The base  302  is longer than the portion  308 . In this embodiment, a width  317  of the base  302  is 5.21 inches and a width  319  of the portion  208  is 2.5 inches. In another embodiment, the width  317  is 6.32 inches and the width  319  is 3.31 inches. A portion  307  of the base  302  extends from the portion  308  to the leading end  310 , and a portion  305  of the base  302  extends from the portion  308  to the trailing end  312 . A width  315  of the portion  307  of is longer than a width  313  of the portion  305 . In this embodiment, the width  315  is about 2.31 inches, and the width  313  is about 0.4 inches. In another embodiment, the width  315  is about 3.02 inches, and the width  313  is about 0.75 inches. The presence of the portions  305 ,  307  provides balance to the blade  300  design by decreasing the rotational forces necessary to pivot the blade  300  about its pitch axis  102  in highly tip  301  loaded conditions. It is contemplated that the portions  305 ,  307  could be of a same length or than the portion  305  could be longer than the portion  307 . 
     The leading edge  304  and the trailing edge  306  extend from the base  302  away from the hub  94 . In this embodiment, the leading edge  304  has a proximal end being the leading end  310  of the base  302 , and a distal end being the trailing edge  306 . It is contemplated that in other embodiments the proximal end of the leading edge  304  could be different from the leading end  310  of the base  302 , and the distal end of the leading edge  304  could be different from the trailing edge  306 . The trailing edge  306  has a proximal end connected to the leading end  310  of the base  302 , and a distal end connected to the leading edge  304 . The distal ends of the leading edge  304  and the trailing edge  306  connect at the juncture point  314 . 
     The leading edge  304  has a rounded convex profile that forms a sharp edge with the base  302  at the leading end  310  of the base  302 . It is contemplated that in other embodiments the leading edge  304  could have a blunt edge with the base  302 . The leading edge  304  connects the trailing edge  306  bluntly, such that the juncture point  314  does not form a discontinuity between the leading edge  304  and the trailing edge  306 . 
     The trailing edge  306  has a first portion  320  towards the leading edge  304 , and a second portion  322  towards the base  302 . The first portion  320  and the second portion  322  connect at point  324 . The first portion  320  forms most of the trailing edge  306 , while the second portion  322  defines a discontinuity in the trailing edge  306  profile, as it will be described in greater details below. 
     The first portion  320  is a rounded and convex, and partially delimits a large rounded tip  301  of the blade  300 . By having the large rounded shape of first portion  320 , water pressure is allowed to distribute on the tip&#39;s  301  surface area in order to control flow of water at the tip  301 . It is contemplated that in other embodiments the first portion  320  would not be designed based on the above criteria. It is also contemplated that in other embodiments the first portion  320  would have a shape different from a rounded convex shape. 
     The first portion  320  represents a majority of the trailing edge  306 . There are several ways to assess the size of the first portion  320  with respect to the size of the trailing edge  306 . As best seen in  FIG. 6A , one way to assess the size of the first portion  320  is by comparing chord lengths with respect to the pivot point  309 . The length of a chord  336  between the distal end of the first portion  320  (i.e. juncture point  314 ) and the pivot point  309  is computed with respect to the length of a chord  337  between the distal end of the first portion  320  (i.e. point  324 ) and the pivot point  309 . In this embodiment, the length of the chord  337  of the first portion  320  is about 45% of the length of the chord  336 . Hence the first portion  320  represents 65% of the trailing edge  306 . In other embodiments the first portion  320  represents 30%, 42%, 54%, 66%, 75%, 70%, or any number in between of the trailing edge  306 . 
     As best seen in  FIG. 6B , another way to assess the size of the first portion  320  is by comparing heights. The height  333  of the first portion  320  is computed as the difference between the height  340  of the juncture point  324  to the pivot point  309  and a height  325  of proximal end  324  of the trailing edge  306  to the pivot point  309  (i.e. a height of the notch  330 ). In this embodiment, the height  333  of the first portion is about 60% of a height  340  of the trailing edge  306  (i.e. height of the blade  300 ). It is contemplated that in other embodiments the height  333  would be 90%, 85%, 80%, 75%, 70%, 65%, or even 40%, or any number in between of a height  340  of the blade  300 . In this embodiment, the height  333  of the first portion  320  is about 3.36 inches and the height  340  is about 4.87 inches. In another embodiment, the height  333  is about 2.95 inches and the height  340  is about 3.7 inches. 
     The second portion  322  extends between the point  324  and the trailing end  312  of the base  302 , and defines a notch  330  in the trailing edge  306  profile and also in the blade  300  profile. As seen in  FIG. 7B , the notch  330  is sized to accommodate the proximal end  310  of the leading edge  304  of an adjacent blade  300  during pivoting about their pitch axis  102 , thereby preventing interference with the adjacent blade  300 . 
     The notch  330  is small compared to the trailing edge  306  and the blade  300 . Similarly to the first portion  320 , one way to evaluate the comparative size of the notch  330  involves comparing heights. In this embodiment, the notch  330  has a height  325  of about 1.24 inches when the height  340  is about 4.87 inches, which represents about 24% of the height  340  of the trailing edge  306 . In another embodiment, the height  325  of about 0.75 inch when the height  340  is about 3.7 inches which represents about 20% of the height  340  of the trailing edge  306 . Another way to estimate the size of the notch  330  is by comparing the length of the chord  337  with the length of the chord  336 , as shown above for the first portion  320 . Yet another way to assess the size of the notch  330  is by comparing surface areas of the notch  330  and of the blade  300 . In this embodiment, a surface defined by the notch  330  has a surface area of about 1.62 square inches for a surface area of the blade  300  of about 27.4 square inches. Therefore, in terms of surface area, the notch  330  represents about 6% of the blade&#39;s  300  surface area. In another embodiment, the surface defined by the notch  330  has a surface area of about 2.5 square inches for a surface area of the blade  300  of about 37.6 square inches. Therefore, in that other embodiment, the notch  330  represents about 6.6% of the blade&#39;s  300  surface area. It is contemplated that the notch  330  could represent anywhere between 2 to 15% of the blade&#39;s  300  surface area. 
     The notch  330  is an indentation (or cut) in the blade  300  that represents a small piece of the blade  300  that has been removed from the blade  300 . The notch  330  forms therefore a discontinuity in the trailing edge  306  profile and in the blade profile  300 . The notch  330  is formed of two generally straight portions  326 ,  328  forming a V, while the first portion  320  of the trailing edge  306  and the leading edge  304  are generally curved. The straight portion  328  extends perpendicularly and slightly inwardly from the leading end  310  of the base  302 , forming a first point of discontinuity at point  312 . The straight portion  326  extends outwardly at an angle  332  (shown in  FIG. 6A ) from the straight portion  328  to meet with the first portion  320  at the point  324 , forming a second point of discontinuity at point  324 . In this embodiment, the angle  332  between the two straight portions  326 ,  328  is about 90 degrees. In another embodiment, the angle  332  between the two straight portions  326 ,  328  is about 122 degrees. It is contemplated that in various embodiments of the invention the angle  332  would be comprised between 70 and 145 degrees. It is also contemplated that the portions  326 ,  328  would not be straight, but would be concave or convex. It is also contemplated that in other embodiments of the invention the second portion  322  would not be V-shaped. For example, the second portion  322  would be formed of one straight portion and one rounded portion, a single straight or rounded portion, or more than two portions. 
     The shape of the notch  330  is dissimilar to a shape of the leading edge  304  of an adjacent blade  300 . By dissimilar it should be understood that the shape of the notch  330  is not matching the shape of the leading edge  304  of the adjacent blade  300 . It is contemplated that in other embodiments of the invention the shape of the notch  330  could be at least partially similar to the shape of at least a portion of the leading edge  306 . 
     As best seen in  FIG. 6B , the blade  300  has a height  340  shorter than a width  342 . The height  340  of the blade  300  is computed by taking half of the difference between the diameter of the variable pitch propeller assembly  54  and the diameter of the hub  94 . The width  342  of the blade  300  is the transversal span of the blade  300 . It is the longest distance between the trailing edge  306  and the leading edge  304  when projected onto the rotation axis  206  of the propeller shaft  56 . In this embodiment, the height  340  is about 4.87 inches, and the width  342  is about 6.44 inches. In another embodiment, the height  340  is 5.26 inches and the width  342  is about 7.23 inches. The height  340  is preferably between 2 inches and 7 inches, and the width  342  is preferably comprised between 4 inches and 12 inches. It is contemplated that in various embodiments the blade  300  could have the width  340  shorter than the height  342 . A chord at the hub  347  is about 6 inches. The chord at the hub  347  is a chord joining the leading end  310  to the trailing end  312 . 
     As best seen in  FIG. 6C , in this embodiment the blade  300  is skewed at 30 degrees. In various other embodiments, the skew angle is between 20 degrees and 35 degrees. For example the skew angle could be 22 degrees, 33 degrees. In this embodiment, the skews angle of the blade  300  is lower than the ones of the prior art blades  300 ′. The blade  300 ′ of the prior art have a higher skew angles (45 degrees and higher) to avoid interference between adjacent blades. Such a skew angle is not necessary in the blade  300  due to the presence of the notch  330 . 
     In this embodiment, the blade  300  has a rake angle  344  of about 27.5 degrees. In various other embodiments, the rake angle  344  ranges from 0 to 45 degrees. For example, the rake angle  344  could be 25 degrees, 31 degrees, or 41 degrees. The blade  300  has a cup  346 . It is contemplated that in other embodiments the blade  300  would not have a cup. 
     In this embodiment, the blade  300  has a pitch of 24 inches. In various other embodiments, the pitch ranges from 5 to 35 inches. For example the blade  300  could have a pitch of 5 inches, 7 inches, 13 inches, 22 inches, or 28 inches per revolution. The pitch is progressive. It is contemplated that in other embodiments the pitch could be constant. 
     The variable pitch propeller assembly  54  has a diameter of 14.5 inches. The diameter of the variable pitch propeller assembly  54  is comprised between 5 inches and 40 inches. For example the variable pitch propeller assembly  54  could have a diameter of 7 inches, 9.3 inches, 12.1 inches, 23 inches or 32.5 inches. The diameter of the variable pitch propeller assembly  54  is twice a radius  351  of the variable pitch propeller assembly  54  (shown in  FIG. 6C ). The hub  94  has a diameter of 4.75 inches. In other embodiment, the hub  94  diameter would be up to 12 inches, and preferably less than 6 inches. For example, the diameter of the hub  94  could be 1.6 inches, 2.73 inches, 3.31 inches, 7.61 inches or 9.23 inches. The diameter of the hub  94  is twice a radius  350  of the hub  94  (shown in  FIGS. 6B and 6C ). 
       FIGS. 7A-7C  show front, left perspective views of the propeller blades  300  mounted on the hub  94 , and having a first pitch angle such as to provide a forward thrust, a second pitch angle during pass-by and a third pitch angle such as to provide a rearward thrust. In one embodiment, the pitch angle range between a maximum pitch angle for which the variable pitch propeller assembly  54  produces a forward thrust, and a maximum pitch angle for which the variable pitch propeller assembly  54  produces a rearward thrust is −60 degrees. It is contemplated that the pitch angle range could be less than 60 degrees. 
     In  FIG. 7A , the leading edges  304  are positioned partially forward from the trailing edges  306 . The variable pitch propeller assembly  54  generates a forward thrust. In  FIG. 7B , the proximal ends of the leading edges  304  enter into the notch  330  defined in the trailing edges  306  without interfering. In  FIG. 7C , the leading edges  304  are positioned partially rearward of the trailing edges  306 . The variable pitch propeller assembly  54  generates a rearward thrust.  FIG. 7B  also shows the blades  300  of the variable pitch propeller assembly  54  area spaced from each other by a clearance  331 . The clearance  331  is defined between the leading edge  304  of one blade  300  and the notch  330  of an adjacent blade. The clearance  331  is preferably designed to be a minimal distance between two adjacent blades  300  in pass-by that takes into account tolerance and deflection. 
     Any one of a number standard techniques that are known in the art in association with convention variable pitch propeller blades for these types of engines are used to manufacture the invention. 
     Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.