Patent Publication Number: US-2023150638-A1

Title: Outboard motor and anti-vibration structure of outboard motor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-185232 filed on Nov. 12, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an outboard motor and an anti-vibration structure of an outboard motor. 
     2. Description of the Related Art 
     Japanese Laid-open Patent Application Publication No. H9-11988 discloses a tilt/trim device of a vessel propulsion apparatus. This device is configured so that hydraulic oil is supplied from a pumping device to a hydraulic cylinder device located between a swivel bracket of an outboard motor and a hull. A propulsion unit of the outboard motor is tilt-operated or trim-operated by the extension and contraction of the hydraulic cylinder device. 
     SUMMARY OF THE INVENTION 
     The inventor of preferred embodiments of the present invention described and claimed in the present application conducted an extensive study and research regarding an outboard motor, such as the one described above, and in doing so, discovered and first recognized new unique challenges and previously unrecognized possibilities for improvements as described in greater detail below. 
     An engine rotates at a low speed during low-speed navigation, such as trolling. At this time, low-frequency vibrations are transmitted from the outboard motor to a hull. If these vibrations can be reduced, a comfortable ride on the vessel can be improved. 
     Preferred embodiments of the present invention provide outboard motors that each significantly reduces or prevents vibrations transmitted to a hull, and anti-vibration structures of the outboard motors. 
     In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment of the present invention provides an outboard motor including an outboard motor main body and a mount assembly to mount the outboard motor main body to a hull. The mount assembly includes a clamp bracket to be fixed to the hull and a swivel bracket that is coupled to the clamp bracket through a tilt shaft, turnable around the tilt shaft, and supports the outboard motor main body. The outboard motor further includes a hydraulic cylinder located between the clamp bracket and the swivel bracket and that is able to turn the swivel bracket around the tilt shaft with respect to the clamp bracket. The outboard motor further includes a gas damper to damp an external force that acts in a telescopic direction of the hydraulic cylinder. According to this structural configuration, an external force that acts in the telescopic direction of the hydraulic cylinder that turns the swivel bracket around the tilt shaft is attenuated by the gas damper. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. Additionally, the gas damper is provided in the cylinder main body and is filled with gas. According to this structural configuration, the gas damper functions as a damper, and, as a result, an external force that acts in the telescopic direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the gas damper includes a free piston that is slidable in the cylinder main body on a side opposite to the rod and that defines a gas chamber filled with the gas in the cylinder main body. According to this structural configuration, gas with which the gas chamber defined by the free piston in the cylinder main body of the hydraulic cylinder is filled functions as a damper, and, as a result, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the cylinder main body includes a contact portion that is contactable with the free piston so as to regulate a position of a stroke end of the free piston and that regulates a minimum volume of the gas chamber. According to this structural configuration, an increase of the pressure in the gas chamber is regulated. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. The gas damper includes a sub-cylinder that is hydraulically connected to at least one of the pair of oil chambers. 
     According to this structural configuration, the sub-cylinder that is hydraulically connected to at least one of the oil chambers of the hydraulic cylinder functions as a gas damper, and, as a result, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the sub-cylinder includes a sub-cylinder main body and a free piston that is slidable in the sub-cylinder main body. The free piston divides a sub-oil chamber in communication with at least one of the pair of oil chambers of the hydraulic cylinder and a gas chamber filled with the gas from each other. According to this structural configuration, the gas with which the gas chamber divided from the sub-oil chamber by the free piston in the sub-cylinder main body is filled functions as a damper, and, as a result, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the sub-cylinder main body includes a contact portion that is contactable with the free piston so as to regulate a position of a stroke end of the free piston and that regulates a minimum volume of the gas chamber. According to this structural configuration, an increase of the pressure in the gas chamber is regulated. 
     In a preferred embodiment of the present invention, the sub-cylinder includes a pair of sub-cylinders. Sub-oil chambers of the pair of sub-cylinders are connected to the pair of oil chambers of the hydraulic cylinder, respectively. According to this structural configuration, each of the sub-cylinders that respectively correspond to the extension and the contraction of the hydraulic cylinder functions as a gas damper. Thus, it is possible to attenuate an extension/contraction bidirectional external force. Therefore, the attenuation effect is high. This makes it possible to further significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the sub-cylinder includes only a single sub-cylinder, and both of the pair of oil chambers of the hydraulic cylinder are in communication with the sub-oil chamber of the single sub-cylinder. According to this structural configuration, the sub-cylinder common to the extension and the contraction of the hydraulic cylinder is able to function as a gas damper. 
     In a preferred embodiment of the present invention, the sub-cylinder includes only a single sub-cylinder, and the gas chamber includes a pair of gas chambers, and the sub-oil chamber includes a pair of sub-oil chambers. The pair of oil chambers of the hydraulic cylinder are in communication with the sub-oil chambers corresponding to the pair of oil chambers, respectively. The free piston includes a rod piston that includes a gas piston that divides the pair of gas chambers from each other and a pair of rods that extend from the gas piston to both sides in a sliding direction of the free piston. Each of the pair of rods functions as a hydraulic piston that defines a portion of the sub-oil chamber corresponding to the rod. According to this structural configuration, the sub-cylinder common to the extension and the contraction of the hydraulic cylinder is able to function as a gas damper. 
     In a preferred embodiment of the present invention, the outboard motor further includes an on-off valve to interrupt a flow of hydraulic oil between at least one of the pair of oil chambers of the hydraulic cylinder and the sub-oil chamber of the sub-cylinder. According to this structural configuration, gas with which the gas chamber of the sub-cylinder is filled functions as a damper in a state in which a flow of hydraulic oil between at least one of the pair of oil chambers of the hydraulic cylinder and the sub-oil chamber of the sub-cylinder is allowed by the on-off valve. Thus, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the outboard motor further includes a prime mover to rotate a propeller, a detector to detect a rotation speed of the prime mover, and a controller configured or programmed to shut off a control valve, which is the on-off valve, when a rotation speed detected by the detector exceeds a predetermined threshold value. According to this structural configuration, gas with which the gas chamber of the sub-cylinder is filled functions as a damper when a detection value of the rotation speed of the prime mover is lower than the threshold value. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. Additionally, the gas damper includes a gas cylinder that slidably houses an end portion of the cylinder main body on a side opposite to the rod and that defines a gas chamber, filled with gas, with the end portion of the cylinder main body. 
     According to this structural configuration, the gas cylinder that slidably houses the end portion of the cylinder main body on the side opposite to the rod functions as a gas damper, and, as a result, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the gas cylinder includes a contact portion that is contactable with the end portion of the cylinder main body. The contact portion regulates a minimum volume of the gas chamber by regulating a position of a stroke end of the end portion of the cylinder main body with respect to the gas cylinder. According to this structural configuration, an increase of the pressure in the gas chamber is regulated. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. Additionally, the piston includes a hollow piston having a cylindrical inner peripheral surface that defines a gas chamber filled with gas. Additionally, the gas damper includes a double piston structure including the hollow piston and an internal piston that is housed in the hollow piston slidably on the inner peripheral surface, coupled to the rod, and partitions the gas chamber into a pair of gas chambers. 
     According to this structural configuration, the internal piston performs a stroke in the hollow piston in response to the extension/contraction of the rod, and, as a result, gas in a corresponding one of the gas chambers in the hollow piston is compressed. Thus, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the hollow piston includes a contact portion that is contactable with the internal piston and that regulates a minimum volume of the gas chamber by regulating a position of a stroke end of the internal piston with respect to the hollow piston. According to this structural configuration, an increase of the pressure in the gas chamber is regulated. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. Additionally, the gas damper includes a pair of independent foam structures located in the pair of oil chambers, respectively. According to this structural configuration, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated by the independent foam structure located in each of the oil chambers. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the hydraulic cylinder includes a cylinder main body, a piston that is slidable in the cylinder main body and that defines a pair of oil chambers in the cylinder main body, and a rod that extends outwardly from the cylinder main body through one of the pair of oil chambers. The hydraulic cylinder extends and contracts the rod by selectively supplying hydraulic oil from a hydraulic circuit to either one of the pair of oil chambers. Additionally, the gas damper includes a movable partition that defines a gas chamber filled with gas in the hydraulic cylinder or in a sub-cylinder that is hydraulically connected to the hydraulic cylinder. 
     According to this structural configuration, gas, for example, in the gas chamber defined by the movable partition in the hydraulic cylinder functions as a damper, and, as a result, an external force that acts in the extension/contraction direction of the hydraulic cylinder is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     In a preferred embodiment of the present invention, the mount assembly further includes an elastic mount that elastically supports the outboard motor main body with respect to the swivel bracket, and the gas damper functions as a gas spring having a spring constant lower than a spring constant of the elastic mount. According to this structural configuration, it becomes possible to extend and contract the gas damper even when an external force that acts in the extension/contraction direction of the hydraulic cylinder is small. Therefore, it is possible to improve an anti-vibration effect, and it is possible to further significantly reduce or prevent vibrations transmitted to the hull. 
     In another preferred embodiment of the present invention, an anti-vibration structure of an outboard motor includes a hydraulic cylinder that is able to turn an outboard motor main body around a tilt shaft coupled to a clamp bracket fixed to a hull and a gas damper to damp an external force that acts in a telescopic direction of the hydraulic cylinder. According to this structural configuration, an external force acting in the extension/contraction direction of the hydraulic cylinder that turns the outboard motor main body is attenuated by the gas damper. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a side view shown to describe an example of an outboard motor according to a preferred embodiment of the present invention. 
         FIG.  2    is a descriptive view showing a state in which an outboard motor main body has been tilted up. 
         FIG.  3    is a schematic view of a mount assembly that attaches the outboard motor main body to a vessel. 
         FIG.  4    is a cross-sectional view showing a cross section along line IV-IV of  FIG.  1   . 
         FIG.  5 A  is a schematic view shown to describe an example of an anti-vibration structure of the outboard motor. 
         FIG.  5 B  is a schematic cross-sectional view that describes a function in the example of the anti-vibration structure of the outboard motor of  FIG.  5 A . 
         FIG.  6 A  is a schematic view shown to describe another example of an anti-vibration structure of the outboard motor. 
         FIG.  6 B  is a schematic cross-sectional view that describes a function in the example of the anti-vibration structure of the outboard motor of  FIG.  6 A . 
         FIG.  7    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  8    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  9    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  10    is a schematic view shown to describe still another example of the anti-vibration structure of the outboard motor. 
         FIG.  11    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  12    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  13    is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  14 A  is a schematic view shown to describe still another example of an anti-vibration structure of the outboard motor. 
         FIG.  14 B  is a schematic cross-sectional view that describes a function in the example of the anti-vibration structure of the outboard motor of  FIG.  14 A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    is a side view shown to describe an example of an outboard motor  1  according to a preferred embodiment of the present invention. The outboard motor  1  includes an outboard motor main body  2 , a mount assembly  3 , and a power trim &amp; tilt mechanism  4  (hereinafter, referred to as “PTT mechanism  4 ”). The outboard motor main body  2  is attached to a rear portion of a hull  5   a  of a vessel  5  by the mount assembly  3 . 
     The mount assembly  3  includes a swivel bracket  7 , a clamp bracket  6 , a steering shaft  8 , a tilt shaft  9 , an upper bracket  10 , a lower bracket  11 , and a plurality of anti-vibration mounts  12 . The steering shaft  8  extends in an up-down direction. The tilt shaft  9  extends horizontally or substantially horizontally in a left-right direction (direction perpendicular to the plane of paper of  FIG.  1   ). The swivel bracket  7  is coupled to the outboard motor main body  2  through the steering shaft  8 . The PTT mechanism  4  is an example of a turning mechanism that turns the outboard motor main body  2  around the tilt shaft  9  and tilts the outboard motor main body  2  with respect to the clamp bracket  6 . The anti-vibration mounts  12  are located on the upper and lower sides. The anti-vibration mount  12  is an example of an elastic mount that elastically supports the outboard motor main body  2  with respect to the swivel bracket  7 . 
     The upper bracket  10  and the lower bracket  11  are each an example of a bracket by which the outboard motor main body  2  is attached to the hull  5   a.  The upper bracket  10  is fixed to an upper end portion  8   a  of the steering shaft  8 . The lower bracket  11  is fixed to a lower end portion  8   b  of the steering shaft  8 . The upper bracket  10  and the lower bracket  11  are each coupled to the outboard motor main body  2  through the anti-vibration mount  12 . 
     The outboard motor main body  2  is attached to the hull  5   a  in a substantially vertical attitude by the mount assembly  3 . The clamp bracket  6  is fixed to the hull  5   a.  The swivel bracket  7  is coupled to the clamp bracket  6  through the tilt shaft  9  so as to turn around the tilt shaft  9 , and supports the outboard motor main body  2 . The outboard motor main body  2  and the swivel bracket  7  are turnable around the tilt shaft  9  upwardly and downwardly with respect to the clamp bracket  6 . The outboard motor main body  2  and the swivel bracket  7  are turned around the tilt shaft  9  upwardly and downwardly by the PTT mechanism  4 . The PTT mechanism  4  is actuated by operating a PTT operation switch (not shown). Therefore, the outboard motor main body  2  is tilted with respect to the clamp bracket  6  by the operation of the PTT operation switch. As a result, it is possible to change the inclination angle of the outboard motor main body  2  with respect to the hull  5   a,  and therefore it is possible to make a trim adjustment and to tilt up/down the outboard motor main body  2 . The outboard motor  1  includes an anti-vibration structure BS of the outboard motor (hereinafter, referred to simply as an “anti-vibration structure BS” if necessary) that fulfills a function to significantly reduce or prevent vibrations transmitted to the hull  5   a  by use of a portion of the PTT mechanism  4 . 
     The PTT mechanism  4  includes a trim cylinder  30  to make a trim adjustment and a tilt cylinder  40  to tilt up/down the outboard motor main body  2 . The trim cylinder  30  includes a cylinder main body  31  and a trim rod  32 . The tilt cylinder  40  includes a cylinder main body  41  and a tilt rod  42 . The tilt cylinder  40  is joined between the clamp bracket  6  and the swivel bracket  7 . The tilt cylinder  40  turns the swivel bracket  7  around the tilt shaft  9  with respect to the clamp bracket  6 . The tilt cylinder  40  is an example of a hydraulic cylinder HC applied to the anti-vibration structure BS. The anti-vibration structure BS includes a gas damper GD to damp an external force that acts in an expansion/contraction direction X (a telescopic direction) of the hydraulic cylinder HC. 
     Additionally, the outboard motor main body  2  is turnable around the steering shaft  8  rightwardly and leftwardly with respect to the swivel bracket  7 . A steering wheel (not shown) is operated, and, as a result, the outboard motor main body  2  is turned around the steering shaft  8  rightwardly and leftwardly. This makes it possible to steer the vessel  5 . 
     The outboard motor main body  2  includes an engine  13  as an example of a prime mover, a drive shaft  14 , a propeller shaft  15 , a propeller  16  as an example of a thrust generating member, a forward-reverse switching mechanism  17  as an example of a clutch, and an ECU (Electronic Control Unit)  18  as an example of a controller. Additionally, the outboard motor main body  2  includes an engine cover  19  and a casing  20 . The engine  13  and the ECU  18  are housed in the engine cover  19 . Additionally, the drive shaft  14  extends upwardly and downwardly in the engine cover  19  and the casing  20 . The propeller shaft  15  extends in a front-rear direction in a lower portion of the casing  20 . An upper end portion of the drive shaft  14  is coupled to the engine  13 . A lower end portion of the drive shaft  14  is coupled to a front end portion of the propeller shaft  15  by the forward-reverse switching mechanism  17 . The propeller  16  is coupled to a rear end portion of the propeller shaft  15 . The propeller  16  rotates together with the propeller shaft  15 . The propeller  16  is rotationally driven by the engine  13 . In other words, the engine  13  rotates the propeller  16 . 
     The engine  13  may be, for example, an internal combustion engine that burns fuel, such as gasoline, to generate power. The engine  13  includes a crankshaft  21 , a plurality of (for example, four) cylinders  22 , and a rotation speed detector  23 . The engine  13  is oriented such that the crankshaft  21  extends vertically. The upper end portion of the drive shaft  14  is joined to the crankshaft  21 . The crankshaft  21  is rotationally driven around a vertical axis by combustion in each of the cylinders  22 . The rotation speed of the crankshaft  21  (rotation speed of the engine  13 ) is detected by both the rotation speed detector  23  and the ECU  18 . The rotation speed detector  23  outputs a detection signal that synchronizes with the rotation of the crankshaft  21 . The ECU  18  calculates the engine rotation speed based on the detection signal. 
       FIG.  2    is a side view showing a state in which the outboard motor main body  2  has been tilted up (a state in which the inclination angle is within a tilt range). The outboard motor main body  2  is turned around the tilt shaft  9  between a substantially vertical attitude and an attitude in which the outboard motor main body  2  is largely inclined while directing a front surface of the outboard motor main body  2  (front surface of the engine cover  19  and front surface of the casing  20 ) toward the lower side as shown in  FIG.  2   . If the inclination angle of the outboard motor main body  2  when a lower end of the drive shaft  14  becomes closest to the hull  5   a  is assumed as zero, the range within which the inclination angle of the outboard motor main body  2  is small is a trim range, and the range within which the inclination angle of the outboard motor main body  2  is larger than an upper-limit boundary value of the trim range is a tilt range. In  FIG.  2   , a state in which the inclination angle of the outboard motor main body  2  is a lower-limit boundary value of the trim range is shown by an alternate long and short dashed line, whereas a state in which the inclination angle of the outboard motor main body  2  is the upper-limit boundary value of the trim range is shown by an alternate long and two short dashed line. Additionally, in  FIG.  2   , a state in which the inclination angle of the outboard motor main body  2  is an upper limit value of the tilt range (full-tilt-up) is shown by a solid line. The upper limit value of the tilt range is, for example, a maximum value of the inclination angle of the outboard motor main body  2 . The outboard motor main body  2  is able to be held at an arbitrary position of the trim range and of the tilt range. 
     “Turning the outboard motor main body  2  upwardly within a trim range” is referred to as “trim up,” and “turning the outboard motor main body  2  downwardly within a trim range” is referred to as “trim down.” As a more functional definition, “turning the outboard motor main body  2  upwardly for a trim adjustment of the vessel” is referred to as “trim up,” and “turning the outboard motor main body  2  downwardly for a trim adjustment of the vessel” is referred to as “trim down.” On the other hand, “turning the outboard motor main body  2  upwardly with the aim of raising the propeller  16  over the surface of water” is referred to as “tilt up,” and “turning the outboard motor main body  2  downwardly with the aim of lowering the propeller  16  under the surface of water” is referred to as “tilt down.” Therefore, there is a case in which “tilt up” and “tilt down” are used for the up-and-down movement of the outboard motor main body  2  in both the trim range and the tilt range. 
       FIG.  3    is a schematic view of the mount assembly  3 , and shows a state of a portion of the mount assembly  3  seen from the rear. The clamp bracket  6  includes a pair of clamp brackets  6  as shown in  FIG.  3   . The pair of clamp brackets  6  are located at a distance from each other in the left-right direction. A portion of the swivel bracket  7  and the PTT mechanism  4  are located between the pair of clamp brackets  6 . 
     The PTT mechanism  4  includes, for example, two trim cylinders  30  and a single tilt cylinder  40 . Each of the trim cylinders  30  and the tilt cylinder  40  are located between the two clamp brackets  6 . The two trim cylinders  30  are located so as to coincide with each other when seen from the left-right direction of the hull  5   a.  The two trim cylinders  30  are located on both the right/left sides of the tilt cylinder  40 . Each of the trim cylinders  30  is obliquely located along the front-rear direction of the vessel so that an upper end of the trim cylinder  30  is placed at a more rearward position than a lower end of the trim cylinder  30 . Likewise, the tilt cylinder  40  is obliquely located along the front-rear direction of the vessel so that an upper end of the tilt cylinder  40  is placed at a more rearward position than a lower end of the tilt cylinder  40 . Each of the trim cylinders  30  and the tilt cylinder  40  are each, for example, a hydraulic cylinder. A tank T 1  in which hydraulic oil is stored and an electric motor M 1  that drives a hydraulic pump that supplies hydraulic oil are located between the two clamp brackets  6  as shown in  FIG.  3   . The outboard motor main body  2  and the swivel bracket  7  are turned around the tilt shaft  9  by each of the trim cylinders  30  and the tilt cylinder  40 . The electric motor M 1  and the hydraulic pump are one example of an electrically-operated actuator in a preferred embodiment of the present invention, and supply a driving force to the PTT mechanism  4 . 
     The cylinder main body  31  of each of the trim cylinders  30  is coupled to a corresponding one of the clamp brackets  6 . The trim rod  32  obliquely upwardly protrudes from an upper end portion of the cylinder main body  31  toward the rear. The trim rod  32  is reciprocated in an axial direction of the trim rod  32  by hydraulic pressure in the cylinder main body  31 . An upper end portion of each of the trim rods  32  comes into contact with the swivel bracket  7  in a state in which the inclination angle of the outboard motor main body  2  is within the trim range as shown by the alternate long and two short dashed line in  FIG.  2   . Therefore, in this state, the outboard motor main body  2  is supported from the front side by the two trim rods  32  through the swivel bracket  7 . Additionally, when the inclination angle of the outboard motor main body  2  is large and reaches the tilt range, the upper end portion of each of the trim rods  32  is separated from the swivel bracket  7 . Therefore, the support of the outboard motor main body  2  by the two trim rods  32  is released. 
     The inclination angle of the outboard motor main body  2  when the trim rods  32  are in contact with the swivel bracket  7  in a maximum expansion state is the upper-limit boundary value of the trim range. In other words, the upper-limit boundary value of the trim range is defined by the upper limit value of the inclination angle that is changeable by driving the trim cylinder  30 . On the other hand, a minimum value of the inclination angle that can be taken by the outboard motor main body  2  when the trim rod  32  is in a minimum expansion state, i.e., in a contracted state is the lower-limit boundary value of the trim range. 
     A lower end portion of the cylinder main body  41  of the tilt cylinder  40  is coupled to the clamp bracket  6 . The tilt rod  42  obliquely upwardly protrudes from an upper end portion of the cylinder main body  41  toward the rear. An upper end portion of the tilt rod  42  is coupled to the swivel bracket  7 . The tilt rod  42  is reciprocated in an axial direction of the tilt rod  42  by hydraulic pressure in the cylinder main body  41 . The upper end portion of the tilt rod  42  is coupled to the swivel bracket  7  even in a state in which the inclination angle of the outboard motor main body  2  is within either of the trim range and the tilt range. Therefore, the outboard motor main body  2  is supported by the tilt cylinder  40  even in a state in which the inclination angle of the outboard motor main body  2  is within either of the trim range and the tilt range. 
     In a state in which the inclination angle of the outboard motor main body  2  is within the trim range, the outboard motor main body  2  is supported by the two trim cylinders  30  and the single tilt cylinder  40 . Additionally, in this state, the outboard motor main body  2  is turned upwardly and downwardly around the tilt shaft  9  by the two trim cylinders  30  and the single tilt cylinder  40 . The inclination angle of the outboard motor main body  2  is larger in proportion to an increase in the amount of protrusion of each of the trim rods  32  and the tilt rod  42 . Additionally, when the inclination angle of the outboard motor main body  2  is large and reaches the tilt range, the support of the outboard motor main body  2  by the two trim cylinders  30  is released, and the outboard motor main body  2  is supported by the single tilt cylinder  40 . In this state, the outboard motor main body  2  is turned upwardly and downwardly around the tilt shaft  9  by the single tilt cylinder  40 . The inclination angle of the outboard motor main body  2  becomes larger in proportion to an increase in the amount of protrusion of the tilt rod  42 . The inclination angle of the outboard motor main body  2  is changeable within a range from the lower-limit boundary value of the trim range to the upper limit value of the tilt range by the extension and contraction of the tilt rod  42 . 
       FIG.  4    is a cross-sectional view along line IV-IV of  FIG.  1   . The upper anti-vibration mount  12  includes a shaft portion  51 , an elastic portion  52 , an outer cylinder portion  53 , and a fastener  54  as shown in  FIG.  4   . The shaft portion  51  is made of metal, such as aluminum, and is integral with the upper bracket  10 . A screw hole  51   a  extending along a central axis of the shaft portion  51  is provided in the shaft portion  51 . The elastic portion  52  is provided in a circular cylindrical shape with an elastic material, such as rubber or sponge. The elastic portion  52  is coaxially fitted and attached to the shaft portion  51  in a state of surrounding the shaft portion  51 . 
     The outer cylinder portion  53  has a circular cylindrical shape and is made of a metal material, such as aluminum. The outer cylinder portion  53  is coaxially fitted and attached to the elastic portion  52  in a state of surrounding the elastic portion  52 . The outer cylinder portion  53  is not in contact with the shaft portion  51 . The elastic portion  52  may be always compressed between the shaft portion  51  and the outer cylinder portion  53 . A portion of the outer cylinder portion  53  is housed in a concave portion  20   a  that is hollowed from a surface of the casing  20 . The outer cylinder portion  53  is fixed to the casing  20  of the outboard motor main body  2  by a fixing member  24 . The fixing member  24  is fastened to the casing  20  by a fastening member  25 , such as a bolt. The outer cylinder portion  53  may be regarded as an element of the casing  20 , and not an element of the anti-vibration mount  12 , because the outer cylinder portion  53  is fixed to the casing  20 . The fastening member  54  is, for example, a bolt screwed into a screw hole  51   a  of the shaft portion  51 . Although not shown, the elastic portion  52  is clamped in the axial direction between a head portion of the bolt and a predetermined portion of the upper bracket  10 . 
     The lower anti-vibration mount  12  has the same configuration as the upper anti-vibration mount  12  except that the shaft portion  51  of the lower anti-vibration mount  12  is integral with the lower bracket  11 . In each of the anti-vibration mounts  12 , the elastic portion  52  is interposed between the shaft portion  51  fixed to the swivel bracket  7  through the steering shaft  8  and the outer cylinder portion  53  fixed to the casing  20  of the outboard motor main body  2 , and is elastically deformable. Therefore, the outboard motor main body  2  is elastically supported by the anti-vibration mount  12 . Vibrations of the outboard motor main body  2  are attenuated by the elastic deformation of the elastic portion  52 , and thus are prevented from being transmitted to the hull  5   a.    
       FIG.  5 A  is a schematic view of a main portion of the PTT mechanism  4 , and shows an example of the anti-vibration structure BS of the outboard motor. The tilt cylinder  40  defining the hydraulic cylinder includes the cylinder main body  41 , the tilt rod  42 , a piston  43 , a free piston  44 , a pair of oil chambers  45  and  46 , and a gas chamber  47  as shown in  FIG.  5 A . 
     The cylinder main body  41  includes a circular cylindrical tube having a bottom. The cylinder main body  41  includes a pair of end portions  41   a  and  41   b,  an outer peripheral surface  41   c,  an inner peripheral surface  41   d,  a contact portion  41   e,  a first port P 1 , and a second port P 2 . A portion of the tilt rod  42  is inserted in the cylinder main body  41 . The piston  43  is housed in the cylinder main body  41 , and defines the pair of oil chambers  45  and  46  in the cylinder main body  41 . The piston  43  slides in the axial direction of the tilt rod  42  along the inner peripheral surface  41   d  of the cylinder main body  41  in the cylinder main body  41 . 
     The tilt rod  42  extends through, for example, the upper oil chamber  46 , which is one of the pair of oil chambers  45  and  46 , and extends outwardly from the cylinder main body  41 . The tilt cylinder  40  is extendable and contractible in the expansion/contraction direction X corresponding to the axial direction of the tilt rod  42  in response to the movement in the axial direction of the tilt rod  42 . The first port P 1  includes an opening that extends through the outer peripheral surface  41   c  and the inner peripheral surface  41   d  of the cylinder main body  41  and that communicates with the lower oil chamber  45 . The second port P 2  includes an opening that extends through the outer peripheral surface  41   c  and the inner peripheral surface  41   d  of the cylinder main body  41  and that communicates with the upper oil chamber  46 . 
     The free piston  44  is housed in the cylinder main body  41 . The free piston  44  is located on the side opposite to the tilt rod  42  with respect to the piston  43 , and defines the gas chamber  47  in the cylinder main body  41 . The free piston  44  is an example of a movable partition KS by which the lower oil chamber  45  and the gas chamber  47  are divided from each other in the cylinder main body  41 . The free piston  44  slides in the axial direction of the tilt rod  42  along the inner peripheral surface  41   d  of the cylinder main body  41  in the cylinder main body  41  on the side opposite to the tilt rod  42 . 
     The free piston  44  has a disk shape, and includes two end surfaces  44   a,    44   b  and an outer peripheral surface  44   c.  The upper end surface  44   a  faces the lower oil chamber  45 . The lower end surface  44   b  faces the gas chamber  47 . A seal  44   e,  such as an O-ring, is held in a circumferential groove  44   d  provided at the outer peripheral surface  44   c.  The seal  44   e  seals a portion between the outer peripheral surface  44   c  of the free piston  44  and the inner peripheral surface  41   d  of the cylinder main body  41 . A gas-filled structure GSS that defines the gas chamber  47  that is filled with gas is provided in the cylinder main body  41  by the free piston  44 . The tilt cylinder  40  is able to fulfill a function of a gas damper GD by the gas-filled structure GSS provided in the cylinder main body  41 . The gas with which the gas chamber  47  is filled may be air, or may be an inert gas, such as a nitrogen gas. An example of the anti-vibration structure BS of the outboard motor includes the tilt cylinder  40  and the gas damper GD. 
     In response to the up-and-down movement of the free piston  44 , the volume of the gas chamber  47  changes and the inner pressure of the gas chamber  47  changes. The inner pressure of the gas chamber  47  generates a force that extends the tilt rod  42  through the free piston  44  and the piston  43 . In other words, the gas damper GD functions as a gas spring in a direction in which the tilt cylinder  40  is extended. 
     The contact portion  41   e  of the cylinder main body  41  is located in the gas chamber  47 . The contact portion  41   e  includes a step that protrudes inwardly from the inner peripheral surface  41   d  of the cylinder main body  41  and that faces the free piston  44 . The contact portion  41   e  is in contact with the lower end surface  44   b  of the free piston  44  when the free piston  44  moves to a lower stroke end as shown in  FIG.  5 B . The gas chamber  47  includes a first portion  47   a  that is located at a higher position than the contact portion  41   e  and a second portion  47   b  that is located at a lower position than the contact portion  41   e.  The position of the stroke end of the free piston  44  is regulated in a state in which the contact portion  41   e  is in contact with the free piston  44 . Additionally, the gas chamber  47  is provided only with the second portion  47   b  in a state in which the contact portion  41   e  is in contact with the free piston  44 . In other words, the contact portion  41   e  fulfills a function to regulate the minimum volume of the gas chamber  47 , and the minimum volume of the gas chamber  47  corresponds to the volume of the second portion  47   b.    
     The PTT mechanism  4  includes a hydraulic circuit  60  that supplies/discharges hydraulic oil to the pair of oil chambers  45  and  46  of the tilt cylinder  40 . It is configured so that the tilt rod  42  is extended and contracted in response to the supply/discharge of hydraulic oil from the hydraulic circuit  60  to the pair of oil chambers  45  and  46 . The hydraulic circuit  60  includes a hydraulic pump  61 , a reservoir tank  62 , and a main valve assembly  63 . The hydraulic pump  61  is driven by the electric motor M 1 . The electric motor M 1  is able to make normal rotation and reverse rotation, and the hydraulic pump  61  is normally rotationally or reversely rotationally driven by the electric motor M 1 . The main valve assembly  63  is connected to two ports  61   a  and  61   b  of the hydraulic pump  61 . 
     The main valve assembly  63  includes a cylinder  64 , a shuttle  65  that slides in the cylinder  64 , and two on-off valves  66  and  67  located at both sides of the cylinder  64 , respectively. Two oil chambers  64   a  and  64   b  are defined at both sides of the shuttle  65  in the cylinder  64 . The two ports  61   a  and  61   b  of the hydraulic pump  61  are joined to the two oil chambers  64   a  and  64   b  through oil passages L 11  and L 12 , respectively. The on-off valves  66  and  67  are check valves, and are opened by an increase in hydraulic pressure in the oil chambers  64   a  and  64   b  corresponding to the on-off valves  66  and  67 . Additionally, the on-off valves  66  and  67  are opened by being pressed by corresponding needles  65   a  and  65   b  joined to the shuttle  65 . In other words, a shuttle valve includes the shuttle  65  and the needles  65   a  and  65   b.    
     The lower oil chamber  45  of the tilt cylinder  40  is joined to the on-off valve  66  of the main valve assembly  63  through the first port P 1  and an oil passage L 1 . The upper oil chamber  46  of the tilt cylinder  40  is joined to the on-off valve  67  of the main valve assembly  63  through the second port P 2  and an oil passage L 2 . The two on-off valves  66  and  67  of the main valve assembly  63  are closed in a state in which the electric motor M 1  has stopped running. Therefore, hydraulic oil is not supplied/discharged from the hydraulic circuit  60  to the oil chambers  45  and  46  of the tilt cylinder  40 . The gas-filled structure GSS provided in the tilt cylinder  40  functions as a gas damper GD in a state in which hydraulic oil is not supplied/discharged from the hydraulic circuit  60  to the oil chambers  45  and  46  of the tilt cylinder  40 . 
     When the hydraulic pump  61  is normally rotationally driven by the electric motor M 1 , the hydraulic pump  61  receives hydraulic oil from the port  61   b,  and discharges hydraulic oil from the port  61   a.  The discharged hydraulic oil is supplied from the main valve assembly  63  to the oil passage L 1 . As a result, the hydraulic oil is supplied to the lower oil chamber  45  of the tilt cylinder  40  through the first port P 1 . Thus, the tilt rod  42  is extended, and turns the swivel bracket  7  upwardly. Hydraulic oil is replenished from the reservoir tank  62  through a one-way valve  68  when hydraulic oil is insufficient. On the other hand, hydraulic oil in the upper oil chamber  46  of the tilt cylinder  40  is drawn into the hydraulic pump  61  through the second port P 2 , the oil passage L 2 , and the main valve assembly  63 . At this time, the on-off valve  66  of the main valve assembly  63  is pressed and opened by the needle  65   b  of the shuttle  65 . 
     When the hydraulic pump  61  is reversely rotationally driven by the electric motor M 1 , the hydraulic pump  61  receives hydraulic oil from the port  61   a,  and discharges hydraulic oil from the port  61   b.  The discharged hydraulic oil is supplied from the main valve assembly  63  to the oil passage L 2 . As a result, hydraulic oil is supplied to the upper oil chamber  46  of the tilt cylinder  40  through the second port P 2 . As a result, the tilt rod  42  is contracted, and turns the swivel bracket  7  downwardly. Hydraulic oil is replenished from the reservoir tank  62  through a one-way valve  69  when hydraulic oil is insufficient. On the other hand, hydraulic oil in the lower oil chamber  45  of the tilt cylinder  40  is drawn into the hydraulic pump  61  through the first port P 1 , the oil passage L 1 , and the main valve assembly  63 . The tilt rod  42  is contracted by being pushed by the swivel bracket  7 . 
     According to the present preferred embodiment, the outboard motor  1  includes the tilt cylinder  40  as an example of a hydraulic cylinder that turns the swivel bracket  7  around the tilt shaft  9  with respect to the clamp bracket  6  as shown in  FIG.  1   . Additionally, the outboard motor  1  includes the gas damper GD that attenuates an external force that acts in the extension/contraction direction X of the tilt cylinder  40  as shown in  FIG.  5 A . Therefore, an external force acting in the extension/contraction direction X of the tilt cylinder  40  is attenuated by the gas damper GD. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull  5   a,  and makes it possible to improve the riding comfort of the vessel. 
     Additionally, the gas damper GD includes the gas-filled structure GSS in the cylinder main body  41  of the tilt cylinder  40  and that is filled with gas. The gas-filled structure GSS in the cylinder main body  41  functions as a damper, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull  5   a.    
     Additionally, the gas-filled structure GSS includes the free piston  44  that slides in the cylinder main body  41  and that defines the gas chamber  47  in the cylinder main body  41 . Gas with which the gas chamber  47  defined by the free piston  44  in the cylinder main body  41  is filled functions as a damper, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull  5   a.    
     Additionally, the cylinder main body  41  includes the contact portion  41   e  contactable with the free piston  44  so as to regulate the position of a stroke end of the free piston  44  as shown in  FIG.  5 B . The minimum volume of the gas chamber  47  is regulated in a state in which the contact portion  41   e  is in contact with the free piston  44 . Therefore, an increase of the pressure in the gas chamber  47  is regulated. 
     Additionally, the mount assembly  3  includes the anti-vibration mount  12  that elastically supports the outboard motor main body  2  with respect to the swivel bracket  7  as shown in  FIG.  1   . The gas damper GD functions as a gas spring having a spring constant lower than that of the anti-vibration mount  12 . Therefore, it becomes possible to extend/contract the gas damper GD even when an external force that acts in the extension/contraction direction X of a hydraulic cylinder (for example, the tilt cylinder  40 ) is small. Therefore, it is possible to improve an anti-vibration effect, and it is possible to further significantly reduce or prevent vibrations transmitted to the hull  5   a.  Particularly, the gas damper GD that functions as a gas spring having a spring constant lower than that of the anti-vibration mount  12  makes it possible to improve an anti-vibration effect when the engine rotates at a low speed during low-speed navigation, such as trolling. 
       FIG.  6 A  is a schematic view shown to describe another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  6 A  mainly differs from the example of 
       FIG.  5 A  as follows. Specifically, the gas damper GD includes a sub-cylinder  70  hydraulically connected to, for example, an oil chamber  45  that is one of the oil chambers  45  of the tilt cylinder  40  and that is an example of a hydraulic cylinder. The sub-cylinder  70  includes a sub-cylinder main body  71 , a sub-oil chamber  72 , a gas chamber  73 , and a free piston  74 . The sub-cylinder main body  71  includes a circular cylindrical tube. The cylinder main body  41  includes a pair of end portions  71   a,    71   b,  an outer peripheral surface  71   c,  an inner peripheral surface  71   d,  and a contact portion  71   e.    
     The sub-oil chamber  72  and the gas chamber  73  are located in the sub-cylinder main body  71 . The free piston  74  is an example of a movable partition KS that is housed in the sub-cylinder main body  71  and by which the sub-oil chamber  72  and the gas chamber  73  are divided from each other. The free piston  74  slides in the sub-cylinder main body  71 . The sub-oil chamber  72  is located at a side of one end portion  71   a  with respect to the free piston  74 . The gas chamber  73  is located at the other side of an end portion  71   b  with respect to the free piston  74 . An oil passage L 3  is provided between the tilt cylinder  40  and the sub-cylinder  70 . The oil chamber  45  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70  are able to communicate with each other through the oil passage L 3 . 
     Two ports P 3  and P 4  are provided at two end portions of the oil passage L 3 , respectively. The port P 3  is located at, for example, the end portion  41   b  of the cylinder main body  41  of the tilt cylinder  40 . The port P 4  is located at the end portion  71   a  of the sub-cylinder main body  71 . For example, an on-off valve V 1  that is manually operable is interposed in the oil passage L 3 . The on-off valve V 1  is able to interrupt the flow of hydraulic oil between the oil chamber  45  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70  in the closed state. The on-off valve V 1  allows the flow of hydraulic oil between the oil chamber  45  and the sub-oil chamber  72  in an opened state. 
     The free piston  74  is the same in configuration as the free piston  44  of  FIG.  5 A , and includes two end surfaces  74   a,    74   b,  an outer peripheral surface  74   c,  and a circumferential groove  74   d  as shown in  FIG.  6 B . The end surface  74   a,  which is one of the two end surfaces, faces the sub-oil chamber  72 , and the other end surface  74   b  faces the gas chamber  73 . A seal  74   e,  such as an O-ring, is held in the circumferential groove  74   d.  The seal  74   e  seals a portion between the outer peripheral surface  74   c  of the free piston  74  and the inner peripheral surface  71   d  of the sub-cylinder main body  71 . A gas-filled structure GSS that defines the gas chamber  73  filled with gas is provided in the sub-cylinder main body  71  by the free piston  74 . The sub-cylinder  70  functions as a gas damper GD. 
     The free piston  74  rises and falls when the tilt cylinder  40  extends and contracts. In response to the up-and-down movement of the free piston  74 , the volume of the gas chamber  73  changes and the inner pressure of the gas chamber  73  changes. The inner pressure of the gas chamber  73  generates a force that extends the tilt rod  42  through the free piston  74  and the piston  43 . A force that extends the tilt rod  42  increases when the inner pressure of the gas chamber  73  increases in response to the contraction of the tilt cylinder  40 . In other words, the gas damper GD functions as a gas spring that resists the contraction of the tilt cylinder  40 . Preferably, the gas damper GD functions as a gas spring having a spring constant lower than that of the anti-vibration mount  12 . 
     The contact portion  71   e  of the sub-cylinder main body  71  is located in the gas chamber  73 . The contact portion  71   e  includes a step that protrudes inwardly from the inner peripheral surface  71   d  of the sub-cylinder main body  71  and that faces a side of the free piston  74 . When the free piston  74  moves to the lower stroke end, the contact portion  71   e  is in contact with the other end surface  74   b  of the free piston  74  as shown in  FIG.  6 B . The gas chamber  73  includes a first portion  73   a  located higher than the contact portion  71   e  and a second portion  73   b  located lower than the contact portion  71   e.  The position of the stroke end of the free piston  74  is regulated in a state in which the contact portion  71   e  is in contact with the free piston  74 . Additionally, the gas chamber  73  is provided only with the second portion  73   b  in a state in which the contact portion  71   e  is in contact with the free piston  74 . In other words, the contact portion  71   e  fulfills a function to regulate the minimum volume of the gas chamber  73 , and this minimum volume of the gas chamber  73  corresponds to the volume of the second portion  73   b.    
     According to the present preferred embodiment, the sub-cylinder  70  hydraulically connected to the oil chamber  45  of the tilt cylinder  40  functions as a gas damper GD, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, in the sub-cylinder main body  71 , the sub-oil chamber  72  and the gas chamber  73  are partitioned by the free piston  74 . The sub-oil chamber  72  is in communication with the oil chamber  45  of the tilt cylinder  40 . Gas with which the gas chamber  73  is filled functions as a damper, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. Therefore, it is possible to significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, the sub-cylinder main body  71  includes the contact portion  71   e  that is contactable with the free piston  74  so as to regulate the position of the stroke end of the free piston  74 . The contact portion  71   e  regulates the minimum volume of the gas chamber  73  in a state of being in contact with the free piston  74 . Therefore, an increase of the pressure in the gas chamber  73  is regulated. 
     Additionally, the on-off valve V 1  that interrupts the flow of hydraulic oil between the oil chamber  45  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70  is able to be manually operated. The sub-cylinder  70  functions as a gas damper GD in a state in which the on-off valve V 1  has been opened. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. For example, during low-speed navigation, such as trolling, it is possible to improve an anti-vibration effect when the engine rotates at a low speed in a state in which the on-off valve V 1  has been opened. It should be noted that the on-off valve V 1  is manually closed when the navigation speed of the vessel increases. 
     Additionally, the gas damper GD functions as a gas spring having a spring constant lower than that of the anti-vibration mount  12 , thus making it possible to improve an anti-vibration effect when the engine rotates at a low speed during low-speed navigation, such as trolling. 
       FIG.  7    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  7    mainly differs from the example of  FIG.  6 A  as follows. Specifically, an electromagnetic control valve V 2  is provided as an on-off valve interposed in the oil passage L 3  by which the oil chamber  45  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70  are in communication with each other. The ECU  18 , which is an example of a controller, calculates an engine rotation speed of the engine  13 , which is an example of a prime mover, based on a detection signal of the rotation speed detector  23 . The ECU  18  is configured or programmed to shut off the control valve V 2 , which is an on-off valve, when the detected rotation speed exceeds a predetermined threshold value. In other words, when the detected engine rotation speed is equal to or below the predetermined threshold value, the control valve V 2  is opened. 
     According to the present preferred embodiment, the same effect as the example of  FIG.  6 A  is achieved. Additionally, when the detection value of the engine rotation speed is lower than the threshold value, the sub-cylinder  70  is able to automatically function as a gas damper GD. This makes it possible to automatically improve an anti-vibration effect when the engine rotates at a low speed during low-speed navigation, such as trolling. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. 
       FIG.  8    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  8    mainly differs from the example of  FIG.  7    as follows. Specifically, a sub-cylinder  70 P hydraulically connected to the upper oil chamber  46  of the tilt cylinder  40  is provided. The sub-cylinder  70 P has the same in configuration as the sub-cylinder  70  hydraulically connected to the lower oil chamber  45  of the tilt cylinder  40 . The elements of the sub-cylinder  70 P are provided with the same reference characters as the equivalent elements of the sub-cylinder  70 . The upper oil chamber  46  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70 P are in communication with each other through an oil passage L 4 . Two ports P 5  and P 6  are respectively located at two end portions of the oil passage L 4 . The port P 5  is located at, for example, the end portion  41   a  of the cylinder main body  41  of the tilt cylinder  40 . The port P 6  is located at the end portion  71   a  of the sub-cylinder main body  71  of the sub-cylinder  70 P. 
     An electromagnetic control valve V 3 , which is an on-off valve, interrupts the flow of hydraulic oil between the upper oil chamber  46  of the tilt cylinder  40  and the sub-oil chamber  72  of the sub-cylinder  70 P. The control valve V 3  is interposed in the oil passage L 4 . The ECU  18  is configured or programmed to shut off the control valve V 2  and the control valve V 3  when the engine rotation speed detected by the rotation speed detector  23  exceeds a predetermined threshold value. When the detected engine rotation speed is equal to or below the predetermined threshold value, the control valve V 2  and the control valve V 3  are opened. 
     When the tilt cylinder  40  that has received an external force in the extension/contraction direction X is extended in a state in which the control valve V 2  and the control valve V 3  have been opened, hydraulic oil flows from the upper oil chamber  46  to the sub-oil chamber  72  of the sub-cylinder  70 P, and gas in the gas chamber  73  of the sub-cylinder  70 P is compressed. Additionally, when the tilt cylinder  40  that has received an external force in the extension/contraction direction X is contracted, hydraulic oil flows from the lower oil chamber  45  to the sub-oil chamber  72  of the sub-cylinder  70 , and gas in the gas chamber  73  of the sub-cylinder  70  is compressed. 
     According to this structural configuration, the same effect as the example of  FIG.  7    is achieved. Additionally, each of the sub-cylinders  70 P and  70  that respectively correspond to the extension and the contraction of the tilt cylinder  40  functions as a gas damper GD. This makes it possible to attenuate an extension/contraction bidirectional external force that acts on the tilt cylinder  40 . Therefore, the attenuation effect is high. This makes it possible to further significantly reduce or prevent vibrations transmitted to the hull. 
       FIG.  9    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  9    mainly differs from the example of  FIG.  8    as follows. Specifically, a single (only one) sub-cylinder  80  hydraulically connected to the two oil chambers  45  and  46  of the tilt cylinder  40  is provided. The sub-cylinder  80  includes two sub-oil chambers  81  and  82  and two gas chambers  83  and  84 . The lower oil chamber  45  of the tilt cylinder  40  and the lower sub-oil chamber  81  of the sub-cylinder  80  are in communication with each other through the oil passage L 3 . The electromagnetic control valve V 2 , which is an on-off valve, is located in the oil passage L 3 . The upper oil chamber  46  of the tilt cylinder  40  and the upper sub-oil chamber  82  of the sub-cylinder  80  are in communication with each other through the oil passage L 4 . 
     The sub-cylinder  80  includes a sub-cylinder main body  85  and a rod piston  86  that functions as a movable partition KS. The rod piston  86  includes a gas piston  89  and two rods  87  and  88  that extend from a central portion  89 H of the gas piston  89 . The rod piston  86  provides an all-in-one piston HGP that functions as a hydraulic piston and as a gas piston. The gas piston  89  divides the two gas chambers  83  and  84  from each other. The rod piston  86  divides the two sub-oil chambers  81  and  82  from each other. The lower rod  87  defines a portion of the lower sub-oil chamber  81 , and functions as a hydraulic piston. The upper rod  88  defines a portion of the upper sub-oil chamber  82 , and functions as a hydraulic piston. 
     The sub-cylinder main body  85  includes a circular cylindrical center tube  90 , two circular cylindrical end tubes  91  and  92 , and two end covers  93  and  94 . The two end tubes  91  and  92  are fitted and fixed to two end portions  90   a  and  90   b  of the center tube  90 . The inner diameter of each of the two end tubes  91  and  92  is smaller than the inner diameter of the center tube  90 . The two end covers  93  and  94  cover end portions of the two end tubes  91  and  92 . 
     The gas piston  89  has a disk shape, and is housed in the center tube  90 . A gas cylinder is provided with the center tube  90  and the gas piston  89 . The gas piston  89  includes two end surfaces  89   a,    89   b  and an outer peripheral surface  89   c.  The gas piston  89  slides on an inner peripheral surface  90   c  of the center tube  90 . A seal  89   e,  such as an O-ring, is housed in a circumferential groove  89   d  provided at the outer peripheral surface  89   c  of the gas piston  89 . A portion between the outer peripheral surface  89   c  of the gas piston  89  and the inner peripheral surface  90   c  of the center tube  90  is sealed by the seal  89   e.    
     The two rods  87  and  88  of the rod piston  86  functioning as a hydraulic piston extend from a central portion  89 H of the gas piston  89  toward mutually opposite sides along a central axis of the center tube  90 . The two rods  87  and  88  are inserted in the corresponding end tubes  91  and  92 . Each of the rods  87  and  88  is slidable along an inner peripheral surface of a corresponding one of the end tubes  91  and  92 . A hydraulic cylinder includes each of the rods  87  and  88  and a corresponding one of the end tubes  91  and  92 . 
     The lower rod  87  includes an outer peripheral surface  87   a,  a step  87   b,  and an axial hole  87   c.  The step  87   b  is located at the outer peripheral surface  87   a.  The axial hole  87   c  has a bottom, and is in communication with the lower sub-oil chamber  81 . In some cases, the axial hole  87   c  is not provided. The upper rod  88  includes an outer peripheral surface  88   a,  a step  88   b,  and an axial hole  88   c.  The step  88   b  is provided at the outer peripheral surface  88   a.  The axial hole  88   c  has a bottom, and is in communication with the upper sub-oil chamber  82 . In some cases, the axial hole  88   c  is not provided. 
     The lower sub-oil chamber  81  is defined by the lower rod  87  and by the lower end cover  93  in the lower end tube  91 . The lower end cover  93  includes a communication hole  93   a  through which the lower sub-oil chamber  81  and the oil passage L 3  are in communication with each other. The port P 4  of the end portion of the oil passage L 3  is located at an end portion of the communication hole  93   a.  The upper sub-oil chamber  82  is defined by the upper rod  88  and by the upper end cover  94  in the upper end tube  92 . The upper end cover  94  includes a communication hole  94   a  through which the upper sub-oil chamber  82  and the oil passage L 4  are in communication with each other. The port P 6  of the end portion of the oil passage L 4  is located at an end portion of the communication hole  94   a.    
     The lower gas chamber  83  is surrounded by the outer peripheral surface  87   a  of the lower rod  87 , the inner peripheral surface  90   c  of the center tube  90 , the lower end surface  89   a  of the gas piston  89 , and an end surface  91   a  of the lower end tube  91 . Thus, a gas-filled structure GSS is provided. The upper gas chamber  84  is surrounded by the outer peripheral surface  88   a  of the upper rod  88 , the inner peripheral surface  90   c  of the center tube  90 , the upper end surface  89   b  of the gas piston  89 , and an end surface  92   a  of the upper end tube  92 . Thus, a gas-filled structure GSS is provided. 
     When the tilt cylinder  40  that has received an external force in the extension/contraction direction X is contracted in a state in which the control valve V 2  has been opened, hydraulic oil flows from the lower oil chamber  45  of the tilt cylinder  40  into the lower sub-oil chamber  81  of the sub-cylinder  80  through the oil passage L 3 . Therefore, the rod piston  86  and the gas piston  89  rise, and gas in the upper gas chamber  84  is compressed. Additionally, when the tilt cylinder  40  that has received an external force in the extension/contraction direction X is extended, hydraulic oil flows from the upper oil chamber  46  of the tilt cylinder  40  into the upper sub-oil chamber  82  of the sub-cylinder  80 . Therefore, the rod piston  86  and the gas piston  89  fall, and gas in the lower gas chamber  83  is compressed. 
     According to this structural configuration, the single sub-cylinder  80  having gas-filled structures GSS that respectively correspond to the extension and the contraction of the tilt cylinder  40  functions as an extension/contraction bidirectional gas damper GD. Therefore, the use of the single sub-cylinder  80  makes it possible to attenuate an extension/contraction bidirectional external force that acts on the tilt cylinder  40 . Therefore, the attenuation effect is high. This makes it possible to further significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, the step  87   b  of the lower rod  87  is in contact with the end surface  91   a  of the lower end tube  91  that is an example of the contact portion, and, as a result, the minimum volume of the lower gas chamber  83  is regulated. Additionally, the step  88   b  of the upper rod  88  is in contact with the end surface  92   a  of the upper end tube  92  that is an example of the contact portion, and, as a result, the minimum volume of the upper gas chamber  84  is regulated. This makes it possible to control an increase of the inner pressure in each of the gas chambers  83  and  84 . 
       FIG.  10    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  10    mainly differs from the example of  FIG.  7    as follows. Specifically, the upper oil chamber  46  of the tilt cylinder  40  is in communication with the sub-oil chamber  72  of the sub-cylinder  70  through an oil passage L 5 . The two oil chambers  45  and  46  of the tilt cylinder  40  are both in communication with the sub-oil chamber  72  of the sub-cylinder  70 . A port P 7 , which is one of two ports P 7 , P 8  of end portions of the oil passage L 5 , is located at the end portion  41   a  of the cylinder main body  41  of the tilt cylinder  40 . The other port P 8  is located at the end portion  71   a  of the sub-cylinder main body  71 . 
     When the tilt cylinder  40  that has received an external force in the extension/contraction direction X is contracted in a state in which the control valve V 2  has been opened, hydraulic oil flows from the lower oil chamber  45  of the tilt cylinder  40  into the sub-oil chamber  72  through the oil passage L 3 . When the tilt cylinder  40  is extended, hydraulic oil flows from the upper oil chamber  46  of the tilt cylinder  40  into the sub-oil chamber  72  through the oil passage L 5 . Either when the tilt cylinder  40  is extended or when the tilt cylinder  40  is contracted, the free piston  74  falls, and gas in the gas chamber  73  is compressed. 
     According to this structural configuration, the single sub-cylinder  70  functions as an extension/contraction bidirectional gas damper GD. Therefore, the use of the single sub-cylinder  70  makes it possible to attenuate an extension/contraction bidirectional external force that acts on the tilt cylinder  40 . Therefore, the attenuation effect is high. This makes it possible to further significantly reduce or prevent vibrations transmitted to the hull. 
       FIG.  11    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  11    mainly differs from the example of  FIG.  6 A  as follows. Specifically, the gas damper GD includes a gas cylinder  100 . The gas cylinder  100  slidably houses the end portion  41   b  of the cylinder main body  41  of the tilt cylinder  40  on the side opposite to the tilt rod  42 . The gas cylinder  100  defines a gas chamber  101 , filled with gas, with the end portion  41   b  of the cylinder main body  41  of the tilt cylinder  40 . When the tilt cylinder  40  receives an external force in the extension/contraction direction X, the cylinder main body  41  of the tilt cylinder  40  slides in the gas cylinder  100 , and the inner pressure of gas in the gas chamber  101  changes. The gas cylinder  100  includes a circular cylindrical cylinder tube  102 , a bottom  103  that closes one end of the cylinder tube  102 , and a contact portion  104 . The bottom  103  of the gas cylinder  100  is coupled to a clamp bracket. 
     The contact portion  104  is located in the gas chamber  101 . A seal  106 , such as an O-ring, is interposed between an inner peripheral surface  105  of the cylinder tube  102  and the outer peripheral surface  41   c  of the cylinder main body  41 . The seal  106  is held in a circumferential groove provided at, for example, the inner peripheral surface of the cylinder tube  102 . The contact portion  104  may include a step that protrudes inwardly from the inner peripheral surface  105  of the cylinder tube  102  and that faces the end portion  41   b  side of the cylinder main body  41  of the tilt cylinder  40 . The gas chamber  101  includes a first portion  101   a  that is located at a higher position than the contact portion  104  and a second portion  101   b  that is located at a lower position than the contact portion  104 . The gas chamber  101  is provided only with the second portion  101   b  in a state in which the contact portion  104  is in contact with the end portion  41   b  of the cylinder main body  41  of the tilt cylinder  40 . In other words, the contact portion  104  fulfills a function to regulate the minimum volume of the gas chamber  101 , and the minimum volume of the gas chamber  101  corresponds to the volume of the second portion  101   b.    
     According to this structural configuration, the gas cylinder  100  that slidably houses the end portion  41   b  of the cylinder main body  41  of the tilt cylinder  40  on the side opposite to the tilt rod  42  functions as a gas damper GD, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, the contact portion  104  of the gas cylinder  100  is contactable with the end portion  41   b  of the cylinder main body  41 , and, as a result, the position of the stroke end of the end portion  41   b  of the cylinder main body  41  with respect to the gas cylinder  100  is regulated. Thus, the minimum volume of the gas chamber  101  is regulated, and an increase of the pressure in the gas chamber  101  is regulated. 
       FIG.  12    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  12    mainly differs from the example of  FIG.  5 A  as follows. Specifically, a hollow piston  110  is included as a piston of the tilt cylinder  40 . The hollow piston  110  includes a cylindrical inner peripheral surface  111  that defines a gas chamber  120  that is filled with gas. In other words, the hollow piston  110  includes a gas-filled structure GSS. The gas damper GD includes a double piston structure DPS including the hollow piston  110  and an internal piston  130 . 
     The hollow piston  110  includes two end walls  112 ,  113  and a peripheral sidewall  114 . A gas chamber  120  is defined in the hollow piston  110  by being surrounded by the two end walls  112 ,  113  and the peripheral sidewall  114 . The inner peripheral surface  111  of the hollow piston  110  includes an inner peripheral surface of the peripheral sidewall  114 . The tilt rod  42  is inserted through the upper end wall  113 , and is coupled to the internal piston  130 . The hollow piston  110  includes a contact portion  115 . 
     The internal piston  130  is housed in the hollow piston  110  so as to be slidable on the inner peripheral surface  111  of the hollow piston  110 . The internal piston  130  coupled to the tilt rod  42  partitions the gas chamber  120  into two gas chambers  121 ,  122 . The internal piston  130  includes two end surfaces  131 ,  132 , an outer peripheral surface  133 , and a seal  134  such as an O-ring. The lower gas chamber  121  is located between the lower end surface  131  of the internal piston  130  and the lower end wall  112  of the hollow piston  110 . The upper gas chamber  122  is located between the upper end surface  132  of the internal piston  130  and the upper end wall  113  of the hollow piston  110 . The seal  134  is held by a circumferential groove provided at the outer peripheral surface  133  of the internal piston  130 , and seals a portion between the outer peripheral surface  133  of the internal piston  130  and the inner peripheral surface  111  of the hollow piston  110 . The tilt cylinder  40  that has received an external force in the extension/contraction direction X is extended and contracted in a state in which the supply/discharge of hydraulic oil from the hydraulic circuit  60  to the oil chambers  45  and  46  of the tilt cylinder  40  have been stopped. In accordance with the extension and contraction, a corresponding one of the gas chambers  121 ,  122  is compressed, and the function as a gas damper GD is achieved. 
     The contact portion  115  includes a step that protrudes inwardly from the inner peripheral surface  111  of the hollow piston  110  in the lower gas chamber  121  and that faces the internal piston  130  side. When the internal piston  130  moves to the lower stroke end in the hollow piston  110 , the contact portion  115  is in contact with the internal piston  130 . The lower gas chamber  121  includes a first portion  121   a  that is located at a higher position than the contact portion  115  and a second portion  121   b  that is located at a lower position than the contact portion  115 . Although not shown, the position of the stroke end of the internal piston  130  is regulated in a state in which the contact portion  115  is in contact with the internal piston  130 . Additionally, the lower gas chamber  121  is provided only with the second portion  121   b  in a state in which the contact portion  115  is in contact with the internal piston  130 . In other words, the contact portion  115  fulfills a function to regulate the minimum volume of the gas chamber  121 , and the minimum volume of the gas chamber  121  corresponds to the volume of the second portion  121   b.    
     According to this structural configuration, the internal piston  130  performs a stroke in the hollow piston  110  in response to the extension/contraction of the tilt rod  42 , and, as a result, gas in a corresponding one of the gas chambers  121 ,  122  in the hollow piston  110  is compressed. Thus, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, the contact portion  115  of the hollow piston  110  is in contact with the internal piston  130 , and, as a result, the position of the stroke end of the internal piston  130  with respect to the hollow piston  110  is regulated. Thus, the minimum volume of the gas chamber  121  is regulated, and an increase of the pressure in the gas chamber  121  is regulated. 
     Although not shown, a contact portion may be added in the upper gas chamber  122 . The contact portion is in contact with the internal piston  130 , and regulates the minimum volume of the upper gas chamber  122  when the internal piston  130  moves to the upper stroke end in the hollow piston  110 . Thus, an increase of the pressure in the gas chamber  122  is regulated. 
       FIG.  13    is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The gas damper GD includes two independent cellular foams  141 ,  142  located in the two oil chambers  45  and  46 , respectively. The independent cellular foams  141 ,  142  provide a gas-filled structure GSS in the cylinder main body  41  of the tilt cylinder  40 . The independent cellular foams  141 ,  142  are made of resin foam or foamed rubber. According to this structural configuration, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated by the independent cellular foams  141 ,  142  located in the two oil chambers  45  and  46 . This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. 
       FIG.  14 A  is a schematic view shown to describe still another example of the anti-vibration structure BS in the outboard motor  1 . The example of  FIG.  14 A  mainly differs from the example of  FIG.  5 A  as follows. Specifically, the gas damper GD includes a movable partition KS that defines the gas chamber  47  in the cylinder main body  41  of the tilt cylinder  40 . The movable partition KS includes, for example, a membrane  150 . The membrane  150  is made of, for example, rubber having elasticity or a metal sheet. The membrane  150  has, for example, a disk shape, and an outer peripheral edge  151  of the membrane  150  is fixed to a step  41   f  of the inner peripheral surface  41   d  of the cylinder main body  41  by an annular fixing member  160 . The gas chamber  47  is defined between the membrane  150  and the end portion  41   b,  which corresponds to a bottom, of the cylinder main body  41 . The membrane  150  divides the oil chamber  45  and the gas chamber  47  from each other. 
     A regulation portion R 1  is provided that regulates a displacement amount when the membrane  150  is displaced so as to protrude toward the side of the gas chamber  47  by elastic deformation. The regulation portion R 1  is provided with, for example, a convex portion  41   g  that protrudes from the end portion  41   b  of the cylinder main body  41  toward the central portion side of the membrane  150  in the gas chamber  47 . According to this structural configuration, gas in the gas chamber  47  defined by the movable partition KS in the tilt cylinder  40  functions as a damper, and, as a result, an external force that acts in the extension/contraction direction X of the tilt cylinder  40  is attenuated. This makes it possible to significantly reduce or prevent vibrations transmitted to the hull. 
     Additionally, the displacement amount of the membrane  150  functioning as the movable partition KS is regulated by the convex portion  41   g  functioning as the regulation portion R 1 . Thus, the minimum volume of the gas chamber  47  is regulated. Therefore, an increase of the pressure in the gas chamber  47  is regulated. The movable partition KS may be a diaphragm having elasticity. 
     In each of the examples described above, as the hydraulic cylinder HC included in the anti-vibration structure BS, the trim cylinder  30  may be used instead of the tilt cylinder  40 , or both the tilt cylinder  40  and the trim cylinder  30  may be used. Additionally, in each of the examples of  FIG.  8   ,  FIG.  9   , and  FIG.  10   , the manually-operable on-off valve V 1  may be used instead of the control valves V 2  and V 3 . Additionally, preferably, the spring constant when the gas damper GD functions as a gas spring is lower than the spring constant of the anti-vibration mount  12  in each of the examples of  FIG.  7    to  FIG.  14 A . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.