Patent Publication Number: US-8979426-B2

Title: Boat lift apparatus

Description:
FIELD 
     The teaching disclosed in this specification relates to one or more methods or apparatuses for raising a boat (or other type of watercraft) from a floating position to a raised position above the water. 
     BACKGROUND 
     U.S. Pat. No. 6,830,002 (Walker) discloses a lift for watercraft that has a raised and lowered positions and is adapted to be mounted in a body of water. The lift has a substantially rectangular base with first and second pairs of vertical corner posts that are connected to and carry longitudinal beams. The base further has two transverse beams connected to the longitudinal beams. A pivoting cradle is attached to the base. Watercraft support bunks are connected to the cradle. A pair of actuators are connected on one end to the pivoting cradle and on the other end to one of the first pair of corner posts. The first pair of corner posts are adapted to be long enough that at least a portion of the corner posts are above water level of a body of water in which the lift is mounted, and the actuators are connected to the first pair of corner posts in the portion of the corner posts above the water level. 
     U.S. Pat. No. 5,908,264 (Hey) discloses a watercraft lift having raised and lowered positions. The lift includes a substantially rectangular base with longitudinal side beams and front, rear, and intermediate transverse beams, connected to the longitudinal beams. The intermediate transverse beam is located between the front and rear transverse beams and at a height lower than the front and rear transverse beams. Forward booms are pivotably connected to the base at a location near the front transverse beam. Rear booms are pivotably connected to the base at a location near the intermediate transverse beam. A watercraft support platform is pivotally connected to the forward and rear booms. The raising and lowering of the lift of the present invention is accomplished by an actuation assembly. In a preferred embodiments, the actuation assembly includes two dual directional high pressure hydraulic cylinders pivotally connected between the intermediate transverse beam and the rear boom. During use, the actuator assembly rotates the booms upward and forward about their pivotable connection to the base further raising the watercraft support platform and the watercraft to an overcenter position. 
     U.S. Pat. No. 5,184,914 (Basta) discloses upwardly extending pivoting booms are supported on a rectangular base which is submerged in water. Watercraft supports on mounting arms are connected to the pivoting booms. A double-acting hydraulic cylinder attached between the rectangular base and pivoting booms swings the pivoting booms upwardly until they are braced by boom supports on the rectangular base at an angle over center. This raising of the pivoting booms lifts the mounting arms and watercraft supports to remove a craft from the water and disposes the booms, mounting arms, and craft in a stable, secure over center configuration. Actuation of the double-acting hydraulic cylinder in the opposite direction forces the booms back out of the over center position and lowers the craft into the water. 
     U.S. Pat. No. 5,890,835 (Basta et al.) discloses a hydraulic lift for raising a boat out of water into a raised storage position is proposed. Pivoting booms are connected to a frame that is supportable by a bed of a body of water. A boat rack is provided at an upper portion of the pivoting booms. A hydraulic cylinder is connected between the frame and a lower portion of the pivoting booms. The pivoting booms are selectively adjustable between a lowered position wherein the rack is submerged in the water and a raised storage position wherein the rack is raised above the water. The position of the pivoting booms is controlled by a ram of the hydraulic cylinder. Importantly, the pivoting booms are maintained in the raised storage position when the ram is in a retracted position which protects the ram from corrosion and fouling. In the preferred embodiment, the pivoting booms are rotated over center when they are in the raised storage position. 
     U.S. Pat. No. 6,830,410 (Davidson et al.) discloses an apparatus for supporting the hull of a watercraft using a flexible bunk beam and a convex cushion attached to the beam using locking elements. The beam has a longitudinal recess with a narrow upper neck portion and a larger lower anchor portion, and the cushion has an elongated cushion locking member lockably insertable into the recess. The cushion locking member has a narrow upper neck portion and a larger lower portion sized to snuggly fit within the recess. The cushion includes internal voids and walls. The beam includes sidewalls with bores forming bearing surfaces. 
     SUMMARY 
     This summary is intended to introduce the reader to the more detailed description that follows and not to limit or define any claimed or as yet unclaimed invention. One or more inventions may reside in any combination or sub-combination of the elements or process steps disclosed in any part of this document including its claims and figures. 
     According to one broad aspect of the invention, a boat lift apparatus can include a base comprising a support surface to rest on the bottom of a body of water. The base can include a first base beam and a second base beam. The second base beam can be oriented generally parallel to and spaced laterally apart from the first base beam. The boat lift can also include a boat support platform having a first lifting beam aligned with the first base beam, a second lifting beam aligned with the second base beam, and at least one cradle support connected to and suspended between the first and second lifting beams. The boat support platform can be moveable relative to the base between a lowered position for receiving a boat and a raised position for lifting the boat out of the water. The boat lift can also include at least two first support struts connecting the first base beam and the first lifting beam. Each first support strut includes a lower end pivotally connected to the first base beam and an opposing upper end pivotally connected to the first lifting beam. The boat lift also includes at least two second support struts connecting the second base beam and the second lifting beam. Each second support strut includes a lower end pivotally connected to the second base beam and an opposing upper end pivotally connected to the second lifting beam. When the boat support platform is in the lowered position the at least two first support struts are oriented generally parallel to both the first base beam and the first lifting beam, and the at least two second support struts are oriented generally parallel with both the second base beam and the second lifting beam. 
     When the boat support platform is in the raised position the at least two first support struts can be parallel to each other and can be generally perpendicular to both the first base beam and the first lifting beam. 
     When the boat support platform is in the lowered position the first support struts can overlie at least a portion of the first base beam and the first lifting beam can overlie at least a portion of the first support struts. 
     Each of the first support struts may include a first bearing surface and an opposing second bearing surface. When the boat platform is in the lowered position a downward facing surface of the first lifting beam may bear against each first bearing surface, and the second bearing surfaces may bear against an upward facing surface on the first base beam. 
     The at least one cradle support may include a lift in surface. When the boat support platform is in the lowered position the lift in surface can be at a lowered height and when the boat support platform is in the raised position the lift in surface can be at a raised height. A lift ratio between the raised height and the lowered height may be greater than 8:1. 
     When the boat support platform is in the lowered position the lift in surface may be less than 10 inches above the support surface. 
     Each first support strut may include a strut axis, and when the boat support platform is in the lowered position the strut axes of the first support struts may be coaxial with each other. 
     The boat lift can also include a first actuator connected between at least one of the first support struts and the first base beam to pivot the at least one of the first support struts relative to the first base beam, and a second actuator connected between at least one of the second support struts and the second base beam to pivot the at least one of the second support struts relative to the second base beam. 
     The first base beam may include an inboard base rail and an opposing outboard base rail. The outer base rail may be laterally spaced apart from and generally parallel to the inner base rail, and a first end of the first actuator may be disposed between the inner and outer base rails and may be pivotally connected to at least one of the inner and outer base rails. 
     The at least two first support struts may include an inboard support arm pivotally connected to the inboard base rail, and an outboard support arm pivotally connected to the outboard base rail. The outboard support arm may be generally parallel to the inboard support arm, and a second end of the first actuator may be disposed between, and pivotally connected to, at least one of the inboard and outboard support arms. 
     The first actuator and second actuator may be positioned on opposite sides of the at least one cradle support, and may be outboard from the at least one cradle support. 
     When the boat support platform is in the lowered position, a lift clearance distance between an upper surface of the lifting beams and the support surface may be between 100% and 150% of the sum of the thickness of one lifting beam, one support strut and one base beam. 
     The boat lift can also include a plurality of bunk assemblies supported on the at least one cradle support and at least a portion of the bunk assemblies can be moveably connected to the at least one cradle support so that the lateral position of the at least some of the bunk assemblies can be adjustable relative to the cradle support. 
     The first lifting beam may be parallel to the first base beam when the boat support platform is in the raised position, when the boat support platform is in the lowered position and when the boat support platform is in an intermediate position between the raised and lowered positions. 
     Each of the first support arms and second support struts may be of variable length and may be securable in a retracted configuration and an extended configuration. 
     The lowered height of the boat support platform may be the same when the first and second support arms are in either the retracted or extended configurations. 
     The boat lift can also include a plurality of support legs for supporting the base above the bottom of the body of water. The plurality of support legs may include a plurality of first support legs connected to the first base beam, and a plurality of second support legs connected to the second base beam. The plurality of first legs may include at least one inboard support leg, positioned laterally between first base beam and the second base beam, and at least one outboard support leg, positioned outboard of the first base beam. 
     The at least one inboard support leg may at least partially underlie the boat support platform. 
     The distance between an outboard surface of the first base beam and an outboard surface of the second base beam may define a base width, and the at least one outboard support leg may be laterally spaced apart from an outboard surface of the first base beam by a leg offset distance that is less than 30% of the base width. 
     At least one of the base and the boat support platform may also include a chamber for containing a gas that is less dense than water. 
     According to another broad aspect of the invention, a boat lift apparatus may include a base configured to rest on the bottom of a body of water and a boat support movably connected to the base. The boat support may be configured to support a boat and may be movable between a lowered position, to receive a boat, and a raised position, to lift the boat out of the water. At least one of the base and the boat support may include at least one first gas-trapping chamber for containing a gas that is less dense than water so that gas within the chamber can exert a lifting force when the at least one of the base and the boat support is submerged under water. 
     The boat lift can also include a first gas fitting having an inlet that is connectable to a gas supply and an outlet that is in fluid communication with at least one first gas-trapping chamber. The gas fitting can be to regulate the flow of gas into the at least one first gas-trapping chamber. 
     The boat lift can also include a first water passage in a downward facing surface of the at least one of the base or boat support. The first water passage can have a first end in communication with the body of water and a second end in fluid communication with the at least one first gas-trapping chamber to allow water to flow out of the first gas-trapping chamber as the gas flows into the first gas-trapping chamber. 
     The base may include a first base beam and a second base beam oriented generally parallel to and laterally spaced apart from the first base beam, and the at least one first gas-trapping chamber may include at least one first gas-trapping chamber in each base beam. 
     The boat support may include at least one second gas-trapping chamber. 
     The boat support may include a first lifting beam oriented generally parallel to the first base beam and a second lifting beam oriented generally parallel to the second lifting beam. The at least one second gas-trapping chamber may include at least one second gas-trapping chamber in each lifting beam. 
     The boat lift can also include a plurality of cradle supports suspended between the first and second lifting beams. Each cradle beam may have a sealed internal gas chamber containing the gas. 
    
    
     
       DRAWINGS 
       For a better understanding of the applicant&#39;s teachings described herein, reference will now be made, by way of example only, to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a boat lift in the raised position; 
         FIG. 2   a  is a side elevation view of the boat lift of  FIG. 1 ; 
         FIG. 2   b  is a side elevation view of the boat lift of  FIG. 1 , in which support struts in a retracted position; 
         FIG. 3  is an end view of the boat lift of  FIG. 1 ; 
         FIG. 4   a  is a side elevation of the boat lift of  FIG. 1  in a lowered position; 
         FIG. 4   b  is an enlarged view of a portion of  FIG. 4   a;    
         FIG. 4   c  is similar to the view of  FIG. 4   a , but showing the support struts in a contracted position; 
         FIG. 5  is a front end view of a hull portion of a boat supported the boat lift of  FIG. 1  in a lowered position; 
         FIG. 6  is a perspective view of a base beam portion of the boat lift of  FIG. 1 ; 
         FIG. 7  is an exploded reverse perspective view of a portion of the boat lift of  FIG. 1 , with the base beam portion shown in a section view taken along line  7 - 7  in  FIG. 6 ; 
         FIG. 8  is a section view of the base beam portion of  FIG. 6  taken along line  8 - 8 ; 
         FIG. 9  is an enlarged perspective view of a boat support platform portion of the boat lift of  FIG. 1 ; 
         FIG. 10  is a section view of a bunk assembly for use on the boat lift of  FIG. 1 ; 
         FIG. 11  is a perspective view of an actuator for use on the boat lift of  FIG. 1  in an extended position; 
         FIG. 12  is a section view of the actuator of  FIG. 11 , taken along line  12 - 12 ; 
         FIG. 13  is a perspective of the actuator of  FIG. 11  in a retracted position; and 
         FIG. 14  is a section view of the actuator of  FIG. 13 , taken along line  14 - 14 . 
     
    
    
     DETAILED DESCRIPTION 
     Various apparatuses or processes will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover processes or apparatuses that are different from those described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or process described below is not an embodiment of any claimed invention. Any invention disclosed in an apparatus or process described below that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such invention by its disclosure in this document. 
     Referring to  FIG. 1 , an example of a boat lift  100  includes a base  102 , and a boat support platform  104  that is movably connected to the base  102 . In the illustrated example, the boat support platform  104  is connected to the base  102  by a plurality of support struts  101 . The base  102  is configured to rest on the bottom of a body of water, such as a lake, ocean or river. Each support strut  101  has a lower end  110  that is pivotally connected to the base  102  and an upper end  108  that is spaced apart from the lower end  110 . The upper end  108  is pivotally connected to the boat support platform  104 . In this configuration, the boat support platform  104  is moveable between a raised position ( FIGS. 1-3 ) and a lowered position ( FIGS. 4   a  and  5 ). 
     Referring to  FIG. 5 , in the lowered position, the boat support platform  104  is below the surface of the water, represented by line  112 , providing sufficient draft so that a boat can generally be moved under its own power onto or off of the support platform  104  when in the lowered position. Referring to  FIG. 2   a , in the raised position, the boat support platform  104  is lifted above the surface of the water  112  so that the boat is supported above the water for storage. 
     Referring to  FIGS. 3 and 5 , for the purposes of this description, the height of the boat lift  100  is the distance between a first reference surface on the boat support platform  104 , for example the lift in surfaces  238  of cradle supports  158  (described in detail below), and a second reference surface on the base  102 , for example the support surfaces  109  of the support legs  103 . The ratio between the height  118  of the boat support platform in its raised position (its raised height,  FIG. 3 ) compared to the height  119  of the boat support platform in its lowered position (its lowered or lift-in height,  FIG. 5 ) defines a lift ratio. As explained in greater detail below, in the illustrated example the lift ratio (i.e. raised height  118 : lowered height  119 ) of the boat lift  100  can be between approximately 5:1 and 15:1, and optionally can be between 8:1 and 14:1. 
     Referring again to  FIG. 1 , the boat lift  100  includes a first end  114  an opposing second end  116 . When moved from the raised position to the lowered position, the boat support platform  104  is in the lowered position it is generally level and, having both ends  114 ,  116  of the platform open, is able to receive a boat from either direction. 
     The boat lift  100  also includes at least one actuator  124 , and preferably at least one actuator  124  per side, for moving the boat support platform  104  between its raised and lowered positions. In the illustrated example, the boat lift  100  includes one hydraulic actuator  124  connected between each support strut  101  and the base  102 , to pivot the support struts  101  relative to the base  102  in the direction indicated using arrow  126 . In the illustrated example, when the boat support platform is in the lowered position, the actuators  124  are in a retracted position. The actuators  124  can comprise a piston/cylinder arrangement connected to a pressurized fluid supply source. Alternatively, the actuators  124  can comprise electric actuators, such as a ball screw and nut arrangement. In the example illustrated, the actuators  124  are in the form of pistons slidably mounted in respective cylinders and connected to a source  128  of pressurized hydraulic fluid (which may include a hydraulic pump driven by an electric motor, a gasoline or diesel motor or other suitable power source) by conduits  130 . While only a single conduit  130  is illustrated for clarity, each actuator  124  can be connected to the hydraulic supply source. The conduits  130  can contain splitters, flow regulators, valves and other hardware that can be used to route hydraulic fluid to all of the actuators  124 . Optionally, the hydraulic supply source  128  can include more than one pump/motor combination, to provide redundancy in the event that one of the pump/motor combinations should fail. Each pump/motor combination can be sized so that it is independently capable of moving a loaded boat support platform  104 . Optionally, the hydraulic supply source  128  can be located in a remote utility box  132  that is positioned out of the water, for example on shore or on a dock. The utility box  132  can also include a power supply  134 , including, for example a battery and/or a solar panel, for providing power to drive the hydraulic supply source. The power supply  134  can also provide power to other devices and accessories that may be mounted on, or used in combination with the lift  100 , including for example, lights. 
     To lift the boat support platform  104  (and any boat thereon) into the raised position, the actuators  124  are moved to the extended positions, thereby pivoting the support struts  101  into their upright positions (see for example  FIG. 1 ). 
     Referring still to  FIG. 1 , the base  102  includes two spaced apart base beams  136   a ,  136   b  that are generally parallel to each other and extend in a longitudinal direction. In the illustrated example, each base beam  136   a ,  136   b  is formed from an inboard base rail member  138   a ,  138   b  and an outboard base rail member  140   a ,  140   b . Each base beam  136   a ,  136   b  has a laterally outboard face  139   a ,  139   b , respectively, facing away from the opposed beam  136   b ,  136   a . The opposing rail members  138   a ,  140   a  and  138   b ,  140   b  in each beam  136   a  and  136   b , respectively, are connected together using end plates  142 . Optionally, the end plates  142  can be permanently connected to the base rails, for example by welding, so that the assembled base beams  136   a ,  136   b  cannot be easily disassembled. Alternatively, the base plates  142  can be detachably connectable to at least one of the base rail members  138 ,  140 , for example using bolts or pins, so that base support rails  138 ,  140  can be detached from each other for transportation and then assembled on site. 
     Referring to  FIG. 3 , the distance between the outboard faces  139   a ,  139   b  of the base beams  136   a  and  136   b  respectively, when assembled as shown, defines a base width  152 . The base width  152  can generally be in the range of about eight feet to about thirty feet. In the example illustrated, the base width  152  is about fourteen feet. Increasing the base width  152  may help increase the lateral stability of the boat lift  100 . The width  152  of the base, and the corresponding length of the cross members  146 , can be selected based on a plurality of factors, including the expected load to be carried by the boat lift, the elevation of the boat support platform in the raised position and the condition and/or composition of the bottom of the body of water (for example sand, rocks, gravel, silt, etc.). 
     Referring again to  FIGS. 1 ,  2   a  and  3 , the lift  100  includes a support surface for resting on the bottom of the lake/ocean. In the illustrated example, the base  102  is supported on a ten height-adjustable support legs  103  that can rest on the bottom of the lake, river or ocean. Each support leg  103  includes an extension member  105  that can be movably connected to the base  102 , and a generally planar foot plate  107  having a support surface  109  for contacting the bottom of the body of water. Each support leg  103  can be fixed in a given extension position using a locking pin, or other suitable locking mechanism. Each support leg  103  is independently moveable relative to base  102  and the plurality of support legs  103  can be independently adjusted so that the base  102  is supported in a generally level orientation even if the bottom of the body of water is uneven, or slopes away from the shore. Each support leg  103  defines a support leg axis  111 , which in the illustrated example is the central axis of the extension member  105 . 
     The support legs  103  on the boat lift  100  are positioned so that each base beam  136   a ,  136   b  is supported by multiple support legs  103 . Referring to  FIG. 3 , in the illustrated example, each base beam  136   a ,  136   b  is supported by at least one outboard support leg  103 , located laterally outboard of the outboard faces  139   a ,  139   b  of base beams  136   a ,  136   b , respectively, and at least one inboard support leg  103 , located laterally inboard of each base beam  136   a ,  136   b . In this configuration, the inboard support legs  103  are positioned beneath the boat support platform  104  and laterally between the base beams  136   a ,  136   b.    
     Providing outboard support legs  103  may help further increase the stability of the boat lift  100 . Increasing the outboard leg offset distance  113 , the distance between the outboard faces  139   a  base beam  136   a , and the outboard support leg axis  111 , may help increase stability of the lift  100  but will also increase the overall width of the boat lift  100 , which may limit the locations in which the lift  100  can be installed. Preferably, the outboard leg offset distance  113  is selected to be between approximately 0-30% of the base width  152 , and optionally is selected to be less than 20% or less than 15% of the base width  152 . 
     Providing inboard support legs  103  may help distribute the load exerted on the base beams  136   a ,  136   b , and may help prevent the base  102  from bowing or deflecting inward when loaded. Preferably, the inboard support legs  103  are positioned close to the inboard surfaces of the base beams  136   a ,  136   b , so that the extension members  105  of the inboard support legs  103  do not hit the hull of a boat on the lift, when the boat lift platform  104  is in the lowered position. Optionally, the inboard leg offset distance  115  can be selected based on the width of the boat that is to be placed on the lift. Alternatively, or in addition, the inboard leg offset distance  115  can be selected based on the lift width  152 , so that the inboard leg offset distance  115  is between approximately 0-30% of the base width  152 . The inboard leg offset distance  115  can be the same as, or different than the outboard leg offset distance  113 . 
     Optionally, the inboard and outboard leg offset distances  115 ,  113  can be selected so that they are each less than the width  137  of the base beams  136   a ,  136   b.    
     Optionally, the boat lift  100  can include more than ten legs  103  or fewer than ten legs. For clarity, some of the support legs  103  have been omitted in some of the Figures in this application. 
     Referring to  FIG. 6 , an example of base beam  136   a  is shown in isolation, with other components of the lift  100  removed. The inboard and outboard base rails  138   a ,  140   a  are generally parallel to each other and are separated by a rail spacing distance  144 . Referring also to  FIG. 7 , the base beam rails  138   a ,  140   a  are, in the illustrated example, formed from hollow, extruded aluminum tubes that have generally rectangular cross sections. Referring to  FIG. 5 , the width  137  of the base beam  136   a  can be between approximately seven and twenty-four inches, and in the example illustrated is approximately twelve inches. 
     Referring again to  FIGS. 1 and 2   a , the base beams  136   a ,  136   b  are connected to each other by a plurality of laterally extending cross members  146 . The cross members  146  are spaced apart from each other along the length of the base beams  136   a ,  136   b , and are generally orthogonal to the beams  136   a ,  136   b . The cross members  146  are hollow, tubular members and are connected to the inboard rail  138   a ,  138   b  of each base beam  136   a ,  136   b . The cross members  146  can help keep the base beams  136   a ,  136   b  generally parallel to each other. The cross members  146  are generally U-shaped, so that the central portion  148  of the cross members  146  is at a lower elevation than the ends  150  that are connected to the inboard rails  138   a ,  138   b . Providing the central portion  148  at a lower elevation than the ends  150  may help prevent interference between the cross members  146  and the boat support platform  104 , when the boat support platform  104  is in the lowered position. Optionally, the cross members  146  can be detachably connected to the base beams  136   a ,  136   b , for example using bolts or pins. In some examples, the cross members  146  can be detached to facilitate transport of the boat lift  100 . 
     Referring also to  FIGS. 3 and 5 , the boat support platform  104  includes a pair of lifting beams  154   a ,  154   b  and a cradle  156  suspended between the lifting beams  154   a ,  154   b . Each lifting beam  154   a ,  154   b  in the boat support platform  104  is positioned vertically above, and is aligned with a corresponding base beam  136   a ,  136   b . In the illustrated example, the upper surfaces  122  of the lifting beams  154   a ,  154   b  are generally flat, planar surfaces that can serve as walkways to allow a user to walk on the boat support platform  104 , beside a boat that is resting on the platform  104 . 
     The cradle  156  includes at least one lateral cradle support  158 . In the illustrated example, the cradle  156  includes four laterally extending cradle supports  158  that are spaced apart from each other along the length of the boat support platform  104  and are connected to lifting beams  154   a ,  154   b . The cradle  156  also includes a plurality of longitudinally extending bunk assemblies  160  for contacting and supporting the hull of the boat  162  on the lift (see  FIG. 5 ). Optionally, the cradle supports  158  are detachably connected to the lifting beams  154   a ,  154   b  and the bunk assemblies  160  are detachably connected to the cradle supports  156 . In some examples, the boat support platform  104  can be shipped to a user as a plurality of separate pieces, and then assembled on site. 
     Referring also to  FIG. 9 , in the illustrated example, the lifting beams  154   a ,  154   b  are each formed from an inboard lifting rail  162   a  and  162   b  and an outboard lifting rail  164   a  and  164   b , respectively. Adjacent lifting rails  162   a ,  164   a  and  162   b ,  164   b  are connected to each other by a plurality of cross-link members  166 . In this example, the lifting beams  154   a ,  154   b  are positioned so that the outboard and inboard rails of each lifting beam  162   a ,  162   b ,  164   a ,  164   b  are aligned with the respective outboard and inboard rails  138   a ,  138   b ,  140   a ,  140   b  of the corresponding base beams  136   a ,  136   b.    
     Referring again to  FIGS. 1 and 3 , in the illustrated example, each support strut  101  comprises an outboard support arm, for example support arm  106   a  that connects outboard lifting rails  164   a  and  164   b  to corresponding base rails  140   a  and  140   b , respectively. Each support strut  101  also includes an inboard support arm, for example support arm  106   b  that is offset from and is generally parallel with the outboard support arm  106   a . The inboard support arms  106  connects inboard lifting rails  162   a  and  162   b  to corresponding base rails  138   a  and  138   b , respectively. The support arms  106   a  and  106   b  in each support strut  101  are, in the example illustrated, connected to each other using at least one cross brace  216 . Connecting the support arms  106   a  and  106   b  in each support strut  101  can help to provide unison of movement of the arms  106   a ,  106   b  in each strut  101  when moving between the raised and lowered positions. At least one of the support arms  106  and  106   b  in each strut  101  is pivotally connected to the upper end of a respective hydraulic actuator  124 . 
     For simplicity, the connection between one representative outboard base rail  140   a  and one outboard lifting rail  164   a  will be described in detail in this description, but it is understood that the other pairs corresponding lifting and base rails are connected to each other in the same manner. 
     Referring to  FIGS. 2   a ,  4   a  and  4   b , in the illustrated example, three support arms  106   a  are used to pivotally connect the outboard base rail  140   a  and the outboard lifting rail  164   a . The support arms  106  are generally identical elongate members, and each defines a corresponding support strut axis  168 . Each support arm  106   a  is positioned vertically between the opposing rails  140   a ,  164   a  and has a lower end  170  that is pivotally connected to the base rail  140   a  and an upper end  172  that is pivotally connected to the lifting rail  164   a . The pivotable connections between the ends  170 ,  172  of the support arms  106  and the rails  140   a ,  164   a  include flanges  176  that are connected to the upper and lower ends of the support arms  106   a . U-shaped seats  178  defined between opposing flanges  176  on the upper and lower ends of the support arms  106   a  can be sized to receive the lifting and base rails  164   a ,  140   a , respectively (see  FIG. 7 ). The flanges  176  include matching apertures  180  that are aligned with a bushing  182  on the lifting and base rails  164   a ,  140   a  and secured to the rails using a pin  184 . 
     Referring to  FIG. 2   a , when the boat support platform  104  is in the raised configuration the support arms  106   a  are arranged in a generally vertical position. In this configuration the support strut axes  168  are generally parallel to each other, and are generally perpendicular to a lifting beam axis  186  and a base beam axis  188 . Each support arm  106   a  has a first surface  190 , facing the first end  114  of the lift  100  when the support arm  106   a  is vertical, and an opposing second surface  192 , facing the second end  116  of the lift  100  when the support arm  106   a  is vertical. 
     Referring now to  FIGS. 4   a  and  4   b , when the boat support platform  104  is pivoted into the lowered configuration, the lifting rail  164   a , support arms  106   a  and base rail  140   a  are aligned with each other and are in a stacked formation, in which the support strut axes  168  are co-axial with each other, and are parallel to both the lifting rail and base rail axes  186 ,  188 . In this configuration, the support arms  106   a  are parallel to both the lifting rail  164   a  and the base rail  140   a , the first surface  190  of each support arm is facing a downward facing bottom surface  194  of the lifting rail  164   a , and the second surface  192  of each support arm is facing an upward facing upper surface  196  of the base rail  140   a . Optionally, the support arms  106   a  can be shaped so that when the boat support platform  104  is in the lowered position, the bottom surface  194  of the lifting rail  164   a  rests on and bears against at least a portion of the first surfaces  190  of the supporting arms  106   a , and at least a portion of the second surfaces  192  of the support arms  106   a  rest on and bear against the upper surface  196  of the base rail  140   a . Alternatively, the support arms  106   a  can be configured so that a gap remains between i) the bottom surface  194  of the lifting rail  164   a  and the first surfaces  190  of the support arms  106   a , and/or ii) the second surfaces  192  of the support arms  106  and the upper surface  196  of the base beam  140   a.    
     Optionally, one or more of the lifting rail  164   a , base rail  140   a  and support arms  106  can include a spacer  198  that can be positioned between the opposing surfaces  190 - 194  and/or  192 - 196  when boat support platform  104  is lowered. The spacers can be any suitable member that can withstand the expected loads transferred from the boat support platform  104  to the base  102 , and can withstand being used underwater. In the illustrated example, spacers  198  can optionally be provided toward the upper end  172  of the support arms  106   a  to account for small size differences between the tubular members used to form variable length support arms  106 , as explained in greater detail below. Optionally, the spacers can be resilient or otherwise deformable to provide cushioning between the rails and the support arms. Examples of suitable spacers include, rubber pads, raised portions of the surfaces themselves (such as bosses) and metal spacers (such as aluminum plates or washers). 
     Optionally, the struts  100  can be of adjustable length to allow a user to vary the lifting height of the boat support platform  104 , relative to the support surface  109 . Referring to  FIGS. 2   a ,  4   a  and  4   b , in the illustrated example, the support arms  106   a  and  106   b  in each strut  101  are telescopically adjustable. Support arm  106   a  includes a boom member  200 , pivotally connected to the base rail  140   a , and an extension member  202  telescopically received in the boom  200 , and pivotally connected to the lifting rail  164   a . The extension member  202  includes a plurality of holes  204  spaced along its length, and can be secured in a desired position relative to a corresponding hole  206  in the boom member  200  using a locking pin  208 . Optionally, a common locking pin can extend between both support arms  106   a  and  106   b  in each strut  101  to lock both support arms  106   a ,  106   b  in their desired extension positions. Alternatively, one or more locking pins can be used to secure each support arm  106   a ,  106   b.    
     Still referring to  FIGS. 2   a ,  4   a ,  4   b  and  5 , when the telescopic support arms  106  are in an extended configuration, the boat support platform  104  is raised to an extended raised height  118   a  ( FIG. 5 ). The extended raised height  118   a  may be in the range of, for example about 60 inches to about 100 inches, or more be greater than 100 inches. In the example illustrated, the extended raised position is approximately ninety-four inches. Referring to  FIGS. 2   b  and  4   c , when the telescopic support arms  106  are in a retracted position, the boat support platform  104  is lifted to a retracted raised height  118   b . The retracted raised height  118   b  is lower than the extended raised height  118   a , and maybe in the range of for example, about 48 inches to about 72 inches, or may be greater than 72 inches. In the example illustrated, the retracted raised height  118   b  is approximately 60 inches. 
     Referring to  FIGS. 4   b  and  4   c , when the boat support platform  104  is in the lowered position, in which the lifting rail  164   a , support legs  106   a  and base beam  140   a  are in the stacked configuration, the lowered height  119  of the boat lift  100  remains the same, regardless of the magnitude of the raised height  118   a ,  118   b . The lowered height  119  can be in the range of, for example, of about 5 inches to about 15 inches, or may be lower than 5 inches or greater than 15 inches. In the illustrated example the lowered height  119  is approximately seven inches. 
     Optionally, the support arms  106   a  can be secured in a plurality of intermediate extension positions, so that the lift ratio of the boat lift  100 , the ratio of the raised height  118   a  or  118   b  to the lowered higher  119  can be in the range of, for example, about 8:1 to about 14:1, or can be greater than 14:1. In the illustrated example, when the support arms  106  are in their extended configuration, the lift ratio (i.e. ratio of extended raised height  118   a : lowered height  119 ) is approximately 13.4:1. When the support arms  106  are in their contracted configuration, the lift ratio (retracted raised height  118   b : lowered height  119 ) is approximately 8.5:1. 
     Referring to  FIGS. 4   a  and  4   c , when the boat support platform  104  is in the lowered position, the distance between the support surfaces  109  and an uppermost surface  122  of the boat support platform  104  defines a lift clearance  120 . In the example illustrated, the lift clearance  120  is generally equal to the distance the boat lift  100  extends above the bottom of the lake. When the boat lift  100  is used in bodies of water that can freeze over during the winter, providing a relatively small lift clearance  120  may allow the boat lift  100  to be left submerged in relatively shallow water (for example close to shore) over the course of the winter without being crushed or otherwise damaged by the winter ice that forms on the surface of the water. When in the stacked configuration, in the illustrated example, the sum of the thickness  155  of the lifting beam  154   a , the thickness  117  of the support legs  106  and the thickness  137  of the base beam  136   a  comprises a majority of the lift clearance  120 , regardless of the degree of extension of the support struts  101 . In this configuration, the lift clearance  120  is in the range of, for example, about 100% to about 150% of the sum of the thicknesses  155 ,  117  and  137 , and optionally can be approximately 125% of the sum. In the illustrated example, the lift clearance  120  is approximately twenty four inches, and the thickness of the lifting beam  154   a  is approximately five inches, the thickness  117  of the support legs  106  is approximately five inches and the thickness of the base beam  136   a  is approximately nine inches. In this example the lift clearance  120  (twenty-four inches) is approximately 125% of the sum (nineteen inches) of the thicknesses  155 ,  117  and  137 . 
     The lifting capacity of the boat lift  100  can vary based on the extension of the support arms  106 , the power of actuators  124  and the materials used to construction the lift. In the illustrated example, when the support struts  101  are in the retracted position, the lifting capacity of the lift  100  can be up to between approximately 20,000 and 25,000 pound, and may be greater than 25,000 pounds. When the support struts  101  are in the extended position the lifting capacity can be up to between approximately 10,000 and 16,000 pounds, and may be greater than 16,000 pounds. Modifying the number of support struts  101  used in the lift  100 , and the number of actuators  124  can also affect the lifting capacity of the lift  100 . For example, a lift  100  equipped with only four support struts  101  and four actuators  124  may have a lifting capacity of up to between approximately 10,000 and 16,000 pounds (taking into account a variety of support arm  106  extension positions). Alternatively, for example, a lift  100  equipped with eight support struts  101  and eight actuators  124  may have a lifting capacity of up to 30,000 pounds or more. 
     Referring to  FIGS. 2   a  and  7 , the actuators  124  include respective piston rods  218  that are slidably mounted in corresponding cylinders  220 . The lower end of each cylinder  220  is pivotably connected between the inboard and outboard base rails  138 ,  140  with a pin joint  222 . The pin joint  222  includes a bushing  226  welded into the base rails  138 ,  140  (see also  FIG. 8 ) and a pin  228  that extends between the rails  138 ,  140  and through a bushing  230  on the cylinder  220 . 
     The outer diameter  224  of the cylinders  220  is selected so that it is less than the lateral spacing  144  ( FIG. 6 ) between the inboard and outboard base rails  138 , 140 . The cylinders  220  can fit between the rails  138 ,  140  and can pivot relative to the rails  138 ,  140  when the boat support platform  104  is moved between the lowered and raised positions. Optionally, portions of the inboard and outboard  138 ,  140  rails surrounding where the cylinder connects to the rails can be reinforced, for example by providing reinforcement plates, to help withstand the forces exerted by the cylinder. Portions of the support arms  106  connected to the upper end of the piston rods  218  can be similarly reinforced. 
     Optionally, referring again to  FIG. 2   a , the mounting flanges  176  connected to the upper and lower ends  172 ,  170  of the support arms  106   a  are shaped so that when the boat support platform  104  is raised, the pivot connections between the support arms  106   a  and the lifting rail  164   a  lie in a first plane  232 , and pivot connections between the support arms  106   a  and the base rail  140   a  lie in a different plane  234 . Plane  234  is longitudinally offset from the first plane  232 . Planes  232  and  234  are located on opposite sides of axes  168 . Preferably, the support arms  106   a  are connected so plane  234  is located closer to the second end  116  of the boat lift  100  than plane  232 . In this configuration, when the boat support platform  104  is in the raised it is in an “over centre” position. 
     In the example illustrated, the lifting beams  154   a , 154   b  are parallel to the base beams  136   a ,  136   b  when the lift  100  is in and moves between the raised and lowered positions. This can help to maintain the boat (supported on the boat support platform  104 ) in a generally level position. 
     Referring to  FIGS. 5 and 9 , each cradle support  158  is a generally U-shaped member having a recessed central portion  236  that is at a lower elevation than the ends  237 . In the illustrated example, the ends  237  are bolted to the inboard lifting rails  162   a ,  162   b . The central portion  236  includes an upper, lift in surface  238  that faces, and underlies the hull of the boat on the lift. When a boat is moved onto the lift, it passes over the lift in surface  238 . In this configuration, when the lifting platform  104  is in the lowered position, the central portions  236  of cradle supports  158  extend below the upper surface  196  of the base beams  136   a ,  136   b , and the lift in surface  238  of the central portion  236  of the cradle support  158  is positioned between the upper  196  and lower  240  surfaces of the base beams  136   a ,  136   b  and a lower surface  242  of the cradle support can be positioned below the lower surface  240 . Optionally, the cradle supports  158  can be configured so that when the boat support platform  104  is in the lowered positions, the lift in surfaces  238  are at a lower elevation than the pivot connections between the actuators  124  and the base beams  136   a ,  136   b.    
     For the purposes of this description, the lift-in height  119  of the boat lift  100  is the elevation of the lift in surfaces  238  of the cradle supports  158  above the bottom of the lake or ocean (which is equivalent to the elevation above the support surfaces  109  of the feet  103 , which are resting on the bottom) in which the lift  100  is being used. Providing a lower lift-in height may enable the boat lift  100  to be positioned in shallower water while still allowing a desired draft clearance  248  between the surface  112  and the cradle supports  158 . The lift-in height  119  can be in the range of, for example, about four inches to about twenty inches. In the illustrated example, the lift-in height  119  is about seven inches. 
     Optionally, a plurality of longitudinal braces  250  can be connected between adjacent cradle supports  158 . The braces  250  may help strengthen the boat support platform  104  and maintain the longitudinal spacing between cradle supports  158 . The longitudinal braces  250  are, in the example illustrated, detachably bolted to cradle supports  158 . This can facilitate transport of the boat lift  100 . 
     Referring again to  FIG. 5 , the bunk assemblies  160  on the boat support platform  104  include a bunk cushion  252  that is supported by an extruded aluminum bunk beam  254 . A mounting bracket  256  connects the each bunk beam to each of the cradle supports  158 . Providing a plurality of mounting brackets  256  along the length of the bunk beam may help limit deflection of the bunk beam  254  when a boat is supported on the lift  100 . Optionally, the mounting brackets  256  can be movably connected to the cradle supports  158  so the lateral position of the bunk assemblies  160  can be adjusted to accommodate different boat hull designs. The number and configuration of the bunk assemblies  160  provided on the boat support platform can be selected based on the hull design of the boat that is to be supported on the platform. 
     Optionally, the bunk beams  254  can be pivotally connected to the mounting brackets  256  so that the bunk assemblies  160  can pivot, in the direction indicated using arrow  257  ( FIG. 3 ). Providing pivotable bunk assemblies may help to accommodate different shaped boat hulls. 
     In the illustrated example, the lifting beams  154   a ,  154   b  and base beams  136   a ,  136   b  are laterally spaced apart so that they are outboard of the boat  162  supported on the lift. Optionally, the lateral spacing between the inboard lifting rails  162   a ,  162   b  can be selected to be between one hundred and one hundred fifty percent of the boat width. Alternatively, in some examples, the configuration of the bunk assemblies  160  may allow a portion of the hull to overhang the lifting beams  154   a ,  154   b  when the boat is resting on the bunks  160 . In such instances, the lateral spacing between the inboard lifting rails  162   a ,  162   b  can be selected to be between approximately seventy five and one hundred percent of the boat width. 
     The example illustrated includes six actuators  124 , with one actuator associated with strut  101 . Alternatively, the boat lift  100  can be configured to include a different number of actuators  124 , and need not have one actuator associated with each strut  101 . For example, each strut  101  can be connected to two or more separate actuator  124 , or only a portion of the support struts  101  can be driven by actuators  124 . 
     In the illustrated example, the structural members the boat lift, including, for example, rails  138   a ,  138   b ,  140   a ,  140   b ,  162   a ,  162   b , and  164   a ,  164   b , cradle supports  158 , support legs  106  and bunk beams  254  are formed from aluminum. The use of aluminum may be preferable because aluminum is relatively light weight and is relatively corrosion resistant when placed in water, compared to an equivalent steel structure. Alternatively, some or all of the members in the boat lift  100  could be formed from other metals having sufficient mechanical properties, such as steel or titanium. 
     In the illustrated embodiment, each rail  138   a ,  138   b ,  140   a , 140   b ,  162   a ,  162   b , and  164   a ,  164   b , is formed from a continuous, extruded tubular member having a generally rectangular cross sectional shape and a hollow interior (see  FIGS. 7 and 9 ). Alternatively, the rails, and other structural members, can be formed from separate plates that are assembled together to form a tubular structure, an I-beam, a C-channel or other suitable structural member that can be used in place of an extruded rail. 
     Referring to  FIG. 10 , a cross sectional view of an example of a bunk assembly  500  that can be used on the boat lift  100  is illustrated. In this example, the bunk beam  502  comprises an extruded aluminum member of constant cross section. The bunk beam  502  includes a pair of T-shaped mounting slots  504  to receive the head of a mounting bolt (not shown) that is used to connected the bunk beam to the mounting brackets, such as mounting brackets  256 . The bunk cushion  506  is an extruded member of constant cross section that is configured to connect to and be supported by the bunk beam  502 . 
     When subjected to the weight of a boat lifted out of the water, the applicant noticed that known vinyl bunk cushions used on traditional boat lifts tend to have undesirable cushioning characteristics (i.e. the vinyl cushions tend to not compress sufficiently or tend to collapse too much), and have limited recovery characteristics (i.e. once crushed, a vinyl bunk cushion may tend to remain crushed). Other known bunk assembly designs, such as covering wood beams with carpet or other such coatings, also tend to have undesirable cushioning and recovery characteristics. 
     In the illustrated example the bunk cushion  506  has an upper portion  508 , that is formed from a resilient material and includes three, longitudinal cavities  510 . The bunk cushion  506  also includes a connecting portion  512  that is configured to connect to the bunk beam  502 . The upper portion  508  is a relatively thin-walled structure and the cavities  510  are filled with inserts  514  formed from a second, resilient material that has a different durometer than the material used to form the upper portion  508 . Optionally, the cavities  510  can have an identical cross sectional shape (although the central cavity can be inverted relative to the outer cavities) so that inserts  514  having a common cross sectional shape can be used to fill each cavity  510 . The outer surface  516  of the upper portion  510  includes three ribs  518  that project above the outer surface  516  to contact the hull of the boat. 
     In the illustrated example, the resilient material used to form the upper portion  508  is an ethylene propylene diene monomer (EPDM) rubber and the insert  514  material is an EPDM closed cell foam. The EPDM foam is relatively less stiff than the EPDM rubber. EPDM rubber and EPDM closed cell foam were selected because they provide desired cushioning and recovery characteristics, as EPDM-based materials can resiliently flex when loaded. The relatively thin walls  520  of the upper portion  508  of the bunk cushion  506  can be sized to provide a desired degree of stiffness, and to deflect after a threshold load has been reached. As the walls  520  deflect, the foam inserts  514  are compressed. Compressing the inserts  514  may provide an additional resistive force, until the cushion  506  reaches an equilibrium position. The bunk cushion  506  may provide a varying, and optionally increasing, level of resistance as it is loaded until the cushion  506  reaches the equilibrium position, for example when the boat initially settles onto the bunk cushions  506 . Applicant also noted that the loading of the bunk assemblies on a boat lift can vary along their length, based on the shaped of the boat and its weight distribution. Because the loading on the bunk cushion can vary along its length, different sections of the cushion  506  may experience different amounts of deflection. 
     Optionally, the stiffness of the bunk cushion  506  can be selected so that the equilibrium compression position (for a rated carrying capacity) is achieved before the inserts  514  are fully compressed. In this configuration, the inserts  514  can further compress and provide increased resistance if the load exerted on the bunk  500  fluctuates or temporarily increases, for example if the boat is jostled while on the lift  100  (for example as a result of wave or wind buffeting on the lift or boat). Providing a varying level of resistance in response to different loading conditions, may help enable the bunk cushion  506  to act as a resilient suspension member that can gently adapt to changes in loading and may help reduce the stress exerted by the cushion  506  on the hull of the boat. 
     This bunk cushion  506  may also be used on other types of boat supporting equipment, including, for example, boat trailers and boat transport railcars or shipping containers. Providing the resiliently deformable bunk cushion  506  on such equipment may act as a suspension system to support the boat above the bunk beams  502  and may help reduce the stress exerted on the boat hull. 
     In the illustrated example, the bunk beam  502  includes a plurality longitudinal grooves  522  separated by cushion retaining members  524 . Each retaining member  524  includes a riser  526  extending from the bunk beam and a head  528  positioned at the distal end of the riser  526 . The head  528  extends laterally beyond the edges of the riser  526  and forms retaining shoulders  530  for engaging the cushion  506 . 
     The connecting portion  512  of the bunk cushion  506  includes a plurality of locking tabs  532 . The tabs  532  can be sized and shaped to fit within the longitudinal grooves  522 . A plurality of longitudinal cushion slots  534  can be configured to receive the heads  528  of the retaining members. The locking tabs  532  include locking barbs  536  that extend laterally away from the locking tabs  532  and are sized to be slightly wider than the spacing between adjacent retaining heads  528 . 
     To assemble the bunk assembly  500 , in the example illustrated, the bunk cushion  506  is placed on the bunk beam  502  so that the locking tabs  532  of the bunk cushion  506  are aligned with corresponding ones of the grooves  522  in the bunk beam  502 , and then compressed against the bunk beam  502  until the barbs  536  laterally compress and the locking tabs  532  are forced into the grooves  522  in a snap-fit manner. After passing between the retaining heads  528 , the locking barbs  536  can return to their original width. When the barbs  536  expand, an upward facing bearing surface  538  on the barbs  536  bears against a downward facing surface  540  of the retaining shoulder  530  to retain the tabs  532  within the grooves  522 . 
     Optionally, some or all of the hollow structural members on the boat lift  100 , including, for example the base rails  138   a ,  138   b ,  140   a , 140   b , the lifting rails  162   a ,  162   b ,  164   a ,  164   b , and the cradle supports  158 , can include internal chambers that can be filled with a gas, for example air, that is less dense than water. When the internal chambers are filled with the gas and submerged in water, the chambers will exert an upward force that can help lift the boat support platform  104  from the lowered position, and optionally can be used to help float the entire boat lift  100  above the bottom of the body of water. 
     Referring to  FIGS. 6-8 , in the illustrated example, the hollow interiors of the inboard and outboard base rails  138   a ,  140   a  are configured to provide air-trapping chambers  262 . The ends of the rails are capped with end plates  142  that are welded to the rails  138   a ,  140   a , and any openings in the sidewalls of the rails, such as bushings  226  for connecting to the hydraulic cylinders  220 , can be sealed using suitable means, including, for example welding the bushings  226  to the sidewalls of the rails  138   a ,  140   a , or using a gasket to seal around the outer perimeter of the bushing. Optionally, the air-trapping chambers  262  in each rail  138   a ,  140   a  can be communicably linked using hollow cross members. Alternatively, each rail  138   a ,  140   a  can form a separate air-trapping chamber  262 . 
     Each rail  138   a ,  140   a  includes a gas fitting  264  that can be connected to an external gas supply, such as, for example, a gas compressor located in the utility box  132  ( FIG. 1 ), using hoses  266 . The gas fitting  264  includes a gas inlet  268  connected to the hose  266 , and a gas outlet  270  in fluid communication with the air-trapping chamber  262 . Optionally, the gas fitting  264  can include a flow control member, such as a valve, to control the flow of gas into and out of the air-trapping chamber  262 . Alternatively, the gas control member can be located upstream from the gas inlet  268  of the fitting, and optionally can be provided at the outlet of the gas compressor or other location that is above the surface of the water, for easier user access. 
     By manipulating the gas control member and/or the gas compressor, the user can selectably transfer air into the air-trapping chamber  262 , to increase the upward force generated by the chamber  262 , or release air from the air-trapping chamber  262  to reduce the upward force generated by the air-trapping chamber  262 . 
     In the illustrated example, each air-trapping chamber  262  also includes a water passage  276  formed in a downward facing surface of the rails  138   a ,  140   a  that provides fluid communication between the interior of the air-trapping chambers  262  and the surrounding water. Each water passage  276  includes an first end  278  in communication with the surrounding water, and a second end  280  in fluid communication with the air-trapping chamber  262 . As pressurized air is pumped into the air-trapping chambers  262  through the fittings  264  in the upper surfaces of the rails  138   a ,  140   a , it can displace any water contained within the air-trapping chambers  262  and cause the water to flow out of the air-trapping chambers  262 , through the water passage  276 , and into the surrounding water. When the gas fitting  264  is sealed, the air within the chambers  262  remains pressurized and exerts and upward lifting force on the boat lift  100 . If the air pressure in the chamber  262  exceeds the surrounding water pressure, excess air may pass through the water passage  276  and bubble out of the chambers  262 . The presence of visible bubbles may alert a user that the air-trapping chamber  262  is full of air. 
     When a user releases the air from the air-trapping chambers  262  (for example by opening the gas fitting  264  or using another type of relief valve) pressure from the surrounding water can urge water through the water passage  276  and into the air-trapping chambers  262 , thereby displacing the air from within the air-trapping chambers  262 . Displacing the air from within the air-trapping chambers  262  can reduce the upward lifting force generated by the air-trapping chambers  262 . If lift  100  is configured to contain the pressurized air within the air-trapping chamber  262  (using the gas fitting  264  or optionally another valve member), the water passage  276  can remain open at all times, as the air pressure will keep water from flowing into the air-trapping chambers  262 . Alternatively, the water passage  276  can include a valve or other flow control member to help control the flow of water into and out of the air-trapping chambers  262 . 
     Similarly, referring to  FIG. 9 , the inboard and outboard lifting rails  162 ,  164  can be configured to provide boat platform air-trapping chambers  272 . The lifting rails  162 ,  164  can also be equipped with gas fittings  264  to allow a user to transfer air into, and out of the boat platform air-trapping chambers  272 . Increasing the amount of upward force generated by the boat platform air-trapping chambers  272  may help reduce the net weight of the boat support platform  104  when it is submerged in water, which may reduce the lifting force required from the actuators  124  to raise the platform  104  from the lowered position. Reducing the lifting force required to lift the boat support platform  104  from the lowered position may be desirable as it may help the actuators  124  rise from the position of least mechanical advantage, and may reduce stress on the pivot joints connecting the actuators to the base beams  136   a ,  136   b  and support arms  106 . 
     Optionally, the cradle supports  158  may also be hollow members that define a sealable internal chamber for containing air, but do not include gas fittings for transferring air into and out of the chamber. In the illustrated example, the cradle supports  158  contain air when they are manufactured, and the ends  237  of the cradle supports  158  can be welded to mounting plates  274 . Optionally, the interior of the cradle supports  158  can be sealed by using solid mounting plates  274 . Alternatively, the mounting plates  274  may not seal the interior of the cradle supports  158 , and when the platform  104  is assembled, the mounting plates  274  can be bolted to the inner lifting rails  162  using a sealing gasket  277 . Using a gasket  277  can help trap air within the cradle supports  158  when the boat lift platform  104  is assembled. A similar connection technique can be used to connect the longitudinal braces  250  to the cradle supports  158 , so that optionally the braces  250  can also retain a quantity of air within their hollow interior chambers. Alternatively, the cradle supports  158  and or longitudinal braces  250  can be equipped with gas fittings as described above. Chambers that do not include gas fittings, for example chambers that are completely sealed by welding need not include water passages  276 , because air is not pumped into, and then released from such sealed chambers. 
     Optionally, a user can fill some or all of the air chambers  262 ,  272  in the boat lift with a quantity of air that is sufficient to generate an upward force that can assist lifting the entire boat lift  100  off the bottom of the body of water. In this configuration, the boat lift  100  may be neutrally buoyant, such that is suspended in the water, or positively buoyant, such that the lift floats at or near the surface of the water. With the boat lift  100  raised off the bottom, the user can reposition the lift on the bottom without requiring a crane or other such heavy lifting device. A user may wish to reposition the lift in response to changes in the water level in the body of water (i.e. if the water level is lower in the fall than it was in the spring), or to move the boat lift into water that is deep enough so that the lift can be sunk and stored (in its lowered position) beneath the ice for the winter. 
     Alternatively, the boat lift  100  may be configured so that with all of its chambers filled with air the boat lift  100  still sinks in the water, but the upward force generated by the air in the chambers  262 ,  272  effectively reduces the net weight of the boat lift  100  to a weight that can be manually lifted by one or more humans (for example approximately 500 pounds), without the need for a crane. 
     Optionally, the air-trapping chambers can include a separate liner or bladder member that is positioned inside the structural members, or other suitable gas containing device. Alternatively, instead of being inside the base beams  136   a ,  136   b  and lifting beams  154   a ,  154   b , the air-trapping chambers can be external tanks or bladders that can be connected to the boat lift  100 . 
     When an unprotected piston/cylinder type actuator, for example actuator  124 , is submerged under water, the sliding seal between the piston rod and the cylinder can be exposed to the water and other contaminants, which may damage the seal. In marine environments minerals, algae and other marine life can coat the piston rod surface and may also cause damage to the seal. If the seal surrounding the piston rod is damaged, dirt, sand, water (possibly salt water), and other foreign material may be able to leak pass the damaged seal and contaminate the hydraulic fluid in the cylinder. Rod scraping mechanisms are an example of devices that are used to clean submerged piston rods, but typically they cannot completely scrap all the accumulated material on the piston rod. 
     Optionally an actuator protection apparatus can be used to insulate the piston rod and hydraulic seals from the surrounding water, and may help prevent seal damage and hydraulic fluid contamination. Optionally, the hydraulic actuators used in the boat lift can include the hydraulic protection system, which may help prolong the useful service life of the actuators. 
     Referring to  FIGS. 11-14 , an example of an actuator  600  including an actuator protection apparatus  602  is illustrated. The actuator  600  can be similar to actuator  124  described above, and is suitable for use with the boat lift  100 . In the illustrated example, the actuator protection apparatus  602  includes a rubber boot  604  surrounding the piston rod  606  of a hydraulic actuator  600 , forming an insulating chamber around the piston rod  606  for containing an insulating fluid. In the illustrated example the insulating chamber is the generally annular cavity  608  between the piston rod  606  and the boot  604 . The actuator protection apparatus also includes a reservoir  610  in fluid communication with the insulating chamber. A quantity of insulating fluid is contained within the apparatus  602  and is transferred between the annular cavity  608  and the reservoir  610  when the actuator is moved. The cavity  608  and reservoir  610  can form a closed fluid circuit. 
     The boot  604  is an expandable bellows-type member that can move between an extended configuration ( FIGS. 11 and 12 ) and a retracted configuration ( FIGS. 13 and 14 ) with the piston rod  606 . The distal end  612  of the boot  604  is coupled to the piston rod  606  to provide a static, water-tight seal  614  between the boot  604  and the surface of the piston rod  606 . The proximate end  616  of the boot is coupled to the cylinder housing  618  of the actuator  600 , to provide an annular, static water-tight seal  620  between the boot  604  and the cylinder housing  618 . In this configuration, the annular cavity  608  is a sealed cavity that is separated from water surrounding the boot. 
     A fluid conduit  622  connects the cavity  608  to the reservoir  610 . In the illustrated example, the fluid conduit  622  includes a passage  624  formed in the cylinder housing  618  and an external pipe  626 . The passage  624  has a fluid inlet  628  in communication with the cavity  608 , and a fluid outlet  630  in a sidewall of the cylinder housing  618  that is connected to the inlet of the pipe  626  using a fitting  632 . The outlet  634  of the pipe  626  is coupled to the reservoir  610  using an outlet fitting  636  ( FIG. 14 ). 
     In the illustrated example, the reservoir  610  includes a resilient, expandable bladder  638  formed from a corrugated rubber tube  640 . One end of the tube is connected to the pipe outlet fitting and the other end of the tube is sealed to contain the insulating fluid in the bladder  638 . The bladder  638  is elastically expandable from a contracted position ( FIG. 12 ) to an extended position ( FIG. 14 ). 
     When the hydraulic actuator  600  is in use, the piston rod  606  is moved between its extended ( FIG. 12 ) and contracted positions ( FIG. 14 ). When the piston rod  606  is extended, the annular cavity  608  has a relatively large volume, and is filled with the insulating fluid. As the piston rod  606  moves toward its retracted position, the volume of the annular cavity  608  decreases, and insulating fluid is forced from the annular cavity  608  into the bladder  638 . As the quantity of insulating fluid in the bladder  638  increases, the resilient bladder  638  expands to accommodate the incoming insulating fluid. 
     When the piston rod is extended, the volume of the annular cavity  608  increases, which can slightly decrease the internal pressure of the cavity  608  and draw insulating fluid from the reservoir  610  into the cavity. In the illustrated example, the resilient nature of the rubber tube  640  may also exert a contractive force on the bladder  638 , which can help urge the insulating fluid from the bladder  638  into the cavity  608 . As the insulating fluid flows from the bladder  638  into the cavity  608 , the bladder  638  can shrink to its contracted configuration ( FIG. 12 ). Optionally, in some configurations, the suction from the extension of the piston rod  606  may be sufficient to draw the insulating fluid into the cavity  608 , and the bladder  638  need not be resilient. 
     In the illustrated example, the reservoir  610  also includes a cylindrical outer shell  642  surrounding the bladder  638 . The cylindrical outer shell  642  is connected to the cylinder housing  618 . The outer shell has a hollow interior  644  that is large enough to accommodate the bladder  638  when the bladder  638  is extended. The outer shell  642  can be water tight, and the interior  644  of the outer shell can be filled with air. In this configuration, the bladder  638  can expand within the outer shell  642 , without encountering resistance from the water surrounding the actuator  600 . Expanding into the interior  644  of the outer shell  642  may also help prevent the bladder  638  from becoming jammed against the support arms  106  or other portions of the lift  100  as the bladder  638  expands. The outer shell  642  can be formed from a rigid material, including for example metal or plastic, to protect the bladder  638  from being impacted by debris in the water. In other embodiments, the bladder  638  can be exposed to the surrounding water, and need not be enclosed in an outer shell  642 , and/or the interior  644  of the shell  642  can be open to the surrounding water. 
     The outer shell  642  can be sized so that when the bladder  638  is fully extended (i.e. when the piston rod  606  is contracted and the boat lift  100  is in the lowered position) the bladder  638  does not contact the end wall of the shell  642 . This can allow for the bladder  638  to over-extend beyond its normal, fully extended position if the pressure of the insulation liquid within the system increases. Such a pressure increase may occur, for example, if some or all of the boot  604  extends above the surface of the water surrounding the boat lift  100 . Optionally, a stopper  646  can be provided within the shell  642 , to support the bladder  638  when it reaches its fully extended position while still allowing for over-extension of the bladder  638  if necessary. Preferably, the stopper  646  is a flexible member that is stiff enough to support the weight of the bladder  638  under normal operating conditions, but yieldable enough to compress and allow the bladder  638  to over-extend if needed. More preferably, the stopper  646  is a resilient member that can return the bladder  638  to its normal, fully extended position when the insulating fluid pressure decreases (for example when the boot  604  is re-submerged in the water). Examples of resilient stoppers  646  can include springs, air bladders, and other biasing elements. Optionally, the stopper  646  can be selected so that it provides a varying, increasing level of resistance in response to increasing extension of the bladder  638  (for example a coil spring having a selected stiffness co-efficient). 
     Optionally, the insulating fluid in the actuator protection apparatus  602  can be pressurized to an operating pressure that is generally equivalent to the hydrostatic pressure of the water surrounding the boot  604 . Pressurizing the insulating fluid within the cavity  608  in this manner can reduce the differential pressure across the static seals  614  and  620 , which may help reduce leakage across these seals. Optionally, the insulating fluid can be pressurized to a pressure that is above the hydrostatic pressure of the water, so that if any leakage does occur at the seals, insulating fluid will leak into the water, instead of allowing water to contaminate the insulating fluid. In the illustrated example, the insulating fluid contained in the actuator protection apparatus is filtered fresh water that is generally free from sand, salt and marine life. Filtered water may be a preferred insulating fluid for use with the boat lift  100 , because it is unlikely to cause environmental damage if it leaks into the surrounding water. Optionally, instead of filtered water, the insulating fluid can be any other fluid that will not damage the actuator  600 , including, for example, hydraulic oil, air, inert gases and other lubricants. 
     Optionally, the insulating fluid within the annular cavity  608  can be selected to have generally the same density as the surrounding water. 
     What has been described above has been intended to be illustrative of the invention and non-limiting and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto. 
     What has been described above has been intended to be illustrative of the invention and non-limiting and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto.