Failsafe watercraft lift with convertible leveling system

Abstract

A watercraft lift for raising and lowering a watercraft in water, including a buoyant pontoon, a lifting cradle and at least one pivot arm. The lifting cradle includes at least one air tank and a support bunk configured to receive and support the watercraft. The air tank has an internal chamber configured to receive and release pressurized air. The internal chamber has sufficient internal volume that when sufficient pressurized air is received therein the air tank has sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the support bunk and that when sufficient pressurized air is released from the internal chamber the air tank loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft. The pivot arm is pivotally connected to the pontoon and pivotally connected to the lifting cradle to guide movement of the lifting cradle between the lowered position and the raised position.

1 . A watercraft lift for raising and lowering a watercraft in water, comprising: 
 a buoyant pontoon;    a lifting cradle including at least one air tank and a support bunk configured to receive and support the watercraft, the air tank having an internal chamber configured to receive and release pressurized air, the internal chamber having sufficient internal volume that when sufficient pressurized air is received therein the air tank has sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the support bunk and that when sufficient pressurized air is released from the internal chamber the air tank loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft; and    at least one pivot arm pivotally connected to the pontoon and pivotally connected to the lifting cradle to guide movement of the lifting cradle between the lowered position and the raised position.    
   
   
       2 . The watercraft lift of  claim 1  wherein the pontoon includes a buoyant port pontoon portion and a buoyant starboard pontoon portion, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween.  
   
   
       3 . The watercraft lift of  claim 2  further including ballast positioned at the port pontoon portion and ballast positioned at the starboard pontoon portion in amounts sufficient to prevent the watercraft lift from rolling when the watercraft is positioned on the support bunk with the watercraft lift in the raised position when all pressurized air is released from the internal chamber the air tank.  
   
   
       4 . The watercraft lift of  claim 3  wherein the ballast is removably attached to the port pontoon portion and the ballast is removably attached to the starboard pontoon portion.  
   
   
       5 . The watercraft lift of  claim 3  wherein the port and starboard pontoon portions are made of a first material and the ballast is made of a second material, the first material being different than the second material.  
   
   
       6 . The watercraft lift of  claim 1  wherein the pontoon includes a buoyant port pontoon portion having an end portion and a buoyant starboard pontoon portion having an end portion, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween, and further includes a buoyant connection pontoon portion having the end portions of the port and starboard pontoon portions attached thereto, the combined buoyancy of the port pontoon portion, the starboard pontoon portion and the connection pontoon portion being sufficient the support bunk in sufficient contact with the watercraft to prevent the watercraft from floating off of the support bunk when the watercraft is positioned on the support bunk with the watercraft lift in the raised position when all pressurized air is released from the internal chamber the air tank.  
   
   
       7 . The watercraft lift of  claim 1  wherein the pontoon includes a buoyant port pontoon portion having an end portion and a buoyant starboard pontoon portion having an end portion, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween, and further includes a buoyant connection pontoon portion having the end portions of the port and starboard pontoon portions attached thereto, the port pontoon portion, the starboard pontoon portion and the connection pontoon portion having upper surfaces arranged to provide a floating dock surface for access to the watercraft from three sides thereof when positioned on the support bunk.  
   
   
       8 . The watercraft lift of  claim 1  wherein the pontoon includes a buoyant port pontoon portion having an end portion and a buoyant starboard pontoon portion having an end portion, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween, and further includes buoyant port and starboard connection pontoon portions, the end portion of the port pontoon portion being attached to the port connection pontoon portion and the end portion of the starboard pontoon portion being attached to the starboard connection pontoon portion, and the port connection pontoon portion and the starboard connection pontoon portion being removably attached together.  
   
   
       9 . The watercraft lift of  claim 1  further including a lock operable to lock the pivot arm relative to the pontoon when the lifting cradle is in the raised position to prevent downward movement of the pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       10 . The watercraft lift of  claim 9  wherein the lock includes a selectively rotatable upright member having an engagement member attached thereto, the rotatable member being rotatable between a locked position and an unlocked position, in the locked position the engagement member being positioned in locking engagement with the pivot arm to prevent downward movement of the pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the pivot arm to allow downward movement of the pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       11 . The watercraft lift of  claim 10  wherein the lock includes a security member which when engaged prevents rotation of the rotatable member out of the locked position.  
   
   
       12 . The watercraft lift of  claim 1  wherein the pivot arm has an upper end portion pivotally connected to the pontoon and a lower end portion pivotally connected to the air tank of the lifting cradle using a laterally extending member, the lower end portion of the pivot arm being connected to an end portion of the member to provide a pivotal connection between the lower end portion of the pivot arm and the air tank.  
   
   
       13 . The watercraft lift of  claim 1  wherein the air tank has a longitudinally extending integrally molded structural rib projecting upward, the member extending laterally through the structural rib.  
   
   
       14 . The watercraft lift of  claim 1  wherein the at least one pivot arm includes first and second spaced apart port pivot arms, each having an upper end portion pivotally connected to the pontoon and a lower end portion pivotally connected to the lifting cradle, and wherein the air tank has a longitudinally extending integrally molded structural rib projecting upward and the lower end portions of the first and second pivot arms are pivotally connected to the structural rib.  
   
   
       15 . The watercraft lift of  claim 14  wherein the support bunk is attached to the air tank in a manner to provide longitudinal rigidity to the air tank.  
   
   
       16 . The watercraft lift of  claim 1  wherein the air tank has a laterally projecting portion extending outward to under the pontoon to engage the pontoon upon the lifting cradle reaching the raised position.  
   
   
       17 . A watercraft lift for raising and lowering a watercraft in water, comprising: 
 a buoyant pontoon having a buoyant port pontoon portion with an end portion, a buoyant starboard pontoon portion with an end portion, and a buoyant connection pontoon portion with the end portions of the port and starboard pontoon portions attached thereto, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween;    a lifting cradle including an air tank assembly and a support bunk configured to receive and support the watercraft, the air tank assembly having at least one internal chamber configured to receive and release pressurized air, the internal chamber having sufficient internal volume that when sufficient pressurized air is received therein the air tank assembly has sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the support bunk and that when sufficient pressurized air is released from the internal chamber the air tank assembly loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft; and    at least one port pivot arm pivotally connected to the port pontoon portion and pivotally connected to the lifting cradle, and at least one starboard pivot arm pivotally connected to the starboard pontoon portion and pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position.    
   
   
       18 . The watercraft lift of  claim 17  wherein the port and starboard pivot arms are pivotally connected to the lifting cradle using a member extending between a port side of the lifting cradle and a starboard side of the lifting cradle, the port pivot arm being connected to a port end of the member and the starboard pivot arm being connected to a starboard end of the member.  
   
   
       19 . The watercraft lift of  claim 18  wherein the member is a torsion bar.  
   
   
       20 . The watercraft lift of  claim 17  wherein the pontoon includes a buoyant port pontoon portion having an end portion and a buoyant starboard pontoon portion having an end portion, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween, and further includes a buoyant connection pontoon portion having the end portions of the port and starboard pontoon portions attached thereto, the combined buoyancy of the port pontoon portion, the starboard pontoon portion and the connection pontoon portion being sufficient the support bunk in sufficient contact with the watercraft to prevent the watercraft from floating off of the support bunk when the watercraft is positioned on the support bunk with the watercraft lift in the raised position when all pressurized air is released from the internal chamber the air tank assembly.  
   
   
       21 . The watercraft lift of  claim 17  further including a first lock operable to lock the port pivot arm relative to the port pontoon portion when the lifting cradle is in the raised position to prevent downward movement of the port pivot arm and thereby movement of the lifting cradle to the lowered position, and a second lock operable to lock the starboard pivot arm relative to the starboard pontoon portion when the lifting cradle is in the raised position to prevent downward movement of the starboard pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       22 . The watercraft lift of  claim 21  wherein the first lock includes a selectively rotatable first rotatable upright member having a first engagement member attached thereto, the first rotatable member being rotatable supported by the port pontoon portion and rotatable between a locked position and an unlocked position, in the locked position the first engagement member being positioned in locking engagement with the port pivot arm to prevent downward movement of the port pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the port pivot arm to allow downward movement of the port pivot arm and thereby movement of the lifting cradle to the lowered position, and a selectively rotatable second rotatable upright member having a second engagement member attached thereto, the second rotatable member being rotatable supported by the starboard pontoon portion and rotatable between a locked position and an unlocked position, in the locked position the second engagement member being positioned in locking engagement with the starboard pivot arm to prevent downward movement of the starboard pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the starboard pivot arm to allow downward movement of the starboard pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       23 . The watercraft lift of  claim 22  wherein the first lock includes a first security member which when engaged prevents rotation of the first rotatable member out of the locked position, and the second lock includes a second security member when engaged prevents rotation of the second rotatable member out of the locked position.  
   
   
       24 . The watercraft lift of  claim 17  wherein the port and starboard pivot arms each have an upper end portion and a lower end portion, the upper end portion of the port pivot arm being pivotally connected to the port pontoon portion and the lower end portion of the port pivot arm being pivotally connected to the air tank assembly of the lifting cradle, and the upper end portion of the starboard pivot arm being pivotally connected to the starboard pontoon portion and the lower end portion of the starboard pivot arm being pivotally connected to the air tank assembly of the lifting cradle.  
   
   
       25 . The watercraft lift of  claim 24  further including a member extending between a port side of the air tank assembly and a starboard side of the air tank assembly, the lower end portion of the port pivot arm being connected to a port end of the member and the lower end portion of the starboard pivot arm being connected to a starboard end of the member.  
   
   
       26 . The watercraft lift of  claim 17  wherein the air tank assembly has a longitudinally extending integrally molded structural rib projecting upward, the member extending laterally through the structural rib.  
   
   
       27 . The watercraft lift of  claim 17  wherein the at least one port pivot arm includes first and second spaced apart port pivot arms, each having an upper end portion pivotally connected to the port pontoon portion and a lower end portion pivotally connected to the lifting cradle, and the at least one starboard pivot arm includes first and second spaced apart starboard pivot arms, each having an upper end portion pivotally connected to the starboard pontoon portion and a lower end portion pivotally connected to the lifting cradle, and wherein the air tank assembly has a longitudinally extending integrally molded structural rib projecting upward and the lower end portions of the first and second port and starboard pivot arms are pivotally connected to the structural rib.  
   
   
       28 . The watercraft lift of  claim 27  wherein the support bunk is attached to the air tank assembly in a manner to provide longitudinal rigidity to the air tank assembly.  
   
   
       29 . The watercraft lift of  claim 17  wherein the air tank has a laterally projecting portion extending outward to under the pontoon to engage the pontoon upon the lifting cradle reaching the raised position.  
   
   
       30 . A watercraft lift for raising and lowering a watercraft in water, comprising: 
 a buoyant pontoon having a buoyant port pontoon portion with an end portion, a buoyant starboard pontoon portion with an end portion, and a buoyant connection pontoon portion with the end portions of the port and starboard pontoon portions attached thereto, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween;    a lifting cradle with support bunks attached thereto and including a port internal chamber configured to receive and release pressurized air, and a starboard internal chamber configured to receive and release pressurized air, the support bunks being configured to receive and support the watercraft, the combined port and starboard internal chambers having sufficient internal volume that when sufficient pressurized air is received therein the lifting cradle has sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the support bunks and that when sufficient pressurized air is released from the port and starboard internal chambers the lifting cradle loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft; and    at least one port pivot arm having an upper end portion pivotally connected to the port pontoon portion and a lower end portion pivotally connected to the lifting cradle, and at least one starboard pivot arm having an upper end portion pivotally connected to the starboard pontoon portion and a lower end portion pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position.    
   
   
       31 . The watercraft lift of  claim 30  wherein a lifting cradle includes at least one port air tank defining the port internal chamber and at least one starboard air tank defining the starboard internal chamber.  
   
   
       32 . The watercraft lift of  claim 31  further including a hydrodynamic brake extending between the port and starboard air tanks to create additional resistance against movement of the lifting cradle.  
   
   
       33 . The watercraft lift of  claim 31  wherein the lower end portion of the port pivot arm is pivotally connected to the port air tank of the lifting cradle, and the lower end portion of the starboard pivot arm is pivotally connected to the starboard air tank of the lifting cradle.  
   
   
       34 . The watercraft lift of  claim 33  wherein the lower end portions of the port and starboard pivot arms are pivotally connected to the port and starboard air tanks of the lifting cradle using a member extending between a port side of the port air tank and a starboard side of the starboard air tank, the lower end portion of the port pivot arm being connected to a port end of the member and the lower end portion of the starboard pivot arm being connected to a starboard end of the member.  
   
   
       35 . The watercraft lift of  claim 34  wherein the member is a torsion bar extending through an aperture in the port and starboard air tanks.  
   
   
       36 . The watercraft lift of  claim 31  wherein the port air tank has a laterally projecting portion extending outward to under the port pontoon portion to engage the port pontoon portion upon the lifting cradle reaching the raised position, and the starboard air tank has a laterally projecting portion extending outward to under the starboard pontoon portion to engage the starboard pontoon portion upon the lifting cradle reaching the raised position.  
   
   
       37 . The watercraft lift of  claim 30  further including a port relief port communicating with the port internal chamber and a starboard relief port communicating with the starboard internal chamber, the port and starboard relief ports being arranged to permit the release of pressurized air in one of the port internal chamber and the starboard internal chamber based on which is least submerged in the water.  
   
   
       38 . The watercraft lift of  claim 37  wherein a lifting cradle includes at least one port air tank defining the port internal chamber and at least one starboard air tank defining the starboard internal chamber, and the port relief port is positioned at a port side of the port air tank and the starboard relief port is positioned at a starboard side of the starboard air tank.  
   
   
       39 . The watercraft lift of  claim 30  further including a hydrodynamic brake extending between the port and starboard air tanks to create additional resistance against movement of the lifting cradle.  
   
   
       40 . The watercraft lift of  claim 30  for conversion for use with a slip having port and starboard side members, wherein the upper end portion of the port pivot arm is removably connected to the port pontoon portion and pivotally attachable to the port side member of the slip, and the upper end portion of the starboard pivot arm is removably connected to the starboard pontoon portion and pivotally attachable to the starboard side member of the slip.  
   
   
       41 . The watercraft lift of  claim 30  wherein the port and starboard pontoon portions are each a rotomolded plastic part.  
   
   
       42 . A watercraft lift for raising and lowering a watercraft in water, comprising: 
 a buoyant pontoon having a buoyant port pontoon portion with an end portion, a buoyant starboard pontoon portion with an end portion, and a buoyant connection pontoon portion with the end portions of the port and starboard pontoon portions attached thereto, the port and starboard pontoon portions being spaced apart sufficient to receive the watercraft therebetween;    a lifting cradle including a port air tank with a port support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, and a starboard air tank with a starboard support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, the port and starboard support bunks being configured to receive and support the watercraft, the combined internal chambers of the port and starboard air tanks having sufficient internal volume that when sufficient pressurized air is received therein the port and starboard air tanks have sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the port and starboard support bunks and that when sufficient pressurized air is released from the internal chambers of the port and starboard air tanks the air tanks loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft; and    first and second spaced apart port pivot arms, each having an upper end portion pivotally connected to the port pontoon portion and a lower end portion pivotally connected to the lifting cradle, and first and second spaced apart starboard pivot arms, each having an upper end portion pivotally connected to the starboard pontoon portion and a lower end portion pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position, the first and second port pivot arms, the port pontoon portion and the lifting cradle being in a 4-bar linkage arrangement and the first and second starboard pivot arms, the starboard pontoon portion and the lifting cradle being in a 4-bar linkage arrangement.    
   
   
       43 . The watercraft lift of  claim 42  wherein the lower end portions of the first and second port pivot arms are pivotally connected to the port air tank of the lifting cradle, and the lower end portions of the first and second of the starboard pivot arms are pivotally connected to the starboard air tank of the lifting cradle.  
   
   
       44 . The watercraft lift of  claim 43  wherein the lower end portions of the first port pivot arm and the first starboard pivot arm are pivotally connected to the port and starboard air tanks of the lifting cradle using a first member extending between a port side of the port air tank and a starboard side of the starboard air tank, the lower end portion of the first port pivot arm being connected to a port end of the first member and the lower end portion of the first starboard pivot arm being connected to a starboard end of the first member, and wherein the lower end portions of the second port pivot arm and the second starboard pivot arm are pivotally connected to the port and starboard air tanks of the lifting cradle using a second member extending between a port side of the port air tank and a starboard side of the starboard air tank and spaced apart from the first member, the lower end portion of the second port pivot arm being connected to a port end of the second member and the lower end portion of the second starboard pivot arm being connected to a starboard end of the second member.  
   
   
       45 . The watercraft lift of  claim 44  wherein the first and second member are a torsion bars, each extending through an aperture in the port and starboard air tanks.  
   
   
       46 . The watercraft lift of  claim 44  wherein the port and starboard air tanks each have a longitudinally extending integrally molded structural rib projecting upward, the first member extending through the structural rib of the port and starboard air tanks, and the second member extending through the structural rib of the port and starboard air tanks.  
   
   
       47 . The watercraft lift of  claim 43  wherein the port and starboard air tanks each have a longitudinally extending integrally molded structural rib projecting upward, and the lower end portions of the first and second port pivot arms are pivotally connected to the structural rib of the port air tank and the lower end portions of the first and second starboard pivot arms are pivotally connected to the structural rib of the starboard air tank.  
   
   
       48 . The watercraft lift of  claim 47  wherein the port support bunk is attached to the port air tank in a manner to provide longitudinal rigidity to the port air tank, and the starboard support bunk is attached to the starboard air tank in a manner to provide longitudinal rigidity to the starboard air tank.  
   
   
       49 . The watercraft lift of  claim 42  further including a port relief port communicating with the interior chamber of the port air tank and a starboard relief port communicating with the interior chamber of the starboard air tank, the port and starboard relief ports being arranged to permit the release of pressurized air in the interior chamber of the one of the port and starboard air tanks that is least submerged in the water.  
   
   
       50 . The watercraft lift of  claim 49  wherein the port relief port is positioned at the port side of the port air tank and the starboard relief port is positioned at the starboard side of the starboard air tank.  
   
   
       51 . The watercraft lift of  claim 42  further including a first lock operable to lock one of the first and second port pivot arms relative to the port pontoon portion when the lifting cradle is in the raised position to prevent downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and a second lock operable to lock one of the first and second starboard pivot arms relative to the starboard pontoon portion when the lifting cradle is in the raised position to prevent downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       52 . The watercraft lift of  claim 51  wherein the first lock includes a selectively rotatable first rotatable upright member having a first engagement member attached thereto, the first rotatable member being rotatable supported by the port pontoon portion and rotatable between a locked position and an unlocked position, in the locked position the first engagement member being positioned in locking engagement with the one port pivot arm to prevent downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the one port pivot arm to allow downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and a selectively rotatable second rotatable upright member having a second engagement member attached thereto, the second rotatable member being rotatable supported by the starboard pontoon portion and rotatable between a locked position and an unlocked position, in the locked position the second engagement member being positioned in locking engagement with the one starboard pivot arm to prevent downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the one starboard pivot arm to allow downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       53 . The watercraft lift of  claim 52  wherein the first lock includes a first security member which when engaged prevents rotation of the first rotatable member out of the locked position, and the second lock includes a second security member when engaged prevents rotation of the second rotatable member out of the locked position.  
   
   
       54 . A watercraft lift for raising and lowering a watercraft in water, attachable to a slip having a port side and a starboard side, the watercraft lift comprising: 
 a lifting cradle including a port air tank with a port support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, and a starboard air tank with a starboard support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, the port and starboard support bunks being configured to receive and support the watercraft, the combined internal chambers of the port and starboard air tanks having sufficient internal volume that when sufficient pressurized air is received therein the port and starboard air tanks have sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the port and starboard support bunks and that when sufficient pressurized air is released from the internal chambers of the port and starboard air tanks the air tanks loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft;    first and second spaced apart port pivot arms, each having an upper end portion pivotally connectable to the port side of the slip and a lower end portion pivotally connected to the lifting cradle, and first and second spaced apart starboard pivot arms, each having an upper end portion pivotally connectable to the starboard side of the slip and a lower end portion pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position, the first and second port pivot arms, the port side of the slip and the lifting cradle forming a 4-bar linkage arrangement and the first and second starboard pivot arms, the starboard side of the slip and the lifting cradle forming a 4-bar linkage arrangement; and    a first lock operable to lock one of the first and second port pivot arms relative to the port side of the slip when the lifting cradle is in the raised position to prevent downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and a second lock operable to lock one of the first and second starboard pivot arms relative to the starboard side of the slip when the lifting cradle is in the raised position to prevent downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position.    
   
   
       55 . The watercraft lift of  claim 54  wherein the first lock includes a selectively rotatable first rotatable upright member having a first engagement member attached thereto, the first rotatable member being rotatable supported by the port side of the slip and rotatable between a locked position and an unlocked position, in the locked position the first engagement member being positioned in locking engagement with the one port pivot arm to prevent downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the one port pivot arm to allow downward movement of the one port pivot arm and thereby movement of the lifting cradle to the lowered position, and a selectively rotatable second rotatable upright member having a second engagement member attached thereto, the second rotatable member being rotatable supported by the starboard side of the slip and rotatable between a locked position and an unlocked position, in the locked position the second engagement member being positioned in locking engagement with the one starboard pivot arm to prevent downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position, and in the unlocked position the engagement member being positioned out of locking engagement with the one starboard pivot arm to allow downward movement of the one starboard pivot arm and thereby movement of the lifting cradle to the lowered position.  
   
   
       56 . The watercraft lift of  claim 55  wherein the first lock includes a first security member which when engaged prevents rotation of the first rotatable member out of the locked position, and the second lock includes a second security member when engaged prevents rotation of the second rotatable member out of the locked position.  
   
   
       57 . A watercraft lift for raising and lowering a watercraft in water, attachable to a slip having a port side and a starboard side, the watercraft lift comprising: 
 a lifting cradle including a port air tank with a port support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, and a starboard air tank with a starboard support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, the port and starboard support bunks being configured to receive and support the watercraft, the combined internal chambers of the port and starboard air tanks having sufficient internal volume that when sufficient pressurized air is received therein the port and starboard air tanks have sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the port and starboard support bunks and that when sufficient pressurized air is released from the internal chambers of the port and starboard air tanks the air tanks loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft;    first and second spaced apart port pivot arms, each having an upper end portion pivotally connectable to the port side of the slip and a lower end portion pivotally connected to the lifting cradle, and first and second spaced apart starboard pivot arms, each having an upper end portion pivotally connectable to the starboard side of the slip and a lower end portion pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position, the first and second port pivot arms, the port side of the slip and the lifting cradle forming a 4-bar linkage arrangement and the first and second starboard pivot arms, the starboard side of the slip and the lifting cradle forming a 4-bar linkage arrangement; and    a first member attached to port and starboard air tanks and extending between a port side of the port air tank and a starboard side of the starboard air tank, the lower end portions of the first port pivot arm and the first starboard pivot arm being pivotally connected to the port and starboard air tanks of the lifting cradle using the first member, the lower end portion of the first port pivot arm being connected to a port end of the first member and the lower end portion of the first starboard pivot arm being connected to a starboard end of the first member; and    a second member attached to port and starboard air tanks and extending between a port side of the port air tank and a starboard side of the starboard air tank and spaced apart from the first member, the lower end portions of the second port pivot arm and the second starboard pivot arm being pivotally connected to the port and starboard air tanks of the lifting cradle using the second member, the lower end portion of the second port pivot arm being connected to a port end of the second member and the lower end portion of the second starboard pivot arm being connected to a starboard end of the second member.    
   
   
       58 . The watercraft lift of  claim 57  wherein the first and second member are a torsion bars, each extending through an aperture in the port and starboard air tanks.  
   
   
       59 . The watercraft lift of  claim 57  wherein the port and starboard air tanks each have a longitudinally extending integrally molded structural rib projecting upward, the first member extending through the structural rib of the port and starboard air tanks, and the second member extending through the structural rib of the port and starboard air tanks.  
   
   
       60 . The watercraft lift of  claim 57  wherein the port support bunk is attached to the port air tank in a manner to provide longitudinal rigidity to the port air tank, and the starboard support bunk is attached to the starboard air tank in a manner to provide longitudinal rigidity to the starboard air tank.  
   
   
       61 . A watercraft lift for raising and lowering a watercraft in water, attachable to a slip having a port side and a starboard side, the watercraft lift comprising: 
 a lifting cradle including a port air tank with a port support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, and a starboard air tank with a starboard support bunk attached thereto and having an internal chamber configured to receive and release pressurized air, the port and starboard support bunks being configured to receive and support the watercraft, the combined internal chambers of the port and starboard air tanks having sufficient internal volume that when sufficient pressurized air is received therein the port and starboard air tanks have sufficient buoyancy to lift the lifting cradle to a raised position with the watercraft out of the water when positioned on the port and starboard support bunks and that when sufficient pressurized air is released from the internal chambers of the port and starboard air tanks the air tanks loses sufficient buoyancy to sink the lifting cradle to a lowered position sufficiently submerged to receive and deploy the watercraft;    first and second spaced apart port pivot arms, each having an upper end portion pivotally connectable to the port side of the slip and a lower end portion pivotally connected to the lifting cradle, and first and second spaced apart starboard pivot arms, each having an upper end portion pivotally connectable to the starboard side of the slip and a lower end portion pivotally connected to the lifting cradle, the port and starboard pivot arms configured to guide movement of the lifting cradle between the lowered position and the raised position, the first and second port pivot arms, the port side of the slip and the lifting cradle forming a 4-bar linkage arrangement and the first and second starboard pivot arms, the starboard side of the slip and the lifting cradle forming a 4-bar linkage arrangement; and    a hydrodynamic brake extending between the port and starboard air tanks to create additional resistance against movement of the lifting cradle.