Low profile lift for watercraft

A low-profile watercraft lift having first and second cantilever arms pivotally mounted to a base at offset pivot points for use in shallow water. The lift includes an actuator connected to the first and second cantilever arms and operable to move the first and second cantilever arms between a collapsed configuration and an extended configuration with uniform application of force and a minimum amount of travel of actuator components. The lift further includes a universal plate affixed to bunk support rails for pivotally attaching hull support bunks to the support rails and to accommodate attachment of accessories including guide posts and a motor stop.

TECHNICAL FIELD
 The invention relates to lifting devices, and in particular to devices for
 lifting watercraft, for example, boats and sea planes.
 BACKGROUND OF THE INVENTION
 Known is U.S. Pat. No. 5,184,914 issued to the inventor of the present
 invention which is incorporated herein by reference and discloses a
 watercraft lifting device having a rectangular stationary base formed of
 two longitudinal parallel beams and two transverse beams, generally
 described as front and rear transverse beams. The rectangular base is
 submersible under water. Pivoting booms connect each of the four corners
 of the rectangular base to swingable mounting arms positioned parallel to
 and coplanar with each of the longitudinal beams to form two pairs of
 pivoting booms, generally described as front and rear pivoting booms. The
 two pair of pivoting booms form with the mounting arms collapsing
 parallelograms on which watercraft supports extended a predetermined
 distance above the mounting arms hold the craft during lifting. A
 double-acting hydraulic cylinder is pivotally connected to the rear
 transverse beam and its piston rod is pivotally connected to the two front
 pivoting booms such that expansive energization of the double-acting
 hydraulic cylinder extends the piston rod and swings front pair of
 pivoting booms upward from a collapsed attitude. The parallelogram linkage
 forces the mounting arms and rear pair of pivoting booms to follow the
 front pair of pivoting booms. Thus, expansive energization of the
 double-acting hydraulic cylinder raises the front pair of pivoting booms
 and lifts the rear pair of pivoting booms, the mounting arms and the
 watercraft supports attached to the mounting arms upward to lift a
 watercraft out of the water. Upward movement continues until the pivoting
 booms pass through a vertical orientation into an overcenter orientation
 whereby the watercraft is supported above the surface of the water.
 Retractive energization of the double-acting hydraulic cylinder retracts
 the piston rod into the piston jacket of the double-acting hydraulic
 cylinder and reverses the motion of the pivoting booms. Thus, retractive
 energization of the double-acting hydraulic cylinder first raises the
 pivoting booms and lifts the mounting arms and watercraft supports
 attached to the mounting arms upward. Upward movement causes the pivoting
 booms to pass back through vertical orientation. Continued retraction of
 the piston rod into the double-acting hydraulic cylinder combined with the
 weight of the latching apparatus and the watercraft collapses the
 parallelograms whereby the watercraft is lowered into the water. The
 piston rod continues to retract into the double-acting hydraulic cylinder
 collapsing the parallelograms, including the mounting arms and watercraft
 supports attached to the mounting arms, until contact between the
 watercraft supports and the watercraft is broken and the watercraft can
 float free.
 Although the apparatus of the prior art operates effectively in many
 practical applications, a need exists for a watercraft lifting apparatus
 which operates effectively in shallow water applications where the typical
 water depth is minimal and the apparatus of the prior art cannot collapse
 sufficiently to break contact between the watercraft supports and the
 watercraft and release the watercraft to float free.
 SUMMARY OF THE INVENTION
 The present invention resolves limitations of the prior art by providing a
 low profile watercraft lifting apparatus. The present invention is a
 watercraft lifting apparatus which includes a generally rectangular base
 adapted to be submerged under water. The base is formed of two
 longitudinal beams joined by two transverse beams generally described as
 front and rear transverse beams. Pivoting booms connect each of the four
 corners of the rectangular base to swingable mounting arms positioned
 generally parallel with the longitudinal beams to form two pairs of
 pivoting booms, generally described as a front pair of pivoting booms and
 a rear pair of pivoting booms. The pivoting booms form with the mounting
 arms collapsing mock parallelograms on which watercraft supports hold the
 craft during lifting.
 According to one aspect of the present invention, the low profile lifting
 apparatus of the present invention provides a self-guiding watercraft
 entry attitude by providing the pivot points for the rear pair of pivoting
 booms at a position above the pivot points for the front pair of pivoting
 booms. Positioning the rear boom pivot points above the pivot points for
 the front pair of pivoting booms provides a mock parallelogram shape in a
 side elevation view wherein the rear pair of pivoting booms and thus the
 rear ends of the mounting arms are positioned at a lower attitude than the
 front ends of the mounting arms and are angled downwardly from the higher
 elevation of the front ends of the mounting arms when the lifting
 apparatus is in a collapsed attitude. In one preferred embodiment, the
 pivot points for the rear pair of pivoting booms at a position above the
 centerline of the longitudinal beams of the base.
 According to another aspect of the present invention, the self-guiding
 watercraft entry attitude provided by the positioning of the rear boom
 pivot points above the front boom pivot points is accentuated by providing
 the pivot points for the front pair of pivoting booms at a position below
 the centerline of the longitudinal beams of the base. Positioning the
 front boom pivot points below the longitudinal beam centerline provides an
 accentuated mock parallelogram shape in a side elevation view by
 accentuating the downward angle of the mounting arms when the lifting
 apparatus is collapsed. Furthermore, varying the lengths of the front and
 rear pivoting booms by the amount of the off-set between the front and
 rear boom pivoting points reduces the downward angle of the mounting arms
 when the booms are fully extended such that mounting arms are essentially
 parallel with the longitudinal beams of the base when the lifting
 apparatus is in an upright or extended orientation.
 According to yet another aspect of the invention, each pair of pivoting
 booms are positioned either inward or outward of the two longitudinal
 beams of the base rather than coplanar with the longitudinal beams. Thus,
 the booms collapse into a side-by-side orientation with the longitudinal
 beams of the base providing a lower profile lifting apparatus as compared
 with the prior art apparatus by providing more complete collapsing of the
 mock parallelogram.
 According to another aspect of the invention, a low profile lifting
 apparatus is provided by providing one or more convex-shaped cross
 supports or cross braces joining the pair of rear pivoting booms. The
 shaped cross supports or cross braces provide a low profile lifting
 apparatus by reducing the dimension by which the watercraft supports must
 be extended above the mounting arms to provide a hull-clearing channel
 portion for shaped boat hulls. At least one cross brace joining the pair
 of rear pivoting booms is positioned adjacent the pivot points on the
 longitudinal beams and provides a boom extension projecting downward
 beneath the level of the pivot points. A double-acting hydraulic cylinder
 or other suitable actuator is pivotally connected between the downward
 projecting boom extension on the rear pair of pivoting booms and the front
 pair of pivoting booms such that expansive energization of the
 double-acting hydraulic cylinder extends the piston rod and swings both
 pairs of pivoting booms upward from a collapsed attitude. Thus, expansive
 energization of the double-acting hydraulic cylinder causes the hydraulic
 cylinder to exert a first rotational force against the front pair of
 pivoting booms which rotates the front pair of pivoting booms upward and a
 second equal and opposite rotational force on the downward projecting boom
 extension of the cross braces on the rear pair of pivoting booms which
 acts over a lever arm distance and causes the rear pair of pivoting booms
 to rotate upward.
 According to another aspect of the present invention, pivotally connecting
 the double-acting hydraulic cylinder to a boom extension projecting
 downward beneath the rear pivoting booms' pivot point on the longitudinal
 beams of the base compounds the rotational action exerted by the
 double-acting hydraulic cylinder providing increased rate of rotation of
 the pivoting booms relative to the base. Thus, the unique mounting of the
 double-acting hydraulic cylinder provided by the invention provides
 increased actuation speeds without an increase in hydraulic pressure.
 Thus, expansive energization of the double-acting hydraulic cylinder
 raises both the front and rear pairs of pivoting booms and lifts the
 mounting arms and the watercraft supports upward to lift a watercraft out
 of the water. Continued expansive energization of the hydraulic cylinder
 causes upward movement to continue until the piston rod is fully extended
 providing a locked upright attitude. Alternatively, upward movement
 continues until the pivoting booms pass through a vertical orientation
 into an over-center orientation whereby the watercraft is supported above
 the surface of the water. According to yet another alternative, upward
 movement continues to some intermediate orientation between the collapsed
 and fully extended orientations and which orientation is maintained by a
 force exerted against both front and rear pivoting booms by the pressure
 in the hydraulic cylinder.
 According to yet another aspect of the present invention, retractive
 energization of the double-acting hydraulic cylinder retracts the piston
 rod into the piston jacket of the double-acting hydraulic cylinder and
 reverses the motion of the pivoting booms. Thus, when the watercraft
 lifting apparatus is locked in an over-center attitude, positive
 retractive energization of the double-acting hydraulic cylinder first
 raises the pivoting booms and lifts the mounting arms and watercraft
 supports attached to the mounting arms upward. Upward movement continues
 until the pivoting booms again pass through a vertical orientation.
 Continued retraction of the piston rod into the piston jacket of the
 double-acting hydraulic cylinder combined with the weight of the lifting
 apparatus and the watercraft collapses the mock parallelograms whereby the
 watercraft is lowered into the water. Positioning the downward projecting
 boom extension of the rear pair of pivoting booms beneath the level of the
 pivot points on the longitudinal beams provides a low profile lifting
 apparatus by providing more complete collapsing of the mock parallelogram
 formed by the two pair of pivoting booms and the mounting arms on which
 the watercraft supports are mounted. Thus, continued retraction of the
 piston rod into the piston jacket of the double-acting hydraulic cylinder
 collapses the mock parallelograms, including the mounting arms and
 watercraft supports attached to the mounting arms into a low profile mock
 parallelogram at which point contact between the watercraft supports and
 the watercraft is broken and the watercraft can float free even in
 relatively shallow water.

DETAILED DESCRIPTION OF THE INVENTION
 FIGS. 1 and 2 show isometric views of the low profile watercraft lifting
 apparatus according to one embodiment of the present invention in an
 upright or extended attitude and a collapsed attitude, respectively. In
 FIGS. 1 and 2 the watercraft lifting apparatus 10 includes an essentially
 rectangular base 12 including a front transverse beam 14 and a rear
 transverse beam 16 connected to opposite ends of spaced-apart longitudinal
 beams 18a, 18b. In one embodiment, longitudinal beams 18a, 18b are
 essentially equal in length and parallel with one another and transverse
 beams 14, 16 extend beyond the connection points with longitudinal beams
 18a, 18b to form "I"-shaped base 12. In a preferred embodiment, base 12
 further includes four sleeves 20. One sleeve 20 is connected to each end
 of transverse beams 14, 16. Each sleeve 20 receives a support post 22
 which is independently adjustable for positioning and leveling base 12 at
 a desired depth submerged under water. Support posts 22 include shoes 24
 which rest on the river or lake bed.
 Four pivoting booms 26a, 26b, 26c, 26d are attached to rectangular base 12,
 one pivoting boom 26 adjacent each of the four corners of rectangular base
 12, with the lower ends of each front boom 26a, 26b pivotally joined to
 base 12 adjacent front ends of each longitudinal beam 18a, 18b and the
 lower ends of each rear boom 26c, 26d pivotally joined to base 12 adjacent
 rear ends of each longitudinal beam 18a, 18b. In a preferred embodiment,
 longitudinal beams 18a, 18b are fitted with brackets 28 which include a
 pivot point 30 extended an off-set distance 32 above the centerline 34 of
 longitudinal beams 18a, 18b. Brackets 28 pivotally join rear booms 26c,
 26d to longitudinal beams 18a, 18b such that rear booms 26c, 26d pivot
 about pivot point 30 relative to longitudinal beams 18a, 18b. In one
 preferred embodiment, pivot point 30 is several inches above centerline
 34. Brackets 28 position rear booms 26c, 26d either between longitudinal
 beams 18a, 18b (shown) or astride longitudinal beams 18a, 18b (not shown)
 such that in a fully collapsed attitude, rear pivoting booms 26c, 26d are
 positioned in a side-by-side orientation with longitudinal beams 18a, 18b.
 One or more cross supports or cross braces 36 provide structural integrity
 to front pair of pivoting booms 26a, 26b. Those of skill in the art will
 recognize that alternative cross support configurations may provide
 structural integrity to front pair of pivoting booms 26a, 26b. The cross
 supports or cross braces 38a, 38b, 38c, 38d provide structural integrity
 to rear pivoting booms 26c, 26d. The cross braces 38 may be formed in a
 hull-clearing convex or channel shape. In one preferred embodiment, the
 cross support 38a is a "V"-shaped member extending between rear pivoting
 booms 26c, 26d which points generally rearward when watercraft lifting
 apparatus 10 is in an extended attitude as shown in FIG. 1 and point
 generally downward when watercraft lifting apparatus 10 is in a collapsed
 attitude as shown in FIG. 2. The hull-clearing "V" shape of cross support
 38a provides increased clearance for watercraft having generally
 "V"-shaped hulls as compared with the lifting apparatus of the prior art.
 Lower cross support 38b is a "V"-shaped member which extends between rear
 pivoting booms 26c, 26d adjacent pivot point 30. In one embodiment, cross
 supports 38c, 38d extend between the outer ends of intermediate cross
 support 38a and the approximate center of lower cross support 38b. Those
 of skill in the art will recognize that other configurations of cross
 supports may be employed, for example, intermediate and lower cross
 supports 38a, 38b may be formed as a straight beam or in a "U" shape or a
 "C" shape, and the cross supports 38c, 38d extending between cross
 supports 38a, 38b may be positioned parallel with the rear booms 26c, 26d
 or at any other suitable orientation whereby the cross supports 38a, 38b
 provide a shape suitable for clearing the bottoms of boats having shaped
 hulls.
 Two mounting arms 40a, 40b are pivotally mounted adjacent the upper ends of
 pivoting booms 26 to rotate about pivot points 42a, 42b and swing with
 pivoting booms 26 as a mock parallelogram. The invention provides an
 essentially parallel relationship between mounting arms 40 and
 longitudinal beams 18 when lifting apparatus 10 is in a fully extended or
 upright orientation. The essentially parallel relationships between
 mounting arms 40a, 40b and longitudinal beams 18a, 18b , respectively, are
 provided by varying the lengths of front pair of pivoting booms 26a, 26b
 relative to the lengths of rear pair of pivoting booms 26c, 26d. When
 front pivoting booms 26a, 26b are adapted to pivot about a pivot axis
 passing through centerlines 34 of both longitudinal beams 18a, 18b , the
 lengths "A" of front pivoting booms 26a, 26b are essentially equal to the
 lengths "B" of rear pivoting booms 26c, 26d plus dimension "C" defined as
 an off-set distance 32 between rear boom pivot point 30 and centerline 34
 of longitudinal beams 18a, 18b. Thus, the relationship between the lengths
 of front pivoting booms 26a, 26b and rear pivoting booms 26c, 26d is given
 by:
EQU A=B+C (Eq. 1)
 A=lengths of rear pivoting booms 26a, 26b defined as the distance between
 pivot point 42a and a pivot axis passing through centerlines 34 of both
 longitudinal beams 18a, 18b,
 B=lengths of rear pivoting booms 26c, 26d defined as the distance between
 pivot point 42b and pivot point 30, and
 C=off-set distance 32 as defined by the vertical distance between rear
 pivot point 30 and centerline 34.
 When lifting apparatus 10 is retracted to a collapsed orientation as shown
 in FIG. 2, mounting arms 40a, 40b are oriented at an angle relative to
 longitudinal beams 18a, 18b. Mounting arms 40a, 40b angle downward toward
 the rear portion of lifting apparatus 10 to provide a self-guiding aspect
 whereby the bow of a boat is guided into the center of lift apparatus 10
 midway between mounting arms 40 by the rising angle of mounting arms 40
 leading toward FRONT of lifting apparatus 10. The downward and backward
 sloping angle of mounting arms 40 is provided in part by the position of
 pivot point 30 relative to the pivot points of front booms 26a, 26b about
 an axis passing through centerline 34 and in part by the shorter lengths
 of rear pivoting booms 26c, 26d relative to the lengths of front pivoting
 booms 26a, 26b. In one preferred embodiment, watercraft supports (not
 shown) attached to mounting arms 40 brace the watercraft during lifting.
 In one embodiment of the present invention, a suitable actuator, for
 example a double-acting hydraulic cylinder 44, extends diagonally across
 the mock parallelogram. Double-acting hydraulic cylinder 44 comprises a
 piston rod 46 extending from and retracting into a piston jacket 48. In a
 preferred embodiment, upper end 50 of piston rod 46 is connected to cross
 rod 52 and cross rod 52 is rotatably fitted in flanges 54 which are
 attached to front pivoting booms 26a, 26b adjacent the upper ends of booms
 26a, 26b. Alliteratively, upper end 50 of piston rod 46 is connected to a
 collar (not shown) rotatable on cross rod 52 as disclosed in prior U.S.
 Pat. No. 5,184,914. Lowering and raising of mounting arms 40 and
 watercraft supports (not shown) is achieved by extension and retraction of
 piston rod 46 of double-acting hydraulic cylinder 44. Those of skill in
 the art will recognize that the present invention may be practiced using
 alternative raising and lowering means or actuator, for example, pneumatic
 cylinders, opposing single-acting hydraulic cylinders, electrically driven
 push/pull rods, or other suitable actuator including chain, cable, or rope
 pulley drives.
 FIG. 3 shows a detail view of the pivotal connection between double-acting
 hydraulic cylinder 44 and rear pivoting booms 26c, 26d according to one
 embodiment of the present invention. A boom extension 56 projects from
 rear pivoting booms 26c, 26d opposite pivot point 30 whereby a lever is
 formed. The lever includes a first lever arm defined by rear pivoting
 booms 26c, 26d; a second lever arm defined by boom extension 56; and a
 fulcrum defined by pivot point 30 positioned between the first and second
 lever arms. In one preferred embodiment, boom extension 56 projects
 downward from the approximate center of lower cross support 38b and
 provides a pivot point 58. The lower end 60 of hydraulic cylinder piston
 jacket 48 is adapted to pivotally connect to boom extension 56 at pivot
 point 58. According to one preferred embodiment, pivot point 58 is located
 at a distance 62 from rear boom pivot point 30. Distance 62 provides the
 lever arm over which the force exerted by hydraulic cylinder 44 acts to
 rotate rear pair of pivoting booms 26c, 26d about pivot point 30. In one
 preferred embodiment of the present invention, pivot point 58 is located
 at a distance 62 from rear boom pivot point 30 selected to provide an
 adequate force movement.
 FIG. 4 shows an operational side elevation view of the watercraft apparatus
 according to one embodiment of the present invention. To lift a watercraft
 from the water, watercraft lifting apparatus 10 is positioned in a first
 retracted or collapsed attitude (shown in solid) with the craft to be
 lifted (not shown) floating above mounting arms 40 and watercraft
 supports, if so equipped. Piston rod 46 of double-acting hydraulic
 cylinder 44 is extended by introduction of water under pressure into the
 lower end 60 of piston jacket 48 as disclosed in prior U.S. Pat. No.
 5,184,914. A piston (not shown) inside piston jacket 48 extends piston rod
 46, forcing cross rod 52 and hence front pivoting booms 26a, 26b to swing
 upwardly and forwardly from their collapsed attitudes to their raised
 attitude (shown in phantom). Simultaneously, lower end 60 of piston jacket
 48 exerts an equal and opposite force on pivot point 58 of boom extension
 56 acting over lever arm distance 62 forcing cross supports 38 and hence
 rear pivoting booms 26c, 26d to swing upwardly and forwardly about pivot
 point 30 from their collapsed attitude to their raised attitude above the
 water surface (shown in phantom). Pivotally attached mounting arms 40
 follow as the mock parallelogram is deployed. Thus, a craft is lifted out
 of the water on mounting arms 40 or watercraft supports, if so equipped.
 In a preferred embodiment of the present invention, full extension of
 watercraft lifting apparatus 10 is achieved when the piston (not shown)
 inside piston jacket 48 extends piston rod 46 to its fully extended
 attitude.
 Prior U.S. Pat. No. 5,184,914 discloses various alternative means of
 defining full extension of watercraft lifting apparatus 10 which are fully
 applicable to the present invention. For example, each longitudinal beam
 18a, 18b may be equipped with boom stops (not shown) located adjacent rear
 transverse beam 16 and/or adjacent front transverse beam 14 engaging sides
 of pivoting booms 26 adjacent their lower pivoting ends to brace pivoting
 booms 26 and mounting arms 40 in their fully extended attitude.
 Alternatively, full extension of hydraulic cylinder 44 may swing booms 26
 from a collapsed or retracted attitude through a vertical attitude into an
 over-center attitude which locks watercraft lifting apparatus 10 in a
 fully extended attitude. Another alternative combines both boom stops and
 an over-center locking position.
 According to one embodiment, the present invention provides an over-center
 locking position including booms stops. The present invention provides
 brackets 66 connected between the ends of each pivoting boom 26 and the
 ends of each mounting arm 40. Each bracket 66 provides pivot point 42 such
 that one mounting arm 40a is oriented in a plane defined by front pivoting
 boom 26a and rear pivoting boom 26c and the other mounting arm 40b is
 oriented in a plane defined by front pivoting boom 26b and rear pivoting
 boom 26d. Brackets 66 are configured to position pivot points 42 such that
 a portion of mounting arm 40 contacts the end of each pivoting boom 26
 when lifting apparatus 10 is in a fully extended upright and over-center
 attitude. Brackets 66 are further configured such that, when lifting
 apparatus 10 is oriented in any attitude other than a fully extended
 upright and over-center attitude, clearance is provided between the ends
 of each pivoting boom 26 and each mounting arm 40.
 Retraction of watercraft lifting apparatus 10 is accomplished by positive
 retractive energization of double-acting hydraulic cylinder 44 which
 retracts piston rod 46 into piston jacket 48. Retraction of piston rod 46
 causes upper piston rod end 50 to pull front pivoting booms 26a, 26b from
 their raised attitude back over-center if an over-center lock is used.
 Simultaneously, the force exerted by retraction of piston rod 46 acts over
 lever arm 62 causes lower piston jacket end 60 to pull boom extension 56
 upwardly which rotates pivoting booms 26c, 26d about pivot points 30 from
 their raised attitude back over-center. After booms 26 pass through their
 vertical over-center attitude, the weight of booms 26, mounting arms 40
 and the supported craft lower watercraft lifting apparatus 10 into its
 collapsed or retracted attitude.
 According to one embodiment of the present invention, longitudinal beams
 18a, 18b are fitted with brackets 70 which include a pivot point 72
 extended a distance "D" defined as off-set distance 74 below centerline 34
 of longitudinal beams 18a, 18b. Brackets 70 pivotally join front booms
 26a, 26b to longitudinal beams 18a, 18b such that front booms 26a, 26b
 pivot relative to longitudinal beams 18a, 18b at pivot point 72. Brackets
 70 position front booms 26a, 26b either between longitudinal beams 18a,
 18b (shown) or astride longitudinal beams 18a, 18b (not shown) such that
 in a fully collapsed attitude, front pivoting booms 26a, 26b are
 positioned in a side-by-side orientation with longitudinal beams 18a, 18b.
 Positioning of pivot points 72 at offset distance 74 below centerline 34
 of longitudinal beams 18a, 18b accentuates the self-guiding watercraft
 entry attitude of the invention by accentuating the downwardly and
 rearwardly sloping angle of mounting arms 40 when lifting apparatus 10 is
 collapsed. Thus, front boom pivot points 72 are off-set a total vertical
 off-set distance "E" defined as vertical off-set distance 76 from rear
 boom pivot points 30 which accentuates the downwardly and rearwardly
 sloping angle of mounting arms 40 when lifting apparatus 10 is in a
 collapsed attitude. Off-set distances 32, 74 in combination with the
 differing lengths of front pivoting booms 26a, 26b relative to the lengths
 of rear pivoting booms 26c, 26d reduces the downwardly sloping angle of
 mounting arms 40 when booms 26 are fully extended such that mounting arms
 40a, 40b are essentially parallel with longitudinal beams 18a, 18b when
 lifting apparatus 10 is in an upright or extended attitude.
 According to this embodiment, the essentially parallel relationship between
 mounting arms 40a, 40b and longitudinal beams 18a, 18b when lifting
 apparatus 10 is in an upright or extended attitude is provided by varying
 the lengths "A" of front pair of pivoting booms 26a, 26b relative to the
 lengths "B" of rear pair of pivoting booms 26c, 26d. The lengths "A" of
 front pivoting booms 26a, 26b minus off-set distance 74 are essentially
 equal to the lengths "B" of rear pivoting booms 26c, 26d plus off-set
 distance 32. Thus, the relationship between the lengths of front pivoting
 booms 26a, 26b and rear pivoting booms 26c, 26d is given by:
EQU A'-D.apprxeq.B+C (Eq. 2)
 where:
 A'=lengths of rear pivoting booms 26a, 26b defined as the distance between
 pivot point 42a and pivot point 72,
 B=lengths of rear pivoting booms 26c, 26d defined as the distance between
 pivot point 42b and pivot point 30,
 C=off-set distance 32 as defined by the distance between pivot point 30 and
 centerline 34, and
 D=off-set distance 74 as defined by the distance between centerline 34 and
 pivot point 72.
 In one preferred embodiment, pivot point 72 is several inches below
 centerline 34.
 Stated differently, the lengths "B" of rear pivoting booms 26c, 26d plus
 vertical off-set distance 76 between rear boom pivot points 30 and front
 boom pivot points 72 are essentially equal to the lengths "A" of front
 pivoting booms 26a, 26b. Thus, the relationship between the lengths of
 front pivoting booms 26a, 26b and rear pivoting booms 26c, 26d is
 alternatively given by:
EQU A'.apprxeq.B+E (Eq. 3)
 where:
 A'=lengths of rear pivoting booms 26a, 26b defined as the distance between
 pivot point 42a and pivot point 72,
 B=lengths of rear pivoting booms 26c, 26d defined as the distance between
 pivot point 42b and pivot point 30, and
 E=off-set distance 76 as defined by the vertical distance between rear
 pivot point 30 and front pivot point 72.
 Referring next to FIGS. 5-7, another embodiment of a lift 100 formed in
 accordance with the invention is shown. The lift 100 includes a
 rectangular base 112 formed from front and rear transverse beams 114, 116,
 respectively, that are each connected to parallel longitudinal beams 118a,
 118b. A sleeve 120 is connected to each of the transverse beams 114, 116.
 Each sleeve 120 is sized and shaped to receive a support post 122. A
 plurality of openings 123 in each sleeve 120 and each support post 122
 enables independent adjustment of the base 12 relative to support shoes
 124, which can rest on a river bed or lake bed.
 Four pivoting booms 126a, 126b, 126c, 126d, are pivotally attached to the
 rectangular base 112 at each of the four corners 127. Ideally, brackets
 128 are connected to the rear booms 126c, 126d and the longitudinal beams
 118a-b such that the rear booms 126c, 126d pivot about a pivot point 130.
 The pivot point 130 is a distance 132 that several inches above a
 longitudinal axis 134 of the longitudinal beams 118a, 118b. In one
 embodiment the pivot point is in the range of five (5) to twelve (12)
 inches above the axis 134. In the embodiment shown, the brackets 128
 position the rear booms 126c, 126d inside the longitudinal beams 118a-b,
 although the brackets 128 can be mounted astride the longitudinal beams
 118a-b such that when in a fully collapsed attitude, the rear pivoting
 booms 126c, 126d are positioned in a side-by-side orientation with the
 longitudinal beams 118a-b. A first pair of cross braces 136 provides
 structural integrity to the front pair of pivoting booms 126a, 126b. A
 second pair of cross braces 138 provides structural integrity to the rear
 pivoting booms 126c, 126d. In the depicted embodiment, the cross braces
 138 are formed to have a v-shape, with the vertex 139 pointing downward
 when the lift 100 is in a collapsed configuration, as shown in FIG. 7.
 This v-shape of the cross support 138 provides increased clearance for a
 watercraft having generally v-shaped hulls. Other configurations of the
 cross brace 138 may also be used as desired.
 Mounted to the top of pivoting booms 126a and 126c is a support rail 140a;
 and similarly mounted to pivoting booms 126b, 126d is a support rail.
 Mounting brackets 142 are fixedly attached to pivoting booms 126a-d and
 provide a pivot attachment point 143 for attachment of the support rails
 140a-b.
 The length and function of the pivoting booms 126a-d is the same as
 described above with respect to the pivoting booms 26a-d in FIG. 1, and
 will not be described in detail herein. As shown in FIG. 6, the support
 rails 140a-b are essentially parallel to the longitudinal beams 118a-b
 when the lift 100 is in the extended configuration.
 An actuator 144, similar to the double-acting hydraulic cylinder 44
 described above with respect to FIG. 1, is connected to the pivoting booms
 126a-d by means of a front T-bar 152 connected to forward pivoting booms
 126a, 126b and a rear T-bar 154 connected to rear pivoting booms 126c,
 126d. The front T-bar 152 is rotatably mounted to support brackets 156,
 each attached to a respective pivoting boom 126a, 126b. The rear T-bar 154
 is similarly pivotally attached to support brackets 158 that are each
 attached to pivoting booms 126, 126d. The actuator 144 is attached to the
 rear T-bar 154 with a sleeve 160 and to the front T-bar 152 by a yolk 162.
 Ideally, the T-bars 152, 154 can be easily replaced to facilitate
 interchangeability of high-pressure and low-pressure activators.
 In a preferred embodiment, a bunk 164a,b is pivotally mounted to each
 support rail 166a,b. The bunks 164a,b can pivot about a longitudinal axis
 that is parallel to the axis 134 of the longitudinal beams 118a-b. The
 bunks 164a,b can either freely pivot or be attached to a fixed
 orientation, thus accommodating hulls of a particular configuration.
 Referring again to FIGS. 6 and 7, the relationship between the actuator 144
 and the pivoting booms 126a-d is illustrated. In FIG. 6, the lift 100,
 working in a cantelever arm arrangement, is in an extended configuration
 wherein the actuator 144 is fully extended. In FIG. 7, the lift 100 is in
 a collapsed configuration wherein the actuator 144 is retracted.
 In a preferred embodiment, the front pivoting booms 126a,b have a pivot
 point 129 that is lower than the pivot point 130 of the rear pivoting
 booms 126c,d. The relative distance between the pivot points 129, 130
 ranges from four inches to ten inches, and in the configuration shown in
 FIG. 6, is eight inches. In other words, the rear pivot point 130 is
 approximately 8 inches higher than the front pivot point 129. It is to be
 understood that these distances can vary according to the size of the lift
 100.
 The actuator 144 provides a linkage through the front and rear T-bars 152,
 154 with the pivoting booms 126a-d. When mounted as shown, the actuator
 144 provides a pushing force on the forward and rear booms 126a-d. The
 pushing action of the actuator 144, in combination with the moving
 mounting points of the actuator 144 on the pivoting booms 126a-d, enables
 lifting of loads with nearly uniform force throughout the travel of the
 pivoting booms 126a-d.
 In addition, as shown in FIG. 7, when the lift 100 is in a retracted or
 collapsed configuration, the bunks 164a,b are angled downward towards the
 rear of the lift 100. This facilitates in loading of watercraft,
 especially in very shallow water.
 Referring next to FIGS. 8-12, shown therein is the lift 100 of FIG. 5
 having optional accessories attached thereto. More particularly, four
 guide-ons 802 are attached near the free ends of the pivoting booms
 126a-d. In addition, a stern stop 804 is connected to the upper ends of
 the pivoting booms 126c,d.
 Each of the guide-ons 802 are formed from tubular members 806 having a
 90.degree. bend to create first and second legs 808, 810, respectively.
 The first leg 808 is attached to the lift 100 by an attachment bracket
 812, which is shown more clearly in FIG. 10.
 Referring to FIG. 10, the attachment bracket 812 comprises a mounting plate
 814 having a pair of mounting holes 816 formed therein. Attached to the
 plate 814 adjacent the holes 816 is a sleeve 818 sized and shaped to
 slidably receive the first leg 808 of the guide-on 802. A pair of set
 screws 820 are threadably engaged with the sleeve 818 such that as the
 screws 820 are threaded into the sleeve 818, they project into the
 internal bore 822 of the sleeve 818 and will bear against the guide-on
 802. Alternatively, holes may be formed in the guide-on 802 to accept the
 screws 820.
 The stern stop 804 is of tubular construction having a U-shaped
 configuration with two legs 824 joined at a 90.degree. bend by a cross
 member 826. The stern stop 804 is attached to the bunk support rails
 166a,b with attachment brackets 828, shown in greater detail in FIG. 9. As
 shown therein, each attachment bracket 828 includes a mounting plate 830
 with openings 832 formed therein, that is attached to or integrally formed
 with a sleeve 834. The sleeve 834 has a longitudinal axial bore 836 with a
 circular cross-sectional configuration. The mounting plate 830 is attached
 at a right angle to the sleeve 834 and reinforced with a gusset 838. A
 pair of set screws 840 (only one shown in FIG. 9) are threadably received
 in the sleeve 834 such that when tightened, they project into the axial
 bore 836 and will bear against the stem stop 804 or be received in
 preformed holes in the stem stop 804, as shown in FIG. 11.
 FIGS. 11 and 12 show the attachment of the guide-on 802 and stem stop 804
 to the bunk support rail 166b on the pivoting boom 126d. To facilitate
 mounting of the brackets 812, 828 and the bunk 166b to the support rail
 164b, a universal plate 842 is provided. As shown more clearly in FIG. 12,
 the universal plate 842 has a substantially rectangular configuration with
 one of its planar sides attached to the support rail 166b, preferably by
 welding, although other attachment means known in the art may be used.
 Mounting holes 844 centrally located on the universal plate 842 are used
 for attachment of the brackets 812, 828. Additional holes 846 are provided
 near the top of the universal plate 842 for attachment of the bunk 164b.
 As shown here, a bunk attachment plate 848 connects the bunk 164b to the
 universal plate 842.
 As shown in FIG. 12, the bunk attachment plate 848 is connected to the
 universal plate 842 through one opening 846 (on the right side) to permit
 rotation of the bunk 164b about an axis that is parallel with the axis 134
 of the longitudinal beam 118b. This permits orienting the bunk 164b to
 accommodate different hull shapes. The bunk 164b can be attached to the
 bunk support rail 166b in a fixed orientation, or it can be freely
 rotatable, as desired.
 To enable the bunk 164b to rotate without interference from the universal
 plate 842, the top corners 850 of the plate 842 are angled downward as
 shown. However, the top edge 852 between the corners 850 remains straight
 to provide a bearing surface for the bottom surface 854 of the bunk
 bracket 848. This prevents the bunk 164b from inadvertently rotating
 counterclockwise (from the orientation shown in FIG. 12) and causing
 damage to a boat hull.
 As shown more clearly in FIG. 11, the guide-on 802 mounting bracket 812 is
 first attached to the universal plate 842 followed by the stern stop
 bracket 828 through the openings 844 with suitable fasteners (not shown).
 The guide-ons 802 and stem stop 804 are inserted into their respected
 sleeves 818, 834 where they are slidably received for adjustable
 positioning to accommodate the watercraft. The guide-ons 802 aid in
 centering the watercraft on the lift 100, while the stem stop 804 is
 contacted by the stern drive or outboard drive to position the boat
 longitudinally on the lift 100.
 Suitable materials for use in a marine environments, as known to those
 skilled in the art, can be used to construct the components of the lift
 100, including the accessories described above, i.e., the guide-ons 802,
 stem stop 804, and associated brackets 812, 828, and universal plate 842,
 and fasteners. The guide-ons 802, as well as the stern stop 804, can be
 formed from sturdy plastic that will help prevent damage to the exterior
 of the boat hull and the stern drive or outboard drive components. While a
 preferred embodiment of the invention has been illustrated and described,
 it will be appreciated that various changes may be made therein without
 departing from the spirit and scope of the invention. Consequently, the
 invention is to be limited by the scope of the claims that follow.