Abstract:
A hull locating arrangement for a multi-hulled vessel having a body at least partially suspended above at least a first and a second hull by support components is disclosed. The hull locating arrangement comprises, for a hull, a first locating linkage and a second locating linkage to together constrain the hull in the lateral, longitudinal, roll and yaw directions relative to the body. The first and second locating linkages are longitudinally spaced, and the first locating linkage includes a first part and a second part, one of the parts of the first locating linkage being pivotally connected to the body about a body end pivot axis, and the other of the parts being pivotally connected to the hull about a hull end pivot axis. The first locating linkage is also arranged to permit relative motion between the first and second parts along at least one sliding axis to thereby permit the first locating linkage to vary in length between the body end pivot axis and the hull end pivot axis. The support components include a first support component adjacent the first locating linkage, the first support component including a first support element providing a support force supporting a portion of the body above the first hull. The support force of the first support element has a line of action that is within thirty degrees of parallel to a first linkage plane extending through the body end pivot axis and the hull end pivot axis.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is generally directed to water craft and in particular to methods of or systems/arrangements for at least partially suspending or supporting a body above at least two movable hulls. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the applicant&#39;s earlier International Patent Application Numbers PCT/AU2011/000557 and PCT/AU2011/000565 there are disclosed various configurations of catamaran, trimaran and quadmaran all having resilient suspension between at least two laterally spaced hulls and central hull or body. Where two longitudinally spaced locating linkages are provided between one hull and the body some form of longitudinal compliance or freedom is required in one of the locating linkages. For example to locate one hull relative to the body, a front fixed length trailing arm is used and a back variable length trailing arm is used. The variable length of the back trailing arm is provided by telescopic or sliding sections in the arm or by a drop link. The support forces are provided by rams which provide a moment on the trailing arms which can require the trailing arms to be of very high strength and large section on some vessels and places a high side load on the sliding joints of the variable length arm which increases the friction of the sliding joints and therefore reduces joint life and ride comfort. Alternatively the rams are provided connected directly between the body and the hull, which can require a large tower on the deck of the body to accommodate the rams which can be difficult to package in some vessel designs and layouts. Also wherever the suspension arms and rams are located in separate areas of the vessel, having separate load paths through respective mounting points into the body and/or the hull(s), the greater the spacing between the respective mounting points, generally the more bending induced into the body and/or hull and the less efficient the design in terms of packaging and weight. 
       SUMMARY OF THE INVENTION 
       [0003]    According to a first aspect of the invention there is provided a hull locating arrangement for a multi-hulled vessel having a body at least partially suspended above at least a first and a second hull by support components, the hull locating arrangement comprising for a said hull a first locating linkage and a second locating linkage to together constrain said hull in the lateral, longitudinal, roll and yaw directions relative to the body, the first and second locating linkages being longitudinally spaced from each other. The first locating linkage includes a first part and a second part, one of said parts of the first locating linkage being pivotally connected to the body about a body end pivot axis, the other of said parts being pivotally connected to said hull about a hull end pivot axis. The first locating linkage being arranged to permit relative motion between the first and second parts along at least one sliding axis to thereby permit the first locating linkage to vary in length between the body end pivot axis and the hull end pivot axis. The support components including a first support component adjacent the first locating linkage, the first support component including a first support element providing a support force supporting a portion of the body above the said hull, wherein the support force of the first support element of the first support component has a line of action that is within thirty degrees of parallel to a first linkage plane extending through the body end pivot axis and the hull end pivot axis. 
         [0004]    The support force of the first support element may have a line of action that is aligned with or at a tighter angle of for example 5, 10 or 20 degrees of parallel to the first linkage plane. 
         [0005]    The line of action of the support force of the first support element may be offset from the body end pivot axis or the hull end pivot axis or the first linkage plane by a distance of less than ten percent of a length of the first hull, for example 2, 5 or 10 percent of the (waterline) length. 
         [0006]    The at least one sliding axis of the first locating linkage may include at least two laterally spaced sliding axes defining a first sliding plane and the line of action of the support force of the first support element may be substantially aligned with said first sliding plane. The line of action of the support force of the first support element may be substantially aligned with both the first linkage plane (and the first sliding plane) to thereby minimise bending moments and side loads in the first locating linkage resulting from the support force. 
         [0007]    The first locating linkage may include bearings or bushings to permit the relative motion between the first and second parts along said at least one sliding axis. 
         [0008]    The hull end pivot axis and the body end pivot axis of the first locating linkage of said hulls may be aligned substantially laterally with respect to the body of the vessel. 
         [0009]    The first support component may further include a second support element. The first and/or second support element may be connected between the first part and the second part. 
         [0010]    The at least one sliding axis may be a single sliding axis. 
         [0011]    Alternatively, the at least one sliding axis may be a pair of parallel first and second laterally spaced sliding axes. Bearings or bushings may be provided between the first part and the second part and may be arranged to effectively constrain the relative motion between the first and second parts to a linear sliding motion along said first and second laterally spaced sliding axes, said sliding axes being parallel to each other and substantially perpendicular to the body and hull end pivot axes. 
         [0012]    The first linkage plane of the first locating linkage may be substantially perpendicular to the body of the vessel during operation. However due to practical limits on the lengths of arms and other geometry considerations, the first linkage plane of the first locating linkage may only be substantially perpendicular to the main body during a part of the total range of operation of the first locating linkage. So alternatively the first linkage plane of the first locating linkage may be within 5, 10, 20, 30 or even 40 degrees of perpendicular to the body of the vessel during operation. 
         [0013]    The second locating linkage may constrain the longitudinal motion of the hull relative to the body. 
         [0014]    The support components may further include second support components for providing support forces in the second locating linkage between said hull and the body. 
         [0015]    The support force of the first support means acts (or has a line of action that is) substantially parallel to the at least one sliding axis of the first locating linkage. Alternatively, the support force of the first support means may act in a direction (or have a line of action) that is substantially aligned with either the at least one sliding axis of the first locating linkage or a plane defined at least in part by the at least one sliding axis of the first locating linkage. 
         [0016]    Where the first locating linkage is variable in length along a first sliding axis, the variable length of the first locating linkage may vary within a range defined as a first locating linkage stroke distance. The first support component may be arranged such that the resultant force has a line of action substantially aligned with the first sliding axis or passing within a distance from the first and second pivot axes of the first locating linkage that is less than twenty-five percent of said stroke distance. 
         [0017]    The first support component may be connected between the first part and the second part. 
         [0018]    One or more forms of the present invention may provide a multi-hulled vessel including a main body and at least one left hull and at least one right hull, each of said at least one left hull and at least one right hull being moveable with respect to the main body. At least one hull of said at least one left hull and at least one right hull is located relative to the main body by a hull locating arrangement according to the first aspect of the invention. 
         [0019]    In an embodiment, the hull locating arrangement is arranged to constrain the hull relative to the main body in the lateral, longitudinal, roll and yaw directions, but to permit motion of the hull in the heave and pitch directions. To this end the hull locating arrangement includes longitudinally spaced first and second locating linkages. The first locating linkage is a variable length arm between the main body and the hull, the second locating linkage is a fixed length arm between the main body and the hull. A first end of the respective first and second locating linkages is rotatably connected to the main body by a respective first joint having a first pivot axis, a second end of the respective first and second locating linkages is rotatably connected to the hull by a respective second joint having a second pivot axis. 
         [0020]    The first locating linkage may vary in length along a first sliding axis and the variable length of the first locating linkage may vary within a range defined as a first locating linkage stroke distance. 
         [0021]    Additionally, the first locating linkage may further include support means providing a support force to support a portion of the main body above the hull. 
         [0022]    The first locating linkage may further include support means providing a support force to support a portion of the main body above the hull, the support force having a line of action substantially aligned with the first sliding axis or passing within a distance from the first and second pivot axes of the first locating linkage that is less than twenty-five percent of said stroke distance (and is preferably substantially parallel to the first sliding axis). 
         [0023]    The first locating linkage may include a first part connected to the first joint and a second part connected to the second joint, the first and second parts sliding relative to each other along the first sliding axis. The support means may be connected between the first part and the second part. 
         [0024]    Additionally or alternatively, the support means may include multiple devices. The resultant force of said multiple devices may have a line of action substantially aligned with the first sliding axis or passing within a distance from the first and second pivot axes of the first locating linkage that is less than twenty-five percent of said stroke distance. 
         [0025]    The first sliding axis may be within 40 degrees of vertical relative to the main body during operation. From a force standpoint, ideally the first sliding axis is substantial vertical with respect to the main body to minimise the longitudinal component of the support force input from the first locating linkage into the main body. However the geometry of the second locating linkage can require the first sliding axis to deviate from vertical. Also from a packaging standpoint it can be preferable to angle the first sliding axis from vertical to reduce the requirement for the first locating linkage to penetrate the hull or a deck area of the vessel. 
         [0026]    It will be convenient to further describe the invention by reference to the accompanying drawings which illustrate preferred aspects of the invention. Other embodiments of the invention are possible and consequently particularity of the accompanying drawings is not to be understood as superceding the generality of the preceding description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the drawings: 
           [0028]      FIG. 1  is a side view of a vessel incorporating a hull locating arrangement according to the present invention. 
           [0029]      FIG. 2  is a perspective view of a first possible arrangement of the first locating linkage of the hull locating arrangement. 
           [0030]      FIG. 3  is a perspective view of an alternate, second possible arrangement of the first locating linkage of the hull locating arrangement. 
           [0031]      FIG. 4  is a side view showing a compression of a suspension system employing the hull locating arrangement of the present invention. 
           [0032]      FIG. 5  shows suspension extension or rebound. 
           [0033]      FIG. 6  shows suspension pitch in the hull nose up direction. 
           [0034]      FIG. 7  shows suspension pitch in the hull nose down direction. 
           [0035]      FIG. 8  is a diagrammatic view of a third possible arrangement of the first locating linkage of the hull locating arrangement. 
           [0036]      FIGS. 9 and 10  are perspective views of a fourth possible arrangement of the first locating linkage of the hull locating arrangement. 
           [0037]      FIG. 11  is a cross sectional view of the arrangement in  FIG. 10 . 
           [0038]      FIG. 12  is a perspective views of a fifth possible arrangement of the first locating linkage of the hull locating arrangement. 
           [0039]      FIG. 13  is a cross sectional view of the arrangement in  FIG. 12 . 
           [0040]      FIG. 14  is a diagrammatic side view of the arrangement from  FIG. 10 . 
           [0041]      FIGS. 15 and 16  are diagrammatic side views of modifications to a first locating linkage of the hull locating arrangement of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0042]    Referring initially to  FIG. 1  there is shown a catamaran  1  with a main body  2  suspended above the independently moveable hulls  3 . The gunnels on the near side are omitted to fully expose the cabin  4  and the hull locating arrangement for the visible hull. The hull locating arrangement comprises a back hull locating linkage  6  and a front hull locating linkage  7 . 
         [0043]    The front locating linkage shown includes a leading arm  8  rotatably connected to the body  2  by pivot  9  such as bearings or bushings and rotatably connected to the hull  3  by pivot  10 . This provides lateral, longitudinal and roll constraints to the motion of the hull relative to the body. Although it can also provide a yaw constraint, the use of a second lateral constraint at a longitudinally spaced position (i.e. the back locating linkage) generally provides most of the yaw reaction. A front support means  11  (such as a spring damper unit or one or more hydraulic cylinders) is provided, packaged inside a suspension tower  12  which can be located in the gunnels or in the cabin structure for example. The front support means is connected to the body by pivot  13  and to the leading arm by pivot  14 . The distance of the connection point  14  of the front support cylinder  11  along the leading arm determines a mechanical advantage on the support cylinder and can be used for many beneficial reasons such as to reduce the total length of the cylinder (or other support means) to reduce the height of the suspension tower  12  and to improve the ratio of buckling strength versus weight of the cylinder  11 . 
         [0044]      FIG. 2  shows the back hull locating linkage  6  in more detail with the body omitted for clarity. Although in the Figures the back hull locating linkage is shown substantially vertical with respect to the body at ride height, the linkage can be inclined to suit the packaging of the vessel, however this increases any longitudinal component of the rear support force acting on both the body and the hull and resolved through the leading arm. This rear locating linkage  6  is variable in length between the body pivot  21  and the hull pivot  22  and as shown in  FIG. 2  includes a sliding frame made up of two laterally spaced sliding members  24  and  25 . The sliding members are shown as cylindrical devices, but as the upper and lower pivots  21  and  22  reduce or remove the bending moment about laterally extending axes (such as the pivot axes of the pivots) the sliding members can be wider in section in a lateral direction than in a longitudinal direction. Each sliding member has two parts, one of which slides inside the other telescopically. The outer part  26  is shown connected to the body and the inner part connected to the hull although the frame can be used inverted so that the inner part  27  is connected to the body. Preferably bushings or bearings are used in pairs between the inner and outer parts of the sliding frame. For ease of maintenance these can be split shell bushes in the outer parts, although using a bush or bearing in the opposite end of each part from the pivots improves bearing spacing which can be beneficial. 
         [0045]    As with the front hull locating linkage, one or more support means is used to provide support to the vessel body. In  FIG. 2  the back support means is two hydraulic cylinders  28  and  29  which are connected either directly between the body and the hull or indirectly by being connected between the inner and outer parts of the sliding frame. The support means may comprise a supporting spring and a damper such as a hydraulic or pneumatic support cylinder and a shock absorber, or part of an interconnected suspension system such as those shown in the applicant&#39;s previously mentioned International patent applications, details of which are incorporated herein by reference. 
         [0046]    The stroke of the suspension system (the vertical travel of the hull relative to the body) together with the lack of mechanical advantage (or lever ratio) of the back hull locating linkage  6  and back support cylinders  28  and  29  can require the top of the linkage to be housed above the deck of the body, such as in a suspension tower  30  which is preferably tied or integrated into the gunnels or the cabin or other superstructure as shown in  FIG. 1 . Alternatively or additionally the lower ends of the sliding frame and support means can be recessed into wells  34  or cut-outs in the hulls as shown in  FIG. 2 . Such a well can be sealed from the buoyant volume of the hull and to prevent water from collecting in the well  34  a vent  35  or other means of drainage can be provided as shown. Alternatively, the support means such as cylinders  28  and  29  can be located in one or more of the sliding members  24  and  25 , in which case it is possible to use a single sliding member which penetrates the hull and is sealed by a flexible membrane between the sliding member and the hull. 
         [0047]    A large stroke of the variable length arm arrangement with no mechanical advantage as shown in the rear linkage can require the support cylinders to be larger than hydraulically necessary to avoid the mechanical risk of buckling. This is particularly necessary when the cylinders are free to rotate at both ends, so a more efficient solution is to fix the ends of the cylinders to the outer and inner parts of the sliding frame. In this case alignment is especially important, so preferably the force of each support cylinder  28 ,  29  is aligned with a plane defined by the sliding axes of the laterally spaced sliding members  24 ,  25  as shown in  FIG. 2 . 
         [0048]    Preferably the outer parts  26  of sliding members  24  and  25  are laterally connected by a beam  41  as shown in  FIG. 3  and the inner parts  27  are connected by a beam  42  to improve the rigidity of the sliding frame and reduce variations in alignment of the inner and outer parts. The operation and life of the sliding frame can be improved by minimising such misalignment and by minimising the side load on the bearings between the inner and outer parts through keeping the line of action of the total force from the support means as close to the pivot axes of the sliding frame as possible. 
         [0049]    Where the width of the back hull locating linkage is limited and two support cylinders are used, the support cylinders can be positioned on either side of the plane defined by the axes of the two sliding members as shown in  FIG. 3 . 
         [0050]    As discussed with respect to  FIG. 2 , the support cylinders of the arrangement shown in  FIG. 3  can be rigidly fixed between the inner and outer parts of the sliding frame even when the support cylinders are not in line with the sliding members, as long as the axes of both cylinders and both sliding members are parallel. 
         [0051]      FIGS. 4 to 7  show the range of displacements possible between the hulls and the body. The suspension is fully compressed in  FIG. 4  and fully extended in  FIG. 5 . In  FIG. 6  the suspension is in a full pitch in the hull nose up direction and similarly in  FIG. 7  hull nose down full pitch travel is shown. When the left and right hulls of the catamaran pitch in opposite directions, a warp mode (not shown) is possible. 
         [0052]    In  FIG. 8 , the inner  27  and outer  26  parts of the laterally spaced sliding members  24  and  25  are not concentric. A lower-mid beam  43  braces between the lower ends of the outer parts  26  and locates bushings  46  around the inner parts  27 . Similarly an upper-mid beam  44  braces between the upper ends of the inner parts  27  and locates bushings  45  around the outer parts  26 . This arrangement can increase the torsional rigidity of the sliding members by forming an upper frame and a lower frame and can improve serviceability by placing all the bushings in more easily accessible locations. The upper (or outer) frame comprises the outer parts  26 , the lower-mid beam  43  and the upper beam  41  between the tops of the outer members. The lower (or inner) frame comprises the inner parts  27 , the upper-mid beam  44  and the lower beam  42  between the bottoms of the inner members. As with the arrangement shown in  FIG. 3 , the support rams  28  and  29  can be mounted directly between the upper beam  41  and the lower beam  42  in any arrangement desired such as the longitudinal spacing shown in  FIG. 3  or the lateral spacing shown in  FIG. 8 . 
         [0053]      FIGS. 9 to 11  show another preferred arrangement of sliding linkage, again shown towards the back of the hull as a back hull locating linkage (although again it could be used towards the front of the hull if desired). The lower end is again shown recessed into a well  34  in the hull in  FIG. 9 . The laterally spaced sliding members ( 24 ,  25 ) are now nesting U-section beams in place of the concentric tubes shown in  FIGS. 2 and 3  or the pairs of adjacent tubes shown in  FIG. 8 . The outer parts  26  of each sliding member are fixed to each other by the top beam  41 , plus a lower brace  47  and two diagonal braces  48  on each side (ie front and back) forming an outer frame  49 . The pivots (bushings or bearings)  21  between the outer frame and the body form the body end pivot axis. The hull end pivot axis is formed by bushings or bearings between the inner parts  27  and the mount brackets  40 . In  FIG. 9  the sliding linkage is shown in the compressed position with part of the hull shown, but the body omitted for clarity. Cover plates  50  are shown between the outer parts  26  and fill much of the rectangular outer frame which they contribute to forming. The cover plates  50  can be used to add stiffness to the rectangular outer frame in place of or in addition to the diagonal braces  48  and/or as shields to simply to protect the hydraulic components from direct exposure to the elements and/or to provide a barrier to deflect water flowing over the top of the hull (for example to provide some protection of an engine air intake if the engine is located in or on the back of the hull). Additionally or alternatively, similar cover plates can be provided between the inner parts  27  to fill some or all of the rectangular frame they form. 
         [0054]    In  FIGS. 10 and 11  the locating linkage is shown in the fully extended position, with the hull omitted for clarity and the cover plates omitted to reveal the support rams  28  and  29  and an optional compression stop tube  55  and compression stop resilient elements  56 . While it is possible to package compression and rebound travel limit stops within the U-shaped vertical beams of the inner and outer frames, it can be more space efficient to package one or both functions separately. 
         [0055]    The inner parts  27  of each sliding member are fixed to each other by the lower beam  42 , plus mid brace  58  and diagonal braces  59  forming an inner frame  60 . Bearing covers  61  on the outer frame  49  shield roller bearings  62  visible in the sectional view of  FIG. 11 . The roller bearings  62  are mounted to the outer frame  49  and bear on the inner frame  60  (or a bearing surface fixed to the inner frame). Similarly roller bearings  64  are mounted to the inner frame and bear on the outer frame  49  (or a bearing surface fixed thereto). The roller bearings  62  and  64  resolve lateral and roll forces between the hull and the body. Similarly roller bearings are provided on the front and back of the inner and outer frames (such as under bearing covers  65  on the outer frame) to resolve bending loads in the locating linkage in a plane perpendicular to the pivot axes of the linkage pivots  21  and  22 . Alternatively, some or all of the roller bearings forming the sliding mechanism between the inner and outer frames can be replaced by sliding bearings and any or all can be adjustable to ensure correct alignment and correct for wear. 
         [0056]    In the example shown in  FIG. 11 , the joints  70  connecting the cylinder portions  71  of each of the rams  28  and  29  to the top beam  41  are shown as a pair of annular bushings  73  around a pin  72 . In each case one bushing is between the top of the cylinder body and the top beam, the other bushing is between the top beam and a washer held on the pin by a nut. The lower ends of the rams (ie the rod ends  77  on the ends of rods  78 ) can be connected to part of the inner frame such as the lower beam  42  or to mounts adjacent to the frame mounts  79 . The mounting axis of the rod ends  77  does not need to be aligned with the joint axis of the frame mounts  40  which form the hull pivots  22 . 
         [0057]    If the braces  58  and  59  are omitted from one side of the lower frame as shown, then support brackets can be added between the lower brace  47  on the outer frame and the lower ends of the cylinder portions  71  to prevent or limit relative motion and protect against buckling of the rams  28  and  29 . 
         [0058]    The cross-section of  FIG. 11  (cut through the locating linkage of  FIG. 10 ) is cut through the pivot axes of the body pivots  21  and the hull pivots  22 , the locating linkage being substantially symmetrical about this plane. The centre-lines of the rams  28  and  29  also lie in this plane and the roller bearings are symmetrical about this plane, ie if the sliding axes of the sliding mechanism between the inner and outer frames are assumed to be though the centroid axes of the outer and/or inner U-shaped beams  26 ,  27 , then in the illustrated example the rams are parallel with the sliding axes and lie in the same plane as the sliding axes and the pivot axes. 
         [0059]    The inner and outer frames can be complex to manufacture to suitable bearing tolerances (due to distortion if welded together for example) and it can be difficult to machine the desired surfaces to the correct tolerances once the frames are assembled, so  FIGS. 12 and 13  show a modified construction of the arrangement from  FIGS. 9 to 11  whereby the bearings and planar running surfaces of the inner and outer frames are replaced by rods and cylindrical bushings. The arrangement combines elements of the arrangements from  FIG. 2  and  FIGS. 9 to 11  with additional modifications. A pair of rods or tubes  85  are positioned in each side of the inner frame  60 , between pairs of side beams  89 . Each pair of rods is fixed to an upper plate  87  at their top end and to a lower plate at their lower end. The upper plates  87  are fixed to the upper ends of the side beams  89 . The lower plates  88  are fixed to the lower beams  42  on the front and back of the frame. The front and back of the inner frame each include a lower beam  42 , an upper beam  57 , two side beams  89  and two diagonal braces  59  and these components can be fixed together into a front and a back assembly prior to mounting of the running surfaces (of the rods). The advantage of this construction is that the alignment of each rod in a pair of rods is easily achieved by the machining of the plates  87  and  88  and that the alignment of the two pairs of rods within the inner frame can be assured through the fixing of the plates to the front and back assemblies of the inner frame after the front and back assemblies have been welded or otherwise fixed together. 
         [0060]    Bearing or bushing blocks  90  can be fixed to the U-shaped beams of the outer frame  49  after the outer frame has been assembled. The outer frame can include additional bracing as shown in  FIGS. 9 to 11 . The outer and/or inner frames can again include covers to brace between the beams of the frames and provide protection and water deflection as previously discussed. The bearing or bushing blocks  90  preferably utilise sliding bushing material sleeves such as PTFE coated shells and can be split to allow replacement of the sliding bushings. Providing the bushings for both adjacent rods of a pair in a single block  90  (or split block that fits around both rods) again allows for the blocks to be machined accurately prior to fitting to the outer frame. 
         [0061]    Each pair of rods  85  with the upper and lower bushing blocks  90  tied together by the U-shaped beams  26  of the outer frame are similar in function to the outer parts  26  and inner parts  27  of the laterally spaced sliding members  24  and  25  in  FIG. 2 , so each pair of rods and bushing blocks could be referred to as one of the laterally spaced sliding members. 
         [0062]    Although not shown, compression stops can be provided in the locating linkage, for example, by placing resilient stops inside the top corners of the outer frame attached to the U-shaped beams  26  and  41 . Such stops can act on the upper plates  87  for the rods or on brackets attached to the upper plates. Alternatively the compression stops between the inner and outer frames can be attached to the upper plates  87  of the inner frame and act on the top beam  41  of the outer frame. The upper plates  87  can be stepped to allow the compression stops to be packaged adjacent to the rods  85  to reduce the dead length in the locating linkage. Similarly rebound stops (not shown) can be provided, for example on brackets attached to the U-shaped beams  26  of the outer frame just above the upper of the bushing blocks  90 , to contact the underside of the upper plates  87  of the inner frame. 
         [0063]    The cross-section of  FIG. 13  (cut through the locating linkage of  FIG. 12 ) is cut through the pivot axes of the body and hull pivots (not shown) in a similar manner to  FIG. 11  and again the locating linkage is substantially symmetrical about this plane and as is preferable, the centre-lines of the rams  28  and  29  also lie in this plane. If the sliding axes of the sliding mechanism between the inner and outer frames are defined as being half way between the primary axis of each rod  85  in a pair forming part of the laterally spaced sliding members  24  and  25 , then in the illustrated example the rams are parallel with and lie in the same plane as the sliding axes. 
         [0064]    The arrangements shown in  FIGS. 9 to 13  provide lower forces in the bearings of the sliding mechanism than offsetting the centre lines of the rams from the plane through the pivot axes of body and hull pivots  21  and  22 .  FIG. 14  shows a diagrammatic side view of the arrangement of  FIGS. 9 to 11 . 
         [0065]    However in any arrangement of the present invention it can be beneficial to maintain a small load on at least some of the bearings in one direction around the static equilibrium point of the vessel at ride height. To this end the centre-line  91  of the rams (and therefore, when the end joints are positioned on the centre-line to minimise bending loads in the rams, the line of action of the rams) can be angled slightly relative to the plane  92  through the pivot axes of body and hull pivots  21  and  22  as shown in  FIG. 15 . The angle (as shown by the arrow  93 ) can be up to 30 degrees at mid stroke or ride height, but is preferably less, for example 20 degrees, 10 degrees, 5 degrees or as in  FIG. 14  substantially zero. Where the plane through the pivot axes of the body and hull pivots is significantly inclined (and it can be up to 40 degrees from perpendicular to the body) the angle of the ram line of action is preferably not greater than another 15 degrees in the same direction, but can be up to 30 degrees back towards perpendicular to the body. The rods  78  can be joined to the lower frame by joints  77  at any position on the inner frame, but preferably near the lower beam  42  as shown. The cylinder portions of the rams can be joined to the body (not shown) or to a bracket or other structure attached to or forming part of the outer frame  49  as illustrated. 
         [0066]    Alternatively or additionally, to maintain a small load on at least some of the bearings in one direction around the static equilibrium point of the vessel at ride height, the line of action or centre-line  91  of the rams can be offset from the plane  92  through the pivot axes of body and hull pivots  21  and  22  as shown by the arrow  94  in  FIG. 16 . The offset  94  is preferably less than 5 percent of the length of the hull, but can be up to 10 percent of the length of the hull where the hull is relatively short (compared to the length of the vessel, as for example in the case of a hull on a quadmaran). Offsets between zero and 5 percent of the length of the hull are also beneficial, for example 1 percent or 2 percent of the length of the hull. Alternatively or additionally the offset  94  is preferably less than 25 percent of the distance between the pivot axes of body and hull pivots  21  and  22  when the locating linkage is at mid stroke or ride height. Again offsets between zero and  25  percent of the distance between the pivot axes of body and hull pivots  21  and  22  when the locating linkage is at mid stroke or ride height can be beneficial, such as 5 and 10 percent. 
         [0067]    Even if the line of action or centre-line  91  of the rams is angled or offset from the plane through the sliding axes and/or from the plane through the body and hull pivots  21  and  22 , preferably the rams are packaged within the arrangement of inner and outer frames. The advantages of this are many, including: frames can be braced and covered to shield the rams from direct exposure to the elements and direct water flowing over the hulls away from engine components; low bending loads in the locating linkage, reducing required weight; single load path (in side view) for the locating linkage so suspension geometry loads and support loads all flow through the same reinforced points on the body or the hulls; packing of the suspension geometry and support components in the same area, minimising the number of intrusions into the body or hulls; and low preload forces on the bearings or bushings of the sliding mechanism between the inner and outer frames, allowing a low running friction of the mechanism allowing the locating linkage to vary in length. 
         [0068]    It should be understood that the sliding arm can be applied to different geometries of hull locating arrangement. For example, the sliding arm can be used in the front locating linkage and a trailing arm could then be used in the rear locating linkage. Alternatively the front leading arm could be replaced with a trailing arm or other suspension geometry. Another alternative for example is to use a pair of sliding arms, one being substantially vertical relative to the body and using a body mount with substantially no rotation so that the vertical sliding arm provides longitudinal location of the hull, the other sliding arm remaining pivoted to the body to permit pitch motions of the hull relative to the body. 
         [0069]    Hydraulic rams  28 ,  29  have been shown in the Figures to support the body of the vessel, but other forms of linear actuator and/or spring could be used. For example a coil or air spring can be used with a linear damper (or shock absorber) and the spring seat can even be adjusted as is known in automotive suspension systems to adjust for example the roll attitude of the body above the hulls. 
         [0070]    Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.