Patent Publication Number: US-10315736-B2

Title: Suspension systems for multi-hulled water craft

Description:
TECHNICAL FIELD 
     The present invention relates to suspension systems for multi-hulled water craft. 
     BACKGROUND 
     The applicant has developed a number of suspension systems for multi-hulled water craft where the hulls are able to move relative to a chassis or body portion, examples of which are disclosed in U.S. Pat. No. 7,314,014, and international publication numbers WO 2011/143692 and WO 2011/143694, details of which are incorporated herein by reference. 
     While underway, in many situations there is little benefit to providing active heave motion compensation, i.e. precisely controlling the overall height of chassis or body portion. However while docked or otherwise engaging with another object, be it a fixed dock or floating pontoon or other vessel, it can be beneficial to provide active heave motion compensation or at least active compensation of the vertical height of a point or region on the vessel, such as the bow. 
     It is known to provide active adjustment of a platform to compensate for vertical motions in addition to roll and pitch motions as disclosed in U.S. Pat. No. 5,822,813. Servos are used to provide the force required to position and support the platform, the support being non-resilient. Therefore the support force must be exceeded to generate an extension of the servo, and so the higher the load being supported, the more energy is required to effect a given displacement adjustment. 
     In U.S. Pat. No. 9,073,605 the support of a platform above two hulls is resilient, with electromagnetic actuators being positioned in parallel with separate pneumatic support springs. The electro-magnetic actuators are used to provide a force to displace the platform in roll, pitch and heave, but a requirement for a continual force due to a manoeuvre, such as centrifugal force during turns or pitch forces due to longitudinal acceleration or deceleration requires continual supply of energy to the actuators to provide the continual force in parallel with the springs. 
     It is to be understood that the prior art publications discussed above does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 
     SUMMARY OF INVENTION 
     According to a first aspect of the invention there is provided a suspension system for a multi-hulled vessel, the vessel including a chassis and at least one left hull and at least one right hull, the suspension system supporting at least a portion of the chassis relative to the at least one left hull and at least one right hull, the suspension system including a front left support arrangement and a back left support arrangement between the at least one left hull and the chassis and including a front right support arrangement and a back right support arrangement between the at least one right hull and the chassis, each front left, front right, back left, back right support arrangement including at least one respective ram, and the suspension system including a first adjustment accumulator having a fluid chamber and a gas chamber; and a first actuator to transfer or effectively transfer fluid (for example, directly or indirectly) between the fluid chamber of the first adjustment accumulator and at least one compression chamber of a respective at least one ram of a first support arrangement comprising one or more of the front left, front right, back left or back right support arrangements, a static pressure in the gas chamber of the first adjustment accumulator being within 25% of a static operating pressure in the at least one compression chamber of the at least one ram of the first support arrangement. 
     Alternatively, the static pressure in the gas chamber of the first adjustment accumulator may be within 20%, or preferably within 15% or more preferably within 10% or more preferably within 5% of the static operating pressure in the at least one compression chamber of the at least one ram of the first support arrangement. 
     Alternatively, the static pressure in the gas chamber of the first supply accumulator may be substantially equal to the static operating pressure in the at least one compression chamber of the at least one ram of the first support arrangement. 
     The first actuator may be controllable to adjust an average stroke position of the at least one ram of the first support arrangement and/or to adjust an average pressure in the at least one compression chamber of the at least one ram of the first support arrangement. 
     The first actuator may include a bi-directional pump and a motor to drive the pump. Alternatively, the first actuator may include a bi-directional pump and a motor-generator to drive and be driven by the pump. 
     The first adjustment accumulator may include a moveable wall between the gas chamber and the fluid chamber, and the first actuator may include the first adjustment accumulator and a first linear motor for at least driving the moveable wall to thereby vary the relative size of the gas and fluid chambers. The moveable wall may for example be a piston in a piston style accumulator or in a bellows or diaphragm type accumulator. The first linear motor for at least driving the moveable wall may be a motor-generator for driving and being driven by the moveable wall. Additionally or alternatively, the first linear motor may be a voice coil linear motor. The voice coil type of linear motor is typically actuable to provide an offset force, in this case on the moveable wall to increase or decrease the compression of the gas chamber of the adjustment accumulator. Alternatively, the first linear motor may be a lead screw driving the moveable wall. The lead screw type of linear motor is typically actuable to provide a displacement, in this case on the moveable wall to increase or decrease the compression of the gas chamber of the adjustment accumulator. 
     One or more forms of the present invention may further include: second, third and fourth support arrangements, each comprising one or more of the front left, front right, back left or back right support arrangements; a second actuator to transfer or effectively transfer fluid between the fluid chamber of a second adjustment accumulator and at least one compression chamber of a respective at least one ram of the second support arrangement; a third actuator to transfer or effectively transfer fluid between the fluid chamber of a third adjustment accumulator and at least one compression chamber of a respective at least one ram of the third support arrangement; and a fourth actuator to transfer or effectively transfer fluid between the fluid chamber of a fourth adjustment accumulator and at least one compression chamber of a respective at least one ram of the fourth support arrangement. 
     A static pressure in a gas chamber of the respective second, third or fourth adjustment accumulator may be within 25% of a static operating pressure in the at least one compression chamber of the at least one ram of the respective support arrangement. Alternatively, a static pressure in a gas chamber of the respective second, third or fourth adjustment accumulator may be within 20%, or 15%, or 10%, or 5% of a static operating pressure in the at least one compression chamber of the at least one ram of the respective support arrangement. Alternatively, a static pressure in a gas chamber of the respective second, third or fourth adjustment accumulator may be substantially equal to a static operating pressure in the at least one compression chamber of the at least one ram of the respective support arrangement. 
     In one or more forms of the present invention, the first support arrangement may comprise the front left support arrangement, the second support arrangement may comprise the front right support arrangement, the third support arrangement may comprise the back left support arrangement, and the fourth support arrangement may comprise the back right support arrangement. The at least one respective ram of each of the respective front left, front right, back left or back right support arrangements may be a respective single ram. The single ram in each of the front left, front right, back left or back right support arrangement may be a single-acting ram. Alternatively, the single ram in each of the respective front left, front right, back left or back right support arrangements may be a double-acting ram including a respective compression chamber and a respective rebound chamber, the front left compression chamber being connected to the front right rebound chamber, the front right compression chamber being connected to the front left rebound chamber, the back left compression chamber being connected to the back right rebound chamber, the back right compression chamber being connected to the back left rebound chamber. 
     Where second, third and fourth actuators and second, third and fourth adjustment accumulators are provided for the respective second third and fourth support arrangements, each of the respective front left, front right, back left and back right support arrangements may include two rams comprising a roll ram and a pitch ram, each ram including a respective compression chamber, the first support arrangement being a front pitch support arrangement including the compression chamber of the front left pitch ram and the compression chamber of the front right pitch ram, the second support arrangement being a back pitch support arrangement including the compression chamber of the back left pitch ram and the compression chamber of the back right pitch ram, the third support arrangement being a left roll support arrangement including the compression chamber of the front left roll ram and the compression chamber of the back left roll ram, and the fourth support arrangement being a right roll support arrangement including the compression chamber of the front right roll ram and the compression chamber of the back right roll ram. 
     Alternatively, in one or more forms of the present invention, each of the respective front left, front right, back left and back right support arrangements may include two rams comprising a roll ram and a pitch ram, each ram including a respective compression chamber, the first support arrangement being a front pitch support arrangement including the compression chamber of the front left pitch ram and the compression chamber of the front right pitch ram, a second support arrangement being a back pitch support arrangement including the compression chamber of the back left pitch ram and the compression chamber of the back right pitch ram, a third support arrangement being a left roll support arrangement including the compression chamber of the front left roll ram and the compression chamber of the back left roll ram, a fourth support arrangement being a right roll support arrangement including the compression chamber of the front right roll ram and the compression chamber of the back right roll ram. 
     Then, each of the respective front left, front right, back left and back right roll rams may be a double acting ram including a respective rebound chamber, the third support arrangement or left roll support arrangement may further include the rebound chamber of the front right roll ram and the rebound chamber of the back right roll ram, and the fourth support arrangement or right roll support arrangement may further include the rebound chamber of the front left roll ram and the rebound chamber of the back left roll ram. Alternatively or additionally, each of the respective front left, front right, back left and back right pitch rams may be a double acting ram including a respective rebound chamber, the first support arrangement or front pitch support arrangement may further include the rebound chamber of the back left pitch ram and the rebound chamber of the back right pitch ram, and the second support arrangement or back pitch support arrangement may further include the rebound chamber of the front left pitch ram and the rebound chamber of the front right pitch ram. 
     A second actuator may be provided to transfer or effectively transfer fluid between the fluid chamber of a second adjustment accumulator and at least one compression chamber of a respective at least one ram of the second support arrangement, a third actuator may be provided to transfer or effectively transfer fluid between the fluid chamber of a third adjustment accumulator and at least one compression chamber of a respective at least one ram of the third support arrangement, and a fourth actuator may be provided to transfer or effectively transfer fluid between the fluid chamber of a fourth adjustment accumulator and at least one compression chamber of a respective at least one ram of the fourth support arrangement. Alternatively, a second actuator may be provided to transfer or effectively transfer fluid between the fluid chamber of a second adjustment accumulator and at least one compression chamber of a respective at least one ram of the second support arrangement, and a third actuator may be provided to transfer or effectively transfer fluid between the at least one compression chamber of a respective at least one ram of the third support arrangement and at least one compression chamber of a respective at least one ram of the fourth support arrangement. 
     In one or more forms of the present invention, each of the respective front left, front right, back left and back right support arrangements may include two rams comprising a roll ram and a pitch ram, each ram including a respective compression chamber, the suspension system further including: a front pitch compression volume including the compression chamber of the front left pitch ram and the compression chamber of the front right pitch ram; a back pitch compression volume including the compression chamber of the back left pitch ram and the compression chamber of the back right pitch ram; a left roll compression volume including the compression chamber of the front left roll ram and the compression chamber of the back left roll ram; and a right roll compression volume including the compression chamber of the front right roll ram and the compression chamber of the back right roll ram. 
     Each of the respective front left, front right, back left and back right roll rams may be a double acting ram including a respective rebound chamber, the left roll compression volume further including the rebound chamber of the front right roll ram and the rebound chamber of the back right roll ram, the right roll compression volume further including the rebound chamber of the front left roll ram and the rebound chamber of the back left roll ram. Alternatively or additionally, each of the respective front left, front right, back left and back right pitch rams may be a double acting ram including a respective rebound chamber, the front pitch compression volume further including the rebound chamber of the back left pitch ram and the rebound chamber of the back right pitch ram, the back pitch compression volume further including the rebound chamber of the front left pitch ram and the rebound chamber of the front right pitch ram. 
     A heave device may be provided, forming the first support arrangement, the heave device comprising a heave piston assembly having four system volume pressure areas and a heave pressure area, the system volume pressure areas of the heave piston assembly being slidable inside respective system volume bores and being fixed relative to the heave pressure area of the heave piston assembly which is slidable inside a heave bore, the four system volume bores each being respectively connected to a respective one of the front pitch, back pitch, left roll and right roll compression volumes, the heave bore being connected to the first adjustment accumulator (for example, a heave adjustment accumulator), such that when the first actuator transfers fluid between the first adjustment accumulator and the heave bore, the heave piston assembly slides inside the heave and system volume bores, thereby effectively transferring fluid between the first adjustment accumulator the compression chambers of the pitch and roll rams of each of the front left, front right, back left and back right support arrangements. 
     A second (for example pitch) actuator may be provided to transfer or effectively transfer fluid between the front pitch compression volume and the back pitch compression volume, and a third (for example roll) actuator may be provided to transfer or effectively transfer fluid between the left roll compression volume and the right roll compression volume. 
     In one or more forms of the present invention, each of the respective front left, front right, back left and back right support arrangements may comprise a single respective single-acting ram, each ram including a respective compression chamber, the suspension system further including: a warp and heave device comprising: a first diagonal device connected to (for example, a first diagonal pair of the support arrangements being) the front left and back right rams; and a second diagonal device connected to (for example, a second diagonal pair of the support arrangements being) the front right and back left rams. Each diagonal device may include a first cylinder axially aligned with a second cylinder, the first cylinder including a piston connected to a rod extending into the second cylinder, forming first, second and third chambers, the rod being accommodated in the first and second chambers being a front system chamber and a back system chamber, the first and second chambers varying in volume in a common direction with motion of the piston and rod and varying in an opposite direction to the third chamber being a diagonal chamber, the front system chamber of the first diagonal device being connected to the compression chamber of the front left ram, the back system chamber of the first diagonal device being connected to the compression chamber of the back right ram, the front system chamber of the second diagonal device being connected to the compression chamber of the front right ram, the back system chamber of the second diagonal device being connected to the compression chamber of the back left ram, and the diagonal chamber of the first diagonal device being connected to the diagonal chamber of the second diagonal device forming a heave volume further including a heave resilience accumulator. This arrangement may allow for warp motions to compress the volume of the diagonal chamber of one of the first or second diagonal devices and increase the volume of the diagonal chamber of the other of the first or second diagonal devices, thus permitting free warp motions, by for example removing the warp stiffness of the rams of the support arrangements, such pure warp motions not requiring use of the heave resilience accumulator. This arrangement may also allow for heave motions to cause the diagonal chambers of both the first and second diagonal devices to compress or cause the diagonal chambers of both the first and second diagonal devices to increase in volume, such heave motions requiring the resilience of the heave resilience accumulator to accommodate the volume changes of the diagonal chambers of both the first and second diagonal devices. The first adjustment accumulator may be connected to the heave volume (either directly if for example the first adjustment accumulator incorporates the first actuator, or indirectly if for example the first actuator is connected between the heave volume and the first adjustment accumulator) such that when the first actuator transfers fluid between the first adjustment accumulator and the heave volume, the piston and rod in each diagonal device is displaced, effectively transferring fluid to the compression chambers of the front left, front right, back left and back right rams in the respective support arrangements. A pitch device may further be provided comprising three axially aligned cylinders, three pistons connected together by two rods each piston being disposed one to each cylinder to divide each cylinder and form a front pitch chamber, a front left chamber, a front right chamber, a back left chamber, a back right chamber and a back pitch chamber and a roll device may also be provided comprising three axially aligned cylinders, three pistons connected together by two rods forming a left roll chamber, a front left chamber, a front right chamber, a back left chamber, a back right chamber and a right roll chamber, the respective front left, front right back left and back right chambers of the pitch device and of the roll device being connected to the respective compression chambers of the respective rams. 
     A pitch actuator may be provided to transfer or effectively transfer fluid between the front pitch chamber and the back pitch chamber, and a roll actuator may be provided to transfer or effectively transfer fluid between the left roll chamber and the right roll chamber. 
     Another aspect of the present invention provides a method of controlling a suspension system for a multi-hulled vessel having a chassis and at least two hulls, the vessel further including front left, front right, back left and back right support arrangements between the chassis and the at least two hulls, each support arrangement including at least one ram having at least one compression chamber the method including the steps of: determining a control mode (for example: docking relative to a moving body such as a pontoon or docking relative to a fixed body such as a wharf or pylon in which cases any or all of the roll pitch and heave modes may be controlled; transit where roll and pitch may be the only controlled modes; or no powered control); sensing any one or more of a control switch position, at least one displacement (of for example a ram, a hull or the chassis), at least one velocity (of, for example a ram, a hull or the chassis), at least one acceleration (of, for example a ram, a hull or the chassis) and at least one force or pressure in the suspension system; controlling at least a first actuator configured to transfer or effectively transfer fluid from a first adjustment accumulator to the at least one compression chamber of the at least one ram of at least one of the support arrangements to adjust a heave, roll and/or pitch of the chassis (relative for example to the at least two hulls or a sensed water surface or other sensed positions such as markers on a pylon), a static pressure in the first adjustment accumulator being within 25% of a static pressure in the at least one compression chamber of the at least one ram of at least one of the support arrangements. 
     The method may further include the step of controlling the static pressure in the first adjustment accumulator (for example, to substantially equalise the static pressure in the first adjustment accumulator with the static pressure in the at least one compression chamber of the at least one ram of at least one of the support arrangements). 
     The method may further include, before controlling the static pressure in the first adjustment accumulator, any of the steps of: checking that the first actuator is not operating; checking that the vessel is in a substantially static or steady state condition; measuring a static, steady state or average pressure in the first adjustment accumulator and measuring a static, steady state or average pressure in the at least one compression chamber of the at least one ram of at least one of the support arrangements. 
     The step of controlling the static pressure in the first adjustment accumulator may include opening a valve in a bypass around the first actuator, the valve selectively communicating the first adjustment accumulator with the at least one compression chamber of the at least one ram of at least one of the support arrangements. 
     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 superseding the generality of the preceding description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic view of a catamaran incorporating a first possible form of the present invention. 
         FIG. 2  is a schematic view of a second possible form of the present invention. 
         FIG. 3  is a schematic view of a third possible form of the present invention. 
         FIG. 4  is a schematic view of a quadmaran incorporating a fourth possible form of the present invention. 
         FIG. 5  is a schematic view of a fifth possible form of the present invention. 
         FIG. 6  is a schematic view of a sixth possible form of the present invention. 
         FIG. 7  is a schematic view of a support arrangement according to one form of the present invention. 
         FIG. 8  is a flow chart showing a portion of a possible control method for the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , there is shown a plan view of a vessel  1  having a body or chassis  2 , a left hull  3  and a right hull  4 . An example of the suspension geometry providing location of the hulls relative to the body is disclosed in the applicant&#39;s international publication number WO 2013/181699, details of which are incorporated herein by reference. Towards the front and back of the left and right hulls are shown rams  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12  which form part of support arrangements between the hulls and the chassis. The support arrangements may together provide all of the support of the chassis, or if for example the chassis includes a hull portion or other wetted area that engages with the water, the support arrangements may provide only a portion of the support of the chassis. 
     In each of the respective front left, front right, back left and back right support arrangements is a respective roll ram  5 ,  6 ,  7 ,  8  and a respective pitch ram  9 ,  10 ,  11 ,  12 . In this example each ram is double acting so includes a compression chamber  13 ,  15 ,  17 ,  19 ,  21 ,  23 ,  25 ,  27  and a rebound chamber  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28 . The compression chambers  13 ,  17  of the front left  5  and back left  7  roll rams are in fluid communication with the rebound chambers  16 ,  20  of the front right  6  and back right  8  roll rams forming a left roll compression volume. A left roll compression (fluid pressure) accumulator  31  is connected to the left roll compression volume to provide resilience and may thus be referred to as a left roll compression resilience accumulator  31 . Similarly, the compression chambers  15 ,  19  of the front right  6  and back right  8  roll rams are in fluid communication with the rebound chambers  14 ,  18  of the front left  5  and back left  7  roll rams forming a right roll compression volume to which is connected a right roll compression accumulator  32 . 
     The compression chambers  21 ,  23  of the front left  9  and front right  10  pitch rams are in fluid communication with the rebound chambers  26 ,  28  of the back left  11  and back right  12  pitch rams forming a front pitch compression volume. A front pitch compression (fluid pressure) accumulator  33  is connected to the front pitch compression volume to provide resilience and may thus be referred to as a front pitch compression resilience accumulator. Similarly, the compression chambers  25 ,  27  of the back left  11  and back right  12  pitch rams are in fluid communication with the rebound chambers  22 ,  24  of the front left  9  and front right  10  pitch rams forming a back pitch compression volume to which is connected to a back pitch compression accumulator  34 . 
     A roll actuator  37  including a driving device such as a (roll) motor  38  powering a bi-directional pump  39  allows fluid to be displaced along conduits  40 ,  41  between the left and right roll compression volumes to provide for example an active roll adjustment of the attitude of the chassis  2  relative to the hulls  3  and  4 . In addition to converting energy (typically electrical energy) into motion of the pump and thereby roll of the vessel chassis, the roll actuator  37  can optionally also convert roll motions allowed to drive rotations of the bi-directional pump  39  into energy if for example the roll motor  38  is a motor-generator. 
     Similarly, a pitch actuator  42  including a driving device such as a (pitch) motor (not shown) powering a bi-directional pump  44  allows fluid to be displaced between the front and back pitch compression volumes to provide for example an active pitch adjustment of the attitude of the chassis  2  relative to the hulls  3  and  4 . 
     Heave device  47  includes a heave displacer  48  comprising four system area pistons and a single large heave area piston  53 , all five pistons being rigidly connected together in a heave piston assembly  54 . Each system area piston slides inside a respective system bore and the heave area piston  53  slides inside a heave area bore  59 , each system bore being connected to the respective system volume by a respective heave conduit, i.e. the front pitch compression bore of the heave device is connected by a front (pitch compression) heave conduit  53  to the front pitch compression volume; the left roll compression bore of the heave device is connected by a left (roll compression) heave conduit  54  to the left roll compression volume; the right roll compression bore of the heave device is connected by a right (roll compression) heave conduit  55  to the right roll compression volume; and the back pitch compression bore of the heave device is connected by a back (pitch compression) heave conduit  56  to the back pitch compression volume. 
     Therefore as the heave piston assembly  54  displaces, fluid is displaced into or out of each of the respective compression volumes and out of or into a heave adjustment accumulator  60 , which is connected to the heave bore  59  in which the heave area piston  53  slides via a heave actuator  61 . The heave actuator  61  includes a (heave) motor (omitted for clarity) powering a bi-directional pump  63  to displace fluid between the heave adjustment accumulator  60  and the heave bore  59  to provide for example an active heave adjustment of the chassis  2  relative to the hulls  3  and  4 . For example, if fluid is pumped from the heave adjustment accumulator  60  into the heave area bore  59 , the volume formed by the heave area bore  59  and the heave area piston  53  is increased. Therefore the heave area piston  53  is axially displaced, as is the rest of the heave piston assembly  54 . This causes the four individual volumes formed by the system area bores and the system area pistons to each reduce, displacing fluid into each of the main system volumes of the suspension system (i.e. into the left roll compression volume, right roll compression volume, front pitch compression volume, and back pitch compression volume). As the support provided by the rams is due to pressure acting over a compression piston face that is larger area than the annular face of the rebound side of the piston, then the pressure in the system compression volumes is related to the load supported by the rams and the rod diameters of the rams. If the load on each ram does not change, then the increase in fluid in all four of the main system volumes is accommodated by the ram chambers, expelling a corresponding volume of rod from the rams. This causes the roll rams  5 ,  6 ,  7 ,  8  and the pitch rams  9 ,  10 ,  11 ,  12  to all extend, raising the chassis  2  of the vessel relative to the hulls  3 ,  4 . Similarly, pumping fluid from the heave area bore  59  into the heave adjustment accumulator  60  will lower the chassis  2  of the vessel relative to the hulls. 
     A primary benefit of using a pressurised heave adjustment accumulator rather than a tank of fluid at atmospheric pressure is that the static pressure differential between the heave adjustment accumulator  60  and the heave area bore  59  can be reduced. The static pressure in the heave area bore  59  is dependent in part on the magnitude of the sprung mass. For example a pressure of 70 bar may be required in the heave area bore  59  to ensure that the pressures in the main system volumes provide a sufficient total push-out force of the rams  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12  to support the mass of the chassis  2  for the vessel. If the heave pump  63  was connected to a tank of fluid at atmospheric pressure, it would need to generate fluid pressure in excess of 70 bar to enable the pressure in the heave area bore to be overcome and for fluid to be pumped into the heave area bore to increase the height (heave) of the chassis, i.e. in addition to any dynamic forces causing motion of the chassis, the heave pump would have to work against the constant load of gravity acting on the chassis, which consumes a considerable amount of energy. Conversely, when releasing fluid from the heave area bore into a tank at atmospheric pressure, the energy of the fluid pressure being released is largely lost, typically through damping. Even if the heave actuator includes a motor generator to extract energy from the release of pressurised fluid to a tank, the losses are not insignificant due to the magnitude of energy involved with working against both dynamic and gravitational forces when using a heave adjustment tank at atmospheric pressure. So the heave adjustment accumulator  60  can have an operating pressure that reduces or minimises the static pressure differential across the heave actuator. A significant advantage can be gained over using a conventional supply fluid tank at atmospheric pressure by using an adjustment accumulator  60  that has a pressure within 25% of the static operating pressure of the volume it is directly controlling by the pump  53 , in this case the fluid inside the heave bore  59 . Further advantages are gained by using an adjustment accumulator at a closer pressure to the static operating pressure of the volume being controlled, such as within 20%, or more preferably within 15% or more preferably within 10% or more preferably within 5% with the ideal being equal static pressure in the adjustment accumulator and the volume being controlled. Ideally, the heave adjustment accumulator  60  has an operating pressure that balances the heave piston assembly  54  at a chosen condition such as vessel ride height for example, or mid-stroke travel of the suspension system, as if the heave bore  59  were directly connected to the accumulator  60 . Such a substantial reduction or removal of the static pressure differential across the heave pump  63  due to gravitational force on the chassis  2  greatly increases the efficiency of any heave compensation motions. 
     Alternative embodiments of the present invention are now discussed. Throughout the drawings, equivalent parts are given like reference numerals. In  FIG. 2  the roll rams and roll compression volumes are unchanged from  FIG. 1 . However the roll actuator  37  is now a linear motor  71  powering a piston  72 , the piston is slidable inside a cylinder  73 , separating the cylinder into a left roll chamber  74  and a right roll volume chamber  75 , such that powering the piston  72  axially inside the roll cylinder  73  generates an effective displacement of fluid between the roll compression volumes (i.e. from one roll compression volume to the other and vice versa). 
     The pitch rams in  FIG. 2  are single-acting rams (i.e. having only a compression chamber). The compression chamber  21  of the front left pitch ram  9  is in fluid communication with the compression chamber  23  of the front right pitch ram  10  by a front pitch compression conduit  83  forming a front pitch compression volume. Similarly, the compression chamber  25  of the back left pitch ram  11  is in fluid communication with the compression chamber  27  of the back right pitch ram  12  by a back pitch compression conduit  84  forming a back pitch compression volume. A front heave actuator  89  is connected to the front pitch compression volume and a back heave actuator  90  is connected to the back pitch compression volume. If both the front and back heave actuators  89  and  90  are controlled to increase the volume of fluid in the front and back pitch compression volumes then the chassis  2  can be adjusted in in a pure heave mode (i.e. the front and the back can be adjusted in height by the same amount). However if, for example, just the front heave actuator is used to adjust the volume of fluid in the front pitch compression volume, then the chassis  2  will be raised or lowered at the front end only, so the adjustment to the chassis will overall involve a heave motion component and a pitch motion component. Conversely, if the front and back heave actuators  89  and  90  are used to drive the opposite ends of the chassis  2  in opposite directions (such as the front upwards and the rear downwards relative to the hulls) then it is possible for a pure pitch motion of the chassis to result with no change in heave displacement of the chassis centre of mass for example). 
     The front heave actuator  89  comprises a voice coil type linear motor  91  which generates a force on the piston  92  (or similar moveable wall) of a front heave adjustment accumulator  93 . The piston  92  divides the accumulator  93  into the fluid chamber  94  and gas chamber  95  typical of a fluid pressure accumulator. The front heave adjustment accumulator  93  is again preferably at a similar static pressure to the operating pressure of the associated system volume, in this case the front pitch compression volume. The piston  92  is able to slide axially so if the voice coil motor  91  is not driven or loaded to influence the position of the piston  92 , the adjustment accumulator  93  will act like a conventional piston accumulator, providing resilience to the associated system volume. 
     Front pitch compression accumulators  33  are in this case optional but shown here connected to the front pitch compression volume to provide resilience, in which case the resilience provided by the front heave adjustment accumulator  89  can be locked off or heavily restricted (so the static pressure in the accumulator and the system volume remain similar) using valve  96 . 
     Various forms of linear motor are known, the coil shown here being a simplification for the purposes of drawing clarity. By supplying appropriate currents to the voice coil  91  or applying a load across it, driving or damping forces can be exerted on the piston  92 . This is in contrast to other forms of actuator in the present invention. The benefit of using a linear actuator to apply a force to the piston of an accumulator that is acting as both a resilience accumulator and an adjustment accumulator is that oscillating motions can be readily controlled and as the heave position of the chassis will not vary very far on average (although waves will cause oscillations around this average heave position) and the need for separate adjustment and resilience accumulators can be negated. The disadvantage of such a force-type actuator rather than a displacement-type actuator is that in the many situations where a constant displacement is required for a period, either while maneuvering (turning, accelerating, decelerating, transitioning from planing to displacement operation of the hulls, etc.) a force must be continually generated to compensate the chassis position for sustained dynamic loads. The valve  96  can be used to assist in these situations, but then separate resilient accumulators such as shown at  33  would be required. 
     An alternative arrangement of heave actuator  90  is shown on the back pitch volume in  FIG. 2 , being a displacement-type actuator. Although the back heave actuator  90  is built into the back heave adjustment accumulator  98  the piston  99  is not free to move, so the adjustment accumulator  98  can never act as a resilient accumulator. Therefore back pitch compression accumulators  34  are connected to the back pitch compression volume to provide resilience. The piston  99  separates the adjustment accumulator  98  into a fluid chamber  100  and a gas chamber  101 . The gas chamber  101  may include a flexible annular container (not shown) such as an annular bellows or bag to contain the gas and reduce the need for adjustment or frequency of servicing of the gas pressure or volume in the gas chamber  101 . Similarly the fluid chamber  100  may include a flexible form of annular container to improve sealing, the flexible container being in fluid communication with the back pitch compression volume. The back heave actuator  90  incorporates a back heave motor  102  driving a lead screw type linear motion device  103  which controls the axial displacement of the piston  99 . 
     Ideally the gas chamber  101  of the back heave adjustment accumulator  98  has a significant pressure pre-charge, so as discussed above in relation to the heave adjustment accumulator  60  of  FIG. 1 , the motor  102  has to provide the required force to displace fluid into or out of the system volume (in this case the back pitch compression volume) to compensate for dynamic motions of the back pitch rams but does not have to work against a high pre-load force, for example due to the constant (static) gravitational loads on the back pitch rams. 
     The respective front heave actuator  89  and the back heave actuator  90  can be used to individually adjust the respective front or back height of the chassis relative to the hulls to thereby provide pitch and heave adjustment. 
       FIG. 3  shows a similar arrangement of double-acting roll and double-acting pitch rams as in  FIG. 1  forming left and right roll compression volumes and front and back pitch compression volumes. However in this example in  FIG. 3  there is a respective actuator for each volume. The left (roll or heave) actuator  110  includes a left (roll or heave) motor (omitted for clarity) to drive a left (roll or heave) bi-directional pump  112  for driving fluid flow between the left (roll or heave) adjustment accumulator  113  and the left roll compression volume. Similarly the right (roll or heave) actuator  115  includes a right (roll or heave) motor (omitted for clarity) to drive a right (roll or heave) bi-directional pump  117  for driving fluid flow between the right (roll or heave) adjustment accumulator  118  and the right roll compression volume. 
     The components  110 ,  112 ,  113  and  115 ,  117 ,  118  are designated as roll or heave components by their naming above since they enable adjustments in both the roll and heave modes, so it may be convenient to refer to them as simply left or right height adjustment components. For example, to roll the chassis  2  to the left (lowering the left side and raising the right side) relative to the left and right hulls  3 ,  4 , the left height adjustment pump  112  is powered to drive fluid out of the left roll compression volume, along conduit  114  into the left height adjustment accumulator  113 , and the right height adjustment pump  117  is powered to drive fluid along conduit  119  into the right roll compression volume from the right height adjustment accumulator  118 . To adjust the heave position of the chassis  2  upwards relative to the left and right hulls  3 ,  4 , the left and right height adjustment pumps  112 ,  117  can be powered to drive fluid out of the left and right height adjustment accumulators  113 ,  118  along the conduits  114 ,  119  into the left and right roll compression volumes. If this is done with no change to the front and back pitch compression volumes, then the proportion of the sprung mass supported on the roll rams  5 ,  6 ,  7 ,  8  is increased and the proportion of the sprung mass supported on the pitch rams  9 ,  10 ,  11 ,  12  is correspondingly reduced. 
     The front (pitch or heave) actuator  89  includes a front (pitch or heave) motor (omitted for clarity) to drive a front (pitch or heave) bi-directional pump  122  for driving fluid flow between the front (pitch or heave) adjustment accumulator  123  and the front pitch compression volume. Similarly the back (pitch or heave) actuator  90  includes a back (pitch or heave) motor (omitted for clarity) to drive a back (pitch or heave) bi-directional pump  127  for driving fluid flow between the back (pitch or heave) adjustment accumulator  128  and the back pitch compression volume. 
     The components  89 ,  122 ,  123  and  90 ,  127 ,  128  are designated as pitch or heave components by their naming above since they enable adjustments in both the pitch and heave modes, so it may be convenient to refer to them as simply front or back height adjustment components. For example, to pitch the chassis  2  to the front (lowering the front and raising the rear) relative to the hulls  3 ,  4 , the front height adjustment pump  122  is powered to drive fluid out of the front pitch compression volume, along conduit  124  into the front height adjustment accumulator  123 , and the back height adjustment pump  127  is powered to drive fluid along conduit  129  into the back pitch compression volume from the back height adjustment accumulator  128 . To adjust the heave position of the chassis  2  upwards relative to the left and right hulls  3 ,  4 , the front and back height adjustment pumps  122 ,  127  can be powered to drive fluid out of the front and back height adjustment accumulators  123 ,  128  along the conduits  124 ,  129  into the front and back pitch compression volumes. If this is done with no change to the left and right roll compression volumes, then the proportion of the sprung mass supported on the pitch rams  9 ,  10 ,  11 ,  12  is increased and the proportion of the sprung mass supported on the roll rams  5 ,  6 ,  7 ,  8  is correspondingly reduced. 
     To adjust the heave of the vessel in response to dynamic inputs, without changing the proportion of the sprung mass supported on the roll versus pitch rams, both left and right height adjustment pumps  112 ,  117  drive fluid into the roll compression volumes from the left and right height adjustment accumulators  113 ,  118  and both front and back height adjustment pumps  122 ,  127  drive fluid into the pitch compression volumes from the front and back height adjustment accumulators  123 ,  128  to raise the chassis relative to the hulls; or conversely to lower the chassis both left and right height adjustment pumps  112 ,  117  drive fluid from the roll compression volumes into the left and right height adjustment accumulators  113 ,  118  and both front and back height adjustment pumps  122 ,  127  drive fluid from the pitch compression volumes into the front and back height adjustment accumulators  123 ,  128 . 
     The preceding suspension arrangements according to various embodiments of the present invention are also applicable to vessels having other numbers of hulls, such as the following quadmaran with four hulls moveable relative to the chassis, discussed with reference to  FIG. 4 . Similarly the suspension arrangements shown in the following Figures are also applicable to vessels having two hulls moveable relative to the chassis. 
       FIG. 4  shows a quadmaran having a body portion or chassis  2  at least partially supported relative to four individual hulls (a front left hull  141 , a front right hull  142 , a back left hull  143  and a back right hull  144 ). Each (front left, front right, back left, or back right) support arrangement includes a double-acting ram  145 ,  146 ,  147  or  148 , respectively. A compression chamber  149  of the front left ram  145  is connected to a rebound chamber  154  of the front right ram  146  forming a front left compression volume to which is connected a front left (resilience) accumulator  157  for providing resilience. Similarly, the compression chamber  150  of the front right ram  146  is connected to the rebound chamber  153  of the front left ram  145  forming a front right compression volume to which is connected a front right (resilience) accumulator  158 ; the compression chamber  151  of the back left ram  147  is connected to the rebound chamber  156  of the back right ram  148  forming a back left compression volume to which is connected a back left (resilience) accumulator  159 ; and the compression chamber  152  of the back right ram  148  is connected to the rebound chamber  155  of the back left ram  147  forming a back right compression volume to which is connected a back right (resilience) accumulator  160 . 
     The front left compression volume can be adjusted by a front left height adjustment actuator  161  including a pump  162  to drive fluid along conduit  164  from or to a front left adjustment accumulator  163 ; the front right compression volume can be adjusted by a front right height adjustment actuator  166  including a pump  167  to drive fluid along conduit  169  from or to a front right adjustment accumulator  168 ; the back left compression volume can be adjusted by a back left height adjustment actuator  171  including a pump  172  to drive fluid along conduit  174  from or to a back left adjustment accumulator  173 ; and the back right compression volume can be adjusted by a back right height adjustment actuator  176  including a pump  177  to drive fluid along conduit  179  from or to a back right adjustment accumulator  178 . 
     An optional valve  181 ,  182 ,  183 ,  184  is shown in the respective actuator conduit  164 ,  169 ,  174 ,  179 . Alternatively (or additionally) each pump may be connected to a motor-generator so that electrical energy can be generated from flow through the conduits  164 ,  169 ,  174 ,  179  as well as used to generate flow through the conduits. 
     All four pumps must be operated to adjust any one of the pure roll, pitch or heave modes of the chassis relative to the four hulls. For example, if the front left and front right pumps  162 ,  167  are used to adjust the front left and front right compression volumes to increase the height of the front of the vessel chassis  2  relative to the hulls, but without any adjustment of the back compression volumes, then both the pitch and heave attitude of the chassis are adjusted relative to the hulls. The roll of the front rams  145  and  146  is now separate to the roll of the rear rams  147  and  148 , so to avoid generating unwanted torsional inputs to the chassis or to provide a desired distribution of roll forces between the front and the rear rams, the pressure in the four compression volumes is a useful control input. 
     These front and rear pairs of cross-connected double-acting rams in  FIG. 4  provide a higher roll stiffness than heave stiffness, but provide a warp stiffness whereas the arrangements in  FIGS. 1 to 3  and  FIG. 6  typically provide substantially zero warp stiffness. 
     Alternatively, each compression volume may only comprise a single compression chamber  149 ,  150 ,  151  or  152  and an accumulator  157 ,  158 ,  159  or  160  for resilience, i.e. if as shown in  FIG. 5 , each of the front left, front right, back left and back right rams  145 ,  146 ,  147 ,  148  is effectively single-acting, without any interconnections between the four support arrangements, then the ram arrangement provides the same roll, pitch, warp and heave stiffness. While the roll, pitch and heave could be controlled as discussed in relation to  FIG. 4 , warp control to reduce or minimise torsional inputs into the chassis can add complexity to the control although the number of rams and conduits is reduced. 
     The suspension system shown in  FIG. 6  uses one single-acting ram  145 ,  146 ,  147 ,  148  in each support arrangement. Each ram is connected to a roll device  230 , a pitch device  240  and a heave and warp device  260 , each device including one or more respective accumulators to provide compliance in the relevant mode. 
     The roll device  230  includes three axially aligned cylinders, each separated by a respective piston into two chambers, the three pistons (one in each of the three cylinders) being rigidly connected by rods forming a piston rod assembly. The compression chamber  149  of the front left ram  145  is connected to the front left roll chamber  232  of the roll device  230 ; the compression chamber  150  of the front right ram  146  is connected to the front right roll chamber  235 ; the compression chamber  151  of the back left ram  147  is connected to the back left roll chamber  234 ; and the compression chamber  152  of the back right ram  148  is connected to the back right roll chamber  233 . 
     As the front left roll chamber  232  and the back left roll chamber  234  expand with motion of the piston rod assembly, the left roll compression chamber  231  of the roll device  230  contracts in size, expelling fluid into the left roll compression accumulator  31 , increasing its pressure and therefore the pressure in the left roll compression chamber  231 . The right roll compression chamber  236  correspondingly increases in size, fluid being supplied from the right roll compression accumulator  32  reducing its pressure and the pressure in the right roll compression accumulator. The change in pressures in the roll compression chambers  231  and  236  is reacted by a change in the pressures in the system roll chambers in the roll device  230 , with the front left and back left roll chambers  232 ,  234  increasing in pressure and the front right and back right roll chambers  235 ,  233  decreasing in pressure. This mechanism provides an increase in roll moment with roll displacement, i.e. a roll stiffness. Similarly, as the front right roll chamber  235  and back right roll chamber  233  expand with motion of the piston rod assembly, the right roll compression chamber  236  of the roll device  230  contracts in size, expelling fluid into the right roll compression accumulator  32 . Although all three cylinders of the roll device  230  are shown the same size in  FIG. 6 , changing the diameter of the centre cylinder relative to the end cylinders changes the distribution of roll loads and warp displacements between the front and back rams. 
     The pitch device  240  similarly includes three axially aligned cylinders, each separated by a piston  247 ,  248 ,  249  into two chambers  241  and  242 ;  243  and  244 ;  245  and  246 , the three pistons in the three cylinders being rigidly connected by rods forming a piston rod assembly. The compression chamber  149  of the front left ram  145  is connected to the front left pitch chamber  242  of the pitch device  240 ; the compression chamber  150  of the front right ram  146  is connected to the front right pitch chamber  244 ; the compression chamber  151  of the back left ram  147  is connected to the back left pitch chamber  243 ; and the compression chamber  152  of the back right ram  148  is connected to the back right pitch chamber  245 . 
     As the front left pitch chamber  242  and the front right pitch chamber  244  expand with motion of the piston rod assembly, the front pitch compression chamber  241  of the pitch device  240  contracts in size, expelling fluid into the front pitch compression accumulator  33 . Similarly, as the back left pitch chamber  243  and back right pitch chamber  245  expand with motion of the piston rod assembly, the back pitch compression chamber  246  of the pitch device  240  contracts in size, expelling fluid into the back pitch compression accumulator  34 . As with the roll device, in the pitch device a displacement of the piston rod assembly generates a change in pressures, with the pitch device providing an increase in pitch moment on the vessel with pitch displacement, i.e. a pitch stiffness. 
     The heave and warp device  260  includes a first pair of axially aligned cylinders  261  and a second pair of axially aligned cylinders  262 . One cylinder of each pair includes a piston  263  or  264  separating the one cylinder into two chambers  265  and  267  or  266  and  268 , each piston  263  or  264  being rigidly connected to a respective rod  269  or  270  protruding into the other cylinder of the respective pair. The compression chamber  149  of the front left ram  145  is connected to the front left heave chamber  271  in the first pair of axially aligned cylinders  261  of the warp and heave device  260 . The other chamber in the first pair of axially aligned cylinders which varies in volume in the same direction as the front left heave chamber  271  with motion of the piston  263  and rod  269  is the back right heave chamber  267  and is connected to the compression chamber  152  of the back right ram  148 . Thus when the front left ram  145  and back right ram  148  (i.e. a first diagonal pair of rams) are compressed, fluid is expelled from their compression chambers  149 ,  152  into the front left heave chamber  271  and the back right heave chamber  267 , expanding those chambers and displacing the piston rod assembly such that the first diagonal heave chamber  265  is compressed. Similarly, the compression chamber  150  of the front right ram  146  is connected to the front right heave chamber  272  in the second pair of axially aligned cylinders  262  of the warp and heave device  260 . The other chamber in the second pair of axially aligned cylinders which varies in volume in the same direction as the front right heave chamber with motion of the piston  264  and rod  270  is the back left heave chamber  268  and is connected to the compression chamber  151  of the back left ram  147 . Thus when the front right ram  146  and back left ram  147  (i.e. a second diagonal pair of rams) are compressed, fluid is expelled from their compression chambers  150 ,  151  into the front right heave chamber  272  and the back left heave chamber  268 , expanding those chambers and displacing the piston rod assembly such that the second diagonal heave chamber  266  is compressed. 
     During a warp motion of the rams  145 ,  146 ,  147 ,  148  of the suspension system, for example when the first diagonal pair of (front left, back right) rams are compressed and the second diagonal pair of (front right, back left) rams are extended, fluid is displaced between the first and second diagonal heave chambers  265 ,  266 , so any pressure changes are minimised, as are load changes in the four rams  145 ,  146 ,  147 ,  148 , i.e. there is substantially no warp stiffness. However during a heave motion of the rams  145 ,  146 ,  147 ,  148  of the suspension system, for example when all the rams are compressed, fluid is displaced out of both the first and second diagonal heave chambers  265 ,  266  into the heave resilience accumulator  273 . 
     Such arrangements are discussed in more detail in the applicant&#39;s international publication numbers WO 2011/143692 and WO 2011/143694 details of which are incorporated herein by reference. In this arrangement, as in the arrangement of  FIG. 1 , where the warp mode of the hydraulic suspension system has substantially no stiffness, the other three modes can be individually adjusted using respective actuators. The roll actuator  37  includes a roll pump  39  connected between the left and right roll compression chambers  231 ,  236  of the roll device  230 . While there may be a difference between the static pressures in the left and right roll compression chambers due to a lateral offset load on the vessel for example, the magnitude of pressure differential is typically much lower than the differential between either of the roll compression chamber pressures and atmospheric pressure. The pitch actuator  42  includes a pitch pump  44  connected between the front and back pitch compression chambers  241 ,  246  of the pitch device  240 . While there may be a difference between the static pressures in the front and back pitch compression chambers due to a difference in front to rear load on the vessel for example (or even suspension geometry effects such as the mechanical advantage on rams), the magnitude of pressure differential is typically much lower than the differential between either of the pitch compression chamber pressures and atmospheric pressure. The heave actuator  61  includes a heave pump  63  connected to the heave and warp device  260 . As in  FIG. 1  a heave adjustment accumulator  60  is provided to reduce or minimise the static pressure differential across the heave pump  63  (between the fluid in the heave adjustment accumulator and the fluid in the first and second diagonal heave chambers  265 ,  266 ). The arrangement of rams and modal (roll, pitch and heave/warp) devices of  FIG. 6  has an inherent zero warp stiffness. 
       FIG. 7  shows a support arrangement towards the front left corner of the vessel, along with associated sensing and control elements. The example is taken from a corner of the suspension system shown in  FIG. 5  with the motor or other drive device  165  shown for the bi-directional pump  162 . In this example the drive device is a motor-generator, i.e. it can convert electrical energy into rotational motion to drive the pump  162 , or convert rotational motion of the pump  162  into electrical energy. 
     A pressure transducer  301  is connected to the front left system volume (including compression chamber  149  of front left ram  145 ) which can be used for example with other system pressure transducers to determine the warp load in this independent support arrangement. The pressure transducer  301  can also be used together with the adjustment accumulator pressure transducer  309  to determine when the pressure may need to be equalised between system volume and adjustment accumulator. The position sensor  305  generates an input to the Electronic Control Unit (ECU)  327  indicative of the front left ram stroke position, i.e. the displacement position of the front left ram  145 . Each sensor from the front left support arrangement is communicated back to the ECU by electrical lines  313 ,  317 ,  321 , as are similar sensors from the other three support arrangements (front right, back left and back right). An Inertial Measurement Unit (IMU)  325  fixed to the chassis and typically able to output chassis accelerations, along with calculated velocities and displacements in a reference system relative to the ground or the chassis is also connected to the ECU  327  by electrical line  326 . The ECU can then calculate a desired output to control the relevant actuator, in this case using electrical line  328  to the front left drive device  165 . The use of electrical lines is just used here to indicate the ability to transfer data and control signals electrically or electronically. Typically a CAN (controller area network) bus is actually used to transfer multiple signals significant distances around a vessel with high fidelity. 
       FIG. 8  shows a flow diagram of a possible control for the actuators of the suspension system. Many sensor inputs  350  are possible and can be acquired, depending on the type of control algorithm used and the desired form of control. For example, when controlling the height of either the entire chassis, or a point on the chassis relative to another fixed or moving object such as a pylon, jetty or mother ship, a reference position input is required indicative of the height delta between at least one point on the vessel and a point on the other fixed or moving object. The form of control such as stabilising the chassis while holding station, docking or while underway can be selected by an operator control or performed at least partially automatically using global positioning data and vessel speed for example. Inputs such as data from chassis or hull inertial measurement units can be used as can the positions and pressures in the support arrangements and the pressure in the adjustment accumulator(s). 
     Given the inputs  350 , adjustment aims can be calculated at  351 , which can include modal calculations such as the displacements required in roll pitch and heave to achieve the desired position and if the support arrangements include warp stiffness, such as those in  FIGS. 4 and 5 , whether adjustments need to be made to reduce torsional loads between the support arrangements. Given conditions such as the existing accelerations of the hulls and chassis, the actual adjustments that need to be made through operation of the actuators (be they for example the individual support arrangement actuators  161 ,  166 ,  171 ,  176  of  FIG. 5 ; the actuators for each edge of the vessel—front back left and right as in  FIG. 3 ; or the modal actuators  37 ,  42  and  61  of  FIGS. 1 and 6 ) to satisfy the adjustment aims of  351  can be determined at  352 . 
     If at the decision point  353  no actuator operation is required, then it may be appropriate to equalise the pressure between a system volume and its adjusting accumulator by communicating for example the front left adjusting accumulator  163  in  FIG. 5  with the front left ram compression chamber  149 , the front left resilience accumulator  157  or any other point in the front left compression volume. This can be done by operating a valve that bypasses the adjusting pump  162  as shown in  FIG. 7 , or by allowing free motion of the pump  162 . Referring again to  FIG. 8 , it can be preferable to ensure that the actuator is not operational and has not been operational for a period of time, such as for example 5 seconds (but can be much shorter), as shown at  354  to prevent unwanted response of the system, before proceeding to equalised the system volume and adjusting accumulator pressures as shown at  355 . Similarly the decision point or check at  354  can include verifying that the accelerations on the chassis are within limits, that one or more pressures in the compression volumes are within limits or not varying by more than a pre-set range, or that the stroke positions of at least one or more rams are within limits. Such checks can be used to indicate that the vessel is not undergoing any significant motions or dynamic loads. The pressure is to be equalised while underway, as long as the variation in the compression volume pressures are not varying by such an amount that the pressure in the adjustment accumulator ends up further away from for example an average of the compression volume pressure or outside the limits set, such as within 25% of the static compression volume pressure. 
     However, if at decision point  353 , there is actuator operation required, the actuator operation determined at  352  can be tested at  356  to ensure it is within required limits, for example to limit acceleration or rate of change of acceleration of the controlled actuator adjustment, or to prevent pressure or travel limits being exceeded. If the actuator operation signal(s) are not within such preset limits when tested at  356 , the adjustment aims and/or the intended actuator operation can be modified at  357  and tested again at  356 . If the actuator operation is within preset limits at  356 , the actuator drive parameters can be set at  358  and the actuators driven or otherwise controlled at  359 . Then the control can resample some or all of the inputs at  350  and new aims be calculated at  351 , and so on. 
     Where the term “static operating pressure” is used herein, it refers to any condition where the vessel is in a steady state condition with the sum of forces on the vessel being negligible, i.e. when stationary, or when in motion at a constant speed in a straight line (with only small or negligible wave inputs). The pressure input used may be an average pressure (i.e. time averaged). 
     In all the examples where the actuator drives fluid between an adjustment accumulator and a main system fluid volume, the actuator can be of the displacement type (as shown in most instances) or the force type (as shown with the front pitch actuator  89  in  FIG. 2 . 
     Wherever the actuator is of the force type where the adjustment accumulator  93  can also act as a resilience accumulator, as in  FIG. 2 , then if the adjustment accumulator is in permanent fluid communication with the associated system volume (such as for example the front pitch compression volume in  FIG. 2  if valve  96  is omitted) then additional separate resilience accumulators may not be required. 
     For each of the exemplary hydraulic or hydro-pneumatic suspension systems shown in  FIGS. 1 to 6 , a separate fluid volume maintenance system can be provided. Such a maintenance system is well known to provide slow speed (low flow) compensation for changes in the typically four main system volumes due for example to temperature changes or a slightly higher flow, but still relatively low speed (i.e. non-dynamic) adjustments in response to payload changes or requested changes in the static or steady state ride height or trim of the vessel for example. Preferably such a maintenance system cooperates with any locks between the system volumes and the adjustment accumulators so that the static pressure in each system fluid volume is balanced with the static pressure in the associated adjustment accumulator. 
     Any of the suspension arrangements described may include additional independent support means providing an element of support providing roll, pitch and warp stiffness corresponding to the heave stiffness. 
     Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.