GB2483401A

GB2483401A - Pivoting deck vessel for allowing transfer to a fixed structure

Info

Publication number: GB2483401A
Application number: GB1121472.3A
Authority: GB
Inventors: David Armstrong
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-11-05
Filing date: 2009-11-05
Publication date: 2012-03-07
Anticipated expiration: 2029-11-05
Also published as: GB201121472D0; GB2483401B

Abstract

The present invention relates to a vessel 1 comprising a deck (6, Fig 1a) including a pivoting portion 8. The pivoting portion has a pivot end 10 connected to the rest of the deck by a pivot having its axis transverse to the bow to stern axis of the vessel. The pivoting portion can rotate about the bow to stern axis (32, Fig 3b) of the vessel and pivot about the axis transverse to the bow to stern axis. The vessel has a docking end 12, at or projecting beyond either the bow or the stern of the vessel, the docking end is distal to the pivot end, and is formed and arranged for connecting to a structure 18 in use of the vessel. The pivoting portion of the deck allows the vessel of the invention to connect with or dock to a structure, such as, for example the mast of an offshore wind turbine device and can allow safer transfer of personnel or equipment from the vessel to the structure.

Description

Pivoting Deck Vessel The present invention relates to vessels such as boats ships and hovercraft that dock to structures such as, but not limited to, offshore wind or wave powered energy generators.

Docking a vessel to a structure in open or relatively open water to allow transfer of personnel or equipment can be a hazardous operation especially if the structure is fixed relative to the motion of the waves. The vessel moves up and down with the wave motion and so moves up and down relative to the structure, making transfer of personnel risky except in calm conditions.

Even if the structure is not fixed but is floating the vessel and the structure may move relative to each other, for example on account of a large difference in size between the vessel and the structure.

There are an ever-increasing number of fixed structures employed in open water, for example oil and gas exploration and extraction platforms, and wind turbines. For larger structures such as oil platforms helicopter landing pads are often fitted for safe transfer of personnel. However helicopter operations are expensive. In the case of offshore wind turbines, several turbines are normally grouped together in an array or farm. Each turbine in the farm is normally fitted on top of a simple vertical mast or pile fixed to the seabed. Each turbine requires regular maintenance and also repair if damaged-Fitting helicopter-landing pads to each turbine assembly is not practical or economically feasible. Therefore maintenance and repair is carried out by means of a vessel, typically a relatively small boat, which docks to a turbine pile, to allow maintenance personnel to transfer to a ladder on the mast giving access to the turbine and associated generator.

Such transfers are hazardous when using a conventional vessel.

Typically the vessel approaches and contacts the structure with its bow. Sufficient engine thrust is applied to keep the vessel in contact with the structure whilst personnel transfer takes place. However, as the structure is fixed to the seabed and the vessel rises and falls with the waves the bow of the vessel will slide up and down on the structure, making transfer dangerous. currently a significant wave height cf about O.9m (O.9m Hs) is regarded as being the upper limit beyond which transfers become too dangerous. The slipping of the vessel bow on the structure becomes unpredictable rendering transfers too dangerous.

As the number of offshore installations such as wind farms, oil installations and wave and tide powered energy generators increases there is an increasing need to be able to safely land maintenance personnel on such installations even when the sea state is not calm.

According to a first aspect the present invention provides a vessel comprising: a deck including a pivoting portion1 said pivoting portion having; a pivot end connected to the deck by a pivot having its axis transverse to the bow to stern axis of the vessel; and a docking end, at or projecting beyond either the bow or the stern of the vessel, said docking end being distal to the pivot end, and formed and arranged for connecting to a structure in use of the vessel.

The pivoting portion of the deck allows the vessel of the invention to connect with (dock to) a structure, such as, for example the mast of an offshore wind turbine device and can allow safer transfer of personnel or equipment from the vessel to the structure; even when waves or swell causes the vessel to rise up and down relative to the structure.

The docking end of the pivoting deck portion remains connected to the structure at or close to the same point even whilst the vessel moves up and down on the waves. As the vessel rises and falls the pivoting portion moves about the pivot at its pivot end to maintain the docking end at the same contact point (height) that it conneots to the structure.

In some vessels of the invention the pivoting portion may be formed to rotate about the bow to stern axis of the vessel as well as pivoting about an axis transverse to the bow to stern axis. A pivoting portion that can move about two axes in this way has the advantage of accommodating at least some rolling action caused to the vessel by waves when connected to a fixed structure.

The pivoting portion may constitute a large proportion or indeed all or substantially all of a deck of the vessel, depending on the size and configuration of the vessel selected for a given application.

Where the docking end is at or projecting beyond the bow of the vessel the pivot end may be proximal the stern and vice versa when the docking end is at or projecting beyond the stern -For example for a relatively small vessel (say of 24m in length) designed for use when repairing or maintaining wind turbines the pivot portion may have a length of more than half, perhaps two thirds or more, of the lengthS of the vessel. As the pivot end is distal to the docking end a long pivoting portion has the advantage that only small deviations from the horizontal result in large up and down movement of the docking end relative to the rest of the vessel. For example with a 24m long vessel having a l6m long pivoting deck portion extending from a docking end close to the bow back to a pivot end towards the rear of the vessel, a pivoting action of only 5.36° (plus and minus) from the horizontal gives a relative vertical movement of the order of 3m to the docking end.

Although the pivoting portion will change in angle to the horizontal as the vessel rises and falls in the waves it can nevertheless provide a comparatively stable platform connected to a structure such as the mast or pile of an offshore wind turbine, allowing personnel to transfer back and forth to carry out construction, repair or maintenance work, even when the water is not calm.

Advantageously the vessel is formed and arranged so that the docking end of the pivoting deck portion may move to below the height of the deck itself as the vessel rises and falls in the waves i.e. the pivoting portion of the deck can pivot downwards as well as upwards. This can be achieved in a number of ways. For example the vessel may include a cavity at its bow or stern, providing a space below the pivoting portion. The pivoting deck portion can pivot downwards into the cavity as the vessel rises in response to wave action.

However the presence of such a cavity, especially in the bow of a craft, may lead to a loss of seaworthiness as water may enter between the deck and the cavity.

Alternatively the vessel may comprise two or more hulls with a deck passing across the space in between at least two of the hulls.

The pivoting deck portion of the deck can be arranged between said two hulls, thereby being able to pivot downwards into the space between the hulls when docking to a structure. conveniently, for many uses, the vessel is a catamaran and the pivoting deck portion of a deck is positioned between the two hulls.

Alternatively, for example where the vessel is a monohull, the pivoting portion may be formed to pass around part of the perimeter of a fixed portion of deck and may have pivot points to either side of the fixed portion of deck as described hereafter with reference to a specific embodiment.

when not docked to a structure and pivoting about the pivot the pivoting deck portion may be held locked in a substantially horizontal position, thereby providing a good working or storage platform as the vessel proceeds about its tasks.

The docking end of the pivoting deck portion may be at the bow or the stern of a vessel of the invention and may project beyond the bow or stern. A docking end projecting beyond the bow or the stern, by at least a short distance, has the advantage of keeping the main body of the vessel distanced from the structure being docked to, even where the vessel is moving up and down significantly. (The arcuate motion of the docking end of the pivoting portion has the effect of moving the main body of the vessel closer to the structure at the extremes of rise and fall in the waves as described hereafter with reference to figure 2b.) The pivot end is normally located between the bow and stern of the vessel with the docking end projecting beyond the bow or the stern by only the minimum amount required to distance the main body of the vessel from a structure being docked to as discussed above. This arrangement has the advantage of providing a vessel where all or almost the entire pivoting portion is within the bounds of the vessel plan. This avoids having large pieces of deck projecting out from the vessel that can interfere with normal sailing operations.

In the design of a vessel of the present invention a suitable amount of projection of the docking end beyond the bow or the stern can readily be calculated depending on the range of pivoting desired and the type (configuration) of structure to be docked to.

A docking end at the bow of the vessel can have certain advantages. The forward thrust and steering action of the screw(s) (or other propulsion system) and rudder at the rear of the vessel can be used to keep the docking end contacted to (docked) at a chosen angle and thrust to a structure. This is more easily done where the screws and rudder are at the opposite end of the vessel to that of the docking end of the pivoting portion.

A convenient configuration for a vessel of the invention is a catamaran with a wheelhouse or bridge towards the rear of a main deck, a pivot forward of the wheelhouse transverse to the bow to stern axis and connecting the main deck to a pivot portion of deck. The docking end of the pivot portion of deck projects slightly forwards of the bows and the pivot portion is sized so that it can pivot downwards in between the two catamaran hulls. However it will be appreciated that other configurations are possible depending on the uses envisaged for the vessel.

Typically the docking end includes a fender or fenders of suitably resilient material to absorb the shock of initial docking and any variation in pressure applied between the docking end and the structure.

The docking end of the pivoting portion is formed and arranged for connecting to a structure. This can be achieved in a number of ways. The docking end may be shaped to rest against the structure or a selected part of the structure whilst the propulsion system of the vessel is used to maintain contact (and suitable pressure) between the docking end and the structure, thus keeping connection.

Alternatively the docking end may comprise means for secure interengagement with a given structure. For example the docking end may include a magnet for engagement with a metallic structure. Conveniently an electromagnet is employed that can be energised when the docking end contacts a metal structure.

Alternatively the docking end may comprise a clamp mechanism or mechanisms that can lock onto or round structures to give secure interengagement.

The pivoting portion of the deck can move up and down about the pivotS However, when the vessel is not docked against a structure it is generally desirable that the pivoting portion is locked in a selected position, generally a horizontally disposed position. This can be achieved by providing a suitable looking mechanism, for example electronically operated latches or pins engaging in slots that lock the pivoting portion of the deck to a non-pivoting portion of deck or some other fixed part of the vessel.

Desirably the pivoting portion is moveable in a controlled fashion, for example by a system of hydraulic rams. The hydraulic rams can be used to move the pivoting portion to a desired position for example the horizontal position for use where the vessel is not docked as discussed above.

Once at a desired position the hydraulic system can be used to fix the pivoting portion in place by preventing fluid from entering or leaving the rams. This hydraulic fluid lock can be used in addition to or instead of a separate locking system. The hydraulic rams can also be used limit or damp excessive or fast movement of the pivoting portion.

In use when docking to a structure the procedure employed may be as follows. The vessel approaches the structure with the pivoting portion locked in a horizontal position.

In some circumstances where it is desired to connect to the structure at a higher or lower point the pivoting portion may be held (for example by hydraulic rams) at a different angle. This may be required if or example where the structure being approached is in tidal waters and the height of the connection point on the structure, varies relative to the water surface.

The docking end of the pivoting portion contacts the structure at a selected location and pressure sensors activate a control system to release any mechanical locks employed and to release the hydraulic lock.

At the same time, if fitted, connecting devices such as magnets or clamps are activated to securely interengage the docking end of the pivoting portion with the structure.

The pivoting portion is now able to pivot. The hydraulic rams are connected to a fluid circuit that allows the pivoting portion to move with the motion of the vessel on the waves. The docking end remains connected to the structure, by means of connecting devices or by the action of the vessels propulsion system. personnel and equipment may then be safely transferred to and from the structure via the pivoting portion, whioh provides a stable bridge.

once operations at the structure are complete the docking end is disconnected and the pivoting portion returned to a horizontal or other neutral position and locked in place.

Advantageously the movement of the pivoting portion is controlled by a control system that includes appropriate safety features. For example the docking end of the pivoting portion may include pressure sensors that interface with a control system for hydraulic rams. If an emergency situation arises as indicated by the pressure sensors then the control system may lock the rams in position. For example if the contact between the docking end of the pivoting portion and the structure is lost then the rams may lock to avoid a rapid change in angle of the pivoting portion. Alternatively the control system may move the pivoting portion to the horizontal or some other preferred "neutral" position, at a controlled rate, when contact with the structure is lost.

Other safety features that may be incorporated in a hydraulic system include multi-channel circuitry to achieve locking in an emergency, and the provision of a hydraulic accumulator to ensure power is available to return the pivoting portion to a neutral position in the event of an emergency such as a loss of power. Typically an emergency button will be provided to allow crewmembers to activate the system immediately to a neutral position in the event they see an emergency situation arising. Jo

The control system may also be used to control the vessels propulsion and/or steering systems to more easily keep the vessel in contact with the structure even in changing weather and sea states.

The vessel of the invention will typically incorporate other safety features that may vary depending on the intended operations. For example fencing will be provided to avoid personnel being struck by or trapped by the movement of the pivoting portion.

The vessel of the invention may be purpose built, with the pivoting deck portion as original equipment. Alternatively an existing vessel may have a deck fitted out with the pivoting deck portion. As a yet further alternative a new deck comprising a pivoting deck portion may be fitted to an existing vessel.

Thus according to a second aspect the present invention provides a deck for a vessel, said deck comprising a pivoting portion, said pivoting portion having; a pivot end connected to the deck by a pivot having its axis transverse to the bow to stern axis of a said vessel in use of the deck; and a docking end, at or projecting beyond either the bow or the stern of a said vessel in use of the deck, said docking end being distal to the pivot end, and formed and arranged f or connecting to a structure in use.

Further preferred features and advantages of the present invention will appear from the following detailed description of some embodiments illustrated with reference to the accompanying drawings in which: Figs. la, lb and Ic show schematic elevation, plan and front elevation views of a vessel of the invention; Figs 2a,2b and 2c show the vessel of figure 1 operating when docked against a wind turbine mast; Figs Sa and 3b show in schematic perspective views another vessel of the invention with an alternative deck arrangement; Fig 4 shows in schematic perspective view a yet further vessel of the invention; and Fig 5 illustrates in elevation the range of motion of a deck of the invention on an example of a vessel of the S invention.

A vessel 1 of the invention is shown in the side elevation and plan views of figure la and lb. The vessel 1 is a catamaran having two hulls 2,3 (see front elevation figure lc) and a deck 6 on which is mounted a wheelhouse 4, towards the stern. Forward of the wheelhouse the deck 6 has a pivoting portion B (shown in dashed line in figure la) that has a pivot end 10 and a docking end 12 that projects slightly forward of the bow 13. The docking end 12 includes fenders 14 that incorporate pressure sensors 16 whose operation is described hereafter. The pivoting portion B is powered and can be controlled by a hydraulic ram system (not shown for clarity) In figure la the vessel is shown on open water 24, heading towards a wind turbine mast or pile 18, which has a ladder for maintenance personnel to climb. The pile 18 and the ladder 20 are protected from collision damage by vertical fender tubes 22,23 mounted either side of the ladder (see also plan view figure 2c) . As shown in figure lb the fender 14 of the docking end 12 of the pivoting portion 8 -l6 includes cut outs 25 that conform closely to the fender tubes 22,23 of the wind turbine pile 18 in use (see figure 2c) Figure ic shows a front elevation of the vessel to more clearly show the twin hulls 2,3 of the vessel 1 and that the pivoting portion 8 of the deck 6 is sized to fit between them.

Figures 2 show the vessel 1 of figures 1 docked to a wind turbine pile 18. In the elevation of figure 2a the vessel is shown with the docking end 12 of the pivoting portion 8 contacting the fender tubes 22,23 of the turbine pile 18.

The docking end is held in contact with (connected to) the fender tubes by maintaining forwards engine power as indicated by the arrow A. The forwards engine power may be automatically controlled in response to the pressure placed on the pressure sensors 16 on the docking end 12 of the pivoting portion 8.

In the docked position the vessel I may ride up and down on the waves with the docking end 12 maintaining its position in contact with the fender tubes 22,23. As shown in figure 2a when the vessel 1 moves downwards as indicated by arrow 26 the pivoting portion 8 pivots about the pivot end 10 to keep the docking end 12 up at the chosen docking point. As shown in figure 2b when the vessel 1 moves upwards on the waves as indicated by the arrow 28 the pivoting portion 8 pivots about the pivot end 10 so that the docking end 12 remains in contact with the fender tubes 22,23 at the same point i.e. the pivoting portion 8 of the deck B is now between the hulls 2,3 of the catamaran.

The double-ended arrow 27 in figure 2b indicates the arcuate range of movement of the docking end 12. At the JO extremes of the motion the vessel 1 will be at its closest to the fender tubes 22,23. This range of movement can be limited and damped by the use of the hydraulic rams and control system described before.

Figure 2c shows the vessel docked to the wind turbine mast 18 in plan view. The pivoting portion B of the deck 6 allows ready access for personnel to the ladder 20 so that repair and maintenance operations may be carried out.

Figures 3a and 3b show in perspective view a vessel 1 of the invention where the pivoting portion 8 of the deck 6 is formed to pivot about two axes. The pivoting portion 8 can pivot about the transverse axis 30 (dashed line) at the pivot end 10 as shown in figure 3a. In addition the pivoting portion can rotate about the bow to stern axis 32 (dashed line) as shown in figure 3b. The rotation about the bow to stern axis 32 allows the vessel 1 to roll relative to the pivoting portion 8 as well as to move up and down relative to the docking end 12.

Figure 4 shows in perspective view a monohull vessel 1 including a deck 6 fitted with an alternative form of pivoting portion 8. In this example the pivoting portion 8 passes around the perimeter of the rest of the deck 6 at the bow 13 and pivots about the rest of the deck at the TO pivot ends 10 (pivot axis 30 shown in dashed line). This arrangement allows the docking end 12 to move both up and down relative to the vessel 1 as indicated by curved arrow 27, without interference from the body of the vessel 1.

Figure 5 shows in schematic elevation and as an example a catamaran vessel 1 of 24n length and having a pivoting portion 8 of 1Gm length running from a docking end 12 close to the bow 13 back to a pivot end 10 proximal the stern of the vessel. As indicated in the figure a pivoting action of only 5.36° (plus and minus) from the horizontal gives a relative vertical movement of the order of 3m at the docking end 12. Thus a vessel 1 of this form can be expected to allow relatively safe transfer to a fixed structure even where the waves may exceed the current acceptable operational limit of a significant wave height of about 0.9m.

It will be appreciated that various modifications can be made to the above described embodiments without departing from the scope of the invention. For example the docking end 12 of the pivoting portion 8 may be fitted with magnets or a clamping system to give more secure engagement to the fender poles 22,23 of the wind turbine pile 18. For example the vessel may have the docking end of the pivot portion at its stern in which case the vessel will approach the structure stern first to dock.

The invention will now be described further in the following, non-limiting numbered clauses.

1. A vessel comprising: a deck including a pivoting portion, said pivoting portion having; a pivot end connected to the deck by a pivot having its axis transverse to the bow to stern axis of the vessel; and a docking end, at or projecting beyond either the bow or the stern of the vessel, said docking end being distal to the pivot end, and formed and arranged for connecting to a structure in use of the vessel.

2. A vessel as described in clause 1 wherein the pivoting portion is formed and arranged to rotate about the bow to stern axis of the vessel as well as pivoting about an axis transverse to the bow to stern axis.

3. A vessel as described in clause 1 or clause 2 wherein the pivoting portion constitutes all or substantially all of a deck of the vessel.

4. A vessel as described in any preceding clause wherein the pivot portion has a length of more than half of the length of the vessel.

5. A vessel as described in any preceding clause wherein the docking end of the pivoting deck portion can move to below the height of the deck itself as the vessel rises and falls in use.

6. A vessel as described in clause 5 including a cavity at its bow or stern, providing a space below the pivoting portion.

7. A vessel as described in any one of clauses 1 to 5 wherein the vessel comprises two or nore hulls with a deck passing across the space in between at least two of the hulls.

8. A vessel as described in any one of clauses 1 to 6 wherein the pivoting portion is formed and arranged to pass around part of the perimeter of a fixed portion of deck and may have pivot points to either side of the fixed portion of deck.

9. A vessel as described in any one of clauses 1 to 7 in the form of a catamaran with a wheelhouse or bridge towards the rear of a main deck, a pivot forward of the wheelhouse transverse to the bow to stern axis and connecting the main deck to a pivot portion of deck, the docking end of the pivot portion of deck projecting slightly forwards of the bows and the pivot portion sized so that it can pivot downwards in between the two catamaran hulls.

10. A vessel as described in any preceding clause wherein the docking end includes a fender or fenders of suitably resilient material to absorb the shock of initial docking and any variation in pressure applied between the docking end and the structure.

11. A vessel as described in any preceding clause wherein the decking end comprises means for secure interengagement with a given structure in use of the vessel 12. A vessel as described in clause 11 wherein the means for secure interengagement includes a magnet or a clamp mechanism or mechanisms that can lock onto or round structures to give secure interengagement.

13. A vessel as described in any preceding clause wherein the pivoting portion is moveable by a system of hydraulic rams.

14. A deck for a vessel, said deck comprising a pivoting portion, said pivoting portion having; a pivot end connected to the deck by a pivot having its axis transverse to the bow to stern axis of a said vessel in use of the deck; and a docking end, at or projecting beyond either the bow or the stern of a said vessel in use of the deck, said docking end being distal to the pivot end, and formed and arranged for connecting to a structure in use.

15. A Vessel substantially as described hereinbefore and with reference to Figs 1 and 2 of the accompanying drawings.

Claims

CLAIMS1. A vessel comprising: a deck including a pivoting portion, said pivoting portion having; a pivot end connected to the rest of the deck by a pivot having its axis transverse to the bow to stern axis of the vesseU and a docking end, at or projecting beyond either the bow or the stern of the vessel, said docking end being distal to the pivot end, and formed and arranged for connecting to a structure in use of the vessel; wherein the pivoting portion is formed and arranged to rotate about the bow to stern axis of the vessel and to pivot about the axis transverse to the bow to stern axis.
2. A vessel as claimed in claim 1 wherein the pivoting portion is moveable by a system of hydraulic rams.
3. A vessel as claimed in claim 2 wherein the system of hydraulic rams is operable to limit or damp excessive or fast movement of the pivoting portion.
4. A vessel as claimed in claim 2 or claim 3 wherein the pivoting portion can be locked in place by means of a hydraulic fluid lock.
5. A vessel as claimed in any one of claims 2 to 4 wherein the system of hydraulic rams includes at least one of: multi-channel circuitry to achieve locking in an emergency; and a hydraulic accumulator to ensure power is available to return the pivoting portion to a neutral position in the event of an emergency such as a loss of power.
6. A vessel as claimed in any preceding claim including a mechanical lock for locking the pivoting portion in place.
7. A vessel as claimed in any preceding claim wherein the pivoting portion has a length of more than half of the length of the vessel.
8. A vessel as claimed in any preceding claim wherein the pivoting portion constitutes substantially all of a deck of the vessel.
9. A vessel as claimed in any preceding claim wherein the pivoting portion can pivot both downwards and upwards from the horizontal, in use.
10. A vessel as claimed in claim 9 including a cavity at its bow or stern, providing a space below the pivoting portion.
11. A vessel as claimed in any one of claims I to 10 wherein the vessel comprises two or more hulls with the deck including the pivoting portion passing across the space in between at least two of the hulls.
12. A vessel as claimed in any one of claims 1 to 11 wherein the pivoting portion is formed and arranged to pass around part of the perimeter of a fixed portion of deck and has pivot points to either side of the fixed portion of deck.
13. A vessel as claimed in any one of claims 1 to 11 in the form of a catamaran with a wheelhouse or bridge towards the rear of a main deck including the pivoting portion, wherein the pivot transverse to the bow to stern axis connects the rest of the main deck to the pivoting portion and is forward of the wheelhouse; and wherein the docking end of the pivoting portion projects slightly forwards of the bows and the pivoting portion is sized so that it can pivot downwards in between the two catamaran hulls.
14. A vessel as claimed in any preceding claim wherein the docking end includes a fender or fenders of suitably resilient material to absorb the shock of initial docking and any variation in pressure applied between the docking end and the structure.
15. A vessel as claimed in any preceding claim wherein the docking end comprises means for secure interengagement with a given structure in use of the vessel
16. A vessel as claimed in claim 15 wherein the means for secure interengagement includes a magnet or a clamp mechanism or mechanisms that can lock onto or round structures to give secure interengagement.
17. A deck for a vessel, said deck comprising a pivoting portion, said pivoting portion having; a pivot end connected to the rest of the deck by a pivot having its axis transverse to the bow to stern axis of a said vessel in use of the deck; and a docking end, at or projecting beyond either the bow or the stern of a said vessel in use of the deck! said docking end being distal to the pivot end, and formed and arranged for connecting to a structure in user wherein the pivoting portion is formed and arranged to rotate about the bow to stern axis of a said vessel and to pivot about the axis transverse to the bow to stern axis, in use.113. A method of docking a vessel to a structure, the method comprising: a) providing a vessel according to any one of claims 1 to 16; b) approaching the structure and contacting the docking end of the pivoting portion to the structure at a TO selected location; and e) holding the docking end connected to the structure by use of the vessel' s propulsion system; whereby the docking end of the pivoting portion remains connected to the structure at or close to the same point whilst the vessel moves up and down on the waves.19. A vessel substantially as described hereinbefore and with reference to Figs 3a and 3b of the accompanying drawings.