LAUNCH AND RECOVERY OF UNDERWATER UNITS OR VEHICLES

Abstract

A launch and recovery apparatus is for a surface vessel, a surface vessel is for use on water, and related methods are for launching or recovering an underwater unit. The unmanned surface vessel may have a ramp member for providing a ramp structure under the water for facilitating launch or recovery of the underwater unit, the ramp structure having an incline for the underwater unit and an aperture or gap penetrating through the ramp structure for communicating a flow of water in the aperture or gap relative to the vessel. The surface vessel is advanced across the water, obtaining a flow of water relative to the vessel in the aperture or gap through the underwater ramp structure, the underwater unit is supported on the incline of the ramp structure, and the underwater unit is moved up or down the incline to facilitate the launch or recovery of the underwater unit.

Claims

1-27. (canceled)

28. An unmanned surface vessel for use on water, the unmanned surface vessel having a hull and at least one ramp member for providing a ramp structure under the water for facilitating launch or recovery of an underwater unit, the ramp structure comprising an incline for moving the underwater unit up or down the incline, and at least one aperture or gap penetrating through the ramp structure for communicating a flow of water in the aperture or gap relative to the unmanned surface vessel, the ramp member being movable with respect to the hull to obtain an operational position in which the ramp member is arranged to provide the ramp structure in use, the unmanned surface vessel further comprising at least one actuator between the hull and the ramp member for moving the ramp member into the operational position with respect to the hull.

29. The unmanned surface vessel as claimed in claim 28, wherein the ramp member is movably coupled to the hull so as to be hingeable about a pivot on the hull.

30. The unmanned surface vessel as claimed in claim 28, wherein vessel has a hull and the ramp member is arranged to protrude outboard from the hull.

31. The unmanned surface vessel as claimed in claim 28, wherein the unmanned surface vessel has a hull and the ramp member is arranged to have an extent underwater in use to provide the ramp structure, the ramp member extending to a depth underwater that is lower than an underside of the hull.

32. The unmanned surface vessel as claimed in claim 28, further comprising at least one sloped deck section, wherein the ramp member together with the sloped deck section provides a slipway for the underwater unit.

33. The unmanned surface vessel as claimed in claim 28, further comprising launch and recovery apparatus comprising any one or more of: the ramp member; a winch; a tether extending between the winch and the underwater unit; and a sheave through which the tether is passed between the winch and the underwater unit, the sheave arranged to be moved forward or rearward along the unmanned surface vessel for positioning the sheave relative to the ramp member and/or the underwater unit.

34. The unmanned surface vessel as claimed in claim 28, wherein the underwater unit is an underwater vehicle.

35. The unmanned surface vessel as claimed in claim 34, wherein the underwater vehicle is either a remotely operated underwater vehicle, ROV, or an autonomous underwater vehicle, AUV.

36. A method of launching or recovering an underwater unit from an unmanned surface vessel, the method comprising the steps of: providing an unmanned surface vessel on water, whereby a ramp structure is provided under the water; advancing the unmanned surface vessel across the water, obtaining a flow of water relative to the unmanned surface vessel in an aperture or gap through the ramp structure; and supporting the underwater unit on an incline of the ramp structure, moving the underwater unit up or down the incline to facilitate in the launch or recovery of the underwater unit.

37. The method as claimed in claim 36, which further comprises providing launch and recovery apparatus which includes: at least one winch provided with at least one tether extending between the winch and the underwater unit; at least one sheave through which the tether passes and which is movably positioned rearward or forward along the unmanned surface vessel relative to the ramp structure; at least one actuator to lower the ramp structure to incline and submerge the ramp structure in the water; operating the launch and recovery apparatus to determine control tension of the tether.

38. The method as claimed in claim 37, wherein said operating step includes positioning the sheave to determine an angle of attack between the underwater unit and the sheave and positioning the ramp structure using the actuator to determine an angle of incline of the ramp structure.

39. The method as claimed in claim 36, which further comprises pulling the underwater unit up the incline of the ramp structure to bring the underwater unit onboard to recover the underwater unit or letting the underwater unit travel down the incline of the ramp structure to launch the underwater unit into the water.

40. A ramp member for an unmanned surface vessel for use on water, the unmanned surface vessel having a hull and at least one ramp member for providing a ramp structure under the water for facilitating launch or recovery of an underwater unit, the ramp structure comprising an incline for moving the underwater unit up or down the incline, and at least one aperture or gap penetrating through the ramp structure for communicating a flow of water in the aperture or gap relative to the unmanned surface vessel, the ramp member being movable with respect to the hull to obtain an operational position in which the ramp member is arranged to provide the ramp structure in use, the unmanned surface vessel further comprising at least one actuator between the hull and the ramp member for moving the ramp member into the operational position with respect to the hull.

41. The ramp member as claimed in claim 40, comprising a structure that is rigid, the structure comprising at least one aperture or gap therethrough.

42. The ramp member as claimed in any of claim 41, wherein the structure comprises a frame comprising at least one pair of parallel bars or parallel bar sections extending in a first direction and having at least one aperture between one parallel bar of the pair of parallel bars and another parallel bar of the pair of parallel bars.

43. The ramp member as claimed in claim 42, wherein the structure comprises a frame comprising at least one pair of parallel bars extending in a second direction, transverse to the first direction, and having at least one aperture between one parallel bar of the pair of parallel bars and another parallel bar of the pair of parallel bars.

44. The ramp member as claimed in claim 43, wherein the structure comprises a panel, and apertures comprising holes through the panel.

45. A launch and recovery apparatus for a surface vessel on water, the apparatus comprising at least one ramp member for providing a ramp structure under the water for facilitating launch or recovery of an underwater unit from the surface vessel, the ramp structure comprising an incline for the underwater unit and at least one aperture or gap penetrating through the ramp structure for communicating a flow of water in the aperture or gap relative to the unmanned surface vessel.

Description

DRAWINGS AND DESCRIPTION

[0042] There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

[0043] FIG. 1 is a representation of an unmanned surface vessel (USV) during recovery of a remotely operated underwater vehicle (ROV); and

[0044] FIGS. 2A to 2D are perspective representations of different ramp variants for the USV of FIG. 1 for use in recovering or launching the ROV.

[0045] Referring firstly to FIG. 1, there is generally depicted an unmanned surface vessel (USV) 1 at the sea surface 2. The unmanned surface vessel 1 is arranged to launch and recover an underwater vehicle, which in this example is a remotely operated underwater vehicle (ROV) 18. The launch and recovery of the ROV 18 is carried out under remote control, e.g. from a control room on land, or under automated control, e.g. preprogrammed.

[0046] In FIG. 1, the ROV 18 is located below the sea surface 2, in the sea 3. The vessel 1 has a hinged ramp member 10 which is submerged in the sea providing a ramp structure having an incline for the ROV. The ROV 18 is tethered and coupled to the USV by a tether 14.

[0047] To recover the ROV 18, the tether 14 is used to pull the ROV 18 up the incline of the submerged ramp member 10 and bring it onboard the surface vessel 1. Conversely, to launch the ROV 18, the ROV is to travel down the incline of the ramp member 10 and into the sea 3.

[0048] The recovery and launch of the ROV 18 is performed during movement of the surface vessel 1 slowly forward, i.e. in the direction as indicated by arrow F. One or more propulsion devices (not shown) of the vessel 1 are operated to produce the appropriate forward motion.

[0049] The ramp member 10 has one or more apertures 17 for letting through seawater, in the flow direction indicated by arrows B, as the surface vessel 1 travels forward through the sea. Examples of the ramp member 10 are described in further detail below. The apertures 17 in the ramp member 10 can facilitate the stability of the ramp structure in rough sea, which in turn may allow vessel speeds allowing tension on the tether to be controlled, e.g. with variations within acceptable limits.

[0050] The water passing rearward along the vessel as indicated by arrows B also act through the apertures 17 against an underside of the ROV 18 when the ROV 18 is supported on the ramp member 10, and at sufficient speeds, can produce a vertical component of force against the ROV which may impart lift to the ROV 18. This may reduce the load or tension, or variations thereof, to which the tether 14 is subjected during the recovery or launch. This in turn can help to control the tension of the tether 14, in particular in rough sea states.

[0051] The ramp member 10 is in this example hinge-wise coupled to a rear end part of a hull 4 of the vessel via a pivot 19 on the hull 4. The ramp member 10 is rotatable about the pivot 19 from an upright position to a lowered, operational position in which the ramp member 10 is located, in this case fully submerged as shown in FIG. 1, in the sea 3 for facilitating the launch or recovery of the ROV. The ramp member 10 can be lowered when the vessel 1 is stationary or moving.

[0052] The ramp member 10 is moved to and arranged in the upright, stowed position when the ROV 18 is onboard and stored in a storage region 9 of the vessel 1. The vessel 1 with the ROV onboard may then transit to another site with the ramp member 10 in the upright position. This can be convenient for removing the ramp from the water so that it does not create drag or interfere below the hull when not required, e.g. during transit. The ramp member 10 in the upright position provides a rear fence for the ROV which may act a safety barrier.

[0053] Further, the ramp member 10 can also be arranged in upright, stowed position whilst the ROV is deployed in the water and performing a work operation underwater. In the course of an ROV work operation and whilst the ROV is deployed, the vessel 1 can then potentially move to other locations on the surface of the sea 3, e.g. according to a predetermined schedule or plan with the ramp raised in the stowed position.

[0054] At least one linear actuator 20, e.g. hydraulic or electrically operable, is arranged and operable to vary in length between a rear end part of the hull 4 and the ramp member 10 for lowering or raising the ramp member 10. Thus, by operating the actuators 20 the ramp 10 can be lowered. In addition, the amount of incline of the ramp member 10 in the water can be varied, by operation of the actuator 20. Accordingly, the angle of incline A of the ramp with respect to the water line 2 can be varied, and the ramp member 10 may be positioned with a predefined and/or desired incline angle A, for example dependent upon parameters such as sea conditions and/or vessel speed and/or tension and/or weight of ROV etc. Setting the incline angle A of the ramp member 10 appropriately may facilitate obtaining the proper conditions and/or control of tension and lift upon the ROV 18 (by water flowing through the one or more apertures 17) during the launch or recovery.

[0055] Apparatus 30 of the vessel 1 for launching and recovering the ROV 18, in addition to the ramp member 10, includes means for paying out the tether 14 from the surface vessel 1 for deployment of the ROV 18 and pulling in the tether 14 for recovery of the ROV 18. To this end, the launch and recovery apparatus 30 includes a winch 22 including a storage reel 23 for storing the tether 14, and a guide member in the form of a sheave 24. In FIG. 1, a length of the tether 14 is payed out from the reel 23. The tether 14 extends from the reel and passes via the sheave 24 to the underwater ROV 18.

[0056] The sheave 24 is mounted on frame 25 which in turn is coupled to a guide rail 26 which extends along the vessel 1. The frame 25 is operable to travel horizontally in forward or rearward direction on the rail 26. In this way, the sheave 24 can be positioned in suitable positions along the vessel for supporting the tether 14.

[0057] The tether 14 passes over a point on the sheave 24 higher than the departure point from the winch 23 and greater than the height of the ROV when onboard the vessel 1 in the storage region. In this case, the frame 25 together with the sheave 24 is moved rearward and protrudes over the sea surface 2 from a rear end of the hull. Furthermore, in the position indicated in FIG. 1, the sheave 24 is arranged, in air, vertically above the submerged and inclined ramp member 10 in the sea 3. By moving the frame 25 forward or backward along the rail 26, the pull angle or angle of attack of the tether 14 upon the ROV can be adjusted. This adjustment of the position of the sheave 24 can help adjust the pull-angle / angle of attack of the umbilical upon the ROV 18, while winch is the main component in addition to the forward movement of the vessel in maintaining tension on the tether. The position of the sheave 24 may be adjusted to take account of and/or in response to heave, roll, or pitch motions of the vessel, also during the movement of the vessel forward during deployment and recovery.

[0058] The tether 14 departs from the sheave 24 to the ROV 18 with suitable angle of attack T relative to the water line for facilitating to recover the ROV. The tether 14 is arranged so as to impart and/or maintain a lifting force component upward on the ROV. The position of the sheave 24 and consequently the angle of attack T may be selected depending also upon the angle of the ramp member 10.

[0059] In the example of FIG. 1, two onboard slope sections 11, 12, are provided for further facilitating the recovery and launch of the ROV 18. The ramp member 10 is hinged at the pivot 19 and lowered relative to the slope section 11, 12 into the position of FIG. 1. The ramp member 10 is inclined, so as to have the same angle of inclination A as a surface of the first slope deck section, although this may not necessarily need to be the case. A surface of the second onboard slope section 12 has shallower angle of incline than the first slope section. The ROV 18 is supported upon the second slope section and is held at rest in the storage region 9 when it is recovered or otherwise stored onboard the surface vessel 1. The tether 14 is wound onto the reel 23 and through its connection to the ROV 18 holds the ROV 18 in position on the on the second slope section 12, e.g. in the storage position, and may hinder or restrict slippage of the ROV 18 back down the slope. It can be appreciated that the ramp member 10 together with the first and second onboard slope sections 11, 12 provide a slipway for launching and submerging the ROV 18 in the water. Indeed, therefore in the case of launching, the releasing the tension on the tether 14 allows the ROV 18 to slip down one or more of the onboard sloped sections and ramp, under the force of gravity.

[0060] In the example of FIG. 1, the vessel 1 includes positioner means 40 for positioning the ROV when onboard the vessel. The positioner means 40 can operate to controllably establish and/or maintain tension on the tether 14 for facilitating launch of the ROV 18. To this end, the positioner means 40 has a mechanism onboard the vessel 1 for urging the ROV 18 out of the storage position. More specifically, the positioner means 40 in this example has a belt 42 provided with a “kicker” 43 in the form of a centred “knee plate” that is moved on the belt 42 down the inboard ramp 12 on the aft of the vessel 1. The belt 42 is rotatable and arranged to be driven (so as to rotate) by a motor 41, e.g. a hydraulic or electric motor. The belt 42 extends longitudinally along the vessel downslope, e.g. following the incline of the inboard sloped section 12. When being driven, the belt 42 section to engage the ROV 18 moves in the longitudinal direction toward the aft end 1b of the vessel. The kicker 43 is configured to engage the ROV so that upon rotation of the belt 42 the kicker 43 is urged against and exerts a force against the ROV 18 toward the rear end 1b of the vessel. Since the ROV 18 is coupled to the tether 14 which in turn is coupled to the winch 23, the tether / umbilical cable 14 is tensioned and/or slack is taken up as the ROV is moved and/or urged rearward with assistance of the belt and kicker. This can facilitate trying to ensure that the correct tension is at all times maintained during the launch and/or recovery of the ROV and/or that snap loading does not occur, noting that loads and conditions may vary as the ROV 18 is move from stationary and then over ramp sections with different angles of incline.

[0061] Alternatively, the same or similar functionality could be achieved by the positioner means instead comprising an arm or other member on the frame 25, e.g. gantry, for pushing and/or urging the ROV away from the storage position 9 by running the gantry with sheave 24 toward the aft end 1b of the vessel 1 during launch of the ROV. Alternatively, a separate pusher e.g. arm, that is operated by an actuator to produce movement that imparts a force against the ROV may be provided to push the ROV 18 out of the storage position, e.g. without or in addition to utilising the movement rearward of the gantry assembly as a whole along the rail.

[0062] By use of such positioner means 40, friction of the ROV 18 against the sloped deck section 12 in the storage position may be overcome and initiation of movement of the ROV 18 out of the storage region 9 and down the slipway may be facilitated. The sheave 24 and frame 25 are simultaneously moved toward the rear 1b during launch, and may take at least some of the load/weight of the ROV 18.

[0063] The tether 14 in this example is an umbilical cable providing the ROV 18 with data, power, and/or fluid communication services. Instructions for operating the ROV 18 are communicated through the umbilical cable. The umbilical cable is permanently connected to the ROV throughout the deployment and use of the ROV 18 during underwater ROV operations. As can be appreciated, the ROV 18 is thus recovered by means of the umbilical cable, and the umbilical cable 14 may thus be reinforced structurally for allowing winding in and paying out for launch and recovery purposes, as well as for the umbilical communications purposes. In other variants however, the tether 14 for launch and recovery does not provide such communications services. A separate or additional umbilical cable, e.g., power, data, and /or fluids can then be provided in some variants for delivery of communications.

[0064] In another variant, a tether management system (TMS) may be connected to the ROV 18. The TMS would then be locked onto the ROV 18 during launch & recovery and unlocked at depth. The TMS includes an internal spool arranged to spool out neutrally buoyant cable.

[0065] The onboard sloped sections 11, 12 are in this example fixed position sections which do not hinge. In other variants however, one or more onboard sloped sections 11, 12 may comprise a hinged ramp or ramp section that can be lowered or raised e.g. to suitable angles, e.g. using actuators or the like.

[0066] It can be noted that the two onboard sloped sections 11, 12 in this example may be combined as one. Furthermore, the one or more onboard sloped sections 11, 12 can provide an onboard ramp or slipway that is curved in the longitudinal direction providing smooth or gradual transitions where the slope of the ramp or slipway changes gradually with length, e.g. convex ramp with monotonically shallowing angle of inclination toward the storage region 9. Alternatively, the onboard ramp can be planar with constant slope.

[0067] In other variants, the vessel 1 may include one or more hinged ramp members. Instead of a hinged ramp 10, the ramp member 10 may be provided and lowered into the sea in another way for example extended or from a slot in the vessel or by other manner.

[0068] As can be seen, the hinged ramp 10 protrudes rearward into the sea outboard from a rear end part 4e of the hull 4 of the vessel 1.

[0069] The ramp member 10 is operated by the actuators 20, which in turn may be operated through vessel control system 35. The positioning of the frame 25 and sheave 24 may also be operated by commands communicated from the control system. Data comprising e.g. commands or instructions, are transmitted to operate a motor or actuator for driving the movement of the frame 25 along the rail 26. The vessel control system 35 may be operated by remote control from shore, locally onboard vessel or according to pre-programmed instructions.

[0070] In use, the surface vessel 1 transits to a site with the ROV 18 onboard. To launch the ROV, the ramp member 10 is lowered into the sea 3 to an operational position in which the ramp member 10 is inclined and submerged in the sea 3. The vessel 1 moves steadily forward through the water. The ROV is urged down the slipway including the onboard slope sections 11, 12 and the submerged ramp member 10. Water passing rearward along the hull passes through the apertures and through the apertures also imparts a force against the ROV facilitating its launch. The tether 14 is paid out and the ROV is supported on the winch via the sheave 24 while simultaneously the ROV is supported on the ramp member 10. Once underwater the ROV is used for operations. To recover the ROV, the tether is pulled in bringing the ROV up the ramp member 10, while the vessel 1 is moving forward, and onboard to the storage region 9 in the surface vessel 1.

[0071] Turning to FIGS. 2A to 2D, various examples of the ramp member 10 are depicted. In FIG. 2A, the ramp member 10 has two side rails 104a, 104b extending in the direction of incline, and several transverse bars 105 extending in a direction transverse to the direction of incline between the side rail 104a on one side and the side rail 104b on the other, opposite side of the ramp 10. The transverse bars 105 are spaced apart, in this example, at regular intervals along the side rails 104a, 104b to define apertures 17 through the ramp 10 between respective adjacent pairs of transverse bars. The ramp member 10 generally comprises a rectangular frame 102 which is elongate in the ramp incline direction. The side rails 104a, 104b thus extend in parallel in the longitudinal direction.

[0072] In FIG. 2B, the ramp member 10 is provided with longitudinal bars 106 instead of the transverse bars 105. The longitudinal bars 106 extend in the direction of incline between a first end bar 104e and a second end bar 104f of outer frame 104. The first and second end bars 104e, 104f both extend in a direction transverse to the direction of incline. The longitudinal bars 106 in this example extend in parallel and are spaced apart from one another at regular distances in the transverse direction. In this way, apertures 17 through the ramp member 10 are defined between adjacent pairs of the longitudinally running bars 106. The end bar 104f is supported on a support and arranged to pivot to provide the hinged coupling and movement, e.g. lowering or raising, of the ramp member 10.

[0073] In FIG. 2C, the ramp member 10 is provided with a lattice of bar sections running longitudinally and bar sections running longitudinally. Individual apertures are defined respectively between two adjacent transverse bar sections and two adjacent longitudinal bar sections.

[0074] In FIGS. 2A to 2C, the apertures are rectangular, although in other examples have other forms.

[0075] In FIG. 2D, the ramp member 10 is provided in the form of a panel 108 within an outer frame 104. Apertures 17 through the ramp member 10 are in the form of circular holes 17 that penetrate through the panel 108.

[0076] The ramp member 10 in the various examples described herein is preferably a stiff rigid structure, which facilitates deployment and use on an unmanned surface vessel. The ramp member 10 in these examples comprises typically metal, e.g. aluminium, steel or the like, e.g. stainless steel. In principle however, any suitable material may be used, selected from any of plastics material, synthetic materials, composite materials, e.g. high-performance composite materials, e.g. carbon fibre composites, etc.

[0077] Furthermore, the ramp members 10 as described are merely examples and provide constant or uniform slope in the water for the ROV. Alternatives include one or more ramp members configured to provide a curved ramp structure which extends on a curved trajectory in a curvature direction and between first and second sides in a direction transverse to the curvature direction. In such an alternative, the ramp member is arranged so as to be supported in the water so that the ROV during launch or recovery is inclined on one part of the ramp structure more steeply than on another part. The angle of the incline of the ramp structure thus increases or reduces, e.g. monotonically increasing or decreasing, in the curvature direction.

[0078] As can be appreciated, the different arrangements of the ramp member 10 in the examples of FIGS. 2A to 2D can give rise to different drag and/or lift characteristics in the water. Configurations using bars, e.g. smooth cylindrical rods, may produce less drag due to hydrodynamic shape. Closer spacing of bars such as in FIGS. 2C or 2D, as compared to FIG. 2B, can increase the surface area into total exposed to the water, which may increase drag. The panel of FIG. 2D has a planar surface portions which is acted upon by the sea upon forward motion of the vessel 1 to use the sea motion to facilitate support of the ROV 18 and channeling of fluid through the holes to produce low friction slip surface on the top of the panel surface for the ROV 18. Larger apertures such as in FIGS. 2A or 2B can facilitate direct lift forces against the ROV structure by fluid flow B through the aperture 17, yet can provide a low resistance structure in the water that can be relatively unaffected by high sea states.

[0079] As can be noted in the above, the underwater vehicle is released down the incline of the ramp structure while the vessel is in forward motion through water (or stationary if weather permits). This can be advantageous, as the underwater vehicle is gradually submerged in water the tension on the umbilical to the underwater vehicle remains high due to drag through water. This can prevent unwanted motion in the underwater vehicle that could destroy the umbilical, vehicle, vessel or other equipment. CT mode (Constant Tension functionality) on the umbilical winch can help to further avoid loss of tension on umbilical between USV and ROV. Loss of tension, also called “snap loads” are undesirable events that may impart damage to umbilical, ROV, LARS, sheave wheels, winch system or other connection points of umbilical. The overhead sheave wheel system is constructed to be able to move aft/forward as required for adjusting the lift point and umbilical angle of attack. This can allow for finding the optimal pull and lift forces on the ROV during launch and recovery, resulting in minimal impact loads seen between ROV and USV ramp.

[0080] Although an unmanned surface vessel 1 is described above, the ramp member 10 could also be employed in the same or similar manner on other types of vessel for launching or recovering an underwater vehicle. Typically, it may be used on vessels both manned or unmanned of up to 50 m in length.

[0081] In other examples, other underwater vehicles may be deployed, e.g. an autonomous underwater vehicle (AUV) may be deployed using the ramp member 10. In such an example, the AUV may be coupled to a launch or recovery tether for purposes of launch and recovery via the ramp 10, but where otherwise during work operations the tether is detached and operable under own motive power and instructions with no cable coupled to it.

[0082] Furthermore, the ramp member 10 may alternatively be used for deployment and recovery of a multitude of different tethered underwater units such as instruments, equipment, or machines instead of or in addition to the underwater vehicles such as ROVs as described herein.

[0083] The various examples can be advantageous in recovering highly sophisticated and complex underwater vehicles to a surface vessel in a simplified and robust method compared with prior art solutions. Forward motion through water can advantageously be used to act as a damping force on the underwater vehicle when being pulled up/into the ramp or pushed down/out the ramp, preventing also the vehicle from damage and snap load scenarios during launch and recovery.

[0084] By way of the present solution, the relative movement between the underwater vehicle and the surface vessel may be minimized without having to use any active dampening devices to allow for use of a small surface vessel (<50 m) as mothership for an underwater vehicle.

[0085] Furthermore, the ramp member may be deployed and/or lowered to the operational position in other ways, e.g. by sliding down a track from the hull or extracting from a slot in the hull or any other suitable manner.

[0086] Various modifications and improvements may be made without departing from the scope of the invention herein described.