1 LOW ANGLE OF ATTACK PRAWN TRAWLING OTTER BOARD FIELD OF THE INVENTION This invention relates to a low angle of attack prawn trawling otter board. This invention has particular application to a low angle of attack prawn trawling otter 5 board for bottom trawling for prawn, and for illustrative purposes the invention will be described with reference to this application. However we envisage that this invention may find use in applications fishing off the bottom for other species. BACKGROUND OF THE INVENTION 10 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia. Traditional prawn-trawling otter boards have a low aspect ratio and are operated at 15 a high angle of attack such as 400 to ensure they remain stable during all phases of the fishing operation. A typical prawn-trawling otter board scrapes the seabed due to a ploughing process occurring along its heavy steel shoe. The material cut from the seabed surface by the edge of the otter board is transferred inward along the shoe towards the wing-end of the net. Because of the close proximity of the 20 trawl net to the otter boards in the trawling of tropical prawn species, the disturbance of the seabed causes benthic material to be shunted into the trawl net, where it accumulates in the cod end as a significant part of the bycatch. British Patent GB 1097971 25 GB 1097971 discloses an arrangement of an uncambered sole edge (shoe) within a "cambered otterboard" (ie shoe orientation relative to a cambered foil section); and achieves a degree of optimal compatibility between water reaction forces generated on the foil component of the otterboard and ground reaction forces generated on the shoe, which contribute to spreading the trawl gear. The optimum 30 compatibility is principally to improve stability. The objective was to ensure that a straight edge in contact with the seabed was established and that it was at a greater angle of incidence than the leading part of the cambered foil, by a sufficient margin, to avoid the cambered otter board's tendency to dig in and lose 2 its correct orientation. This situation occurs when trawling in high cross-current circumstances such that the effective angle of incidence between the shoe and the seabed can become negative if the shoe edge incidence is set relative low. 5 The general principle of the disclosure is to ensure that the angle of incidence of the contact shoe is always significantly positive during all feasible trawling situations and it distinguishes two typical cases, namely: 1. High aspect ratio cambered otter boards that ideally work at low angle of 10 attack (about 20 degrees); and 2. Low aspect ratio cambered otter boards that work best at high angle of attack (about 40 degrees), of which there are two implementations of the principle defined for each case. 15 The principle defect of this arrangement is relatively low efficiency (that is, high spread force relative to drag force) in circumstances where the trawl nets connected to the otter boards are spread to the typically found for tropical prawn trawling operations, where the span typically exceeds 70% of the headline length). 20 USSR Inventor's Certificate SU 743657 The disclosed otter board is designed to allow the incidence angle of the hydrodynamic-foil to be set independently to the degree that the trawl net is spread. This is achieved by bringing the connection points of the towing wire and the trawl to a selected longitudinal location along the foil. Adjustment of the angle 25 of incidence of the hydrodynamic-foil on the disclosed otter board is by moving the tow-wire attachment point for/aft around a circular disk, while the trawl connection point is fixed via a otter board body to an effective line relative to the foil, which is a pivot around which they are free to rotate relative to each other in the horizontal plane. 30 The otter board body will align with the wing end angle of the trawl, which for a fish trawl will typically be an incidence angle between 10 and 15 degrees, while for a tropical prawn trawl will be an incidence angle of between 25 and 35 degrees.
3 Therefore, for the disclosed otter board the shoe of the otter board body will significantly scrape the seabed, causing unnecessary drag and long-lasting seabed impacts. The incidence angle of the shoe of the disclosed otter board is not zero, therefore it will create significant ground reaction drag and long-lasting 5 impacts to the seabed. SUMMARY OF THE INVENTION In one aspect the invention resides broadly in a low angle of attack prawn trawling otter board including: 10 an otter board body having a heavy seabed contact shoe; an otter board pole extending upward of said otter board body and providing a common longitudinal position on said otter board body for a towing wire attachment at an upper location, and upper and lower net connections comprising a strain equalizing bridle passing through rope slides and having tail 15 portions connecting to the respective headline and foot line of the trawl; a hydrodynamic wing supported on the otter board body and comprising a sail on a freestanding mast and operated at an angle of attack of about 200, a skirt along the bottom of the wing whereby the wing floats on a cushion of high pressure water during operation, and providing an aspect ratio of about 1, serving 20 to spread the trawl net and limit friction with and ploughing of the seabed by the seabed contact shoe. The upper and lower net connections may comprise respective turning elements articulated to the pole adjacent the trawl attachment point and adjacent the lower 25 end of the pole respectively. The strain equalizing bridle may pass through upper and lower turning elements comprising the rope slides and having an upper tail portion and two lower tail portions, the tail portions having terminals for attachment to respective upper and lower frame lines of a trawl net.
4 The spar may comprise a luff spar whereby a high aspect flexible foil may have a luff supported on the luff spar and a foot tensioned along the boom. The spar may be raked aft to locate the centre of effort. The foil may have a head and leech supported by a batten tensioned in to the luff adjacent the step of said spar. 5 The hydrodynamic assembly may be movable between a first stowing position substantially closed against the board body and the open operable position maintained hydrodynamically in use and limited by stop means comprising a chain, cable or folding strut. 10 The board setup may be selected to provide that the shoe in use has an angle of attack on the seabed tending to spread the net of about 00. The otter board assembly is preferably high aspect. Accordingly the aspect ratio 15 defined by the length of the body and the height from the contact shoe to the top of the foil is preferably about 1. The otter board body may apply the working vertical contact pressure on the seabed, through the contact shoe, of similar magnitude to existing otter boards, 20 and preferably has a zero angle of attack so that no ground reaction force of a type that help spread the gear is produced, nor any associated drag force. Zero degree orientation of the main contact shoe also produces minimum physical impact on the seabed. 25 The orientation of the otter board body, and its associated seabed contact shoe is fixed relative to the foil during operation (although it is collapsible for stowage purposes). The relative angle set between the foil and the otter board body for the present otter board is the desired angle of attack of the foil as it is a requirement of the present otter board that through adjustment of the for/aft position of the foil 30 relative to the otter board body, the incidence of the otter board body relative to the tow direction can be set to zero degrees.
5 The second non-cambered contact shoe exists on the base of the high-aspect ratio cambered foil of the present otter board and has a relative orientation to the foil similar to that specified in GB1097971 for the low-aspect ratio otterboard. Additionally this second shoe does not incorporate a sole "edge" that cuts into the 5 seabeds surface, but rather incorporates a flexible flap that slides over the seabed surface. Correctly adjusted, the second non-cambered shoe of the present otter board applies low vertical pressure to the seabed and due to the water flow pattern around the cambered foil the underlying flap is separated from the seabed by a layer of fluid moving under the foil from the high pressure side to the low pressure 10 side. Through this mechanism, the foil rides on a layer of fluid between itself and the seabed in much the same way as a hovercraft as it moves across a terrestrial or water surface. By applying this physical phenomenon to the otterboard foil, the drag force on the second contact shoe is very low and there is minimal physical disturbance to the seabed during operation. 15 By minimising the intensity of seabed interaction, the present otter board also achieves good stability on the seabed as it is not susceptible to variations in seabed incidence angle. The present otter board has two non-cambered shoes that provides a wide stable base during operation of the seabed. The 20 orientation of the uncambered shoes are for one aligned with the chord of the cambered foil and for the other at zero incidence. For the present otter board, the trawl is connected to the otter board body at the same longitudinal location as the tow-wire and the foil is fixed relative to the otter 25 board body by inter-connections at the fore and aft ends of the foil and otter board body. As stated above, there is a set divergence between the foil and the otter board body that is equivalent to the desired angles of attack of the foil, and the foil can be shifted fore/aft relative to the otter board body to achieve adjustment of the orientation of the foil/sled-frame arrangement, with respect to incidence to the tow 30 direction. For the invented otter board, the heavy contact shoe is aligned with the direction of tow. This removes the scraping action of the board and any serious disturbance to 6 the seafloor. The invented otter board is also short for its size (higher aspect ratio) and operates at a low angle of attack. These features substantially reduce the "footprint" of the Batwing board on the seabed and its towing resistance. The invented board has a different rigging arrangement that maintains stability during 5 all phases of the fishing operation, even though the angle of attack is reduced to 20 degrees. The towing wire (bridle) and the net are connected to the board at the same longitudinal location. This responds to the idea that the difficulties in achieving high 10 fuel-efficiency along with user-friendly operation for traditional otter boards is due to the longitudinal separation of the towing (bridle) point and net attachment points, since this causes the angle of attack to vary depending on operational circumstances. Bringing these points to a common longitudinal position produces an otter board arrangement that is much improved because the angle of attack is 15 fixed at a certain set value. The otter board may also incorporate a strain-equalising system between the headline and the footline of the trawl. This may comprise the use of turning elements each comprising a tubular guide shackled or laced to the pole in lieu of a 20 conventional block shackled or laced to the pole. The turning elements form a rope slide at each end of the otter board pole and through these pass a bridle comprising length of rope such as 10mm DYNEEMA that is connected to the framelines of the trawl. This arrangement for connecting the trawl to the otter board always ensures that the tension in the headline is the same as the tension in 25 the footline. This situation is necessary for correct operation of the otter board during shooting away. The hydrodynamic wing component of the otter board is constructed like a sail on a freestanding mast. Since it is operated at a low angle of attack (about 20 30 degrees) it has an aspect ratio of 1 to ensure that it produces a large spreading force at that angle. Incorporated along the bottom of the wing is a skirt that functions in the same way as a skirt on a hovercraft. This produces the situation where the wing floats on a cushion of high-pressure water during operation. This 7 removes direct physical contact between the bottom of the wing and the seabed and any associated friction/ploughing that is usually the case with traditional otter boards. 5 BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described with reference to the following non-limiting embodiment of the invention as illustrated in the drawings and wherein: Fig. 1 is a perspective view of apparatus in accordance with the present 10 invention. In the figures there is provided a low angle of attack trawling otter board including an otter board body 10 having an anti-plough leading portion 11 and a shoe portion 12 and a longitudinal stiffener 13. An attachment pole 14 is mounted 15 vertically on the otter board body 10 and is braced against towing loads by forward brace 15. The attachment pole 14 is braced as a structure with the otter board body 10 by aft brace 16, aft buttress 17 and diagonal brace 20. At the top of the pole 14 is the otter board towing eye 18. 20 A headline swivel-mount 21 is mounted on the upper portion of the attachment pole 14 and has multiple attachment holes to allow selective attachment by shackle of an upper guide 22. A lower guide 23 is shackled to the otter board body 10 directly below the upper guide 22. The guides 22, 23 carry and distribute loads in a rope link 24 including a headline tail 25 having an upper terminal 26 and 25 footline and ground chain tails 27 having respective footline and ground chain terminals 30. A boom pivot bracket 31 is braced by a boom pivot bracket support strut 32 and forms a boom pivot 33 for a boom 34. The included angle between the otter board 30 body 10 and the boom 34 is selected and set by a limit chain 35, in this case top 200. The boom 34 has mounted on its forward end a spar 36 supporting a foil 37 by a luff pocket 41. The foot of the foil extends along the boom 34 from the luff pocket toward the boom pivot 33 and the foot is tensioned by foil-tensioner 42.
8 A boom skirt 43 is deployed beneath the boom 34 and secured thereto by a clamp strip 44. 5 The attachment pole 14 has an alternative Sandakan trawl warp swivel-mount 45. A foil batten 46 supports the head and leech of the foil 34. The open foot of the sail is lapped by a foot seal 47 and the top of the spar 36 has an end cap 50. 10 Apparatus in accordance with the foregoing embodiment recognises that ground reaction forces are very inefficient (involves a large amount of friction) compared to water reaction forces, and also create substantial long-lasting physical impacts on the seabed that are not acceptable to the overarching sustainability objective, which is increasingly imposed on commercial fishing by management agencies. 15 The main instability issue solved by the above embodiment was to maintain stable and effective operation while the gear is being deployed rather than while the gear is on the seabed. To ensure stability on the seabed in circumstances that are the subject of GB1097971, the above embodiment applies the principle of minimising seabed interaction as much as possible so that the performance of the otter board 20 is little affected by possible variations, rather than accentuating interaction with the seabed so that it plays a dominant role in controlling orientation. Apparatus in accordance with the above embodiment removes the intense disturbance of the otter board on the sea floor (by at least 90%). It substantially 25 reduces the damage to benthic ecosystems during the operation of prawn trawling gear. It reduces the bycatch of benthic organisms in the cod end of the trawl, which reduces trawl gear wear, processing effort when the catch is landed on the vessel and damage to the target catch. The drag of the otter board is reduced by 70%. The fuel consumption while trawling is reduced by about 20%. The energy 30 intensity of prawn trawling and the carbon footprint of associated seafood products are significantly reduced.
9 It will of course be realised that while the above has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is set forth in the claims 5 appended hereto.