GB2147182A - Improvements in or relating to trawl doors - Google Patents

Abstract

A trawl door comprises a door body having two main body portions (31,32) each of which has a camber in at least one plane on at least one surface thereof, the two body portions (31,32) being inclined to one another across a line of separation (33) between the said two portions (31,32), means (34,35) for attaching a warp or towing line (20) to one face of the door and means (42) for attaching a bridle line (16) to the other face of the door. <IMAGE>

Description

SPECIFICATION Improvements in or relating to trawl doors The present invention relates to trawl doors. The term "door" is applied, as far as trawling is concerned, to components oftrawl gear which are attached in the drawing lines between a towing vessel and a trawl net, and which serve, by being deflected by the flow of water there past, to hold the trawl mouth open.

Trawling, as a method offishing, has been known foraverylong time and although very earlytrawls employed a wooden beam mounted at each end on steel shoes to hold the net open, modern techniques utilisethe so called trawl doors to hold the nets open by interacting with the water current generated by movement ofthe towing vessel through the water.

Early trawl doors were simply flat structures made from wooden planks and having a keel or shoe along one edge which, in use ofthe door, ran along the sea bed as a result of which it was subject two substantial wear fro sand, gravel and stones. The points of attachment between the trawl door and the vessel, by means of which the door is towed, usually comprised triangular attachment brackets allowing the doorto pivot on its attachment with the warp or towing line.

The lines betweenthetrawl door and the trawl net itself, known as bridles, were usually attached at the rear edge ofthe door by one or more bridle attachement points.

Although effective in drawing the net open, flat trawl doors have a number of disadvantages, one of which is the fact that it has to be inclined to the direction of movement in order to generate the lateral tension on the bridles, and this inclination increases the resistance to movement exerted by the door on the warp.In orderto increasethe lateral displacement ofthe bridles these are sometimes fitted to the "rear" face of the door, that is,theface opposite that which is directed more or less forwardly in use, and this has the effect of increasing the angle of attack ofthe door namelytheangle between the plane of the door and the direction of approach ofthe water current. At an effective angle of attack for transverse displacement, however, the rearface experiences substantial turbulence as the water displaced by the front face flows around the edges and creates a low pressure area behind the door, and this substantially increasesthe drag so thatthefuel consumption required whilst towing the net is substantial.It has been established that the trawl doors can account for as much as 40 per cent ofthe drag exerted by the gear combination on the towing vessel and this obviously increases the strain on the warps as well as the fuel consumption during trawling.

Another disadvantage of flat trawl doors is that great care has to be taken in ensuring thatthey are set up in a substantially vertical plane when "shooting" the net. Although the majority oftrawlers are adapted for stern shooting, a proportion shootthe trawl gear overthe side and considerable precautions have to be taken in orderto ensure that the trawl doors experi encetheappropriate conditions to ensure that they "swim" away from one another and open the net whilst retaining their substantially vertical orientation so thatthe forces exerted by the boards are generally horizontal.Any vertical component in the force which is exerted bythe boards due to their motion through the water could result in the net being raised from the bottom (if the force is upward) thereby failing to catch many ofthefish nearthe sea bed, or if the force is downward increasing the riskthatthe boards will be tilted from their required vertical orientation to fall "flat", that isto lie horizontally on the sea bed, from which position they cannot be returned to their required vertical orientation without hauling the net and shooting the gear again, a time consuming and costly operation.

Afinal disadvantage of flattrawl doors is the tendencytheyexhibitto plough along the bottom of the sea bed. It is forthis reason thatthey are usually fitted with an extremely strong keel or shoe, but this increasesthestiffness ofthe boardsandthe danger of damage to the nets and lines by the trawl door striking a fixed object and causing a shock loading on the net or warps.

Another known type of trawl door which is very popularwith many trawling vessels istheso-called V-door which is a door resembling the flat door, but having two substantially flat portions inclined with respectto one another about a longitudinal line which is substantially horizontal in use of the board, each door portion being inclined away from the attachment points. Such doors are inherently more stable and can even be returned to their required orientation if knocked flat by striking rough ground at the sea bed or for example when turning or shooting.

V-doors, however, still require a substantial amount of energyto draw them through the water and although they can operate with a smaller angle of attack than flat doors (say, about 35" where flat doors require an angle of attack of 40 ) they do not exert as much lateral force on the bridles as does a flat door.

Other special purpose doors for trawlers are also known, one incorporating a slot transverse its length and another in the form of a flat "wing" having a fore-to-aft camber and a high aspect ratio, that is a ratio between the height from top to bottom of the door in its working orientation with respect to the length from frontto rearofthe door. The aspect ratio of such doors in normally in the region of 1 .8to 2.0 and because of this they are not suitable for conventional demersal trawling (carried out on the sea bed) but only for mid-watertrawiing (raised from the sea bed) because oftheirtendencyto fall flat if towed on the bottom. Being flat in a transverse direction they are impossible to raise once knocked flat.Since mid-water trawling is only undertaken for a small proportion of the year such trawl doors have had limited use, and it has hithertofore been necessaryto change these doorsfor conventional flat or V-doors as discussed above for demersal trawling during the rest of the year.

The present invention seeks to provide a trawl door The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

which will not suffer from the disadvantages discussed above of conventional trawl doors, which will be useablewith a smaller angle of attack than conven tionaldoorswhilststill offering a high transverse spreading force, being easy to shoot without any risk oftwisting, and which will be self-righting in the event of being knocked flat or towed flat during turning.

According to one aspect ofthe present invention, therefore, a trawl door comprises a door body having two main body portions each ofwhich have a chamberin at least one plane on at least one surface thereof, the two body portions being inclined to one another across a line of separation between the said two body portions, means for attaching the warp or towing lineto oneface ofthe door and meansfor attaching a bridle line or lines to the otherface ofthe door.

Because the two body portions are inclined to one another it is convenientforthe warp and bridle lines to lie in the same plane although, as will be discussed below, other alternative configurations may have advantages and enable the trawl doors to be used in a different way from hithertofore.

In the preferred embodiment ofthe invention both opposite faces of a door body portion are substantially parallel to one another, each door body portion being made from flat sheet material which is curved to the required camber. It is within the scope of the present invention, however,to provide a trawl door in which the face of each door body portion opposite the said one face is substantially flat so that the trawl door is thicker in its mid region than it is atthe leading or trailing edge.

As the door is towed through the water, therefore, it generates a deflection due to the cambered stream line over its convexly curved surface, and because the two body portions are inclined with respect two one anotherthe forces generated by the reduction in pressure over the convexly curved surface draws the dooroutwardlyin a stable mannerto hold the net mouth open. The requiredangle of attack is lessthan that required for a flat door because the hydrodynamic pressure differential generated by the movement of water over the curved surface more efficiently translates into the lateral force.Because ofthe inclination of the two body portions along the line of separation, if thedoorshould be knocked flat,forexample by impactwith stones or when shooting orturning, it will tend to right itself because ofthe asymmetry of forces then occurring. This inclination of the two body portions, combined with the hydrodynamic "lift" generated by the cambered surface can also be used in anotherway by providing fortwo or more attachmentsforawarportowing line for which purpose the net lint may be used; at least one such attachment on each side of the said line separating the twodoorbodyportionsfrom oneanotherareenvis- aged.Such a towing line may replace the single pivoted towing line and by applying a different tension to one orthe other a vertical component offorce can be generated by the trawl door so that the pressure which this exerts on the sea bed can be reduced once it has been positioned, or the door can be elevated for mid-watertrawling without requiring to be changed for a special purpose mid-water door.

Instead of splitting the towing line into two or more parts attached to the door in such a way as to be ableto control the movement of the door a single towing line orwarp may be used with the net line or back-strops at the back ofthe door being attached at two orfour points spaced on either side ofthe line separating the two body portions. For example, two back-strops each about one and ahalftirnesthe door's length, may be attached to the door and joined to a single bridle. This increases stability.

In one embodiment oftheinvention the leading edge of each door body portion is substantially rectilinear although it is within thescope ofthe present invention to provide curared orcranked leading edges. Likewise, the trailing edge of each door body portion may be rectilinear irk tie preferred embodimentalthough,again curved orcrankedtrailing edges may alternatively be provided in more complex and sophisticated embodiments.

Likewise, additional control over the position ofthe trawl door may be obtained if there are provided subsidiary flaps on one or each door body portion, and means for controlling the inclination of such flaps with respectto the general plane ofthe body portion on which it is mounted. Such control means may include auxiliary lines orwarps, electrical or electronic control, pneumatic or hydraulic control lines or any other suitable control system.

Any section through a door body portion perpendicularto the line separating the said two body portions may be substantially rectilinear and general ly parallel to the leading and trailing edges, or embodiments having curved or cranked cross-sections may be constructed within the scope ofthe invention.

One edge ofthe door body portions parallel to the line of separation between the two door body portions may be provided with a removable wear resistant shoe or keel, orthis edge may be treated to make it more resistantto wearthan the remainder ofthe trawl door, for example, by special purpose metallurgical hardening techniques in the case of a doorthe body portions of which are both made entirely of steel. Such a shoe or keel mayfollowthecurveofthegeneral plane of that body portion, or may be specially shaped for ease of manufacture and replacement, with a substantially rectilinear shape.

The hydrodynamic lift effectwill be obtained to some extentwhateverthe nature of the camber curvature in the convex face ofthe door body portions.

However, for maximising the transverse liftwhilst minimising the forward drag itisconsidered that an asymmetrical curve, for example, one in which the shortest radius of curvature occurs approximately one-third ofthechordfromthe leading edge may prove to be the most efficient. For ease of manufacture, of course, a regutar or part-circularcurve may be preferred.

Tne aspect ratio ofthe trawl door according to the invention, that is the ratio between the dimension transverse the separation line with respect to the dimension parallel to the separation line is preferably in the region of 0.5 to 0.6, that is, thetransverse dimension or "height" ofthe door (height being considered vertically in the normal position of use) is between 50 per cent and 60 percent ofthe "length" of the door in the direction of intended travel.

The camber ofthe door is preferably in the range of 10 per centto 15 percent, that is, the perpendicular distance from the line joining the leading and trailing edge to the crest of the curved convex surface in relation to the length ofthe said line joining the leading and trailing edges.

The present invention also comprehends a trawl door comprising two guide vane portions in the form of "wings" having a camber from front to backwith reference to the direction of intended movement, the "wings" being inclined at a shallow angle with respect to one another across a junction line between the two "wings".

According to a further aspect ofthe present invention fishing equipment comprises a trawl net, two trawl doors as herein defined attached to bridle lines leading from the mouth ofthe trawl net and at leasttwowarp lines leading from the trawl doors to a towing vessel.

Further, the present invention provides a trawl door having a camber in the direction of intended movement and an aspect ratio (as herein defined) sufficiently lowto allow demersal trawling and mid-water trawling to be effected using the same trawl doors, the configuration ofthe doors being such that they have a degree of inherent stability so thattheywill be substantially self-righting if they should fall or be knocked flat during demersal trawling.

Two embodiments ofthe present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure lisa plan view illustrating the arrangement of a trawl net with trawl doors and a trawling vessel; Figure 2 is an enlarged view of the trawl door; Figure 3 is a plan view of a sidetrawling configuration; Figure 4 is a perspective view of a trawl doorformed as an embodiment ofthe present invention; Figure 5 is a sectional viewtaken on the line V-V of Figure4; Figure 6 is a sectional viewtaken on the line VI-VI of Figure 4; Figure 7 is a view of the front face of a second embodiment ofthe invention; Figure 8 is a view ofthe rear face of a second embodiment; Figure 9 is a sectional view taken on the line IX-IX of Figure 8;; Figure 10 is a sectional viewtaken on the line X-X of Figure 8; and Figure 11 is a rearviewon a smallerscale illustrating the position of back-strops attached thereto.

Referring firstto Figure 1, a conventional rear trawling configuration comprises a trawl net generally indicated 11,which is generally funnel shaped having a closed rear end 12 and an open mouth 13 along the upper edge 140fwhich are placed a numberoffloats 15 causing this to rise.On each side of the mouth 14 are respective bridles 16, l7leadingforwardlyto be attached to trawl doors 18,19, conveniently at the rear edge of these or a point spaced slightly forwardlyfrom the rear edge and on the "outer" face in relation to the configuration of trawl net and doors; that is the "outer" doorface is thatwhich faces awayfrom the other door ofthe pair, and the "inner" face is that face directed generally towards the other door of the pair.

Two warp lines 20, 21 are attached, one to each of the doors 18,19 on the inner faces thereof and at a position spaced about one-third of the length ofthe door from the leading edge, and these warp lines 20, 21 lead to the rear of a trawling vessel 22. Because of the way the warp lines 20,21 and the bridles 16,17 are attached to the doors 18,19 the hydrodynamic forces experienced by the doors causes these to "swim" awayfrom one anotherthereby holding the mouth 14 of the trawl net 11 open. This can be appreciated more readilyfrom the largerscale illustration in Figure 2 which shows the trawl door 18 of Figure 1.

Referring now particularlyto Figure 2,the trawl door 18 illustrated is shown as a prior art flat door which is attached to the warp 20 by means of a hinged "triangle" 23 at a point, as mentioned above, about one-third ofthe length ofthe doorfrom its leading edge 24. The bridle 16 is attached to the outerface 25 of the door at a point about one-quarter ofthe length ofthe doorfrom the trailing edge 26. The direction in which the trawl door 18 moves is represented by the arrow a and the angle of inclination alpha between this direction of movement and the plane ofthe door 18 constitutes the angle of attack which, as illustrated in Figure 2, is about 40". Because of this relatively large angle and the linear construction ofthe door, the turbulence generated by the door creates a substantial drag.

Figure 3 illustrates a sidetrawling configuration in which, as opposed to the stern trawling configuration illustrated in Figure 1,thetrawler27 hasforeand aft attachment points 28, 29forthewarps 20,21. Apart from the variation in length of these warps during shooting, the net gear configuration is the same as in Figure 1. However, as can be seen from Figure 3, difficulties due to the potential fouling of the gear on shooting are increased with side trawling configu rations, forwhich reason the trawler 27 has to follow a large curving path whilst shooting, until the trawl has been left astern by a substantial distance such as illustrated in Figure 1.With conventional flat boards having no inherentstabilitytheturbulence of the water closely adjacent the vessel, into which water the doors enter upon shooting the net, can cause these to turn over one another and become tangled in the lines.

This problem is overcome using the trawl doors ofthe present invention which are illustrated in Figures 4,5 and 6. In Figure 4there is illustrated a trawl door generally indicated 30 comprising upper and lower trawl door body portions 31,32 separated along a separation line 33. The warp 20 is attached to the door 30 by means of a "triangle" connection 23 similar two that illustrated in the prior art configuration of Figure 2, the triangle 23 having two cylindrical pivots 34, 35 on the separation line 33, allowing the triangle 23 to turn about a generally horizontal axis. The term "horizontal" will be understood to relate to the door in Its normal position of use as illustrated in Figure 4 wherethe separation line 33 lies in a generally horizontal plane.In other embodiments this attachment is replaced by a single attachment coupling on the door itself, allowing the necessary movements to take place.

Each ofthe trawl door body portions 31,32 has a respective leading edge 36,37 and a respective trailing edge 38,39 and a respective "outer" edge 40,41 the "outer" edges being those longitudinal edges parallel to but spaced from the separation line 33 atwhich the two body portions 31,32 are joined.

As can be appreciated from Figures 4 and 5the separation line 33 is curved with a regular part-circular curvature and each body portion 31,32 likewise has a camber constituted by a part-circular planar curvature formed by rolling a flat sheet of steel. The two body portions 31,32 are thus manufactured separately and welded together along the separation line 33 with the leading edges 36,37 ofthe two body portions 31, 32b inclined with respect to one another by an angle ss as illustrated in Figure 4.Any transverse section, transverse that isto the separation line 33, will have the form illustrated in Figure 6, comprising two straight line portions inclined by the angle t3 with respect to one another whilst any longitudinal section, as can be seen from Figure 5, taken parallel to the separation line 33 will have a regular part-circular curvature.

An attachment for a bridle 16 is shown in broken outline on the rearface of the door 30 and generally indicated with the reference numeral 42. Possible alternative warp affachment point 43,44 are illustrated in broken outline on the forward face of the door 30, these being provided fortwo warp lines which would replace the warp line 20 and allow differential tensions to be applied to the two body portions 31,32.

Two further attachment points (not shown) may additionally be provided in positions symmetrically located with respect two the attachments 43,44 about a line of symmetrytransverse to the junction line 33.

The four attachment points could then be used for multiple backstrops.

Because ofthe curved or cambered configuration the turbulence generated previously by a flat trawl door is absentthereby reducing drag.

Furthermore, because of its cambered configuration the door 30 will exert hydrodynamic lift forces transverse the line 33 as the door30 is drawn through thewaterandthe required angle of attack in orderto generate a given transverse force is substantially less than that required for a flat doorthereby again substantially reducing the drag exerted bythe door on the warp 20.

The separation ofthe door into two body portions 31,32 inclined atthe angle (3to one anotherensures that the door is stable since the "I ift" forces generated hydrodynamically by the movement ofthe door through the water converge as illustrated by the arrows B of Figure 6.Should the door be knocked down by contact of its lower edge 41 with additional resistancesuch as stones orthe liketheliftforce generated upon continued movementthrough the water by the body portion 32 will be in a generally upward direction causing the door 30 to rise from the sea bed,andthe liftforcegenerated by the body portion 31 will be inclined to the horizontal at an angle of((3-90)0therebygenerating aturning moment tending to return the doorto its upright configuration.

One edge ofthe trawl door may be provided with removable weights, usually termed "ballast weights", which can be fitted to the half ofthe doorwhich is intended to be lowermost in use. Such weights may be fitted at any point on the lower half ofthe door although it may be considered preferable forthem to beadjacentthelowermostor"keel"edge. By making such weights removable they can be adjusted to meet various different criteria such as the vessel horse power and depth of water.

Further embodiments may include the provision of flotation chambers fitted to the half of the door intended to be uppermost in use. Such flotation chambers would not normally be required but in special circumstances where added stability is of paramount importance they may be found convenient.

In addition to the removable weights referred to above a section of heavier gauge metal might be fitted to the edge ofthe door intended to be lowermost in orderto improve the wear resistance of the door. Such a "keel" or shoe need not necessarily be removable although, again for convenience, a removable shoe may have advantages in practice allowing itto be replaced when worn or removed when not required.

Referring now to the second embodiment illustrated in Figures 7 to 11,the same or corresponding components have been identified with the same reference numerals raised by 100. The embodiment of Figures 7 to 11 differs from that of Figures 1 to 6 in that the leading edges 136,137 are rounded where they meetthe lateral or 'outer' edges 140,141 ofthe door body portions 131,132 respectively. The edge 141, which is intended to be lowermost in use, has a keel 163 and two transverse strengthening bars 164,165 extend across both body portions 131,132 from the edge 141 to the edge 140.

The primary difference between the embodiment of Figures7to 11 and that of Figures 1 to 6 lies in the provision of a reinforced central rib 166 along the separating line between the two body portions 131, 132, which rib has a plurality of holes 167 for receiving an attachmentfor a towing line 120. On the rearface of the doorthere are four attachment lugs 168,169 and 170,171 in pairs across the line of symmetry defined bythe rib 166. The provision ofthese lugs on the rear face ofthe door permits two back-strops 172,173 (Figure 11) to be attached between the trawl door and the bridle for additional stability. The provision for such lugs allows the doorto be used either on the lefthand or righthand side ofthe trawl with the keel 163 lowermost in each case.

Claims (15)

1. Atrawl door comprising a door body having two main body portions each of which have a camber in at least one plane on at least one surfacethereof, the two body portions being inclined to one another across a line of separation between the said two portions, means for attaching a warp ortowing line to one face of the door and means for attaching a bridle line to the otherface ofthe door.

2. Atrawl door as claimed in Claim 1, in which both opposite faces of a door body portion are substantially parallel to one another.

3. Atrawl door as claimed in Claim 1 in which the face of each door body portion opposite the said one face is substantially flat.

4. Atrawl door as claimed in any of Claims 1 to 3, in which there are two or more attachments for a warp or towing line, at least one on each side of the said line separating the two door body portions from one another.

5. Atrawl doorasclaimed in any preceding Claim, in which the leading and/ortrailing edge of each door body portion is substantially rectilinear.

6. Atrawl door as claimed in any of Claims 1 to 4, in which the leading and/or trailing edge of each body portion is curved in profile and/or in cross section.

7. Atrawl door as claimed in any preceding Claim, in which there are provided subsidiary flaps on one or each door body portion and means for controlling the inclination of such flaps with respect to the general plane of the body portion on which it is mounted.

8. Atrawl door as claimed in any preceding Claim, in which any section through a door body portion perpendicularto the line separating the said two body portions is substantially rectilinear.

9. Atrawl door as claimed in any preceding Claim, in which one edge of one of the door body portions parallel to the line of separation between the two door body portions is provided with a removable wear resistant shoe or keel.

10. Atrawldoorasclaimedin any preceding Claim, in which the camber is an assymetrical curve.

11. Atrawl door as claimed in Claim 10, in which the camber is an aerofoil curve.

12. Atrawl door as claimed in any preceding Claim, in which the aspect ratio ofthe door is in the region of 0.5 to 0.6.

13. Atrawl doorasclaimedinanypreceding Claim, in which the camber is in the region of 10-15 per cent.

14. Atrawl door as claimed in Claim 13, in which the camber is 12 per cent.

15. Atrawl door substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 6 or 7 to 11 of the accompanying drawings.