NO343896B1 - Trawling arrangement - Google Patents

Description

TRAWLING ARRANGMENT

Technical Field

The present invention relates to a trawling arrangement, a method of operating a trawl, a traction device for towing a trawl and a trawl door.

Background and prior art

Trawling as a fishing technique has existed in its current form since the end of the 19th century. It is based on a bag shaped net that is towed through the water by a boat, i.e. a trawler. To keep the trawl net open, a pair of trawl doors are connected to the trawl some distance in front of the trawl opening. The doors have an angle of attack with respect to the draw direction, so that a horizontal spread is created.

Over the centuries the trawling technique has developed to include larger and larger trawls. The large trawl creates a substantial drag and hence ever larger trawlers have had to be employed to be able to pull the trawl. The result of this is that the size of the catch has increased, but also that the fuel consumption has increased. The fuel consumption is one of the greatest expenses in trawling. The fuel consumption is largely unrelated to the catch, but is substantially depending on the distance the trawl is towed, the speed of the towing and the depth at which the trawl is being towed. The fuel consumption will increase with these factors, but not necessarily the catch.

One of the above factors, the trawling depth, is highly depending on the type of catch. For catching species, that are close to the bottom, such as flatfish or shrimp, the trawl has to be towed close to the bottom. This means that very long warps, i.e. the trawl wires that connect the trawl doors to the boat, have to be deployed. The length of the warps is several times the depth of the trawl. These warps create a substantial drag, which makes bottom trawling very fuel demanding, and also require the use of high power engines.

Bottom trawling has been criticized from an environmental perspective because the trawl doors and weights of the trawl are dragged along the bottom and make long and wide trails, which disrupt and destroy the environment for seabed plants and animals. Consequently, there are restrictions on where and how much bottom trawling is allowed.

There have been many solutions suggested to improve trawling. Most of these are modifications of the conventional principle of a trawler that tows a trawl net with trawl doors. However, there have been some attempts to deviate from this principle.

One of these bold attempts is shown in GB 695641, which shows a trawl net that is being towed by a pair of underwater crafts. The crafts may be electrically driven and are connected to a mother ship by electrical power lines. The mother ship may also control the depth and side steering of the crafts. The mother ship is connected to the cod end of the net by guide lines, which means that the ship trails the trawl net. The guide lines are kept suitably taut during the trawling. The purpose of the guide lines is to pull the cod end up to the ship when the net is full.

The invention of GB 695641 was made in 1949, at a time when the technology for such underwater crafts was not mature enough to be used efficiently for trawling.

DE 1181484 also shows a suggested solution that uses underwater crafts for towing the net. In this solution the crafts are connected to each other by a boom or similar that keeps the crafts at a mutual fixed distance. The boom ensures that the net is held open. However, the boom structure will create a substantial extra drag for the trawl.

WO 2010/015254 shows a trawl configuration that uses one underwater craft that tows one side of the trawl net while the trawler tows the other side. As the underwater craft has a fixed length of wire connecting it to the net, there is little possibility of varying the trawling depth and it would not be possible to trawl below a certain depth.

NO320863 shows a more conventional trawling arrangement. It differs from a conventional arrangement in the fact that the trawl doors can be steered to a certain degree, to control the spread and depth of the trawl. However, as conventional trawling arrangements, the doors and the trawl must be towed by the vessel. This means that the vessel will be situated far forward of the trawl (the distance may vary depending on the depth but would be from a couple of hundred meters to more than one kilometer), and hence the echo sonar field of the vessel cannot possibly see the trawl.

JPH03198731 also relates to such a conventional trawling arrangement with a trawl eye.

Summary of invention

The present invention has as its main objectives to be able to tow a trawl without the need of warps between the boat and the trawl to create the pulling force or with a reduced need for pulling force from the boat.

The invention also has as an object to provide greater freedom for the boat to move and be position relative to the trawl.

It is also an object by the present invention to provide a traction device that is easy to manoeuvre and is stable even when subjected to side currents.

These and other objects are achieved by the appended claims.

In a first aspect, the invention is defined by a trawling arrangement, comprising a trawl net with an opening and a cod end, and at least two propelled and steerable traction units that are coupled to the trawl net forward of the trawl net, by sweep lines, in order to tow the trawl net, one said traction unit being arranged on each lateral side of said trawl net, said traction units being coupled to said vessel at the surface by at least one respective electrical power cable, characterised in that said vesselhas an echo sonar with an echo sonar field, said at least one respective electrical power cable extending directly from said vessel to said traction unit, said trawling arrangement being configured so that the opening of the trawl net is within said echo sonar field of said vessel mounted sonar, enabling a supervision of said trawl net by said sonar.

In a preferred embodiment, the traction unit comprises at least one propeller, a motor coupled to said propeller, said motor being arranged in a motor housing, said motor housing having an attachment bracket for attachment of sweep lines, a duct being arranged to encircle said at least one propeller, wherein the device comprises at least one steering fin that is hingedly attached relative to said motor housing and that said at least one steering fin is situated at the opposite side of the hydrodynamic centre of said traction device relative to said attachment bracket.

Further preferably, said at least one propeller is arranged on an opposite side of the motor housing from the attachment bracket. This will ensure optimum flow towards the propeller without the flow being disturbed by the motor housing.

Preferably, said duct is attached to said motor housing by webs, said at least one steering fin being attached to one of said webs. This provides a simple and robust construction.

Preferably the traction device has four steering fins that are arranged at an angular distance of 90°. This enables manoeuvrability in all directions, port, starboard, up and down.

In an alternative embodiment traction unit comprises a trawl door that is attached to said motor housing or said duct. The traction unit may be arranged in an opening in the trawl door.

Preferably, a main plane of said trawl door is parallel to or coincides with a rotational axis of said propeller.

In a further alternative embodiment, the traction unit comprising a plurality of propellers that are arranged in substantially the same plane, a motor coupled to a respective one of said propellers, said propellers being separately controllable to manoeuvre said traction unit in all directions.

Brief description of drawings

The invention will now be described in detail, referring to exemplary embodiments shown in the accompanying drawings, where:

Figure 1 shows a first embodiment of the trawl arrangement according to the present invention in side elevation view,

Figure 2 shows the trawl arrangement of figure 1 with the trawl being pulled downwards towards the seabed,

Figure 3 shows the trawl arrangement of figure 1 with the trawl being pulled upwards from the seabed,

Figure 4 shows the trawl arrangement of figure 1 in planar view,

Figure 5 shows the trawl arrangement of figure 4 with the trawl being pulled sideways in a first direction with respect to the towing direction,

Figure 6 shows the trawl arrangement of figure 4 with the trawl being pulled sideways in a second opposite direction with respect to the towing direction,

Figure 7 shows the trawling arrangement of the present invention in isometric view,

Figure 8 shows a second embodiment of the present invention with combined trawl doors and pods, in side elevation view,

Figure 9 shows the second embodiment of figure 8 in planar view,

Figure 10 shows a close-up of the trawl door and pod combination according to the embodiment shown in figures 8 and 9,

Figures 11a – g show a first embodiment of a pod suitable for the trawl arrangement according to the embodiment of figures 1-7,

Figure 12 shows a second embodiment of a pod suitable for the trawl arrangement according to the embodiment of figures 1-7, the pod having counter rotating propellers,

Figures 13a – f explain schematically the force and moment arm view of the pod,

Figure 14 shows a further alternative pod where the manoeuvring is done by varying the differential speed of a plurality of propellers,

Figure 15 shows a first configuration of a double trawl,

Figure 16 shows a second configuration of a double trawl,

Figure 17 shows how the pods are manoeuvring the trawl close to the seabed,

Figure 18 shows that the trawl and the boat can follow different tracks, and

Figure 19 shows that the spread of the trawl can be varied.

Detailed description of the invention

In figure 1 is shown a trawling arrangement of the present invention in side elevation view. Figure 4 shows the same arrangement in planar view and figure 7 shows the arrangement in isometric view.

The trawl arrangement comprises a boat 1, which is situated at the sea surface 2. From the boat 1 a pair of cables 3a, 3b extend to a respective pod 4a, 4b. From the pods 4a, 4b a sweeps 5a, 5b, 5c, 5d extends to the trawl net 6 and are attached to a respective corner 7a, 7b, 7c, 7d of the net 6.

The configuration may deviate somewhat from the above, e.g., instead of two sweeps from the pod 4a, b to the corners of the net 6, there may also be one sweep that connects with a bridle of two lines, as is generally known in the field of trawling.

The net 6 comprises a headline 8 and a footrope 9. Between the headline 8 and the footrope 9 is defined an opening 10 into the net 6. At the trailing end of the net is a narrow part called a cod end 11.

When trawling it is important to keep the area of the opening 10 of the net 6 as large as possible and to keep the opening as perpendicular as possible to the direction of travel. This ensures the largest catch as possible. Especially, for bottom trawling it is also important to keep the opening as wide as possible. To ensure this, is conventionally used trawl doors (sometimes also called otter boards) to create a sideways force that stretches the corners 7a, 7b, 7c, 7d outwards from the direction of travel.

In the embodiment shown in figures 1 – 7, the trawl doors have been replaced by traction units 4a, 4b, which in the following are called pods. A first embodiment of the pods 4a, 4b is shown in figures 11a-g, showing the pod in the following views: 11a: front elevation, 11b: rear elevation, 11c: longitudinal section, 11d: cross section, 11e: side elevation, 11f: rear isometric view and 11g: front isometric view.

The pod 4a, b has a similar design as azimuth propellers that are conventionally used on boats and ships to increase manoeuvrability, such as the azimuth propeller described in EP 2468624. The pod comprises a cylindrical duct 12, which encircles a propeller 13. The propeller 13 is mounted at one end of a shaft (not shown) that is supported in a motor housing 14. The duct 12 is attached to the motor housing 14 via four webs 16. The propeller 13 is facing the direction of travel, while the motor housing 14 is at the trailing side of the direction of travel.

A fin or flap 17 is hingedly attached to the trailing side of each web 16. At the trailing end of the motor housing 15 is an ear 18 that functions as an attachment means for the sweeps 5a, b, c, d.

The motor housing 14 contains an electric motor (not shown) and may also contain a battery package (not shown). The battery package may be exchangeable, so that one package can be charged while another is in use.

The duct 12 serves both to protect the propeller 13 and to increase the efficiency of the propeller.

To prevent the pod as such from rotating about its longitudinal axis in the opposite direction of the propeller, the fins 17 are set at an angle that counteracts the torque from the propeller 13.

As the propeller 13 is arranged at the front of the pod 4a, b in the direction of travel, there are no obstacles that will decrease the efficiency. In the prior art, except for WO 2010/015254, the propeller is arranged at the rear side of the pod. In that case the pod body will be in front of the propeller and hinder the flow of water to the propeller.

The cables 3a and 3b may be electrical cables that supply the electrical motor in the pod 4a, 4b with electricity. A generator and an engine on board the boat generate the electricity. A battery package in the pod may functions to level out the peaks of the energy consumption, so that the engine and generator on board the boat can run at a constant speed. The boat may also have batteries for peak loads.

It is also conceivable that the battery package in the pods can supply all the energy for propulsion, as the battery technology advances.

The electrical cables may be reinforced to be used to pull up the trawl after fishing has been completed or if a failure causes one or both of the pods to stop functioning. Alternatively, a separate wire (not shown) may be used for this. A separate wire is not desirable as this will increase the drag.

Figure 12 shows an alternative embodiment of the pod. This deviates from the embodiment of figures 11a-g by having a double propeller 19 with two propellers 19a and 19b that rotates in opposite directions. This ensures that the pod as such is not rotating along its longitudinal axis.

By changing the angle of the fins 17, the pod can be steered in different directions. Figure 2 shows that the fins 17 are angled to steer the pods 4a, 4b downwards towards the seabed 20. Figure 3 shows that the fins 17 are angled to steer the pods 4a, 4b upwards from the seabed 20.

Figures 5 and 6 shows that the fins 17 are angled to steer the pods 4a, 4b sideways to the port and starboard side, respectively, relative to the direction of travel of the boat 1.

With the trawling arrangement explained above, the boat 1 does not impose any towing force on the trawl. The trawl and boat can therefore move independently of one another within the constraints that the length of the cables 3a and 3b gives. This makes it possible to position the boat slightly forward of the net 6 and so that the trawl opening is within the field of the sonar or echo sounder of the boat. Thereby the boat can follow closely both the condition of the trawl, such as spread of the pods, and the amount of and type of fish or other catch that enters the trawl.

Figures 8 and 9 shows an alternative embodiment of the trawl arrangement according to the invention. In this case the pod is a combined trawl door and pod 22, as shown in figure 10. The trawl door and pod combination 22 can best be described as a conventional trawl door 23 in which an opening has been made to receive the pod 24. The pod may be of any of the types shown in figures 11a-g and figure 12. The door/pod combination 22 can be attached to the boat 1 by a warp 25 in a per se conventional. The sweeps 26 can be attached to the pod 24 or the door 23.

The door/pod combination 22 can create the necessary propulsion to tow the trawl 6, so that the warps are used only to keep the door/pods 22 at a certain separation from each other. The propulsion may also be distributed between the door/pods 22 and the boat 1 to a selected ratio. It is also possible to use the pods only to steer the doors upwards, downwards and sideways and let the boat create the towing force.

In figures 14a and 14b is shown an additional alternative pod 30. This pod comprises four propellers 13a, b, c, d that are arranged in substantially the same plane. The propellers have each a separate duct 12a, b, c, d, but it is also conceivable to have one duct surrounding all propellers. The ducts are rigidly coupled to each other, such as by welding. The propellers have each a separate motor housing 15a, b, c, d. The motor housings are connected to the ducts by webs 16. At the end of each motor housing is an ear 18 for connecting a sweep (not shown). In this embodiment it will be an advantage to arrange a bridle between the ears 18 and the sweeps.

In this embodiment the fins may be omitted. Instead the pod 30 can be manoeuvred by differentiating the speed of the propellers 13a-d. For example, if it is desired to steer the pod 30 towards port, the propellers 13a and 13d on the starboard side are sped up, or alternatively or in addition, the propellers 13b and 13c on the port side is slowed down.

It is very convenient if two propellers rotate in one direction and two rotate in opposite direction to counteract the torque of the propellers.

The pod 30 preferably has four propellers, but it is also possible to manoeuvre the pod in all direction with as few as three propellers. The pod 30 may also have more than four propellers.

Figure 13a -f show the force and moment arm view of one pod 4a. In these figures the propeller 13, one fin 17, the motor housing 15, and the duct 12 are shown schematically. Also shown are the sweeps 5a, 5b. The sweeps are connected to the pod 4a at a connection point 27. The hydrodynamic centre of the pod is denoted by 21. In figures 13a-c, the propeller 13 is arranged in front of the hydrodynamic centre 21 in the travelling direction of the trawl. In the figures 13d-f the propeller 13 is arranged behind the hydrodynamic centre 21 in the travelling direction of the trawl.

In a force and moment arm view, the connection point 27 may be considered as a fixed point and the pod 4a is allowed to swing about this fixed point 27.

The hydrodynamic centre 21 is the point where the resultant force of all hydrodynamic forces, especially lift and drag, is acting. This point will be the same irrespective of the position of the fins 17.

Starting with figure 13a, is shown a pod 4a that is travelling straight forward. The fin 17 is in neutral position. The forces acting on the pod is the pulling force F1 of the propeller 13, the resultant force F2 from the sweeps 5a, 5b, the resultant upward hydrodynamic force F3 and the resultant downward hydrodynamic force F4. If the pod is travelling at a constant speed, the forces F1 and F2 are equal. Unless, there is a water current in any direction transverse to the direction of travel the hydrodynamic forces F3 and F4 are also in balance.

Referring to figure 13d, is shown a pod 4a’, which has the propeller 13 and the duct 12 arranged at the rear of the motor housing 15, similar to the arrangement shown in GB695641. This means that the hydrodynamic centre 21 is in front of the propeller. The similar forces as in figure 13a, acting on the pod 4a’ when travelling straight ahead at constant speed and with no transverse currents is also shown. Also here are the forces F1 and F2 in balance and the forces F3 and F4 in balance.

In figure 13b, the fin 17 has been adjusted to create a downward force F5. This will tilt the pod 4a about the point 27, as the force F5 created a momentum about the point 27 with an arm A5. Hence the pulling force F1 from the propeller will be acting obliquely downward, as shown. Due to the changed orientation of the pod 4a relative to the direction of travel, the hydrodynamic forces will no longer be in complete balance, as the downward force F4 will be slightly larger than the upward force F3. This will create an increased momentum about the point 27 as the resultant force of F3 andF4 acts with an arm A4. To compensate for this, the fin has to be adjusted back somewhat. However, due to the much shorter arm A4 than the arm A5, the effect of the resultant force F4 – F3 is not large.

Referring now to figure 13e, is shown the same situation as in figure 13b involving the pod 4a’. The fin 17 is set at an angle that creates a downward force F5, which acts with an arm of momentum A5 about the point 27. This will tilt the pod 4a’ downwards. Due to the change of direction, the hydrodynamic forces will change and result in a positive downward acting force F4 – F5. This resultant force acts with an arm A4 about the point 27. Since the arm A4 is much longer than the arm A5 the contribution from the hydrodynamic forces will be much greater than is the case with the pod 4a in figure 13b. The fin therefore has to be adjusted back quite a bit to that the pod is tilted too much downward. This may create an unstable situation where the fin is adjusted too much back so that the pod is tilted upwards again. Then the fin has to be adjusted in the first direction again, and so on. This may result in a wobbly behaviour of the pod 4a’. The wobbly behaviour will both slow down the speed of the trawl and consume unnecessary power. In some cases the fin may not be able to counteract the momentum of the hydrodynamic forces so that the pod ends up travelling in another direction than intended.

Referring now to figure 13c, is shown a situation where a current C transverse to the direction of travel pushes the pod away from the direction T of travel. The current may be upwards or downwards in the sea, but may also be a side current that pushes the pod 4a sideways. In this case only the resultant hydrodynamic force F6 has been shown. In such a case the fin 17 can be adjusted to compensate for the hydrodynamic force F6 and create an opposite force F5. As the force F5 is acting on a longer arm A5 than the arm A6, the force F5 necessary to compensate for the force F6 is much smaller than the force F6.

Referring now to figure 13f, is shown the same situation as for figure 13c but with the pod 4a’. As is evident from figure 13f, the resulting hydrodynamic force F6 is acting on a much longer arm A6 than the adjusting force F5 of the fin 17. This means that the necessary compensating force F5 must be much larger than the resulting hydrodynamic force F6.

The above shows clearly the benefit of arranging the propeller and the steering fins as far forward in the travelling direction as possible.

In addition a propeller in forward of the motor housing will have a better efficiency than a propeller mounted rearward of the motor housing, as the motor housing will hinder or disrupt the flow of water to the propeller.

Figure 15 shows a possible configuration of a double trawl. Here two trawl nets 6a and 6b are coupled together either by connecting nearest corners 7a, 7b or by connecting their nearest sweeps 5a, 5b to a common pod 4b. The result is a trawl with two nets 6a and 6b and three pods 4a, 4b, 4c.

Figure 16 shows an alternative double trawl arrangement. Here the two trawl nets 6a, 6b are connected to separate pods 4a, 4b, 4c and 4d. The nets 6a,b can therefore to a certain degree be manoeuvred independently.

Figure 18 shows how the pods can be manoeuvred to follow the seabed when the seabed is undulating up and down. The hatched area is the area covered by the trawl.

Figure 19 shows that the trawl can be manoeuvred along a different track (hatched area) than the boat (single line).

Figure 20 shows that the spread of the pods 4a, 4b and hence the spread of the trawl opening can be varied (hatched area) as the trawl is travelling.

With the pods of the present invention it is very convenient to arrange a flow meter on the pod. This will provide an accurate measurement of the flow of water past the pod. By equipping the pod with a sensor that measures the speed in relation to the seabed, an absolute value for the sea current can be obtained.

The pod may also have a load cell to measure the tension in the sweeps. This can also be used to calculate the flow of water.

One or both pods may also be equipped with a sonar or echo sounder, which may send soundwaves at an angle forward of the pod, to determine the state of and distance to the seabed and detect fish or other species.

The distance to the seabed may be detected by the sonar or by other per se known sensors and be used to determine the angling of the fins to keep the trawl at a certain distance above the seabed.

The pods may be equipped with an accelerometer that detects the orientation of the pod, so that the fins can be adjusted to bring the pod back to a predetermined orientation.

Other per se known sensors may also be included in the pods, such as spread sensors to determine the distance between the pods, temperature sensors, depth sensors, salinity sensors, etc.

Claims (9)

Claims

1.

A trawling arrangement, comprising a vessel (1), a trawl net (6) with an opening (10) and a cod end (11), and at least two propelled and steerable traction units (4a, 4b) that are coupled to the trawl net (6) forward of the trawl net (6), by sweep lines (5a, 5b, 5c, 5d), in order to tow the trawl net (6), one said traction unit (4a, 4b) being arranged on each lateral side of said trawl net (6), said traction units (4a, 4b) being coupled to said vessel (1) at the surface by at least one respective electrical power cable (3a, 3b), characterised in that said vessel (1) has an echo sonar with an echo sonar field, said at least one respective electrical power cable (3a, 3b) extending directly from said vessel (1) to said traction unit (4a, 4b), said trawling arrangement being configured so that the opening (10) of the trawl net (6) is within said echo sonar field of said vessel (1) mounted sonar, enabling a supervision of said trawl net (6) by said sonar.

2.

The trawling arrangement of claim 1, characterised in that said traction unit (4a, 4b) comprises at least one propeller (13), a motor coupled to said propeller, said motor being arranged in a motor housing (14), said motor housing (14) having an attachment bracket (18) for attachment of said sweep lines (5a, 5b, 5c, 5d), a duct (12) being arranged to encircle said at least one propeller (13), and at least one steering fin (17) that is hingedly attached relative to said motor housing (14) and that said at least one steering fin (17) is situated at the opposite side of the hydrodynamic centre (21) of said traction unit (4a, 4b) relative to said attachment bracket (18).

3.

The trawling arrangement of claim 2, characterised in that said at least one propeller (13) is arranged on an opposite side of the motor housing (14) from the attachment bracket (18).

4.

The trawling arrangement of claim 3, characterised in that

said duct (12) is attached to said motor housing (14) by webs (16), said at least one steering fin (17) being attached to one of said webs (16).

5.

The trawling arrangement of according to any of the claims 2 - 4, characterised in that said traction unit (4a, 4b) has four steering fins (17) that are arranged at an angular distance of 90°.

6.

The trawling arrangement of any of the preceding claims 2 - 5, characterised in that said traction unit (4a, 4b) comprises a trawl door (23) that is attached to said motor housing or said duct.

7.

The trawling arrangement of claim 6, characterised in that a main plane of said trawl door (23) is parallel to or coincides with a rotational axis of said propeller (13).

8.

The trawling arrangement of claim 7, characterised in that said traction unit (4a, 4b) is arranged in an opening in the trawl door (23).

9.

The trawling arrangement of any of the preceding claims 2 – 8, characterised in that said traction unit (4a, 4b) comprises at least three propellers (13) that are arranged in substantially the same plane, a motor coupled to a respective one of said propellers (13), said propellers (13) being separately controllable to manoeuvre said traction unit in all directions.