Patent Description:
In trawl fishing a spreading device is used to control the opening of the fishing trawl and in seismic surveys in order to maintain a distance between the lines pulling the seismic survey devices in order to obtain as broad a survey zone as possible.

From <CIT> is known a spreading device comprising two foils arranged such that a flow channel is created between the two foils as the spreading device is towed through the water, as described in the document. A number of other prior art documents disclose similar foil arrangements.

A further example is known from <CIT> wherein is disclosed a spreading device with a number of flat foils arranged at an angle relative to the pulling/towing direction. The foils are arranged such than in use the flat foils will be arranged substantially vertically between a lower shoe and an upper shoe. At distal rear edges (relative to the direction of motion during use) of the foils the plate material from which the foils are manufactured are bent back creating spoilers along trailing edges of the foils.

Below the invention will be discussed with reference to a trawl door, but it shall be understood that the principles and the increased spreading effect obtained with the present invention also applies to other devices or industries where it is interesting to pull a device be it a fishing net, a seismic survey device or the like through the water and obtain a spreading effect between the pulling wires connecting the boat to the spreading devices in the water.

For trawl fishing one of the main issues is to obtain the desired spreading effect while the trawl doors cause the least resistance through the water. When talking about spreading effect, this will be referred to as the trawl door's lift, i.e. the trawl door's ability to travel sideways with respect to the forward pull through the water. One way of obtaining this is to arrange the attachment point of the pulling wire from the boat to the trawl door such that the trawl door will be arranged at an oblique angle to the pulling direction thereby urging the trawl door to travel sideways.

Due to the oblique angle of the trawl door relative to the pulling direction a substantial resistance is created and therefore increased force is needed by the pulling boat in order to pull the trawl through the water at a desired speed and at the same time obtain the spreading of the trawl opening such that the trawl's fishing capabilities are achieved.

In the art the trawl doors have further been improved by arranging various foils in the trawl doors such that a number of channels are created allowing the water to stream through the trawl door, but at the same time urging the trawl door sideways relative to the pulling direction. This of course decreases the resistance and thereby the force needed to pull the trawl doors through the water, but still a significant resistance is present when using trawl doors and at the same time when it is desired to have a substantial spread between the pulled trawl doors in order to open the trawl to achieve optimum fishing capabilities.

To this problem the invention presents a spreading device which provides extra lift, i.e. extra spreading force without increasing the pulling force and as such provides increased efficiency with less use of pulling force.

The invention achieves this by a spreading device for trawl fishing, seismic survey operations and other water activities where it is desirable to keep distance between two sides of a towed device, said spreading device comprising the features of independent claim <NUM>.

Due to the provision of the bulb, the water speed on the two sides of the spreading device will be different. The bulb will cause the water to travel longer, and consequently due to the bulb an under-pressure will occur on the aft side of the bulb. This under-pressure will create lift, such that the spreading device will be urged in the direction of the bulb.

The spreading device has at least one section, which at least one section comprises at least two foils, in use a front foil and a rear foil, where both foils has a front surface and a rear surface, said surfaces each being defined by two edges and two side limitations, in use being front and rear edges and upper and lower side limitations and a material thickness between said front and rear surfaces. In this manner between the rear surface of the front foil and the front surface of the rear foil is defined a channel between said two foils, where a predetermined distance from the front edge, on the front surface inside the channel, is provided a bulb at least spanning a majority of the distance between the upper and lower side limitations.

Extensive testing, as is evident from table <NUM> below has clearly indicated that by providing a bulb as described above clearly increases the spreading device's lift ability and thereby its spreading ability without increasing the needed pulling force such that increased spreading is obtained with use of less energy, e.g. less pulling force.

The bulb is mounted extending from a front surface inside the channel in order to limit the cross section of the water flow area through the channel created between the rear side of a front foil and the front side of a rear foil such that water is forced to increase speed going through the channel thereby creating the extra lift.

Within the context of this description and claims as well as the figures the term bulb shall be understood to be a member having an extent in three dimensions such that the bulb will have length, a depth and also a thickness. The length is to be understood as a length for at least a part of the distance from the front to the rear of the foil and the thickness may/will vary along the depth, but will extend in use from the surface of the front side of a foil such that the channel in which the bulb is mounted in the area where the bulb is present has a diminished water flow area.

Extensive testing in tanks has indicated that various configurations of the aspect ratios between the various dimensions of the bulb have various effects as is evident from the table below.

Consequently, in some preferred embodiments of the invention the bulb reduces the distance between the front surface of the rear foil to the rear surface of the front foil by between <NUM>% to <NUM> %, more preferred <NUM>% to <NUM>% most preferred <NUM>% to <NUM>% where the distance perpendicular to the foil surface to the bulb is shortest.

According to the invention, and with respect to the arrangement of the bulb on the rear foil, the front of the bulb is displaced between <NUM>% and <NUM>% of the distance between the foils edges from the front edge, more preferred <NUM>% to <NUM>% of the distance between the foils edges from the front edge and most preferred <NUM>% to <NUM>% of the distance between the foils edges, from the front edge of the foil.

In a further advantageous embodiment the bulb has a front edge and a rear edge, substantially parallel to the edges of the foil, and where the distance between the bulb's front and rear edge is between <NUM>% to <NUM>%, more preferred <NUM>% to <NUM>%, and most preferred <NUM>% to <NUM>% of the distance between the foils front and rear edges.

Naturally the shape of the bulb is also important in order to achieve an optimum flow through the channel and thereby achieve the desired lift, and therefore the bulb in a further advantageous embodiment has an upper side and a lower side, said lower side being flush with the front surface of the foil, and said upper side in a cross-section perpendicular to the front surface of the foil, between the front and rear edge of said bulb has either a convex curvature shape, a triangular shape, part of a circle or ellipse or a combination of geometrical figures.

As the invention has influence on the water flow in one channel limited between two adjacent foils and there will create an extra lift compared to not having a bulb in the channel, the invention is also usable and will create extra lift where the spreading device has more than two foils creating two or more channels where each channel is provided with a bulb.

Likewise, by adding more sections to the spreading device, a linear increase corresponding to the number of channels providing added lift will be achieved such that by adding two sections double lift will be achieved, three sections will triple the lift etc..

The invention will now be described with reference to the accompanying drawings wherein.

In <FIG> is illustrated a fishing vessel <NUM> having two lines <NUM>, <NUM>' connected to a trawl (not illustrated). In order to keep the trawl spread out such that it will have an active opening larger than the back of the fishing vessel's stern <NUM> trawl doors <NUM>, <NUM>' are arranged on the wires <NUM>, <NUM>'.

By arranging the trawl doors <NUM>, <NUM>' at an oblique angle α relative to the pulling direction <NUM> it is achieved that the trawl doors <NUM>, <NUM>' will be urged outwards as indicated by the arrows <NUM>, <NUM>'.

By arranging the trawl doors <NUM>, <NUM>' at an oblique angle α an increased drag is put on the wires <NUM>, <NUM>' such that the fishing vessel must exert more force in order to pull the trawl.

In <FIG> and <FIG> are illustrated plain views of a typical simple trawl door. In the examples the trawl doors comprise two sections <NUM>, <NUM> where the sections are connected at <NUM>. Each section has three foils <NUM>, <NUM>, <NUM>, see <FIG>. In this context the foil <NUM> will be denoted the front foil and the foil <NUM> the rear foil with respect to the channel <NUM> delimited by the rear surface <NUM>' of the front foil <NUM> and the front surface <NUM>' of the rear foil <NUM>.

Each foil <NUM>, <NUM>, <NUM> has a front edge and a rear edge where the front edges are at the channel openings <NUM>, <NUM>' and the rear edges adjacent the channel outflows <NUM>, <NUM>'. In the illustrative embodiment illustrated in <FIG> and <FIG> three foils are provided thereby creating two channels <NUM>, <NUM>' having similar definitions with respect to the surfaces as already explained above with reference to the channel <NUM> between the front foil <NUM> and a rear foil <NUM>.

Where the second channel <NUM>' is concerned, the foil <NUM> is to be considered as the front foil and the foil <NUM> as the rear foil delimiting the channel <NUM>' in the same manner as the channel <NUM> is delimited by the front foil <NUM> and the rear foil <NUM>. In the channel <NUM>, <NUM>' between two foils <NUM>, <NUM> or <NUM>, <NUM> at a predetermined distance from the front edge on the front surface <NUM>', <NUM>' inside the channel <NUM>, <NUM>' is provided a bulb <NUM>, <NUM>'.

In a likewise manner more/additional foils may be added.

The plain view of a trawl door as illustrated in <FIG> and <FIG> comprises two sections <NUM>, <NUM> which have side limitations for each section. A common side limitation is the assembly flange <NUM> for the two sections <NUM>,<NUM>, but in upper and lower ends further side limitations <NUM>, <NUM> are provided.

In <FIG> is illustrated a prior art trawl door with more or less the same construction as the present invention, but without the bulbs in order to illustrate in an asymmetric view the various prior art features also present with the present invention.

The bulb <NUM>, <NUM>' is arranged on the front surface <NUM>', <NUM>' of the rear foils <NUM>, <NUM> and covers at least a major part of the foils <NUM>, <NUM> between the side limitations <NUM>, <NUM> and <NUM>, <NUM>.

In <FIG> is illustrated a trawl door substantially identical to the trawl door described above with reference to <FIG>. The difference, however, between the two embodiments is clearly identifiable when comparing <FIG> and <FIG>. The position of the bulbs <NUM>, <NUM>', <NUM>, <NUM>' relative to the front edges of the rear foils <NUM>, <NUM> is different thereby creating a different water flow through the channels <NUM>, <NUM>'.

Furthermore, the sizes of the bulbs <NUM>, <NUM>' is different from the bulbs <NUM>, <NUM>' described with respect to the embodiment related to <FIG>. It is clear that the trawl doors in the two different embodiments relating to <FIG> and <FIG> will provide a different lift and that further modifications, i.e. by placing the bulb at other positions on the surfaces of the foils and the physical size and geometrical shape of the foils may be varied and still create different lift conditions.

These different lift conditions are interesting for different types of tasks which the trawl doors are designed to solve. For example relatively shallow bulbs may be used at higher speed and still create a very distinct extra lift compared to a trawl door without the bulbs whereas for lower speeds and thereby lower water speeds through the channels <NUM>, <NUM>' thicker bulbs, i.e. bulbs that will reduce the distance between the front foil and the rear foil, i.e. the cross-flow area of the channel <NUM> may be used.

Also depending on the angle α, see <FIG>, which it is desired to arrange the trawl doors at, will have an impact on the size and position of the bulbs such that for each desired situation a particular foil design with respect to the position of the bulb on the top surface, the bulb's extent between the front edge and rear edge of the foil and the thickness of the foil may be determined specifically for particular tasks.

In <FIG> the water flow through the channels <NUM>, <NUM>' is illustrated. For illustrative purposes only flow lines <NUM>-<NUM> are illustrated, but naturally the entire water flow cross the section of the channels <NUM>, <NUM>' will be effected by the bulbs <NUM>, <NUM>'. From this illustration it is clear that the water flow in the channels <NUM>, <NUM>' where the bulb <NUM>, <NUM>' is positioned is influenced by the bulb <NUM>, <NUM>', see the water-flow lines <NUM>, <NUM>, <NUM>', <NUM>' whereas the water-flow along the rear-side of the foil <NUM>, <NUM> indicated by water-flow lines <NUM>, <NUM>' is more or less unaffected.

Consequently, as the water speed in the channels <NUM>, <NUM>' is forced to be increased due to the bulb <NUM>, <NUM>', the bulb will create an extra lift indicated by the arrows <NUM> whereby the overall trawl door is provided with extra lift as compared to for example the trawl door illustrated with reference to <FIG>.

In the examples illustrated in <FIG> the height <NUM> of both bulbs <NUM>, <NUM>' is <NUM>% of the length <NUM> of the bulb. This causes with respect to the foil <NUM> that the water travelling along the front surface <NUM>' of the foil, for example illustrated by flow-line <NUM> has to travel a distance which is <NUM>% longer than the water flow <NUM> along the rear-side <NUM>" of the same foil.

With respect to the rear foil <NUM> the extra distance the water-flow <NUM>' has to travel with the same bulb construction <NUM>' as the bulb <NUM> is <NUM>% longer.

In table <NUM> are listed actual test results for a scale model of a trawl door tested with the present invention. The result listed with reference <NUM> is the trawl door where a bulb has been mounted and the results listed under <NUM> is the same trawl door without the bulb.

The column headed by "AOA" indicates the angle of attack, that is to say the angle of the trawl door relative to the travelling direction, and the column headed by "CL" indicates the lift force and the column headed by "CD" indicates the resistance, i.e. extra drag when the trawl door is travelling through the water. The third column headed by "CL/CD" gives an over-all ration of the efficiency of the trawl door such that it is possible to directly compare a trawl door with a bulb and a trawl door without a bulb.

Claim 1:
A spreading device (<NUM>,<NUM>') for trawl fishing, seismic survey operations and other water activities where it is desirable to keep distance between two sides of a towed device, said spreading device (<NUM>,<NUM>') having at least one section (<NUM>,<NUM>), where each section (<NUM>,<NUM>) has at least two foils (<NUM>,<NUM>,<NUM>), in use a front foil (<NUM>) and a rear foil (<NUM>), where both foils (<NUM>,<NUM>) has a front surface (<NUM>",<NUM>') and a rear surface (<NUM>',<NUM>"), said surfaces (<NUM>',<NUM>",<NUM>',<NUM>") each being defined by two edges and two side limitations (<NUM>,<NUM>,<NUM>), in use being front and rear edges and upper and lower side limitations and a material thickness between said front and rear surfaces, such that between the rear surface (<NUM>') of the front foil (<NUM>) and the front surface (<NUM>') of the rear foil (<NUM>) is defined a channel (<NUM>,<NUM>') between said two foils (<NUM>,<NUM>),
characterized in that at a predetermined distance from the front edge, on the front surface (<NUM>') of the rear foil (<NUM>) inside the channel (<NUM>,<NUM>'), is provided a bulb (<NUM>) at least spanning a majority of the distance between the upper and lower side limitations (<NUM>,<NUM><NUM>), wherein the front of the bulb is displaced between <NUM>% and <NUM>% of the distance between the foils edges from the front edge of the rear foil (<NUM>), more preferred <NUM>% to <NUM>% of the distance between the foils edges from the front edge of the rear foil (<NUM>), and most preferred <NUM>% to <NUM>% of the distance between the foils edges, from the front edge of the rear foil.