Patent Description:
Trawl doors for trawling have been used for more than half a century. The trawl doors are hydrodynamic wings which upon being towed by the towing vessel (trawler) will generate outwards directed forces, such that the starboard trawl door will be forced outwards in the starboard direction and the port trawl door will be forced outwards in the port direction. The trawl doors are connected to the towed trawl by bridles, and as the trawl doors are forced away from each other by the hydrodynamic spreading forces the trawl will gradually open laterally until the desired trawl opening spread is achieved.

Over the past decade developments towards controllable trawl doors have appeared, such that the skipper on the bridge has remote control of the trawl doors and can adjust their position laterally and/or vertically in the water during the trawling operation. The potential advantages being that the trawl spread can be kept constant at all times, and that the trawling depth can be quickly adjusted to catch the fish located on the ship's sonar equipment. Different control actuation concepts have been proposed. Common for most concepts is the implementation of at least two actuators on each trawl door: one on the upper half and one on the lower half of the vertically oriented trawl door, such that individual actuation of the actuators can enable the door to increase or decrease the spreading force on both upper and lower half, or decrease spreading force on lower half while increasing on upper half, such that the trawl door will rotate slightly (i.e. around the horizontal axis) and deflect the spreading force direction slightly downwards so the trawl door will go deeper in the water, and vice versa if the trawl door is to go higher in the water. Examples of existing or previously described control concepts are:.

The pivotable flaps is the most used control concept and has its roots in the aviation industry where it has been used on aircrafts for more than a century. A disadvantage of the pivotable flaps is that the flap pivoting motion is directed perpendicular to the flap surface, and hence parallel to the water pressure forces acting on the flap. Water is a heavy medium and at typical towing velocities of <NUM>-<NUM> knots substantial pressure forces will impact a pivotable flap. Consequently, when the flap rotates in one of its two actuation directions, the mechanical work - and thus the discharge energy needed from the battery power supply - needed to overcome the water pressure forces is substantial. A disadvantage of the sliding surfaces is the friction, wear, and tear involved in the sliding motion, which is also why this concept is mostly of academic interest and has not yet been implemented on full scale trawl doors. A common disadvantage of both pivotable and slidable flaps and slidable foil extensions is the mechanical vulnerability. Flaps and foil extension are lifting-force surfaces that contribute to the overall spreading force of the trawl door, and their exposed position on the trawl door is therefore difficult to protect from possible bumps and accidental mechanical impacts, hence, it is an object of the present invention to provide a trawl door with a robust energy-efficient control concept.

<CIT> discloses a spreading device in the form of a trawl door, deflector, vane or paravane constructed with adjustable panels and driving units for controlling the water flow through the spreading device during towing through the water with remote control where driving units are used to adjust the position of the panels to control the water flow through the spreading device and maneuver the spreading device in optimum position in the towing direction, horizontally or vertically, wherein the panels are adjustable independently to give enhanced performance of the operating systems, fishing trawl or seismic survey system, without having to pull the system to the towing vessel to adjust manually.

Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which this invention pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined in the present specification.

All directions in regards to the trawl door as disclosed herein, such as upper, lower, horizontal forward/mid and aft end, all relate to the positions in or on the trawl door when the trawl door is in its operational position, such as in the water during trawl operation.

Disclosed herein in a first aspect of the invention defined in claim <NUM> is a trawl door comprising a plurality of hydrofoils extending in parallel in a longitudinal direction of the hydrofoils and being arranged in an overlapping sequence so that one or more flow channels are formed between neighbouring hydrofoils, one or more shutters extending in the longitudinal direction of the plurality of hydrofoils and arranged at a pressure side of the plurality of hydrofoils, drive means for rotating the one or more shutters about a pivoting axis extending in the longitudinal direction of the plurality of hydrofoils, wherein the drive means are arranged to rotate at least one of the shutters between a disengaged position and an engaged position, in which the engaged position of the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door.

The disclosed trawl door has an energy-efficient control concept. The drive means is placed to control the position of the one or more shutters. Each shutter can be controlled by the drive means and rotate around the pivoting axis. The energy amount required to make the shutters pivot around their pivoting axis is kept minimal by orientating said controllable shutters such that they, upon pivoting, move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters. As disclosed herein, the term "substantially parallel" is used to describe a flat or curved plate motion, which is in-planar within a +/- <NUM> degree tolerance.

As disclosed herein, the term "substantially perpendicular" is used to describe pressure force directions that are oriented <NUM> degrees off the flat or curved plate motion within a +/- <NUM> degree tolerance.

As used herein, the term "energy efficiency" is used in the sense that very little mechanical work is needed to pivot the parts of the trawl door, which change the trawl door lift forces and positional equilibrium. This is obtained by letting the shutters pivot, not slide, around a rotational centre, such that said shutters main surfaces move substantially perpendicular to the water pressure forces instead of substantially parallel to the water pressure forces, when the trawl door is used during trawl operations. Via this setup the main surfaces of the shutters must overcome only small water friction forces, however the otherwise dominating water pressure forces are reduced to a minimum, and hereby the trawl door obtains an energy-efficient control concept.

In physical terms, the mechanical work (i.e. energy) needed to move an object; say from point A to point B in a pressure field, is described by the vector integration from A to B of pressure force times distance increment. If the pressure force vector is aligned (parallel) with the moved distance, then mechanical work is performed. Contrarily, if the pressure force vector is perpendicular or almost perpendicular to the moved distance, then the mechanical work remains zero, or almost zero, such that the movement requires no or very little energy.

The term "robust" is used in the sense that the moveable shutters main surfaces are easily positioned inside the trawl door in a manner that protects them and prevents them from bumps and mechanical hard impacts, e.g. when the trawl door is dumped into the seabed, during trawl door transportation, or when the trawl door is hauled up from the sea and wire-pulled to a hard stop against the back end of the ship hull.

Disclosed herein in a second aspect of the invention defined in claim <NUM> is use of a set of trawl doors comprising one starboard trawl door and one port trawl door, for trawling operations, wherein each of the trawl doors comprises a plurality of hydrofoils extending in parallel in a longitudinal direction of the hydrofoils and being arranged in an overlapping sequence so that one or more flow channels are formed between neighbouring hydrofoils, one or more shutters extending in the longitudinal direction of the plurality of hydrofoils and arranged at a pressure side of the plurality of hydrofoils, drive means for rotating the one or more shutters about a pivoting axis extending in the longitudinal direction of the plurality of hydrofoils, wherein the drive means are arranged to rotate at least one of the shutters between a disengaged position and an engaged position, in which engaged position the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door.

Disclosed herein in a third aspect of the invention defined in claim <NUM> is use of a set of trawl doors comprising one starboard trawl door and one port trawl door, for towing a fishing trawl net, wherein a fishing vessel is towing the set of trawl doors, and wherein each of the trawl doors comprises a plurality of hydrofoils extending in parallel in a longitudinal direction of the hydrofoils and being arranged in an overlapping sequence so that one or more flow channels are formed between neighbouring hydrofoils, one or more shutters extending in the longitudinal direction of the plurality of hydrofoils and arranged at a pressure side of the plurality of hydrofoils, drive means for rotating the one or more shutters about a pivoting axis extending in the longitudinal direction of the plurality of hydrofoils, wherein the drive means are arranged to rotate at least one of the shutters between a disengaged position and an engaged position, in which engaged position the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door.

Disclosed herein in a fourth aspect of the invention defined in claim <NUM> is use of a set of trawl doors comprising one starboard trawl door and one port trawl door, for towing seismic equipment for seabed measurements for measuring the seabed composition, and wherein each of the trawl doors comprises a plurality of hydrofoils extending in parallel in a longitudinal direction of the hydrofoils and being arranged in an overlapping sequence so that one or more flow channels are formed between neighbouring hydrofoils, one or more shutters extending in the longitudinal direction of the plurality of hydrofoils and arranged at a pressure side of the plurality of hydrofoils, drive means for rotating the one or more shutters about a pivoting axis extending in the longitudinal direction of the plurality of hydrofoils, wherein the drive means are arranged to rotate at least one of the shutters between a disengaged position and an engaged position, in which engaged position the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door.

The term "comprising", "comprises", or "to comprise" is to be interpreted as specifying the presence of the stated parts, steps, features, or components, but does not exclude the presence of one of more additional parts, steps, features, or components.

In the following, exemplary embodiments of the invention are described in more detail with reference to the figures, of which.

Detailed description of the invention The invention relates to a trawl door comprising a plurality of hydrofoils extending in parallel in a longitudinal direction of the hydrofoils and being arranged in an overlapping sequence so that one or more flow channels are formed between neighbouring hydrofoils, one or more shutters extending in the longitudinal direction of the plurality of hydrofoils and arranged at a pressure side of the plurality of hydrofoils, drive means for rotating the one or more shutters about a pivoting axis extending in the longitudinal direction of the plurality of hydrofoils, wherein the drive means are arranged to rotate at least one of the shutters between a disengaged position and an engaged position, in which engaged position the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door.

The disclosed trawl door has an energy-efficient control concept. The drive means is placed to control the position of the one or more shutters. Each shutter can be controlled by the drive means and rotate around the pivoting axis. The energy amount required to make the shutters pivot around their pivoting axis is kept minimal by orientating said controllable shutters such that they, upon pivoting, move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters.

Hence, the shutters are orientated such that they, upon pivoting during operation of the trawl door, move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters. This means that very little mechanical work is needed to pivot the parts of the trawl door, which change the trawl door lift forces and positional equilibrium. This is obtained by letting the shutters pivot, not slide, around a rotational centre, such that said shutters main surfaces move substantially perpendicular to the water pressure forces instead of substantially parallel to the water pressure forces, when the trawl door is used during trawl operations. Via this setup the main surfaces of the shutters must overcome only small water friction forces, however the otherwise dominating water pressure forces are reduced to a minimum, and hereby the trawl door obtains an energy-efficient control concept. Hence, the shutters pivot around the pivoting axis, such that the shutters main surfaces move substantially perpendicular to the water pressure forces during trawl operations.

In one or more embodiments, the shutters are orientated such that the water pressure forces are directed substantially through the shutter pivoting axis during trawl operations.

The disclosed trawl door may comprise an upper half and a lower half when vertically positioned, as during normal trawl operation, hence, in one or more embodiments, the trawl door is made of a first half and a second half. The trawl door may however, not be limited to be made of only two halves.

If the trawl door comprises an upper half and a lower half, a first drive means is placed to control the position of one or more shutters placed in the upper half of the trawl door, and a second drive means is placed to control the position of the one or more shutters placed in the lower half of the trawl door.

In one or more embodiments, the trawl door is made of a first half and a second half, wherein the first half comprises a first set of one or more shutters and the second half comprises a second set of one or more shutters, and wherein the first drive means is placed to control the position of the first set of one or more shutters and the second drive means is placed to control the position of the second set of one or more shutters.

By having at least two sets of one or more shutters, arranging them in two halves of the trawl door, and by being able to engage/disengage only one, or both of them, individually (via the drive means), the trawl door is able to not only be controlled in one direction, but in two directions, i.e. side-to-side and up-and-down.

In one or more embodiment, the trawl door comprises additional drive means, such as a third and a fourth drive means, such as a third, a fourth, a fifth, and a sixth drive means, or such as a third, a fourth, a fifth, a sixth, a seventh, and an eighth drive means.

In one or more embodiments, the trawl door further comprises one or more front wire attachment holes and one or more rear bridle attachment holes. The front wire attachment holes and rear bridle attachment holes may be in the horizontal forward/mid and aft end of the trawl door, respectively.

In one or more embodiments, the trawl door comprises at least two shutters, such as at least three, such as at least four, such as at least five, or such as at least six shutters.

Each shutter can rotate around a pivoting axis, where a fixation device may structurally connect the shutters to the pivoting axes. The energy amount required to make shutters pivot around their pivoting axis is kept minimal by orientating said shutters such that they, upon pivoting, move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters. Hence, the shutters are configured to move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters, and said forces pointing substantially through the shutter pivoting axis, when the shutters pivot around their pivoting axis during operation of the trawl door.

Further, the shutters are configured to move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters, and said forces pointing substantially through the shutter pivoting axis, when the shutters pivot around their pivoting axis during operation of the trawl door.

The pivoting around the axis may be limited, such that entire revolutions of the shutters around the pivoting axis is not possible. By an entire revolution of the shutters around the pivoting axis is meant that the shutters is rotated <NUM> degrees around the axis without being blocked of forced into other parts of the trawl door.

In one or more embodiments, the trawl door comprises at least two fixation devices, such as at least three, such as at least four, such as at least five, or such as at least six fixation devices.

In one or more embodiments, the one or more shutters are split into a first set of one or more shutters and a second set of one or more shutters, and wherein a first drive means is arranged to control the position of the first set of the one or more shutters and a second drive means is arranged to control the position of the second set of the one or more shutters.

The first and the second drive means arranged to control the position of the one or more shutters may be actuators, which means that when the actuator is extended, the shutters are rotated around the pivoting axis in one direction, and when the actuators are retracted, the shutters are rotated around the pivoting axis in the opposite direction.

In one or more embodiments, the drive means are one or more actuators arranged to rotate at least one of the shutters between the disengaged position and the engaged position. In another embodiment, the drive means are one or more gear devices comprising at least two gear parts.

In one or more embodiments, the trawl door further comprises one or more front wire attachment holes. Front wire attachment holes are used to connect one or several wires from the fishing vessel to the trawl door, so the trawl door may be used during trawling operations.

In one or more embodiments, the one or more front wire attachment holes are positioned in the horizontal forward/mid of the trawl door.

In one or more embodiments, the trawl door further comprises one or more rear bridle attachment holes. Rear bridle attachment holes may be used to connect one or several wires from the fishing net/trawling net to the trawl door, so the trawl door may be used during trawling operations for dragging a fishing net/trawling net. The trawl door may also be used for towing seismic equipment for seabed measurements used for measuring the seabed composition.

In one or more embodiments, the one or more rear bridle attachment holes are positioned in the aft end of the trawl door.

In one or more embodiments, the trawl door further comprises section endplates and stiffeners.

The drive means is arranged to operate the shutters from a disengaged positon to an engaged position and vice versa. In one or more embodiments, the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door by at least <NUM>%. A <NUM>% reduction may also be seen as a reduction of the flow by a factor of two, i.e. halving the flow through the flow channel in the engaged state compared to the disengaged position.

In one or more embodiments, the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door by at least <NUM>%. A <NUM>% reduction may also be seen as a reduction of the flow by a factor of four, i.e. only a quarter of the flow is let through the flow channel in the engaged state compared to the disengaged position.

In one or more embodiments, the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door by at least <NUM>%. A <NUM>% reduction may also be seen as a reduction of the flow by a factor of ten, i.e. only a one tenth of the flow is let through the flow channel in the engaged state compared to the disengaged position.

In one or more embodiments, the at least one shutter substantially reduces or prevents a fluid flow through one or more of the flow channels during operation of the trawl door by at least <NUM>%. A <NUM>% reduction may also be seen as a reduction of the flow by a factor of one hundred, i.e. only a one hundredth of the flow is let through the flow channel in the engaged state compared to the disengaged position.

The shutter may also almost completely block the flow through the flow channel, hence, in one or more embodiments, the at least one shutter substantially blocks a fluid flow through one or more of the flow channels during operation of the trawl door.

In one or more embodiments, the pivoting axis is arranged to allow rotational movement of the at least one shutter of up to <NUM> degrees.

In one or more embodiments, the shutter is not allowed full revolutions around the pivoting axis.

In one or more embodiments, the shutter is configured to prevent full revolutions around the pivoting axis.

In one or more embodiments, the pivoting axis is arranged to allow rotational movement of the at least one shutter of up to <NUM> degrees, such as up to <NUM> degrees, such as up to <NUM> degrees, such as up to <NUM> degrees, such as up to <NUM> degrees, such as up to <NUM> degrees, but without allowing a full revolutions around the pivoting axis.

The pivoting around the axis may be limited, such that entire revolutions of the shutters around the pivoting axis is not possible. By an entire revolution of the shutters around the pivoting axis is meant that the shutters are rotated <NUM> degrees around the axis without being blocked or forced into other parts of the trawl door.

The trawl door may further comprise one or more fixation devices, which may be used to structurally connect the shutters to the pivoting axes. Hence, in one or more embodiments, the trawl door further comprises one or more fixation devices, wherein the one or more fixation devices are arranged to structurally connect the at least one shutter to the pivoting axis.

In one or more embodiments, the plurality of hydrofoils at least comprises the first hydrofoil defining the leading edge of the overlapping sequence of the plurality of hydrofoils and the last hydrofoil defining the trailing edge of the overlapping sequence of the plurality of hydrofoils. The plurality of hydrofoils, as disclosed herein, may be multiple hydrofoils, such as <NUM>, <NUM>, <NUM>, <NUM> hydrofoils. The sequence of hydrofoils will be defined by a leading edge, the foremost edge when dragging/trawling the trawl door through water during trawling operation, and a trailing edge, the rearmost edge when dragging/trawling the trawl door through water during trawling operation.

In one or more embodiments, the plurality of hydrofoils at least comprises the first hydrofoil defining the leading edge of the overlapping sequence of the plurality of hydrofoils and the last hydrofoil defining the trailing edge of the overlapping sequence of the plurality of hydrofoils and one or more middle hydrofoils positioned between the first hydrofoil and the last hydrofoil.

In one or more embodiments, the one or more middle hydrofoils comprises at least two hydrofoils.

In one or more embodiments, the plurality of hydrofoils comprises at least three hydrofoils. In one or more embodiments, the plurality of hydrofoils comprises at least four hydrofoils.

In one or more embodiments, the trawl door further comprises at least one round-shaped elongated element, wherein the at least one round-shaped elongated element extends in a longitudinal direction parallel to the longitudinal direction of the hydrofoils.

It is known from fluid dynamic theory, (see e.g. <NPL>), that a circular tube ideally has the highest potential of all shapes for deflecting a flow, i.e. for creating lift force. The word "ideally" means that it is assumed there is no hydrodynamic/aerodynamic stall. However, in real life stall this is unavoidable. Any non-streamlined geometry, such as box-shaped, circular, or otherwise blunt bodies will create flow separation and subsequent stall when the flow fails to run smoothly over the geometry's entire surface.

Furthermore, a circular tube has an omni-directional cross-section, so lift-forces will appear symmetrically around the tube thereby cancelling each other.

A round-shaped elongated element as disclosed herein is an element, which in its cross-section is circular or elliptically shaped. This does not mean that the element needs to be perfectly elliptical, but can have minor variation in its cross-sectional shape. However, the element as disclosed herein is not a hydrofoil or has a hydro-profile shape with a defined leading edge and trailing edge.

Compared to standard multi-element trawl doors, the use of a round-shaped elongated element can improve the lift coefficient by approximately <NUM>%.

An additional benefit is that the absence of hydrofoil characteristics such as a leading edge and a trailing edge on the tube element causes the tube element to be quite insensitive to incidence angle between the flow passing the trawl door and the trawl, which is also known as the Angle-of-Attack (AoA). A trawl door with a tube element will therefore be able to operate well and create a high lift force over a large AoA range of up to <NUM> degrees. For standard known trawl doors, the operative AoA range is approximately <NUM> degrees. A higher operative AoA range is a benefit in general, and specifically when the trawl doors experience different AoAs, e.g. when fishing in crosscurrent conditions and/or when the trawler makes a <NUM> degree turn.

In one or more embodiments, the at least one round-shaped elongated element is positioned at a pressure side of the plurality of hydrofoils and at a rear part of the overlapping sequence of the plurality of hydrofoils in such a way that the at least one round-shaped elongated element forms a structurally connected integrated part to the plurality of hydrofoils such that at least one flow channel is formed between the at least one round-shaped elongated element and the last hydrofoil of the overlapping sequence of the plurality of hydrofoils.

Using a circular tube as an ideal lift-creating hydrodynamic element, by combining it with a series of regular hydrofoils, such that the flow passing the tube, which would otherwise separate and create stall, is guided inside a flow channel between one side of the circular tube and one or several of the regular hydrofoils, such that stall is completely suppressed or at least substantially delayed. Elimination of stall on one side of the circular tube will create a high deflection of the flow around the tube, in fluid dynamic theory known as "circulation", thereby creating lift.

The circular tube will not only create lift acting on the circular tube itself, but the circulation will also increase the lift forces created by the regular hydrofoils, such that the lift coefficient of the entire trawl door is increased.

The technical term "lift coefficient" is a dimensionless coefficient that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a complete foil-bearing body such as a fixed-wing aircraft or a trawl door.

The flow channel formed between the round-shaped elongated element and the last hydrofoil of the overlapping sequence of hydrofoils is arranged to guide a flow of water and is arranged to suppress hydrodynamic stall, which would otherwise be generated around the round-shaped elongated element, if the overlapping sequence of hydrofoils were not there.

The width of the flow channel formed between the elongated element and the last hydrofoil may be defined in terms of the width of the other flow channels formed between neighbouring hydrofoils. Hence, in one or more the width of the at least one flow channel formed between the at least one round-shaped elongated element and the last hydrofoil of the overlapping sequence of the plurality of hydrofoils is between <NUM>% and <NUM>% of the width of the one or more flow channels formed between neighbouring hydrofoils.

By the width of the one or more flow channels formed between neighbouring hydrofoils is meant that if there is multiple channels between neighbouring hydrofoils, and if the width of these flow channels are not substantially the same, and average of the width of these flow channels is taken and used as "the width of the one or more flow channels formed between neighbouring hydrofoils".

If it is assumed that the cross-section of the elongated element is perfectly elliptical, that means that the minor variation in its cross-sectional shape can be disregarded, the elongated element can further be described via the eccentricity. The eccentricity of an ellipse has a well-known mathematical meaning/definition, which a person skilled in the art is fully aware of, and knows how to calculate. If the cross-sectional shape is not perfectly elliptical, the eccentricity (e) may be defined as e = (<NUM>-b^<NUM>/a^<NUM>)^(<NUM>/<NUM>), where a is the length of the major axis and b is the length of the minor axis.

In one or more embodiments, the at least one round-shaped elongated element has an eccentricity at or below <NUM>, such as at or below <NUM>, such as at or below <NUM>, such as at or below <NUM>, such as at or below <NUM>.

In one or more embodiments, the at least one round-shaped elongated element has an eccentricity at or above <NUM>, such as at or above <NUM>.

In one or more embodiments, the at least one round-shaped elongated element has an eccentricity between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>.

In one or more embodiments, the at least one round-shaped elongated element has an eccentricity between <NUM> and <NUM> such as between <NUM> and <NUM>.

In one or more embodiments, the at least one round-shaped elongated element has an eccentricity between <NUM> and <NUM>, such as between <NUM> and <NUM>.

In one or more embodiments, the trawl door has a chord length defined as the chord length measured from a leading edge of a first hydrofoil of the overlapping sequence of the plurality of hydrofoils to a trailing edge of a last hydrofoil of the overlapping sequence of the plurality of hydrofoils.

In one or more embodiments, the at least one round-shaped elongated element has an elliptically shaped cross-section, having a minor axis and a major axis, and wherein the major axis is between <NUM>% and <NUM>% of the chord length and the ratio between the minor axis and the major axis is between <NUM> and <NUM>.

The size of the elongated element is further defined by the length of the major axis of the elongated element compare to the chord length of the trawl door. The chord length of the trawl door is defined as the chord length measured from the leading edge of the first hydrofoil to the trailing edge of the last hydrofoil. In one or more embodiments, the major axis is between <NUM>% and <NUM>% of the chord length, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, of the chord length.

In one or more embodiments, the major axis is between <NUM>% and <NUM>% of the chord length, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>% of the chord length.

In one or more embodiments, the major axis is between <NUM>% and <NUM>% of the chord length, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>%, such as between <NUM>% and <NUM>% of the chord length.

The cross-sectional shape of the elongated element is further defined by the ratio between the minor and major axes of the elongated element. In one or more embodiments, the ratio between the minor axis and the major axis is between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>.

In one or more embodiments, the ratio between the minor axis and the major axis is between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>.

In one or more embodiments, the ratio between the minor axis and the major axis is between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>, such as between <NUM> and <NUM>.

The pivoting axis for rotating the one or more shutters about, which extends in the longitudinal direction of the plurality of hydrofoils, may be located inside the elongated element. Hence, in one or more embodiments, the pivoting axis is located inside the at least one round-shaped elongated element.

Further technical equipment may be positioned inside the elongated element. In one or more embodiments, technical equipment for control of the trawl door is positioned inside the at least one round-shaped elongated element.

In one or more embodiments, the at least one round-shaped elongated element comprises an interior cavity.

An interior cavity as disclosed herein is to be understood as a cavity, which are in fluid connection with water during trawling operation, i.e. the cavity is to be flooded with water whenever the trawl door is submerged in water, e.g. during normal trawling operation. This means, that the cavity is not an interior closed cavity, but at least comprises one opening in which it can be in fluid connected with water.

In one or more embodiments, the interior cavity is in fluid connection with water surrounding the at least one round-shaped elongated element during operation of the trawl door.

In one or more embodiments, the interior cavity extends through the at least one round-shaped elongated element in the longitudinal direction of at least one round-shaped elongated element. The interior cavity of the at least one round-shaped elongated element may comprise electro-mechanical devices, such as sensor equipment, battery power, or hydraulic systems. The cavity may also comprise other components, such as buoyancy or gravity elements.

In one or more embodiments, a first part of the interior cavity comprises one or more buoyancy elements. In one or more embodiments, a second part of the interior cavity comprises one or more gravity elements.

A buoyancy element is an element, with a mass density lower than the mass density of water, such as seawater. The buoyancy element may e.g. be made of syntactic foam encapsulated in a polyethylene external skin. The buoyancy element can be optimized to reach the required buoyancy at operating water depth.

A gravity element is an element, with a mass density higher than the mass density of water, such as seawater. The gravity element may e.g. be made of iron or another density heavy material. The gravity element can be optimized to reach the required volume gravity mass at operating water depth.

The elongated element may comprise additional holes being in fluid connection with the surrounding water during trawling operations. Hence, in one or more embodiments, the at least one round-shaped elongated element comprises an interior cavity and wherein one or more holes extends perpendicular to the longitudinal direction of the at least one round-shaped elongated element and extends from the interior cavity through the at least one round-shaped elongated element. In one or more embodiments, the one or more holes are elliptically shaped.

By having these holes extending from the cavity of the elongated element and through the element easy and quick flooding of the cavity is obtained when the trawl door is submerged in water. Furthermore, these holes enable easy access to, and maintenance of, any electro-mechanical equipment being located inside the round elongated element cavity.

In one or more embodiments, the interior cavity comprises electro-mechanical devices, such as sensor equipment, battery power, or hydraulic systems.

In one or more embodiments, the at least one round-shaped elongated element further comprises a fixed add-on flap. A fixed add-on flap may be attached to the element to create extra fluid circulation. This may result in a slightly better lift coefficient, or help in preventing stall.

The shutters may be pivoted around the pivoting axis such that they are positioned inside the round-shaped elongated element when in disengaged position. Hence, in one or more embodiments the shutters, pivoting axis, and the round-shaped elongated element are all configured to pivot the shutters around the pivoting axis such that they are substantially positioned inside the round-shaped elongated element when in a disengaged position. By substantially is meant that a smaller part, such as <NUM>% or less, such as <NUM>% or less, such as <NUM>% or less of the total area of the shutters are extending outside the circumference of the round-shaped elongated element.

In order for the round-shaped elongated element to be configured to pivot the shutters around the pivoting axis such that they are substantially positioned inside the round-shaped elongated element when in a disengaged position, requires that there is an opening in the longitudinal direction of the round-shaped elongated element. The opening should at least have a size corresponding to the size of the shutters, preferably between <NUM>% and <NUM>% larger in circumference compared to the circumference of the shutters. Although the longitudinal opening in the round-shaped elongated element makes the element slightly weaker structurally, this is not seen as a significant weakening as the construction is plenty strong and the opening is in a zone with only small structural tensions.

Further, by pivoting the shutters around the pivoting axis and into the inner of the round-shaped elongated element, the inactive shutters does not interfere with the flow at all or only interfere to a minor degree. Additionally, relative short shutters limit the force needed for actuation/movement around the pivoting axis when going from disengaged to engaged position or vice versa. Lastly, the disengaged shutters are well protected inside when located inside the round-shaped elongated element when the trawl door is inactive, e.g. during transport or other handling or when the trawl door hang inertly on the rear of the fishing vessel.

The trawl door may be divided into two or several sections. In one or more embodiments, the trawl door is divided into a first half and a second half, wherein the first half comprises a first half of the plurality of hydrofoils and a first half of the one or more shutters, and wherein the second half comprises a second half of the plurality of hydrofoils and a second half of the one or more shutters. The trawl door may also be divided into three parts, four parts, five parts, etc..

In one or more embodiments, the trawl door is divided into a first half and a second half, wherein the first half comprises a first half of the plurality of hydrofoils, a first half of the one or more shutters, and a first half of the at least one round-shaped elongated element, and wherein the second half comprises a second half of the plurality of hydrofoils a second half of the one or more shutters, and a second half of the at least one round-shaped elongated element.

If the trawl door is divided into two halves, the cavity in one-half of the element may contain buoyancy elements. In one or more embodiments, the interior cavity of the first half of the at least one round-shaped elongated element comprises one or more buoyancy elements. The cavity in the other half of the element may contain gravity elements. In one or more embodiments, the interior cavity of the second half of the at least one round-shaped elongated element comprises one or more gravity elements.

The shape of the trawl door in the span wise direction may vary. In one or more embodiments, the trawl door is linear along the span wise direction. In another embodiment, the trawl door is curved, bent, or angled, along the span wise direction. In yet another embodiment, the trawl door comprises two halves, wherein the two halves are linear along the span wise direction of each half, and wherein the two halves are connected to form a non-linear angle of the trawl door along the span wise direction of the trawl door.

By "span wise" is meant the dimension along the length of the trawl door perpendicular to the passing water when in use. When the trawl door is vertically erected, the span wise direction is the vertical direction.

In practice, a trawl door will probably never be curved in the span wise direction, as it will require double-curved surfaces and thus impossible to make in a simple manufacture using rolled steel sheets. Many trawl doors used today have one or more bends of <NUM>, <NUM>, <NUM>, or <NUM> degrees. This may make the span wise direction of the trawl door appear curved, but in reality, it is constructed of multiple linear pieces connected with bends between each piece.

In one or more embodiments, the trawl door comprises plurality of linear pieces connected with bends of between <NUM> and <NUM> degrees between each piece in the span wise direction.

The shape/arrangement of the trawl door may further be defined. In one or more embodiments, the plurality of hydrofoils are one-sided.

By one-sided is meant that the hydrofoil is made by just one flat or curved plate, such that the pressure side and suction side are offset only by the plate thickness.

In one or more embodiments, the plurality of one-sided hydrofoils are made by rolled metal sheets.

In one or more embodiments, the plurality of hydrofoils are double-sided.

In one or more embodiments, the plurality of hydrofoils are hybrids between single-sided and double-sided.

The trawl door as disclosed herein may be used for multiple variations of trawl operation. Hence, in one or more embodiments, the trawl door is of pelagic type, semi-pelagic type, bottom-door type, or combinations hereof.

The hydrofoils, shutters, and/or the elongated element may be made of the same or different material. In one or more embodiments, one or more of the plurality of hydrofoils, and/or the one or more shutters, and/or the at least one round-shaped elongated element are made from metal or a lightweight material such as plastic or rubber, wood, or fibreboard material.

In one or more embodiments, one or more of the plurality of hydrofoils are made from metal or a lightweight material such as plastic or rubber, wood, or fibreboard material.

In one or more embodiments, one or more of the shutters are made from metal or a lightweight material such as plastic or rubber, wood, or fibreboard material.

In one or more embodiments, the at least one round-shaped elongated element is made from metal or a lightweight material such as plastic or rubber, wood, or fibreboard material.

When describing the embodiments, the combinations and permutations of all possible embodiments have not been explicitly described. Nevertheless, the mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. The present invention envisage all possible combinations and permutations of the described embodiments.

Various examples are described hereinafter with reference to the figures. It should also be noted that the figures are only intended to facilitate the description of the examples. In addition, an illustrated example needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular example is not necessarily limited to that example and can be practiced in any other examples even if not so illustrated, or if not so explicitly described.

<FIG> is a perspective view of a conventional trawling operation comprising a (towing) fishing vessel <NUM> towing a set of a starboard and a port trawl doors <NUM>, which provide spreading force for the fishing trawl net <NUM> being towed by said set of trawl doors <NUM>. The figure shows, how the trawl doors <NUM> are connected and trawled by the fishing vessel <NUM> via a wire <NUM> connected to each trawl door <NUM>. These wires <NUM> may be attached as seen in the figure in the mid-section of the trawl door <NUM> e.g. via one or more attachment holes. The fishing trawl net <NUM> is connected to each trawl door <NUM> via two bridles <NUM>. These may be, as shown here, be connected to the trawl door <NUM> in the aft top and bottom of the trawl door <NUM>.

<FIG> is a cross-sectional view of a trawl door with energy-efficient shutters where the pivoting axis <NUM> is inside a round-shaped elongated element <NUM> and the shutters <NUM> are shown in a) disengaged position, b) mid-position, and c) engaged position. The trawl door comprises several hydrofoils <NUM>, each with a leading edge and a trailing edge, and a round-shaped elongated element <NUM> positioned on the pressure side of the several hydrofoils <NUM>. A notable feature is the mix of a round shaped leading edge of the hydrofoils <NUM> and a curved-sheet "single-sided" trailing edge of the hydrofoils <NUM>. The mixed hydrofoils <NUM> still contain the usual hydrofoil characteristics: A leading edge, a trailing edge, slenderness, and camber. The depicted round-shaped elongated element <NUM> comprises the pivoting axis <NUM> on which the shutters <NUM> pivot around when moving from the engaged position c) to the mid-position b) and further to the disengaged position a) (c) <NUM> b) → a)), or vice versa (a) → b) → c)). The shutters <NUM> are shown connected to the pivoting axis <NUM> by fixations devices <NUM>. The embodiment shows how the shutters <NUM> closes two of the flow channels, the flow channel between the last hydrofoil <NUM> and its neighbouring hydrofoil <NUM> and the flow channel between the last hydrofoil <NUM> and the round-shaped elongated element <NUM>. The flow channels are here closed so the substantially blocks all fluid flow through the two flow channels, however a small amount of flow may continue to flow "below" the shutter <NUM> and up through the flow channels.

<FIG> is a cross-sectional view of a trawl door with energy-efficient shutters <NUM> where the pivoting axis <NUM> and drive means <NUM>, are inside a round-shaped elongated element <NUM>, and the shutters <NUM> are shown in a) disengaged position, b) mid-position, and c) engaged position. The drive means <NUM> here comprise of a spindle arrangement used for actuation, which will then pivot the shutters <NUM> around the pivoting axis <NUM>. The shutters <NUM> and drive means <NUM> are shown connected to the pivoting axis <NUM> by fixations devices <NUM>.

<FIG> is a perspective view of the trawl door <NUM> from <FIG>, as seen from two sides a) and b). The trawl door <NUM> first half <NUM> and the trawl door <NUM> second half <NUM>, both comprising hydrofoils <NUM>, leading edges <NUM>, trailing edges, section endplates and stiffeners <NUM>, rear bridle attachment holes <NUM>, and a round-shaped elongated element <NUM>. The trawl door <NUM> is seen from the pressure (concave) side. The bridles (not shown) towing the trawl may be attached to one of the upper and one of the lower rear bridle attachment holes <NUM>, or one bridle will be attached to one of mid rear bridle attachment holes <NUM>. Different types of bridle rigging are possible depending on the type of fishing trawl net to be used and what kind of fish to catch. The main wire (not shown) connecting the trawl door to the towing vessel may be attached to one of the front wire attachment holes <NUM>. Alternative front wire attachment hole <NUM> designs may be used, such as the possibility of having front wire attachment holes <NUM> in different vertical heights slightly offset from the shown span wise mid-pos ition.

Further shows how the round-shaped elongated element <NUM> has an interior cavity with openings towards the surrounding environment. The embodiment shows one opening in the first half <NUM> or the trawl door <NUM>. The opening can be seen extending through the section endplates <NUM>. Further are the elliptically shaped openings (best seen in b)) extending perpendicular to the longitudinal direction of the round-shaped elongated element <NUM>. These holes extends from the interior cavity of the round-shaped elongated element <NUM> through the round-shaped elongated element <NUM>, hereby making the interior cavity being in fluid connection with the surrounding environment.

<FIG> is a cross-sectional view of a trawl door comprising several hydrofoils <NUM>, each with a leading edge <NUM> and a trailing edge <NUM>. The leading edge of the foremost hydrofoil <NUM> is the leading edge <NUM> of the trawl door, and the trailing edge <NUM> of the rearmost hydrofoil is the trailing edge <NUM> if the trawl door. The plurality of hydrofoils <NUM> are arranged so that flow channels <NUM> are formed between neighbouring hydrofoils <NUM>.

In conjunction with the several hydrofoils is a round-shaped elongated element <NUM> positioned such that it forms a flow channel <NUM> with one or several of the hydrofoils <NUM>, and at least form it with the last hydrofoil <NUM>. Said flow channel <NUM> will suppress the flow separation and stall that would otherwise occur when the water flows past the round-shaped elongated element <NUM>, i.e. in the absence of the one or several of the hydrofoils <NUM>. The so-called "circulation" created by the round-shaped elongated element <NUM> will contribute to the overall deflection of the water passing the trawl door, thereby increasing the lift coefficient and enabling the trawl door to provide more spreading force to the towed trawl.

Also shown on <FIG> are the trawl door's imaginary chord line C defined between the leading edge <NUM> of the first hydrofoil <NUM> of the overlapping sequence of the plurality of hydrofoils <NUM> to the trailing edge <NUM> of the last hydrofoil <NUM> of the overlapping sequence of the plurality of hydrofoils <NUM>. Further shown are also the minor D1 and major D2 axes lengths of the round-shaped elongated element <NUM>. When the round-shaped elongated element is perfectly circular, then the minor axis D1 and major axis D2 will have the same length, namely the circle diameter.

The hydrofoils <NUM>, and the trawl door, is arranged so the pressure side <NUM> of the hydrofoil <NUM> is on the concave (inner) side of the cambered hydrofoils <NUM>, and the suction side <NUM> is on the convex (outer) side of said hydrofoils <NUM>. The round-shaped elongated element <NUM> is positioned on the pressure side <NUM> of the hydrofoils <NUM>.

<FIG> is another cross-sectional view of a trawl door comprising several hydrofoils <NUM>, each with a leading edge <NUM> and a trailing edge <NUM>, and a round-shaped elongated element <NUM> positioned on the pressure side <NUM> of the several hydrofoils <NUM>. A notable feature is the mix of curved-sheet "single-sided" hydrofoils <NUM> and regular "double-sided" hydrofoils <NUM>. Both the single-sided and the double-sided hydrofoils <NUM> contain the usual hydrofoil characteristics: A leading edge <NUM> and a trailing edge <NUM>. The round-shaped elongated element <NUM> is not a hydrofoil, and thus characterized by the lack of hydrofoil characteristics such as leading edge, trailing edge, slenderness, and camber. The depicted round-shaped elongated element <NUM> is in the embodiment of <FIG> not perfectly circular but rather elliptic. The elliptical shape may be described by the ratio between the minor axis D1 and the major axis D2 of the round-shaped elongated element <NUM>. Alternatively, it may be described by the eccentricity as disclosed herein. Even though the embodiments as shown in the figures show a round-shaped elongated element <NUM> with a perfect round shape, it is envisioned that the round-shaped elongated element <NUM> may have small curvature, bumps, or creases, which will yield a non-perfect round shape. <FIG> is a cross-sectional view of a trawl door comprising hydrofoils <NUM>, leading edges <NUM>, trailing edges <NUM>, and a round-shaped elongated element <NUM>. One notable feature of the embodiment, as shown in <FIG>, is the exemplified hydrofoil shape being a hybrid between the before mentioned "single-sided" and "double-sided" types. Another notable feature is the flow visualization streamlines <NUM>, which exactly shows the paths of the water in passing the trawl door when towed. The undisturbed direction of the water relative to towed trawl door is horizontally from left towards right. From a fluid dynamics viewpoint, it should be appreciated how the flow channel <NUM> between the round-shaped elongated element <NUM> and the nearest hydrofoil(s) <NUM> guides the flow around said round-shaped elongated element <NUM>, thereby creating fluid dynamic circulation and lift.

<FIG> is a perspective view of the trawl door <NUM> first half <NUM> and the trawl door <NUM> second half <NUM>, both comprising hydrofoils <NUM>, leading edges <NUM>, trailing edges <NUM>, section endplates and stiffeners <NUM>, rear bridle attachment holes <NUM>, and a round-shaped elongated element <NUM>, seen from the suction (convex) side of the hydrofoils <NUM> (trawl door <NUM>). The bridles (not shown) towing the trawl may be attached to one of the upper and one of the lower rear bridle attachment holes <NUM>, or one bridle will be attached to one of mid rear bridle attachment holes <NUM>. Different types of bridle rigging are possible depending on the type of fishing trawl net to be used and what kind of fish to catch.

<FIG> further shows how the round-shaped elongated element <NUM> has an interior cavity with openings towards the surrounding environment. The embodiment shows one opening in the first half <NUM> or the trawl door <NUM>. The opening can be seen extending through the section endplates <NUM>.

<FIG> is a perspective view of an embodiment of the first half <NUM> and second half <NUM> of the trawl door <NUM> similar to the one disclosed in <FIG>. Here the trawl door <NUM> comprises hydrofoils <NUM> with leading edges <NUM> and trailing edges (not shown), section endplates and stiffeners <NUM>, rear bridle attachment holes <NUM>, and a round-shaped elongated element <NUM> with and inner cavity extending through the section endplate <NUM> to be in fluid connection with the surrounding environment. The trawl door <NUM> is seen from the pressure (concave) side. The main wire (not shown) connecting the trawl door to the towing vessel may be attached to one of the front wire attachment holes <NUM>. Alternative front wire attachment hole <NUM> designs may be used, such as the possibility of having front wire attachment holes <NUM> in different vertical heights slightly offset from the shown span wise mid-position. The embodiment of <FIG> further shows a fixed add-on flap <NUM> attached to the round-shaped elongated element <NUM>.

<FIG> is a cross-sectional view of an embodiment of a trawl door with energy-efficient shutters where the pivoting axis <NUM> is inside a round-shaped elongated element <NUM> and the shutters <NUM> are shown in a) disengaged position, b) mid-position, and c) engaged position, and where the shutters <NUM> are pivoted around the pivoting axis <NUM> such that they are positioned inside the round-shaped elongated element <NUM> when in disengaged position a). The trawl door comprises several hydrofoils <NUM>, each with a leading edge and a trailing edge, and a round-shaped elongated element <NUM> positioned on the pressure side of the several hydrofoils <NUM>. A notable feature is the mix of a round shaped leading edge of the hydrofoils <NUM> and a curved-sheet "single-sided" trailing edge of the hydrofoils <NUM>. The mixed hydrofoils <NUM> still contain the usual hydrofoil characteristics: A leading edge, a trailing edge, slenderness, and camber. The depicted round-shaped elongated element <NUM> comprises the pivoting axis <NUM> located inside the round-shaped elongated element <NUM> on which the shutters <NUM> pivot around when moving from the engaged position c) to the mid-position b) and further to the disengaged position a) (c) → b) → a)), or vice versa (a) → b) → c)). The shutters <NUM> are shown connected to the pivoting axis <NUM> by a fixations device <NUM>. The embodiment shows how the shutters <NUM> closes two of the flow channels, the flow channel between the last hydrofoil <NUM> and its neighbouring hydrofoil <NUM> and the flow channel between the last hydrofoil <NUM> and the round-shaped elongated element <NUM>. The flow channels are here closed, substantially blocking all fluid flow through the two flow channels. The embodiment also shows how the shutters <NUM> are pivoted into the round-shaped elongated element <NUM> when being in the disengaged position through a longitudinal opening in the round-shaped elongated element <NUM>.

Although the longitudinal opening in the round-shaped elongated element <NUM> makes the element slightly weaker structurally, this is not seen as a significant weakening as the construction is plenty strong and the opening is in a zone with only small structural tensions.

Further, by pivoting the shutters <NUM> around the pivoting axis <NUM> an into the inner of the round-shaped elongated element <NUM>, the inactive shutters <NUM> does not interfere with the flow at all. Further, relative short shutters <NUM> limits the force needed for actuation/movement around the pivoting axis <NUM> when going from disengaged to engaged position or vice versa. Even further, the disengaged shutters <NUM> are well protected inside when located inside the round-shaped elongated element <NUM> when the trawl door is inactive, e.g. during transport or other handling or when the trawl door hang inertly on the rear of the fishing vessel <NUM>.

<FIG> is a perspective view of embodiment of the trawl door from <FIG>, as seen from two sides a) and b), and where the round-shaped elongated element <NUM> is made invisible in b) for the sole purpose of better illustrating the energy-efficient shutters <NUM> and the pivoting axes <NUM>. The trawl door <NUM> first half <NUM> and the trawl door <NUM> second half <NUM>, both comprising hydrofoils <NUM>, leading edges <NUM>, trailing edges, section endplates and stiffeners <NUM>, rear bridle attachment holes <NUM>, and a round-shaped elongated element <NUM>. The trawl door <NUM> is seen from the pressure (concave) side. The bridles (not shown) towing the trawl may be attached to one of the upper and one of the lower rear bridle attachment holes <NUM>, or one bridle will be attached to one of mid rear bridle attachment holes <NUM>. Different types of bridle rigging are possible depending on the type of fishing trawl net to be used and what kind of fish to catch. The main wire (not shown) connecting the trawl door to the towing vessel may be attached to one of the front wire attachment holes <NUM>. Alternative front wire attachment hole <NUM> designs may be used, such as the possibility of having front wire attachment holes <NUM> in different vertical heights slightly offset from the shown span wise mid-pos ition.

The embodiment further shows how the round-shaped elongated element <NUM> has an interior cavity with openings towards the surrounding environment. The embodiment shows one opening in the first half <NUM> or the trawl door <NUM>. The opening can be seen extending through the section endplates <NUM>. Further are the elliptically shaped openings extending perpendicular to the longitudinal direction of the round-shaped elongated element <NUM>. These holes extends from the interior cavity of the round-shaped elongated element <NUM> through the round-shaped elongated element <NUM>, hereby making the interior cavity being in fluid connection with the surrounding environment.

The embodiment further shows how interior cavity with openings towards the surrounding environment are able to comprise the shutters <NUM> and that the shutters <NUM> are able to be rotated such that they are substantially located inside the round-shaped elongated element <NUM> when in a disengaged position.

<FIG> is a cross-sectional view of an embodiment of a trawl door with energy-efficient shutters where the pivoting axis <NUM> and the drive means <NUM>, are inside a round-shaped elongated element <NUM>, and the shutters <NUM> are shown in a) disengaged position, b) mid-position, and c) engaged position, and where the shutters <NUM> are pivoted around the pivoting axis <NUM> such that they are positioned inside the round-shaped elongated element <NUM> when in disengaged position. The drive means <NUM> here comprise of a spindle arrangement used for actuation, which will then pivot the shutters <NUM> around the pivoting axis <NUM>. The shutter <NUM> and drive means <NUM> are shown connected to the pivoting axis <NUM> by fixations devices <NUM>.

<FIG> is a cross-sectional view of another embodiment of a trawl door with energy-efficient shutters <NUM> where the pivoting axis <NUM> and the drive means, in this embodiment a gear device <NUM>, <NUM>, are inside a round-shaped elongated element <NUM>, and the shutters <NUM> are shown in a) disengaged position, b) mid-position, and c) engaged position, and where the shutters <NUM> are pivoted around the pivoting axis <NUM> such that they are positioned inside the round-shaped elongated element <NUM> when in disengaged position. The gear device comprises a rotation gear <NUM> located inside the round-shaped elongated element <NUM> and a shutter gear <NUM> located on the shutters <NUM>. When the rotation gear <NUM> is rotated (e.g. via a motor) it will move the shutter <NUM> from a disengaged position to an engaged position or vice versa via the shutter gear <NUM>. As depicted in the cross-sectional view of the embodiment, if the rotation gear <NUM> is rotated clockwise the shutters <NUM> will pivot around the pivoting axis <NUM> towards the disengaged position, while if the rotation gear <NUM> is rotated counter clockwise the shutters <NUM> will pivot around the pivoting axis <NUM> towards the engaged position. This configuration can be switched around by adding an additional rotation gear between the rotation gear <NUM> and the shutter gear <NUM>, or in other ways, which are within the skilled person's knowledge.

Claim 1:
A trawl door (<NUM>) comprising
a plurality of hydrofoils (<NUM>) extending in parallel in a longitudinal direction of the hydrofoils (<NUM>) and being arranged in an overlapping sequence so that one or more flow channels (<NUM>) are formed between neighbouring hydrofoils (<NUM>),
one or more shutters (<NUM>) extending in the longitudinal direction of the plurality of hydrofoils (<NUM>) and arranged at a pressure side (<NUM>) of the plurality of hydrofoils (<NUM>), and drive means (<NUM>) for rotating the one or more shutters (<NUM>) about a pivoting axis (<NUM>) extending in the longitudinal direction of the plurality of hydrofoils (<NUM>),
wherein the drive means (<NUM>) are arranged to rotate at least one of the shutters (<NUM>) between a disengaged position and an engaged position, in which engaged position the at least one shutter (<NUM>) substantially reduces or prevents a fluid flow through one or more of the flow channels (<NUM>) during operation of the trawl door (<NUM>),
said trawl door being characterized in that the shutters (<NUM>) are orientated such that they, upon pivoting during operation of the trawl door (<NUM>), move substantially parallel to their own main surfaces such that water pressure forces are directed substantially perpendicular to the moving direction of the shutters (<NUM>).