The invention relates to a system and a method for monitoring a twin trawl being towed by a vessel.

A number of sensor modules are available for use in connection with fishing gear.

Examples of such modules are; symmetry sensor, Trawl Eye sensor, trawl sounder, trawl speed sensor, distance sensor, grid sensor, depth sensor, quantity sensor etc. These modules provide information regarding the position of the fishing gear relative to the seabed, its speed, its direction of travel in relation to water flow, the volume offish inside the fishing gear, etc.

In order that fishing gear should perform efficiently, it is important that during a fishing operation it has a geometry that is as optimal as possible, for example, that the trawl opening is sufficiently large, that the trawl opening is symmetrical, that the trawl bag is spread and is not twisted or skewed etc.

Moreover, the fishing gear will create a certain acoustic image as it moves in the water. Incorrect positioning of the fishing gear could result in an acoustic image that scares away the fish.

It is therefore important to be able, at all times, to check that the geometry of the fishing gear is correct and that the net meshes are taut, and to be able to take the right decisions and actions from the boat in order to achieve efficient and effective fishing. The geometry of the trawl is normally understood as the size of the trawl's opening (height and width), but may also comprise other values such as water velocity and direction of water flow into the trawl during a fishing operation.

In the case of fishing with two interconnected trawls, a so-called "twin trawl", there are additional challenges associated with the trawl geometry. In a twin trawl, each of the trawls is connected to a trawl door at its outer edge, whilst the adjacent edges of the trawls are connected to a "clump" (centre clump). The trawl doors ensure the trawl bags are kept spread and provide stability of the trawl, whilst the clump is a central weight or a lead, for example, a roller weight, which helps to keep the trawl bags spread.

An optimal geometry for a twin trawl requires that the trawl doors and clump should be essentially in alignment with one another. Optimal spreading of the trawl doors, optimal opening of both trawls and a stable movement of the trawls are then achieved. Another requirement for a trawl to fish efficiently and effectively is that the trawl should move through the sea in such a way as to be symmetrical relative to the water flow entering it. When there is no side current, the trawl will be towed with an opening at 90° to the direction of travel of the vessel. When there is a side current, the trawl must be "towed skewed" such that the opening is not at 90° to the direction of travel, but at an angle thereto determined by the strength of the side current and the speed of the vessel.

The object of the invention is to provide a system and a method for monitoring and checking the trawl geometry of a twin trawl so as to have a basis on which to take the right decisions during a fishing operation and to achieve efficient and effective fishing.

The object of the invention is achieved with the aid of the features set forth in the patent claims.

In an embodiment, a system for monitoring a twin trawl being towed by a vessel, the twin trawl consisting of two trawl bags, two trawl doors and a clump, comprises at least one sensor arrangement for measuring distance between the doors and between a door and the clump, and a processing unit for the computation of geometric indicators based on the measured values.

A method for monitoring a twin trawl being towed by a vessel, the twin trawl comprising two trawl bags, a first and a second trawl door and a clump, comprises the following steps:

- transmitting an acoustic signal from the first trawl door to the second trawl door and to the clump;

- transmitting an acoustic signal from the second trawl door and from the clump after reception of said acoustic signal transmitted from the first trawl door;

- measuring the lengths of time Tl and T2 it takes from when the acoustic signal is transmitted from the first trawl door until the first trawl door receives the acoustic signal transmitted from the second trawl door and the clump, respectively;

- registering the time t2 for reception at the clump of the acoustic signal transmitted from the second trawl door;

- calculating the distance S 1 between the first trawl door and the second trawl door and the distance S2 between the first trawl door and the clump, based on the lengths of time Tl and T2;

- calculating the distance S3 between the second trawl door and the clump, based on the time t2 and the length of time T2;

- comparing the distances S2 + S3 with S 1.

The sensor arrangement and sensor modules and means for transmitting and registering reception of acoustic signals and measurement of other values may, for example, be catch sensors, which are sensor modules for use in connection with fishing gear. They comprise, in many embodiments, an outer main body containing a sensor device for measuring different values/properties, and fastening devices for attaching the main body to the fishing gear. The sensor device is either fully encased by the main body, arranged on the surface of the main body, or partly contained within the main body. The sensor modules are as a rule secured to the fishing gear by means of wires, elastic cords or rope, one end of the wire, elastic cord or rope being fastened to the fishing gear and the other end being fastened to the fastening devices. Depending on the type of sensor module, two or more wires, elastic cords or ropes will be used.

The system may comprise a user interface, for example, comprising a screen or monitor for display of data. In an embodiment, geometric indicators are shown on the monitor. The system may further comprise a communications system for transmission of signals between sensor devices, sensor arrangement and processing unit. In an embodiment, the communications system comprises one or more hydrophones, but other wireless communication or communication over one or more cables may also be used. The processing unit may be arranged on the vessel, for example, in a bridge system in a catch monitoring system. A catch monitoring system of this kind may, for example, consist of receiver units, display units/monitors, hydrophones and one or more sensors such as trawl sensors, door sensors, bottom contact sensors, flow sensors, Trawl Eye sensor, trawl sounder etc. The processing unit may be connected to other units and be capable of controlling these units, as for example, winches, propeller pitch, rudder, engine speed, and also angles of trawl doors and weights/clumps.

In an embodiment, the system comprises a sensor device for measuring the angle of the water flow into the opening of at least one of the trawl bags. An angle of 90° indicates that there are no side currents, and that the trawl has a desired geometry, whilst an angle different from 90° indicates that the length of the wires/lines connecting the trawl doors and the clump to the vessel ought to be changed.

In an embodiment, the distances S2+S3 are compared with SI and the angle of water flow into the opening of at least one of the trawl bags is registered and, based on this, geometric indicators are determined. Geometric indicators are a number /figure/value that gives an indication of the geometry of the trawl, for example, whether the trawl is being towed symmetrically through the water in the direction of travel of the vessel, indicated, for instance, by an angle or an angular deviation (from 90°), or a difference in distance. When the distances S2+S3 are equal to SI, this is an indication that the trawl doors and clump are in alignment with each other. When these distances are not equal, the geometric indicator will tell an operator or a control system that the geometry is not optimal, so that actions can be taken to change the geometry.

By adjusting and positioning the doors and the clump, the geometry of the trawl, including symmetry, can be changed. This can be done by controlling the winches that regulate the length of the wire or line connecting the doors and clump to the vessel.

In an embodiment, the doors and clump are adjusted such that S2+S3 is essentially equal to S 1 and/or such that the angle of the water flow into the opening of at least one of the trawl bags is 90°. In an embodiment, the depth of at least two of the two doors and the clump is measured. This can be done by arranging depth gauges on the trawl doors and/or the clump or in that sensor modules comprise depth gauges. In that case, the doors and the clump can be adjusted and positioned such that the measured depths are essentially identical.

In an embodiment, the temperature of the water is measured using temperature- measuring devices arranged on the doors/clump or in sensor modules, and where calculation of distance is corrected by taking into account the speed of sound at the measured temperature.

The invention will now be described in more detail with the aid of examples and with reference to the attached figures.

Figure 1 illustrates a vessel towing a twin trawl.

Figure 2 illustrates distance measurement for monitoring a twin trawl.

Figure 1 illustrates a vessel 11 towing a twin trawl consisting of two trawl bags 10, two trawl doors 12 (a fust and a second door), and a centre clump 14. Sensor modules 13, 15 are arranged on the doors 12 and a sensor module 16 is arranged on the clump 14. The sensor modules comprise means for transmitting and receiving acoustic signals. The sensor modules 16 on the clump and 13 on the starboard trawl door may, in one embodiment, be a simple transponder, but may also comprise several types of sensor devices and other desired tools. The sensor module 15 on the port trawl door may, for example, be a double distance sensor capable of transmitting an acoustic signal and calculating the distance based on the time it takes from when the acoustic signal is transmitted until a signal is received from the transponders on the clump 14 and the starboard trawl door 12. In addition, the sensor module 15 may comprise further sensors. Examples of sensors that can be included in the sensor modules 13, 15 and 16 aie depth sensors and temperature sensors.

In addition, a symmetry sensor 17 is arranged on the port trawl. A symmetry sensor is a sensor that indicates the angle of the jet of water entering the opening of the trawl bag.

Often the vessel 11 will be equipped with a catch control system capable of

communicating with the sensor modules. Communication may be acoustic by means of hydrophones, cabled or by using another type of wireless signal transmission. The catch control system may comprise a processing unit able to receive raw data and calculated distances or to calculate distances itself. Based on received signals, the processing unit is able to display information on a monitor or other type of user interface. The information displayed to the user may be processed information, for example, calculated distances, comparison of distances, such as addition or subtraction, or other types of geometric indicators, or raw data can be displayed directly to the user. The processing unit may be connected to other equipment for sending signals to or controlling the equipment. In the figure both trawl bags are out of the desired position and the trawl doors 12 and clump 14 are not in alignment with each other. In a desired position to obtain optimal spreading of the doors, optimal opening of the trawls and a stable movement of the trawls, the trawl doors 12 and the clump 14 will be in alignment with each other.

Figure 2 illustrates an example of measurements for monitoring a twin trawl and providing one or more geometric indicators that provide an indication or description of the geometry of the trawl.

An acoustic signal is transmitted from the first trawl door 12 to/in the direction of the second trawl door 12' and to the clump 14. When they register reception of the acoustic signal, an acoustic signal is transmitted from the second trawl door 12' and from the clump 14, i.e., after reception of the said acoustic signal transmitted from the first trawl door. At the first trawl door, the lengths of time (Tl and T2) it takes from when the acoustic signal is transmitted from the first trawl door 12 until the first trawl door 12 receives the acoustic signal transmitted from die second trawl door 12' and the clump 14 are measured. In addition, the time t2 for reception at the clump of the acoustic signal sent from the second trawl door 12' is registered. This registration of the time t2 can be effected at the clump and sent to the first trawl door 12 or it can be sent to another position, for example, the vessel towing the trawl.

The measured lengths of time Tl and T2 can then be used to calculate the distance SI between the first trawl door and the second trawl door and the distance S2 between the first trawl door and the clump. The distance S3 between the second trawl door and the clump can also be calculated, based on the time t2 and the length of time T2. A comparison of the distances calculated will then give an indication of the geometry of the trawl. By, for example, comparing the distances S2 + S3 and SI, an indication can be obtained of whether the trawl doors and the clump are in alignment with each other. If S2+S3=S1, they are alignment with each other, whilst if S2+S3≠S1, they are not in alignment with each other, and there may be reason to act, for example, paying out one of the lines.

In some cases, the lines can be connected to controllable winches which regulate the length of the wire or line connecting the doors and the clump to the vessel. In these cases, the processing unit may send signals to the winches in order to adjust and position the doors and the clump by controlling the winches so as to obtain the desired distances. In other cases, a bridge system or catch control system on the vessel can display computed geometric indicators or calculated distances, thereby enabling an operator to take suitable actions.