French twine, netting and fishing gear manufacturer Le Drezen told World Fishing how it has taken digital simulation software to successful level in its research and application strategies.
It has uses two packages. One is for purse seiners, software which it developed in-house. The other package it uses, for trawls, was developed by the French National Institute for Exploitation of the Sea (Ifremer).
Jean Roullot from Le Drezen said the advantages of the trawl software is that "it allows us to simulate the behaviour of a net and take into account various parameters such as the trawling speed, the water depth, the length of the sweeps, the length of the warps, the weight, the number of floats and so forth...
Our comprehensive database has been built up over years of and allows us to simulate very precisely the trawl and its rigging when fishing. To test how good our simulation software is, we used data collected at sea and we tested this against the results of the calculations made by the computer.
"To produce greater reliability, we worked on different trawl designs used in a variety of fisheries. One of the tests was done for 46m vessels with a towing capacity of 40 tonnes. We defined both vertical and horizontal openings, along with the gap between doors and the tension on the warps even before manufacturing the trawls.
"The performance of the trawls has been validated at sea. This gear showed its efficiency when fishing and is perfectly suited to the requirements of the skipper," he added.
Choose your style
There is much more he says, because they can extrapolate from an existing database and produce a whole range of models.
"With our accurate database and a good correlation with what really happens at sea, we can extrapolate these results and apply them to different gear. For example, using the parameters of a 64m bottom trawl, we were able to project the performance of a 72m one. We could calculate very precisely the additional effort when using this type of gear. We can also take knowledge acquired by experience and confirm this against our simulation program. So, most of new gear is now designed according to these computer studies."
The other advantages of computer modelling include:
Specific adjustments: "After a careful analysis of the results of our computer simulations, a vast number of adjustments can be made. We enter the parameters of different kinds of gear for which the adjustments have not been optimised and then simulate how they perform. The results mean we can then improve the design of the gear.
"For example, the length of the legs must be adjusted with great precision. Several simulations are often necessary to regulate the tension and get an optimum vertical opening.
"Another example would be the weight of the footrope. Very often the wings of the trawl lift up. It is then necessary to ballast them. All these parameters can be precisely determined using Le Drezen computer modelling.
"Other examples speak for themselves. When twin trawling, any lack of symmetry on the gear will show up when shooting. In this case, the difference between the inside and the outside of the sweeps has to be adjusted for the trawl to keep a straight position. Lack of symmetry causes abnormal tensions, which can reduce the catch, increase fuel consumption and of course damage the gear itself.
"We are also able to tackle the geometry of the trawl. Nowadays everyone has an eye on the fuel gauge. Therefore, the design of the trawl must be optimised to reduce fuel consumption. Computer modelling has shown that by reducing the surface of the netting and using modern fibres, the trawl performs better as drag is reduced.
"We have also worked on the design of bottom pair trawls. When optimised, the manufactured gear has proved to be much more efficient in terms of catch and drag (10 per cent improvement).
Two plus four equals two
"Take the example of pelagic two and four doors. Some vessels carry different kind of trawls onboard. Also, the rigging must be adapted to the trawl. At times, manoeuvres can be tricky when one type of trawl is swapped for another. For example, the pelagic trawl with four doors (two bottom doors and two small mid-water doors) performs very well, but it requires additional manoeuvres. This is why we have studied another kind of trawl called a "progressive trawl". In this case, the usual four doors are replaced by two doors.
"Our testing proved that this was a very good alternative which avoided tedious manoeuvring on big vessels. With the help of our computer simulation, we can study the possibility of rigging a pelagic trawl on a bottom-trawl vessel --with its existing doors," he said .
