Fishing – trapping – and vermin destroying – Fishing – Nets
Patent
1991-05-30
1993-04-13
Rowan, Kurt C.
Fishing, trapping, and vermin destroying
Fishing
Nets
A01K 7304
Patent
active
052011372
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a hydrodynamic device, more particularly for bottom trawls, for ensuring satisfactory opening thereof, whatever the trawling speed.
A traditional bottom trawl, such as shown schematically in FIG. 1 of the accompanying drawings, comprises a pocket 1 whose opening is bordered on the lower side by a weighted pad 2 dragging on the bottom, on the upper side by a back rope 3 to which floats are attached and on the sides by extensions of the net or wings 4. The rigging of the trawl comprises on each side a lower rope 5a and an upper rope 5b attached respectively to a low point (end of pad 2) and a top point (end of the back rope 3) of a wing 4. The other ends of ropes 5a and 5b are joined together, and their junction is connected to a divergent panel 6 by an arm 7, the panel 6 being itself connected to the boat by a trawl warp 8. A portion of net 9 adjacent the back rope 3 and situated in the centre is called "back square".
The purpose of said floats (not shown) attached to the back rope 3 is to raise the latter and provide vertical opening of the trawl. As shown in FIG. 1, the upper part of the pocket is then incident so that, when moving, the play of the forces at the level of the back rope 3 is such as shown by the diagram of FIG. 2 where f is the buoyancy of the floats, t their drag, R the trawling force (via the upper and lower ropes) on the back rope and T the pull exerted thereon by the net.
Of course, T, t and R are increasing functions of the speed so that when the speed increases, angle .alpha. decreases, which results in flattening of the trawl and so a reduction in height of the opening, the vertical opening force (buoyancy f of the floats) being constant.
Now, for good efficiency of the trawl, the stability of the shape of its mouth is determinant, so that many solutions have been developed for overcoming this tendency of the trawl to close again when the speed increases.
One solution consists in increasing the number of floats. This results in a substantial increase of the drag, influencing the consumption of the ship. In addition, during storage, these floats are very cumbersome, all the more so since they are rolled up with the trawl, and to be able to withstand hard element (chains, cables), they have a tendency to be chosen oversize. Thus, in practice, even for shallow bottoms (50 m), it is floats designed for withstanding 1200 m of depth which are used.
Profiled floats have also been used, such as described particularly in the French patent 1 245 612. These floats combine buoyancy and hydrodynamic operation, allied with reduced drag. However, during storage of the trawl, these floats have the same drawback of being cumbersome as the above floats, even an increased disadvantage because of their larger size.
A particular rigging brings satisfaction in some fishing conditions. In this rigging, named "fork rigging", the front ends of the forks which replace the upper ropes, instead of being fixed to the arms are fixed to the trawl warps, so that when the trawl is fully open, the tractive force on the back rope is substantially horizontal.
However, it is a more complicated mounting than the upper and lower rope rigging. Moreover, it is difficult to adjust and can only be used for shallow bottoms (up to 150 m) with little variation of level. In practice, that means that it can only be used alternately with a conventional rigging (depending on the fishing location), with handling and time consuming requirements at each change.
As for the hydrodynamic operation systems, besides the above mentioned profiled floats, there exist rigid lift panels providing vertical opening of the trawls. They are generally rectangular plates made from wood, light alloy or resin, whose area depends on the power of the trawler. Floats are disposed at the front (leading edge) of the panel, so as to ensure the angle of incidence thereof at the time of lowering into the water. The panel may be fixed on the trawl in two ways: either directly on the back rope, or at a distance therefrom, connecte
REFERENCES:
patent: 3299560 (1967-01-01), Luketa
Guigo Herve
Kerrest Bernard
Theret Francois
Institut Francais de Research pour l'Exploitation de la Mer
Plottel Roland
Rowan Kurt C.
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