A good tuna purse seine net must have two main qualities : The net must sink very quickly to avoid the fish to escape underneith the net.
To improve the sinking speed of a net, we must diminish the water resistance by making a net with big meshes and thin twine. However, in that case the net is not very strong.
The net must be strong to avoid tears: Tuna vessels are heavier nowdays, also the tensions on the net are bigger.
When the underwater currents are very strong, there is a big tension on the purse line and also on the netting. Tears can then appear.
To make a net stronger, it must be made with thick twine or smaller mesh sizes. Then, the water resistance is big and the net sinks slowlier.
To be able to make a good net, a netmanufacturer must mix these twodivergent objectives.
This is why Le Drezen has developed a simulation software that digitally simulates a haul. This new tool, unique in the world, allows the company to design a seine that integrates the captain or ship owner's needs and demands-- A good design is the best possible compromise between different parameters including the purse seine size, the size of the net and the real fishing conditions in which the vessel sails around.
The numerical simulation allows to take all of this into account.
There are 3 phases in the numerical simulation :
1. We enter all the parameters of the net in the computer :
n The type of material
n The runnage per kilo of the twine
n The mesh size
n The floatline caracteristics (buoyancy of floats )
n Parameters of the chain (size, pitch, length...)
n Hanging ratio
n Towline
At that time, we also enter the vessel parameters (size of the boat, power of winches, speed of the boat at the beginning of the shooting, action of the propeller, action of the skiff...) and the real fishing conditions in which the boat fishes (strength, direction and depth of currents,...)
2. Then we leave the computer to do the calculation for a few hours
3. We analyse the results in 2 forms:
n Graphs
n Video.
During the seine design phase, we can simulate the net performance to varying parameters:, twine diameter and type, mesh size, hanging ratio, purse line or towline lengths, the speed of the boat, the power of winches. We can observe the tension and the sinker impact on every point of the seine net or on the purse line, during each phase: shooting or hauling. The different performance for the same purse seine net can also be analyzed and compared: by changing parameters like the shooting speed, by taking into account one or several currents at different intensities, directions and depths, by introducing one incident during the shooting phase like one or several brakes on the purse line. We can observe the impact of different fishing winch powers and boat hulls on the hauling speed of the purse line.
Today, this tool allows to assist the shipowners and the captains on their net design while taking into account the demands of oceanic tuna fishing.
This program also allows the Captains to better grasp the reaction of their boat and their net in their own fishing conditions.
When fishing and using their gear, the captains observe that the results of the simulation are matching the real performance of the seine.
