During towing there are only two things you can do to affect the trawl geometry and fishing ability (apart from changing course):
- Change the towing speed (water flow into the trawl)
- Change warp lengths
On the other hand there are many factors over which you have no control that influence trawl geometry:
- Bottom conditions
- Depth variations
- Underwater currents
- Fish influx and filling
Although you cannot control these conditions you can immediately detect them through changes in door distance, towing speed or door angles. You must therefore focus on door angles and towing speed in order to obtain the correct door distance. If you have rigged the doors correctly then you can focus entirely on ensuring that the doors have the correct tilt angle. This you correct easily by adjusting the towing speed and in some cases by a small adjustment of the winches.
Scanmar has developed a program that processes and displays information so you instantly see if everything is on line or if you must take action to prevent problems from occurring.
Towing speed Door distance
Follow the white line;
Over the line: Increase towing speed
Under the line: Decrease towing speed