Scanmar has delivered Trawl Speed / Symmetry sensors for 25 years and they are absolutely indispensable for those who have begun to use them. In the first years sales were modest, but with extensive use of Distance sensors, Door Angle sensors, and Angle sensors on the trawl and the bag it has become obvious to many fishermen how incredibly important it is to have a correct towing speed and water speed (the angle of the water entering the trawl, which should be 90°).
Proper towing speed leads to correct door distance, correct door angle, and stable doors. This also means the sweeps are functioning properly and that the trawl is stable. Problems with the doors immediately spread backwards and lead to loss of capture.
Proper towing speed also means that the trawl is towed at optimal speed, not so slowly to allow fish to escape in front of the trawl and not so fast as to create a bucket effect in the trawl allowing fish to escape through the panels.
Everyone understands the importance of symmetry in the trawl: without symmetry fish disappear directly out through the side panel opposite the direction of the inflow.
What many do not think of is that a cross current strikes each door differently, one therefore has a different angle, stability and spread than the other. This causes the doors of a pelagic trawler to be at different depths, the doors of a bottom trawler to easily lose bottom contact, and all the associated consequences that follow.
(1) Reduced trawlspeed leads to (2) the doors laying down inwards and (3) reduced door distance.