Calculating the Spread of Trawl Doors
One of the most important pieces of information for any trawler skipper is to know what his door spread is and to be aware if there is any appreciable changes to this dimension. To ensure that trawl gear is towed at its optimum efficiency it is necessary for the skipper to know how far apart his trawl doors are. It is the spread of the trawl doors, in conjunction with the length of sweep and bridle, and the dimensions of the trawl that determines the angle that the sweeps and bridles take up to the direction of towing. This angle, known as the sweep angle or bridle angle, has to be set at a suitable angle for the target species at the chosen towing speed
Ideally door spread can be monitored using one of the proprietary trawl monitoring systems, such as Scanmar, Notus and Simrad. These systems can be costly, particularly for the smaller class of vessels but they will give the skipper a convenient and reliable readout of the door spread in the wheelhouse. If the vessel does not have any of these systems fitted, there are 2 basic ways to calculate the spread of the trawl doors. This document gives skippers of vessels without electronic door sensors advice on the best method of calculating the door spread of their gear for a selection of towing arrangements so that they have a better understanding of the spread of their gear.