Twin-rig demersal trawl has taken over to a large extent from single otter demersal trawl within Scottish waters. The main differences being that one boat deploys two nets (three have also been tried)
There is also a large stabilising clump-weight bridled between and level with the trawl doors.
This clump-weight is often equal to the weight of one door, it may be constructed as one, two or 3 ball model, or as a chain mat.
All sizes of vessels are now using twin trawl from 200 – 1500 hp
Towing speeds are from 2-3kts Duration 4-7hrs Warp ratio 2·5x water depth.
Normally the gear is towed on 3 warp wires, but in early days vessels with only 2 drummed winches towed 2 warps splitting into 4 at 200m.
Originally this method was used mainly catching prawn (nephrop) and deploying nets with light footropes , but now more powerful vessels deploy nets with heavy rock-hopper ground gear.
All intermediate gauges of footrope are used in correspondence with the seabed conditions.
Vessel ability to manoeuvre is almost as good as single demersal trawler.