Selecting the proper trawl door requires careful consideration of the following criteria:
Horsepower – Bollard Pull available at the propeller. If the vessel is taking power off the main propulsion system through shaft generators or other machinery, horsepower is the minimum power remaining at the wheel when all machinery is at full load.
Towing Speed – This will determine how fast the vessel needs to tow in order to capture the target species. This is, of course, species dependent. Trawl drag and door size are the two main factors that effect towing speed.
Size – What size door can the vessel handle? To answer this question you must consider both the drag of the door when towing and the limitations of onboard stowage space. For instance, does the door need to fit up the stern ramp for servicing? NETsystems frequently designs doors to suit these special vessel requirements.
Weight – The trawl door weight when submerged is of critical importance, as this is the environment in which the door is operating. For a vessel fishing in very shallow water, lighter-weight doors allow for more trawl warp (higher scope) to be used, which improves the spread of the net. In very deep water, heavier doors may be an advantage in bringing the gear quickly to the fishing depth. NETsystems designs and manufactures doors of various densities and displacements for different fishing requirements; using proprietary foam cores encased in alloy steel sheathing, it is even possible to create doors that are almost buoyant.
Aspect Ratio – Simply stated, aspect ratio is a comparison of a trawl door’s height to its width. For example, a door with a height of 2 meters and a width of 1 meter would have a corresponding aspect ratio of 2 and would be considered “high aspect” (because it’s aspect ratio is greater than 1). Conversely, a door with a height of 1 meter and a length of 2 meters would have an aspect ratio of 0.5 and would be considered “low aspect” (less than 1). High-aspect ratio doors offer higher spread per meter of surface area and better fuel economy. NETsystems specializes in high-aspect ratio trawl doors, which are carefully engineered to provide exceptional performance and consistent quality.
Bottom Tending – Bottom tending is a consideration for some types of doors and some types of seabed. Low-aspect ratio trawl doors receive a large portion of their spreading force from “ground shear,” the force resulting from the door shoe tracking on the seabed at an angle. High-aspect ratio trawl doors do not require this force and can be fished effectively just off the bottom. This reduces the drag of the system, stabilizes the doors, and minimizes the wear on the door shoes.
Fishing Depth – At what depth will the gear be fished? As mentioned in the discussion of weight, the ratio of trawl warp to depth (“scope”) is an important factor in the design of a trawl system. For a system to operate well in shallow fishing depths, sufficient main wire should be set out to give the doors a “long leash,” which allows them to spread to their maximum potential. A door with a light in-water weight is advantageous in this situation. Conversely, when fishing very deep, a heavy door will require less main wire to get it to the bottom and will stay on bottom more predictably.
Trawl Drag – When discussing door performance, the drag of the trawl (expressed as “twine surface area,” or “TSA”) is useful information. The drag of the trawl contributes to setting the door’s angle of attack. A trawl that has very low drag will do little to set the angle of attack, while a trawl with too much drag may be difficult to spread properly.
Center of Gravity – The center of gravity of the door should be as low as possible, given the design of the door. A low center of gravity results in better stability, easier turning, and a more upright door attitude.