SCANMAR says that, following comprehensive trials of its Angle Sensor on doors, they are now moving to commercial production. It says it is the first sensor to feature brand new functionality since the mid-90s.
It says its quest revolved around finding a solution to the difficult of finding "the correct balance between the trawl doors, the ground gear, the sweep and the net. To say nothing of fishing at different depths and at different tow speeds." The rationale behind the work of the sensor, says Scanmar is that: "For pelagic trawling, the trim angle is critical for the trawl doors' lifting capability. In principle, an inward-leaning door gives the door (and the trawl) lift, while an outward-leaning one has the opposite effect. In side currents, this may result in great differences in depth between the doors. The door angles are also highly significant for any rapid alterations to the trawl depth. Any change in speed or current will tend to either elevate or depress the doors, depending on the trim angle. When fishing near the surface especially, the doors' motion is critical in preventing the doors from collapsing inwards and breaking the surface. For a pelagic door, the other angle, the tilt angle, is highly significant. Here, we look for the doors to be vertical in the tilt plane in order to achieve optimum spreading effect and minimum towing resistance.
"For bottom trawling, a change in the trim angle will affect bottom pressure and stability. An acute warp angle reduces the bottom pressure. This can be counteracted by allowing the door to lean outwards. Changes in towing speed, currents and towing on slopes also affect the trim angle. This can be counteracted by allowing the door to lean outwards and so increase bottom pressure.
It says the answer lies with its Angle Sensor not only showing the angles, but also the ScanFactor, i.e. the doors' stability in the trim and tilt plane. For many bottom trawlers, the most significant aspect of the Angle Sensor will be the ScanFactor; if during the tow phase, situations arise where the bottom pressure is reduced, because of underwater currents, the fill of the trawl, the heave of the vessel, and so on, door instability will increase so as to lose bottom contact. Preventing this is important, because it leads to reduced door distance, to the sweeps lifting and to the trawl gear losing bottom contact, especially out on the wings -- and fish then escape.
It adds that "by examining the other angle, the tilt, you can see if the entire shoe is down, or if the door is sitting up. A door riding on its entire shoe is very stable, both in the tilt and trim planes, but one sitting on its heel is very unstable in both planes, that is to say, it has a high ScanFactor. This may be a sign that it will readily lose bottom contact. Both the Angle measurement and the ScanFactor reveal a lot about the bottom conditions, but what is particularly interesting is that one can instantly register even small snags and so always get a warning of possible snagging or friction."
Scanmar claims they "have also taken the first step towards a multifunctional sensor.
"So as to avoid multiple variants, the sensor comes in a single housing for use on either door or as a bottom contact sensor in that it features three built-in swingers for different alignment to the vessel. With the new Door Sensor, many users will probably opt to combine it with Depth or Temperature in the same sensor."
