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About Water Speed and Trawl Geometry
Along Water Speed
The along water speed is the trawl longitudinal speed through water (relative to the water). It is different from the GPS speed. The GPS speed of the vessel is the speed of the vessel relative to the ground. The along water speed depends on the current, bearing, pitch and roll of the trawl or vessel.
The trawl and fish are in the same frame of reference: water. Therefore, it is important to know the speed of the trawl through water. Each type of fish has his own speed and behaviour. If you know the water speed in the trawl, you will be able to adapt the speed for the fish you are looking for.
For example, you want the trawl speed to be at 5 knots relative to the water, and you have a GPS speed of 5 knots:
- If the along water speed is inferior to the GPS speed, it means there is a co-current. If the trawl does not go fast enough, fish could go faster and escape the trawl. So, for example, if the along water speed is at 4 knots, it means there is a co-current of 1 knot. You need to increase the GPS speed of at least 1 knot to make the trawl go faster.
- If the along water speed is superior to the GPS speed, it means there is a counter-current. If the trawl goes too fast against the current, there will be pressure inside the trawl and fish may get damaged or ejected. You will also increase your fuel consumption. So, for example, if the along water speed is at 7 knots, it means there is a counter-current of 2 knots. You need to decrease the GPS speed of 2 knots, to be at 5 knots through water.
A. The along water speed is 4 knots (1) and the GPS speed of the vessel is 5 knots (3). It means there is a co-current of 1 knot (2). You need to increase the GPS speed of 1 knot.
B. The along water speed is 7 knots (1) and the GPS speed of the vessel is 5 knots (3). It means there is a counter-current of 2 knots (2). You need to reduce the GPS speed of 2 knots.
Across Water Speed
Your trawl opening must be perpendicular to the current in order to catch more fish. The sensors measure the across water speed, that allows you to know if the trawl opening is perpendicular or not.
If the trawl is not perpendicular to the current, it can impact its geometry and create non-uniformity in the mesh size. As a result, you could catch non-targeted species.
When the across speed displayed on Scala2 is 0, it means the trawl is perpendicular. If it is superior, it means the trawl is not well positioned and you may loose fish. You need to adjust the warp length to make the trawl face the current.
To see the water speeds and identify the trawl position, from Scala2, look at the Trawl Speed 3D view or Trawl Speed dial.
