Scanmar has always focused on developing equipment that will help fishermen increase efficiency, save costs, prevent damage to the gear and reduce wear and tear.
Today, when skippers are offered instruments such as trawlspeed/symmetri for observing the waterflow, not only in the opening of the trawl, but also in the belly; door/angle sensors for optimal efficiency of the trawl doors; twist sensors for trawl efficiency; and bottom contact sensors for controlling how hard the gear goes on the bottom, it is quite a new situation.
Based on reports, and the fact that most fishermen are fishing in a very traditional way, Scanmar says that it does not have any doubts that saving of 10-20% on the fuel bill is possible for almost everyone.
Many studies have been carried out over the years in order to analyse what the elements are that account for most of the fuel consumption in the towing phase. Due to the generally low towing speed, the vessel itself accounts only for a minor part; the trawl, the doors and the bottom gear are the elements that count for 80-90%. Obviously new designs could make a lot of changes, but it is a difficult process due to all the various elements that have to be taken into consideration.
Scanmar has been involved in many different tests and trials that show how great savings in fuel consumption may be made by optimal rigging and full focus on a couple of elements during catching. It is incredible to see how great results come out of small adjustments.
Focusing on fuel saving, the most important elements to control are:
• The trawl doors
• The net
• The ground gear
Trawl geometry
Several years ago Scanmar introduced the term 'trawl geometry'. In order to fish efficiently the trawl must have optimal horizontal and vertical opening, must not be skewed, towed with correct water flow into the trawl and the ground gear going just hard enough on the bottom for the species being fished for.
Vertical opening
The vertical opening of the trawl can be easily controlled by distance sensors at the wing ends. Used on the doors the optimal spread must be calculated and controlled based on the length of the sweep lines, the design of the trawls and also consider the right angle of the sweep lines for efficient catching.
Trawl doors
There are big differences in towing resistance for trawl doors; the results from trials have been 10-30% of the total resistance. Most fishermen are fishing with the trawl doors leaning too far inwards. This causes inefficient operation, compared to doors standing practically vertical, due to less spreading force from water and reduced bottom contact.
To reduce fuel consumption, the doors should be practically vertical and the angle of attack should be reduced as much as possible, still having the correct door spread.
Quite often the doors and the trawl do not correspond very well. Fuel is always saved when the door angles are adjusted correctly and their bottom contact is no more than necessary.
Trawl net
The trawl construction and material is what counts most for the fuel consumption (40-90%). The most important thing is to have the correct towing speed. All trawls are different and the resistance in the net itself may change over time.
It is very important to find the optimal towing speed for every trawl; even small variations from that will have great impact on the efficiency and fuel consumption. In general one may say that the towing resistance changes by the square of the towing speed (doubling the towing speed increases the resistance four times). This means that even small changes, 0.3-0.4 knots, caused by underwater currents will have considerable impact.
The wrong towing speed will also cause a bucket effect in the trawl, if towing to hard, or that fish disappear in front of the trawl if the towing speed is too low.
A skewed net (the trawl opening not in symmetry) will mean deformation of the meshes, and will result in increased fuel consumption, as well as reduced catches.
A skewed trawl may be caused by underwater currents as well as wind and currents on the surface.
With the new ScanSensors (traditional catch- depth- and temperature sensors with built in angle meters), Scanmar has discovered that more or less all nets are being twisted to some extent. This has the same effect as described above.
Ground gear
The ground gear (bottom trawling) may count for between 10-20% of the towing resistance. Today many vessels are towing very heavy bottom gear, and especially on soft bottom this will create a lot of resistance. By using the bottom contact sensor, measuring how hard the gear is in the bottom, Scanmar has observed how it is possible to trim the gear in order to reduce the resistance, and at the same time avoid the gear losing the bottom contact.
In order to save fuel, the gear's weight on the bottom should be adjusted according to the bottom conditions and the specie being fished for.
The total picture
The three areas mentioned above are where it is most possible to save during fishing, and even without modern equipment fishermen should try some of the elements mentioned, and see if they get results. But it is important to understand that making changes to one of the elements will impact on the others.
