Features and Benefits
Ground Effects and Sea Bottom Damage
Conventional trawl doors are typically weighted to run on the sea bottom. On a hard surface they sometimes heel
inwards or outwards or fall over, thereby upsetting the fine balance of forces necessary to maintain a stable trawl. On
soft muddy bottoms the trawl doors dig in and act like ploughs. This increases the spreading force but also increases
the power needed to tow the trawl. In addition, this ploughing action disturbs the sea bottom, for instance by
creating huge mud clouds, and in certain areas can destroy sensitive corals and other seabed features. This has
resulted in trawling and bottom trawling in particular, being portrayed as one of the most environmentally destructive
fishing methods around today.
The Active Trawl System can operate in a bottom-contour (bottom-hugging) mode in which the trawl doors are
controlled to maintain a fixed height above the seabed while the trawl net makes light contact with the bottom or
floats a little above it. This can significantly reduce damage to the seabed as well as eliminate troublesome ground
effects. This mode does require a change in the rigging and buoyancy of the trawl net. It also requires responsive,
powerful and controllable trawl doors to be fully effective - such as the Active Trawl System's VTVD's.
Potential Savings
With typically 30% of total fuel costs accounting for travel to and from the fishing grounds and 70% used for the
actual trawling operation, fuel efficiency while fishing is a very important consideration. This became very evident
when the oil price peaked at $147/barrel in June 2008. While the current oil price is considerably lower, analysts
expect it to climb again sometime in the future. The ATS, by virtue of its off-seabed operation, dynamic optimization
and compensation features, its hydrodynamic efficiency as well as its superior fishing performance, can significantly
reduce fuel costs.
In addition, as constant winch and engine adjustments are not used by the ATS to effect control, maintenance and
replacement costs can also be appreciably reduced.
User Friendliness
The good performance of conventional trawl doors depends on careful setting of the rigging during manufacture and
also during trawling operations. Stretch of chains and bending or wear of the warp attachment brackets can
dramatically influence the running attitude of the trawl doors. It is also important to ensure that the boards are
matched i.e. the lift forces and running attitudes of the port and starboard trawl doors are the same at all times.
Monitoring of the trawl and/or examination of the trawl doors after a trawl are required to ensure this.
The Active Trawl System on the other hand, requires no mechanical adjustments as it constantly monitors and
controls the magnitude and direction of the lift force of each device. In fact it is possible to have VTVD's of a different
size on each warp and still have a stable trawl.
To match the VTVD's to a trawl net is a simple operation:- input parameters such as device size, net size and type,
bridle length, warp diameter and length, trawler type and available power to the control system via a keyboard. For
the initial installation, a programmed test trawl is required to check, modify (if necessary) and store the system
constants.
Operation at Greater Depths
Using conventional trawl gear, the four parameters that dictate the trawl depth are: length of warp, diameter of warp,
trawl speed and weight of trawl door. Physical limitations on all of these, place a restriction on the maximum depth
attainable.
With the VTVD's, a larger component of force can be vectored downwards, forcing the trawl to operate at greater
depths without having to pay out more warp.
Real Time Fault Identification and Correction
Fouled or crossed gear are problems that normally require hauling the gear in. Damaged or mismatched trawl doors
can also degrade the catching ability of the trawl. Observing the wear pattern on the trawl door shoe and net-borne
sensors can provide information with respect to actual trawl gear parameters. Not all problems can be rectified during
trawling operations.
The Active Trawl System with its simpler rigging, onboard sensing system and control software can prevent most
problems occurring by taking early corrective action. It can also provide predictive and real time warning of non-
optimal or dangerous conditions.
Copyright Active Fishing Systems 2016
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