Investigation of thrust loss due to steering while trawling
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- Institutt for marin teknikk 
This thesis investigates the potential for a reduction in required propeller power by optimizing the transverse position of attachment points for trawl warps, simulating a system of movable trawl blocks. Full scale measurements from the 65 meter long stern trawler F/T Prestfjord is used as a basis. A surge, sway and yaw manoeuvring model based on Lagrangian mechanics is used together with control forces from trawl, rudder and propeller is put into a bounded optimization algorithm to find the transverse position of trawl blocks requiring the lowest thrust. The bounds are the original placement of the port-side and starboard blocks, i.e 5.08 meters. The result is a small reduction in power. For the operational profile from the data-set, the mean reduction in power is 0.64%. During trawling, the mean reduction in power is 1.13% when pulling a double trawl, and 1.11% when a single trawl is pulled. This is smaller than earlier reports have found, and it is not certain that such a system will be beneficial in an economical perspective when investment and maintenance cost is included. Differences to earlier reports seem to be based on rudder size and efficiency. The power reduction is greatest in relation to manoeuvring, while it is more stable when trawling on a straight course with a drift angle.