Feasibility Study of the ECO Trawl Concept
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- Institutt for marin teknikk 
The objective of thesis is a preliminary feasibility study of the ECO Trawl Concept, proposed by ECO Trawl AS. The concept aims at replacing the traditional trawl door solution commonly used in trawling, and instead use two submarine thrusters (ECO Trawlers), powered by a mother ship. This will increase the efficiency of trawling by both reducing the drag forces on the system and by increasing the maneuvering capabilities of the trawl. The ECO Trawl concept is referenced against traditional trawling vessel Sille Maire.To determine the feasibility a design procedure is carried out, where two main categories are studied; propulsion and maneuvering. With regards to the first category a propulsion system using contra-rotating propellers and duct is presented as the best alternative for the concept. Then the two propellers operating in said propulsion system is designed using numerical propeller design program, AKPD, in close cooperation with numerical propeller analysis program, AKPA. The final design is a propulsors producing a total of 85.22 kN of thrust using 343.4 kW of power; an efficiency of 0.511. The thrust is below the benchmark of the reference vehicle, but allows for the utilization of the more common 200 kW electrical motor. Due to the design program not being built to take ducts into account it's believed that even better performance can be achieved, and a proposal for further propeller design is included in the further work section.Within the maneuvering part of the design process resources has been allocated to defining the forces that acts on the system with the ultimate goal of studying the static balance and stability of the ECO Trawler using a bridle control system to steer. The duct lift force is identified as a potential problem to the system stability, the lift force that occur when the ECO Trawler travels with oblique angle on inflow is established based on experimental work. The umbilical drag force is found numerically using the cross-flow drag equations, with adequate safety factors to account for possible vibrations. Design considerations regarding the umbilical diameter and shape is presented, but no conclusion is made. The gravitational and buoyancy forces are determine to not be of importance at this point, and instead a requirement of 100 kg net positive buoyancy is made.The balance point was intended to be found using a code which altered a set of input variables connected to inflow angle and propeller thrust to make the forces and moments sum zero. However due to errors the balance point was not found in this thesis. The system was however found to be unstable.The feasibility of the concept, with main focus on the hydrodynamics, relies heavily on finding a solution to the instability problem.