Theoretical and experimental investigation of a free running fishing vessel at small frequency of encounter
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- Institutt for marin teknikk 
The hydrodynamic behaviour of a fishing vessel in a seaway was studied. The focus was on small frequencies of encounter, corresponding to following and stern quartering seas, where fishing vessels are susceptible to capsize. Experiments and simulations were performed with the model of a 21m modern fishing vessel, called the Lady Marianne, with a small ship length-to-beam ratio. A mathematical model for simulating ship manoeuvring in a seaway was developed. The mathematical model was based on a simplified modular method, combining a ‘blended’ six degrees of freedom seakeeping model with a four degrees of freedom non-linear manoeuvring model. In the seakeeping part of the simulation model, the radiation and diffraction potentials were computed by WAMIT and used in the Salvesen-Tuck-Faltinsen strip theory (Salvesen et al., 1970) to obtain the forward speed dependent radiation and diffraction loads. The memory effects of the radiation loads were computed by convolution integrals (Cummins, 1962). The non-linear Froude-Krylov and hydrostatic loads were obtained by integration of the pressure (in the undisturbed wave) on the instantaneous wetted surface of the ship hull, up to the exact incident free surface. The incoming regular waves were described by second order Stokes waves. The non-linear manoeuvring model was based on a combination of slender body theory and added mass theory (S¨oding, 1982) and can simulate the ship slow-down in a turn. The resistance, propulsion, rudder, and viscous loads were modelled by separate modules based on a mix of empiricism, simplified theoretical methods, and experimental results. Captive calm water experiments were performed with the Lady Marianne to measure the ship resistance, the wake factor, the propulsion forces, hull-lifting forces, and the viscous cross-flow forces in calm water. The wake factor varied strongly with the forward speed due to the wave system generated by the ship. The measured rudder lift coefficient indicated stall at large rudder angles. The viscous cross-flow principle was applied to the ship hull and gave good results, even for drift angles < 20, where the applicability of the principle is questionable. Calm water free running experiments were performed with the Lady Marianne. The simulated and experimental characteristics of the turning circle and the zigzag manoeuvre with the Lady Marianne were comparable, such that the simulation model was judged adequate for predicting any manoeuvre in calm water. The random uncertainty in the experiments was small, and the main part of the systematic uncertainty was due to uncertainties in the rudder position and ship yaw angle. The wave-induced surge forces in following seas were studied for three different fishing vessels: the Lady Marianne, a Japanese fishing vessel (Ayaz, 2003), and an Australian trawler (Thomas and Renilson, 1992). Captive experiments with the Lady Marianne in following seas and with forward speed were performed. The ship resistance was computed based on the space-averaged relative forward speed (between the ship and the particles in the incident waves) along the ship length to account for some of the incoming-wave effects on the ship resistance. The simulations over-predicted the wave-induced surge forces for the Lady Marianne and the Japanese vessel. For the Success trawler, the prediction of the wave-induced surge forces was good. Experiments were performed with the free-running Lady Marianne navigating on a straight line in stern seas. The ship forward speed was over-predicted by the simulation model due to the excessive wave-induced surge force. The overpredicted wave-induced surge force caused surf-riding and broaching to occur in the simulations, while it was not observed in the experiments. By reducing the waveinduced surge forces based on the results of the captive experiments in following seas, good prediction of the ship forward speed and the maximum yaw angle was possible, and surf-riding and broaching were no longer simulated. The influence of the waves on the rudder forces, propulsion forces, and hull-propeller interaction coefficients was studied because one of the reasons for broaching is the reduced rudder effectiveness when the ship is surf-riding (Faltinsen, 2005; Renilson, 1982). Including the influence of the incoming waves on the inflow to the rudder had little influence on the simulated ship behaviour. Experiments were performed with the free running Lady Marianne performing turning circle and zigzag manoeuvres at small frequency of encounter. The simulated wave-induced surge forces were reduced based on the captive experiments in following seas. The path of the ship in the turning circle manoeuvre was correctly simulated for the first half of the turn, and there was good agreement between the simulated and the experimental path of the ship during the zigzag manoeuvres.