Model Experiments on Capsize and Its Prevention For a Small Fishing Boat in Waves

Abstract

There have been frequently occured miserable accidents of capsize for small fishing boats in the northern sea area near Japan, which were attended in the most cases with loss of human lives. Such accidents occured on the salmon fishing boats in high percentage in comparison with others.The purpose of this paper is to investigate the important factors affecting to capsize for such a small fishing boat as well as the effectiveness of the mean for preventing the accidents, through the model experiments in heavy seas.In the experiments, the 1/7 scale model was navigated in regular and irregular waves of high steepness generated in the model basin so that the capsize of the model could be expected to occur at some special conditions of the model and sea.The model ship was equipped with the weight shifting device for simulating cargo shift which could occur by heavy rolling motion of ship and would promote the capsize, while, on the other hand the inflatable rubber bulges were also attached on the both sides of the model hull which will lend an additional buoyancy and the righting moment to the ship in the emergency cases.A great number of running was carried out in the experiments under the various ship conditions as well as sea conditions, and as the results some capsizing phenomena were observed in case of poor stability condition. It was deduced through analysis of the experiments that the trapped water on deck with cargo shift act most effectively to capsize the vessel in beam and quartering seas, while the inflated bulges were so efficient to prevent the capsize that no capsize was observed when the inflation of the bulges was timed in spite of the trapped water and weight shift.The theoretical analysis of ship motions, such as rolling, accelerations and relative motions in irregular waves can explain well the experiment results of capsizing phenomena which attracted attention to the danger of navigation in quartering seas having high steepness.