Effect of freeboard and metacentric height on capsizing probability of purse seiners in irregular beam seas

The probability of capsize of purse seiners in irregular beam seas and the effect of freeboard height and metacentric height on trapped water on the deck was investigated. The aim was to quantify a safety level that can be achieved by direct stability assessment for this type of fishing vessel. The amount of trapped water on deck was numerically estimated using a hydraulic flow assumption. The long-term capsizing probabilities were estimated using a piecewise linear approach together with wave statistics from major Japanese fishing areas. The estimated safety level of capsizing probability was compared with that obtained by the IMO weather criterion and by the water-on-deck criterion of the IMO Torremolinos Convention. Numerical results for four typical Japanese purse seiners indicated that the effect of freeboard, on the amount of trapped water on deck, is more important than that of the metacentric height. Besides the metacentric height and the freeboard, it was shown that the danger of capsizing is a function of the rise of floor. The safety level obtained by the capsizing probability approach is generally higher than that based on the IMO weather criterion. However, the water-on-deck criterion provides a higher safety level than the capsizing probability approach for ships with a low rise of floor.     

Prediction of capsizing probability for a ship with trapped water on deck

The authors have already examined a method for evaluating the capsizing probability of a ship in the dead ship condition based on a piecewise linear approximation of the restoring arm. Here, this method is extended to ships with trapped water on deck. This is because the stability of ships having a relatively high bulwark, such as fishing vessels, could substantially deteriorate due to trapped water on deck. First, the mean amount of water trapped on deck was estimated as a function of the significant wave height and the mean wave period using a model experiment in irregular beam seas. Second, the restoring arm curve with trapped water on deck was calculated hydrostatically and then approximated with a piecewise linear curve. Third, the roll angle was estimated using a nonlinear and uncoupled equation of absolute roll angle under stochastic wave and wind exciting moments. The short-term and long-term capsizing probabilities were calculated for a fishing vessel operating off Kyushu. Numerical results quantitatively demonstrated that the effect on capsizing probability of trapped water on deck cannot be ignored when accurately evaluating the stability of fishing vessels.