Investigations on the Strength of Ships Going in Waves by Model Experiments

Abstract

On the problems of the dynamical strength of sh ips going in waves, especially on the problems of the slamming of ships, we have tried the experiments in the experimental tank by towing the six metre model ship, made of brass, and the influences of the wave lengths and heights were investigated in the former reports.In the present paper, the experiments on various ship draughts and trims when wave length and height are constant are investigated. From the experiments, the following principal conclusions may be drawn, (1) Slamming occurs as the encounter period of ship and wave Te closes to the period of pitching and heaving coupled motion of ship Tph, and abates as it recedes.(2) The cause of slamming is due to the coupled motion of pitching and heaving, but pitching is preeminent.(3) The pitching angle, the maximum rise of bow, the phase lag between pitching and heaving, the phase lag between wave and pitching, all these factors have the important influences on the cause of ship slamming.(4) The statical pressure difference of the maximum and minimum bottom pressure at ship bow increases in the slamming zone.(5) The impact bottom pressure indicates the maximum value when d/L=0.0325, and the impact pressure ratio at ship bow amounts to 11.6.(6) The maximum impact bottom pressure changes linearly with the change of ship trim. It is given approximately as the following formulaMax.ζ0=11.0+176(Trim/L)(7) The mean hogging stress at deck, having no consideration for the impact stress due to slamming, shows a tendency to increase remarkably with the occurrence of slamming ; while the mean sagging stress scarcely increases. This property is the same in any ship draughts and trims.(8) In slamming zone, the impact compressive stress is recognized on ship deck, and indicates the maximum value when d/L=0.0325. This impact stress also changes linearly with the change of ship trim.(9) The maximum ship draught which causes the slamming throughout all ship speeds is ds/L=0.062.(10) The minimum ship speed which causes the slamming throughout all ship draughts is Vms=0.54M/sec (F=0.42). This value perfectly coincides with the value which were obtained in the wave length experiments.(11) By comparing the measured stresses with calculated ones, the effective wave height ratios η are obtained.(i) Hogging ηH indicates the maximum value when d/L=0.0325.(ii) Hogging ηH indicates the maximum value when even keel.(iii) Sagging ηS has a tendency to increase with increase in ship draught.(iv) Sagging ηS indicates the minimum value when even keel.(v) η in high speeds become fairly large compared with η in low speeds, while the increasing percentage of η owing to the Smith's correction are nearly the same, irrespective of ship speeds in any ship draughts and trims.