Numerical Simulation and its Application on the Falling Motion of Freefall Lifeboats

Abstract

In the first report the authors presented a numerical simulation method on the falling motion of a freefall lifeboat with experimental verification and proposed, as its application, tentative criteria on geometries of the freefall lifesaving system for the safe entry into the water.In the present report they develop further a numerical method to simulate the falling motion of a freefall lifeboat from a cradle on a ship oscillating in the vertical plane. The method is verified through an experiment where a wooden block slides off from a slope on a swinging frame. In the discussion of the results they deal with the interaction between motions of the wooden block and frame. They calculate, using the method, the effect of the ship motion on the entry attitudes of the boat. The followings are concluded from the calculation : (1) The present method is of practical use for predicting the falling motion of a freefall lifeboat from a cradle on an oscillating ship in waves.(2) The angular motion of the cradle does not effect so much on the entry attitudes of the boat in the practical ranges of amplitude and period of pitching motion of the ship.(3) The surging motion of the cradle effects more on the entry attitudes of the boat than the heaving motion of same amplitude.(4) The combination of phases releasing the boat in surging and heaving motion brings about changes in the entry attitude from the stationary cradle. It is roughly assumed, however, that the simple superposition of those independent motions gives a general tendency of the changes.(5) A combination of those releasing phases makes a zone of the safer entry than the entry from the stationary cradle. A zone of particular combination of phases can lead to the dangerous entry.(6) The real ship motion concerned is sufficiently described on a “ship motion phase line”, which is defined in the present paper, when the ship and incident waves are given. The entry attitude calculated along this line provides useful information for the design and usage of the freefall lifesaving system for the ship.