Numerical Simulation and its Application on the Falling Motion of Freefall Lifeboats (1st Report)

Abstract

Freefall lifeboat systems were developed in 1970s by Scandinavian countries as a life saving system which ensured safe and quick evacuation from a wrecked ship even in adverse weather conditions. The international regulation requires that the system launches a lifeboat into water wituhout any harmful acceleration to occupants and effectively clears the ship. Before studying these important matters for safety, the attitude of the boat must be defined at the water entry. The falling motion of the boat is a pure dynamic phenomenon in the air and then a mathematical model should be useful for describing it. No theoretical investigation, however, has been presented yet about the falling motion.In this paper the authors present a numerical simulation method on the falling motion of freefall lifeboats. After validating the method through experiments, they discuss the relation between the attitude and launching conditions, i. e. geometries of the boat and cradle, friction between the boat and cradle, starting position and fall height, H, of the boat, etc. They further propose tentative criterion for the safety entry of freefall lifeboats as an application of the method.The followings are some of the conclusions from the discussions : (1) The present method is of practical use for predicting the attitude of freefall lifeboats at the water entry and providing the criterion of safety entry for given boat and cradle geometries.(2) There is a range of launching angle where the attitude of the boat at the water entry dose not change seriously.(3) The friction of the boat and cradle is negligible in the practical range of rolling friction. They are warned, however, against unexpected stumbling on the cradle which increases the rotational motion of the boat.(4) In practical ranges of the radius of gyration and height of C. G. of the boat, their changes give no serious influence on the attitude.(5) The plunging speed is assumed in practice to be √2gH when the F. P. of the boat is close to the lower end of the cradle at the starting position.(6) The length of guide rails after C. G. of the boat is one of the most important factors for defining the falling motion and it is, to some extent, possible to control the attitude by changing the length.