Numerical investigations on roll decay of a lifeboat in calm water and waves

Lifeboat serves an important water rescue equipment. Capable of self-righting and being stable in free roll decay is crucial for a safe and efficient rescue for a lifeboat. To better understand the roll decay of lifeboat in complex environments, this work performs systematic numerical investigations on roll motion of a lifeboat in both calm water and periodic wave conditions. Investigations of lifeboat in calm water show that it is capable of selfrighting with the initial heel angle equal or less than 120 degrees . Although the initial heel angle not affects the final stable status, it has remarkable influence on the self-righting process. Lifeboat with a large initial heel angle subjects to more violent roll motion in the early stage, and thus has weaker stability. Analysis also shows that the nonlinear component of the roll damping coefficient becomes the major part of the total damping coefficient with an increasing initial heel angle. On the other hand, in waves conditions, lifeboat undergoes periodic roll motion in beam waves, and the periodic rolling motion is primarily associated with wave frequency. When the wave frequency approaches the natural frequency of the lifeboat, the rolling motion increases drastically. The present work helps to better understand the self-righting process of a lifeboat in complex environments and also to provide guidance in design and safe operation of a lifeboat.