Motion Response of a Full-Scale Life Raft in Laboratory Tow Experiments

Abstract

A 16-person full-scale life raft was towed in a tow tank in calm water, regular and irregular waves. The objectives were to assess the raft motion response, occupant motion, tow force, effect of tow speed, effects of different test variables (drogue deployment, floor inflation, weight distribution and ballast), and the likelihood of occupant motion sickness. Comparisons of RAOs obtained in regular and irregular waves demonstrated that irregular waves could be used as a cost effective means to determine raft response with a high degree of confidence. They also show that the life raft tow performance is different in waves than in calm water. For example, mean tow force is 20% higher in the sea state tested than in calm water. Floor inflation, drogue deployment, even weight distribution and tow speed increase mean tow force and tow force variation about its mean. The data also show that the same ballast types should be used to access the effects of different variables because manikin and water bag ballast produce different results. Measured occupant heave acceleration was about the same as the raft heave acceleration. From occupant heave acceleration, it was estimated that after 20 hours in the raft, 20% of occupants would vomit. Formulae were proposed to predict tow force in different sea states. Mean tow forces predicted using calm water tow resistance and RAOs derived from regular wave tow tests compared well with measured mean tow force in irregular waves.