A sensitivity study of bow variants on the distribution of sea spray in regular head seas

Abstract

There has always been a need and desire to improve upon the operability of ships at sea. The driving force behind making improvements can be safety, economic, or militarily oriented. This paper deals with improving a ship's operability by studying the effects of bow flare on the quantity and distribution of spray across the main deck under varying environmental conditions. A 1:36 scaled model of a 3600 LT on displacement ship, resembling a U.S. Navy FFG-7 class combatant, was used throughout this study. The model was tested with four different bows with varying degrees of flare. A surfactant was added to the towing tank water to reduce the surface tension and increase the Weber Number in order to better simulate spray at the model scale. Environmental conditions imposed were regular head seas of a mean sea state 6 and generated true wind equivalent to 32 knots. One bow form was first tested in ordinary tank water so that a comparison could be made between the two surface tension conditions. A 64% reduction in surface tension was achieved through the addition of the brand name surfactant AEROSOL OT-75. Though this value is relatively great, it corresponds to a Weber Number that is 22 times smaller then the required full scale value. The visual effect on the spray was to cause a finer droplet size and break up of the water sheet that normally is present rolling off the bow. With respect to the measurements taken, the reduction in surface tension resulted in; (a) a smaller volume of spray water being captured, (b) a change in the density distribution of the spray across the main deck, and (c) an increase in the wetted area on the main decking. In the absence of any specific spray criteria in which to judge each bow's performance against, the general trend was to reduce the quantity of spray water delivered and limit its distribution with an increase in the bow flare. The one knuckled bow that was tested performed much worst then any of the conventionally flared bows