An Experimental Investigation on Bow Water Shipping

The water on deck caused on a restrained ship model without forward speed in head waves is studied experimentally by using a transient-test technique. A single water-shipping event is induced by the wave packet, and the severity of the interaction is controlled by the wave-packet steepness. Three different bow geometries are considered. Two of them are analytical hull forms, and the last is the ESSO-Osaka tanker. The models are equipped with a transparentmaterial deck to study the flow-field evolution by image analysis. A vertical wall is placed at a certain distance from the forward perpendicular to mimic the presence of deck structures. Velocity of the shipped water along the deck, pressure field on the deck and horizontal impact force on the wall are measured. The main fluid-dynamic aspects of the green-water phenomenon are highlighted. For the tested cases, water shipping starts always with the free surface exceeding the freeboard, plunging onto the deck, and forming complex cavities entrapping air inside. The geometry of the air cavity depends on the hull form and the wave steepness. Then the water propagates along the deck. In general, the water front is strongly three dimensional because of the water entering along the deck contour. The interaction of the shipped water with the vertical structure consists in: impact, run up-run down cycle, and backward plunging of the water onto the deck, still wetted. The evolution of the pressure field follows that of the water front. Pressure peaks are associated with the impact against the vertical wall, and by the backward plunging of the water on the deck, at the end of the run up-run down cycle of the water. It is shown that both these stages can be of importance from the structural point of view.