Towing tank tests were carried out on three slender models in order to obtain more information on the hydrodynamics of such shapes, and in particular, how the shapes could generate side force when operating at a leeway angle. Slender hulls are of much interest for multi-hull vessels. There is increased interest in such vessels at present due to the use of wing-sail multi-hulls in the recent America's Cup races in San Francisco and Bermuda, although the AC yachts use foils to generate a lot of the side force, and only at low speeds are the hulls in the water. On the other hand, ancient Polynesian multi-hull vessels did not appear to have keels, and so the side-force had to be generated by the hulls. The authors speculates that the earliest vessels had rounded hulls (from trees) and were probably used mainly for sailing downwind. However, with time, it would have been realized that sail powered vessels could also maneuver across the wind, and then the importance of side force as well as drag would have begun to be appreciated. Modern Polynesian multi-hull vessels often have one or both hulls which are Vee-shaped in cross-section. Hence, it appeared that evolution may have caused a change in shape from circular to Vee, presumably because such shapes are better able to generate side force.

A CFD study with ANSYS-CFX using three different hulls was carried out as suggested by the first author and it showed that sharper Vee sections were better at generating side-force than a rounded hull. The purpose of the present tests is to investigate whether such behavior could also be observed in physical testing. Three models were manufactured and were tested in the Towing Tank at Newcastle University in July and August 2013. One set of tests were carried out with fixed sink and trim, and the August tests had free sink and trim, with a thrust moment applied which approximated the sail force at part height up the mast.

It was found that there was good agreement between the CFD and tank test results, and indeed that the hypothesis that narrower Vee shaped hulls would generate more side-force when at leeway than a rounded hull was proved.

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