Observation of full size yachts sailing upwind in a seaway has shown that, because of the presence of the sails, the yacht is constrained to move in body axes (parallel to the mast) rather than in earth axes (normal to the water). It is thought that this is due to the effect of the sails in the air and the keel and other appendages in the water providing a large damping force which resists any motion normal to the mast line. An experimental project has been carried out therefore to investigate the effect of this change in motion axes on the forces and motions induced by the seaway.

The experiments were carried out on a model of an IACC class hull in regular head waves for a range of wave heights in both earth and body axes using a recirculating water channel. The magnitudes and phase angles of resistance, side force, pitch, heave and heel moment were measured. Comparisons between the results of the present work and previous experimentation, with motion in earth axes, showed similar trends. However the results from the experiments using motion in body axes showed marked changes in the measured motion and resistance characteristics when compared to the earth axes data. It is thought that this difference could well affect the order of merit when comparing the performance of two hulls and it was concluded therefore that the change to measurement in body axes is important for the correct prediction of the performance of a yacht in a seaway.

The effect of the pitch moment of inertia on the motion and forces on the model was then studied, first using motion in earth axes and subsequently using motion in body axes. In general it was found that in both earth and body axes there was a strong cross coupling between the pitching motion and the heave motion and that a low gyradius resulted in smaller motions and a reduced added resistance; however the effect of any change in pitch moment of inertia was more significant in body axes. The results showed that a yacht optimised for low pitch moment of inertia would have superior performance in comparison with a yacht which has a high inertia under identical wave conditions.

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