Experimental measurement of sideforce and induced drag on catamaran demihulls

A catamaran comprises two demihulls and although the flow about the catamaran centre line is symmetric the flow about the centre lines of the individual demihulls is not. The asymmetric nature of the fluid crossflow around the demihulls causes sideforce and hence induced drag to be experienced on the demihulls. The sideforces generated by each demihull act in opposition and cancel whereas the induced drags of both demihulls act together to resist the forward motion of the vessel. Experimental procedures used to estimate the sideforce and induced drag are presented together with results for one hullform at two demihull separations (S/L = 0.225 and S/L = 0.329) and at several Froude Numbers. It is shown that the induced drag generated by the demihulls is negligible despite the generation of significant sideforce. The sideforce produced was found to reduce rapidly with increasing demihull separation.