Free-Surface Effects of Variations in Appendage Vertical Volume Distribution: Where does a Bulb not See the Free-Surface?

The vertical distribution of appendage volume for a sailing yacht is predominantly driven by the desire for the lowest possible centre of gravity due to the dependence of sail drive force on stability. This results in volume being pushed downwards regardless of hydrodynamic effects. The increase in aspect ratio of the appendages is also increased with greater draft, resulting in a higher efficiency as well as greater drive force. However, some modern designs have reached the limit of draft in terms of restricted access to ports, structural and rule limitations. To counter these restrictions it is not uncommon to incorporate lifting or swing keels to reduce draft at specified times. The effects on aspect ratio and stability are relatively easy to approximate for such a vertical shift in volume, the effects on the free surface are much more difficult to accurately predict. For this purpose some novel experiments using extreme shifts in vertical volume distribution have been performed and compared with numerical predictions. Limits on submergence beneath which appendages effectively “see” no wave resistance have been established based on numerical and experimental results.