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Keel-centerboards seem a natural solution for sailors who want the windward performance of a deep-draft keel with the shoal-waters capability of a, well, shoal-draft keel. The problem has always been that wedding a thickish keel that houses ballast with a thinnish foil or plate that retracts inside it creates all kinds of problems with interference drag. This occurs in a fluid wherever there's a significant change in angle of a surface. You also have the drag problems of water being forced up into the centreboard slot on the high-pressure side, which necessitates flexible closure plates to minimize. Also, unless the board is shaped with an arc for a trailing edge, when it's fully lowered, there's a huge length of exposed slot churning up drag behind it, which again requires a close plate arrangement.
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