A keel/centerboard combination is, like probably everything on a boat, a compromise. It offers mostly improved lateral resistance for windward performance in exchange for reduced board-up draft, the possibility of reasonable stability, and an uncluttered cabin (trunk-free). Some boats, like the Montgomerys, were designed with them. Others, like the Compac 16, have been modified from a shoal keel in an effort to improve windward performance. It only improves the "stiffness" of the boat in a momentary dynamic manner in much the same way that lowering the centerboard on an unballasted dinghy makes it roll slower to a boarder at the dock. Tod