require a large, watertight trunk to hold them in place when retracted, and this occupies what otherwise might be useful space in the cabin or cockpit of the boat. Use of leeboards, while it adds complexity to the process of tacking, leaves the floor of the boat unobstructed. According to traditional design principles, a boat utilizing a leeboard either needs to have two retractable leeboards, one on each side, or a method of removing the leeboard and attaching it to either side while under way, because nearly all sailboats tack to work their way upwind (with the exception of the proa). However, some small sailboat designers claim to have created seaworthy vessels with only a single, fixed leeboard. On a traditional two-leeboard (or non-fixed leeboard) design, the boards are designed so that the windward board can be retracted, to reduce drag. Unlike centreboards, which are symmetric along the boat's axis, leeboards are often asymmetric, so that they more efficiently provide lift in one direction. Some fast racing scows use bilgeboards, which are mounted between the centreline of the hull and the sides, so they can use a pair of asymmetric foils for maximum lift and minimum drag. See also Bruce foil References External links Chinese inventions Sailing rigs and rigging Sailing ship components