A kind of small capstan, consisting of an upright shaft, having several holes at the top, through which long bars or levers are thrust. The name of crab is likewise given to a simple portable crane, on the wheel-and-axle principle, and chiefly used for raising building materials to the tops of houses, etc. There is a machine, likewise called a crab, that is used in launching ships, or heaving them off or on to their ways or slips.