ropes, cables, nets and plastics. British narrowboats invariably have a weed hatch over the propeller, and once the narrowboat is stationary, the hatch may be opened to give access to the propeller, enabling debris to be cleared. Yachts and river boats rarely have weed hatches; instead they may fit a rope cutter that fits around the prop shaft and rotates with the propeller. These cutters clear the debris and obviate the need for divers to attend manually to the fouling. Several forms of rope cutters are available: A simple sharp edged disc that cuts like a razor; A rotor with two or more projecting blades that slice against a fixed blade, cutting with a scissor action; A serrated rotor with a complex cutting edge made up of sharp edges and projections. Propeller variations A cleaver is a type of propeller design especially used for boat racing. Its leading edge is formed round, while the trailing edge is cut straight. It provides little bow lift, so that it can be used on boats that do not need much bow lift, for instance hydroplanes, that naturally have enough hydrodynamic bow lift. To compensate for the lack of bow lift, a hydrofoil may be installed on the lower unit. Hydrofoils reduce bow lift and help to get a boat out of the hole and onto plane. See also Propeller characteristics Propeller phenomena Other Materials and manufacture Notes Citations External links Titanic's Propellers Theory calculation propellers and wings: detailed article with blade element theory software application "What You Should Know About Propellers For Our Fighting Planes", November 1943, Popular Science extremely detailed article with numerous drawings and cutaway illustrations Archimedes Screw History: The story of marine propulsion propellers history: The story of propellers Propulsors and gears: Wartsila Marine Propellers Propeller Drop: Measured by feeler gauge Scientific American, "History of the Screw Propeller", 1881, pp. 232 Watercraft components Swedish inventions