Known also as a ‘screw’ or a ‘prop’ it is the familiar two-or three-bladed device which propels a boat by screwing its way though the water. Water not being a solid, there is a certain amount of ‘slip’, defined as the difference between the actual movement of a water-screw and the theoretical movement of a similar screw working in the solid. A boat propeller is right-handed if it turns clockwise when viewed from astern: left-handed if it turns the other way. Most propeller blades are at a fixed Pitch, but some have Variable pitch (VP) blades, and some have reversible pitch for going astern. With reversible pitch, the shaft turns always in the same direction, and there is no need for a reversing gear. It is often possible to Feather the blades of a VP prop, so that they stand edge-on to the water flow and have minimum Drag when the boat is under sail. Some propellers fold to reduce drag – that’s to say the blades hinge backward and come together like the palms of the hands in prayer and so present a form somewhat like a fish’s tail. (See also: Pitch.)