A propeller converts the rotary output of a boats engine into an accelerated stream of water. This acceleration creates thrust that pushes the boat through the water. To work efficiently, propellers must be properly matched to their engines and transmissions. Other important factors include the boats displacement and waterline length and the clearance between the propeller shaft and the bottom of the hull. The terms and measurements used to describe a propeller are explained in the figure below. Specifying propellers for particular boats and engines is a job for experts, but owners can still choose between various types and brands.
Fixed-blade propellers are strong and cheap. They are most efficient when operating at the rpm and boatspeed they were specified for, and are less efficient under conditions such as motorsailing or powering into headwinds. Fixed blades also create a lot of drag under sail, whether they are locked in place or freewheeling. Sailors are generally better off with folding or feathering propellers that reduce drag and present a smaller, more streamlined shape to the water when sailing. Most folding and feathering props will fit standard propeller shafts and are available in two-, three-, and (in some cases) four-bladed versions.
Folding propellers (Gori, Martec, Varifold, and Flexofold are a few established brands) have long been popular with racers and are also favored by some cruisers. Some folding props have gears that ensure the blades all open together, but the simplest have blades that pivot independently at the hub. In both cases the blades are pressed backwards by the passing water and nest neatly against one another. When the engine is engaged, the blades are opened by centrifugal force, and in forward gear are held strongly against their stops by thrust pressure. In reverse, the water flow instead tries to fold the blades, so relatively high engine revolutions are needed to hold them open. The performance of folding propellers in reverse is notoriously poor. A rubber band is sometimes fitted to prevent blades opening due to gravity.
Two-bladed folding props have the least drag compared to all other types and are least likely to catch passing weed or debris. Three-bladed versions generally fold less compactly and create a little more drag.