Whether mounted on a boat or an airplane, a propeller is basically a fan responsible for providing propulsion to the craft on which it's mounted. Although superficially it may appear that a propeller consists simply of a number of blades attached to a shaft, propellers have a number of different parts that are essential to their operation. The propeller parts, the shaft, and the connection between the two are carefully crafted to provide maximum efficiency.
The main propeller parts are the blade and the hub. The blade’s tip is the very end of the blade furthest from its root, which is attached to the propeller hub, or boss. Some boat propellers’ hubs will have inner and outer portions, connected by three or four ribs much like a bicycle wheel’s spokes, creating an opening for the exhaust gases of those engines that discharge their exhaust through the hub. The blades are attached to the hub, which is attached to the propeller shaft. The shaft is connected to the engine and transmits the power to the propeller.
A propeller generally will have at least two blades, to balance the forces produced when it rotates. Most small boats, such as motorboats, have three-, four- or five-bladed propellers; larger crafts’ propellers may have up to six blades. In general, the more blades a boat propeller has, the quieter it will run; many submarines have seven-bladed propellers.
A boat propeller blade’s leading edge is the part that cuts into the water. The opposite edge is the trailing edge. The leading edge is fairly sharp, and it forms a definite edge to streamline the blade’s movement in the water. The blade’s face is the side that faces away from the boat, sometimes called the positive pressure side of the blade; the side of the blade that faces the boat, the negative pressure side, is called the blade back.
One of the propeller parts unique to boat propellers is called the cup, which is a small lip or curve on the trailing edge of the blade. The cup allows the blade to hold water better and also adds to the propeller’s pitch, which is the forward distance the boat would travel under ideal conditions for a single complete turn of the propeller. Some propeller blades are also cupped on their tip or leading edge; this is done primarily to modify a boat's handling characteristics.
Airplane propeller parts are similar, but an airplane propeller’s blades are long and narrow, rather than the much broader blades found on most maritime propellers. In addition, the leading edge isn’t sharp at all, but rounded. This is because an airplane propeller is essentially a wing, in that it utilizes aerodynamic principles, and thus is shaped like an airfoil with a more rounded leading edge and a sharper trailing edge. In general, the broader the leading edge of the blade, the greater the amount of thrust the propeller will produce.
When a boat propeller is revolving at full speed, any contact it has with anything at all — rocks, logs, stumps, fish or people — will result in severe damage not only to the object struck, but also to the propeller. For this reason, many boat propellers are outfitted with guards of varying design; strong metal bars or cages are often placed under or around the propeller to reduce the likelihood of the blades striking any foreign object. Airplane propellers generally aren't protected in this manner.
Whether for maritime or aviation use, propeller parts must be constructed from strong materials to withstand the forces they’ll be placed under, often making thousands of revolutions per minute for extended periods of time. In addition, propellers and engines must be carefully matched, both for each other and for the craft they’ll be operating. For maximum efficiency and performance, the engine and propeller should be neither too small nor too large for the boat or plane being driven.