A folding propeller is a modern propeller designed for sail boats so that the pressure of the water pushing on the propeller blades causes them to fold down into a stream-lined shape as the boat moves forward under sail. The blades of a folding propeller are hinged, not fixed, allowing the fold down action. The folded propeller has significantly less surface area forcing against the water compared to a conventional fixed blade propeller.
The main benefit of using a folding propeller is reduced drag and faster speed. In broad terms, a sail boat fitted with a folding, instead of a fixed, propeller can boost the speed of a boat traveling under sail at 6 knots by as much as 0.5 of 1 knot. The precise speed gain will vary mainly according to the size and design of the boat. Other benefits of a folding propeller include less vibration and noise, since a folding propeller does not rotate like a fixed blade propeller when forced through water.
There are several negatives of a folding propeller. It is more expensive than a fixed blade propeller, and it renders the boat more difficult to steer when it is motoring in reverse. Additionally, folding propellers must be kept clean of marine growth, otherwise their benefits will be eliminated since the growth may prevent them from folding.
Folding propellers are suitable for all sized sailing boats larger than about 20 feet (6 meters), up to mega-yacht length. They are available with two, three, or four blades folding. In most cases, two-blade propellers are appropriate for smaller boats, while more blades are used on larger boats.
Folding propellers are similar to self-feathering propellers. Both designs provide the benefit of a more stream-lined shape, and thus less drag. A self-feathering propeller, however, allows the pitch of the blades to be better controlled when moving both forward by sail and in reverse by motor. This controlled pitch allows more accurate steering when the sail boat is motoring in reverse.
In the case of a self-feathering propeller, as the boat moves forward under sail, the force of the water swivels the propeller blades and feathers them to lie directly in line with the water flow. In other words, the edge of the propeller blade, not the broad face of the blade, faces in the direction of forward movement so that drag is decreased.
Virtually all high-performance sail boat use either a folding or self-feathering propeller. These propellers are suitable for both racers and cruisers. Generally speaking, for any given sized sail boat, the optimum sized self-feathering propeller will be more expensive than a folding propeller, which in turn will be more expensive than a fixed blade propeller.