Folding propellers reduce drag and increase speed for sailing boats, but are more expensive and difficult to handle in reverse. They are suitable for boats over 20 feet and come with 2-4 blades. Self-propelled propellers offer even less drag but are more expensive.
A folding propeller is a modern propeller designed for sailing boats so that the pressure of the water pushing on the propeller blades causes them to fold in a current fashion as the boat moves under sail. The blades of a folding propeller are hinged, not fixed, allowing for the folding action. The folded propeller has significantly less surface area forced against the water compared to a conventional fixed-blade propeller.
The main benefit of using a folding propeller is reduced drag and faster speed. Generally speaking, a sailing boat equipped with a folding propeller, rather than a fixed propeller, can increase the speed of a boat sailing at 6 knots by up to 0.5 of 1 knot. The precise speed gain will vary mainly depending on the size and design of the boat. Other benefits of a folding propeller include less vibration and noise, since a folding propeller does not spin like a fixed-blade propeller when forced through the water.
There are several negative aspects of a folding propeller. It is more expensive than a fixed-blade propeller, and it makes the boat more difficult to handle when driving in reverse. Also, collapsible propellers must be kept free of marine growth, otherwise their benefits will be removed as growth can prevent them from collapsing.
Folding propellers are suitable for sailing boats of all sizes over 20 feet (6 meters), up to mega yacht length. They are available with two, three or four folding blades. In most cases, two-bladed propellers are appropriate for smaller boats, while more blades are used on larger boats.
Folding propellers are similar to self-propelled propellers. Both designs provide the benefit of a more fluid shape and therefore less drag. However, a self-firing propeller allows better control of the pitch of the blades when moving forward with the sail and in reverse with the engine. This controlled pitch allows for more precise steering when the sailboat is sailing in reverse.
In the case of a self-complete propeller, as the boat moves under sail, the force of the water spins the propeller blades causing them to fall directly in line with the flow of the water. In other words, the blade edge of the propeller, not the broad face of the blade, faces in the direction of forward motion, so drag is decreased.
Virtually all high performance sailboats use folding or self-propelled propellers. These propellers are suitable for both racers and cruisers. Generally speaking, for any given size sailboat, the optimally sized self-feather propeller will be more expensive than a folding propeller, which in turn will be more expensive than a fixed-blade propeller.
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