2-blade / 3-blade / 4-blade props are a function of power available from the engine versus the physical space available to mount the propeller
- - With a given amount of power from the engine a 2-blade propeller
is the best form for a sailboat as it is possible to "hide" the propeller blades behind the keel
when the engine is off. However if the space available for the propeller is not sufficient - i.e., there is the main hull
in the way then you need to go with a propeller with shorter blades.
- - The 3-blade propeller for the same "given engine power" will have a smaller blade length. And a 4-blade will have an even smaller blade length. What you get into when going with a shorter blade length and/or more blades is more resistance to water
flow (drag) as the blade hang outside the shade of the vertical keel. Additionally as the blades length gets shorter there is less "clear water" for the propeller to bite into that is not affected by the flow of water
around the vertical keel.
- - So you first determine how much physical space there is available for a propeller - including blade tip clearance from the bottom of the main hull
- and then match the power available from the engine. With small hp engines a 2-blade is probably best because you want to get the blades out away from the shadow of the keel. As the engine power available increases the 2-blade propeller will have to have longer blades and there might not be sufficient room. So you move to the 3-blade and again match the power to blade length. With large engines - say a motor-sailor - a 4-blade may be what is needed due to limited space available.
- - Pitch is used to "fine-tune" the available power to the boat
characteristics so that you do not "lug" the engine (black smoke) and cannot achieve max rpms. Or you get max rpm's too quickly/easily. The reason to use pitch to fine tune is that you are limited in pitch range by the propeller manufacturers. They do not want to make too many different pitch "versions" of the same size propeller so generally when the pitch vs size of a propeller overlaps the next size propeller they discontinue making the previous size&pitch. Which boils down to there is a limited number of pitches available for a given size propeller.
- - Best advice I have is research
what that make/model/size boat
(engine) had originally installed before the folding propeller was installed - the manufacturer's recommended fixed propeller size. Use that as your starting size and find a used propeller place that will lend you the same size prop in different pitches. Try each one to which works best on your particular boat. Then buy that one.