Everything you need to know about Propellers but were afraid to ask!!!
PROPELLERS - Now here is a difficult topic to discuss.
So often people come in asking for a new prop but few understand how important it is to get the correct prop for the particular application in order to obtain peak performance. Not all props are created equal. There are many factors that go into a propeller - Pitch, Cup, Rake, Diameter, Number of Blades and of course Material.
A proper pitch prop MUST be used in order to allow the engine to turn the proper RPM at Wide Open Throttle. Most, (but not all) engines today are made to run at an operating range of 5000 to 6000 RPM WOT. Manufactures of 4 stroke engines that we carry, tell us that they want their engines to rev 6000 RPM with a light load in the boat. That way when there is a heavy load, it still runs in the operating range.
A proper pitch prop MUST be used in order to allow the engine to turn the proper RPM at Wide Open Throttle. Most, (but not all) engines today are made to run at an operating range of 5000 to 6000 RPM WOT. Manufactures of 4 stroke engines that we carry, tell us that they want their engines to rev 6000 RPM with a light load in the boat. That way when there is a heavy load, it still runs in the operating range.
Pitch is what allows us to achieve the correct RPM.
So what is pitch?
The easiest way to explain pitch is to think that pitch is how many inches a prop would travel in one revolution through a solid object. A 20 Inch prop would travel 20 inches, right? Not always.
So what is pitch?
The easiest way to explain pitch is to think that pitch is how many inches a prop would travel in one revolution through a solid object. A 20 Inch prop would travel 20 inches, right? Not always.
If a 20 pitch prop is made of Aluminum, there is a loss of approximately 25% due to slip and flex going through water. Water is not a solid object. Because of the material aluminum is soft and flexible. Aluminum blades need to be thicker than stainless and therefore are not as efficient. So a 20 Pitch prop actually moves about 15 inches per revolution. That's a significant loss.
Stainless Props are thinner and stronger. They do not slip and flex like a aluminum prop does. They say that a stainless prop only has 10% loss to slip and flex. So that same 20 pitch prop moves 18 inches per revolution. Much lore efficient but of course more expensive.
Because a stainless prop is more efficient, that 20 pitch prop will lug the engine down, not allowing it to achieve the RPM that an aluminum would. Therefore when going from aluminum to stainless, you must drop down a pitch to maintain that 6000 RPM at WOT. Of course stainless being so much stronger, there is more of a likely-hood you could do damage to the gears and bearing inside the lower unit when hitting submerged objects.
Cup, Rake, Number of Blades and Diameter also play an important part of propeller selection, but to keep things simple I'll leave those topics for another day.
Happy Boating and see you on the water
Tom Rieschl
Family Marine
Stainless Props are thinner and stronger. They do not slip and flex like a aluminum prop does. They say that a stainless prop only has 10% loss to slip and flex. So that same 20 pitch prop moves 18 inches per revolution. Much lore efficient but of course more expensive.
Because a stainless prop is more efficient, that 20 pitch prop will lug the engine down, not allowing it to achieve the RPM that an aluminum would. Therefore when going from aluminum to stainless, you must drop down a pitch to maintain that 6000 RPM at WOT. Of course stainless being so much stronger, there is more of a likely-hood you could do damage to the gears and bearing inside the lower unit when hitting submerged objects.
Cup, Rake, Number of Blades and Diameter also play an important part of propeller selection, but to keep things simple I'll leave those topics for another day.
Happy Boating and see you on the water
Tom Rieschl
Family Marine
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