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- Mar 9, 2005
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In order for the engine to attain its optimum operating RPM, you need to know what wide-open-throttle RPM range the manufacturer recommends and a functioning tach.
As the example, we'll use the common small block WOT range of 4400-4800 rpms.
Two things you need to know about props: 1) diameter 2) pitch. The stamping in your prop hub may look like this: 14.50X21 Diameter is stated first.
Without going into the scientific equations and theories, I'll simplify...A 1" change in pitch will create about a 200 rpm change. (Going from a 21 pitch to a 23 pitch will drop the rpms by about 400 rpm...comes out of the hole slower, but has more top end speed, and vice-versa).
Changing from a three-blade to four-blade prop will decrease rpm about 50-100 rpm.
A stainless 21 prop will produce higher performance than the equivalent aluminum because stainless has thinner blades and is stronger with less flex. A bad strike with stainless will probably trash your lower gear set whereas an aluminum blade will sacrifice itself.
If you have Bravo Three props, diameter X pitch is a fixed equation and you have no options in the set. Pitch is determined by diameter and the pitch is incremental in even numbers (ie: 22, 24,26,28,30) This data applies to B3's only.
Back to our 4400-4800 example from above:
Example prop is an aluminum three-blade 14.50X21. Under good weather conditions - breeze with very light surface chop, 1/2 tank of fuel, functioning tach, and no passengers...you pull a couple of speed runs and determine that your current prop is turning at 4200 rpms. You're over-propped, and engine damage CAN occur because it's being overworked, or, "lugged".
Answer: find a wheel that's about a 18" pitch. The net change will be an increase in rpm of about 600...which means you're right near the top end...perfect.
Finding "the perfect size" may be almost impossible in some cases,and this is where a competent prop shop can fine tune your prop by adding cup, changing the rake, or balancing the blades.
A marine engine produces its maximum shaft horsepower at wide-open throttle, and that is why the object here is to find a propeller that will allow the engine to run nearest to the top end of the WOT recommendation.
As the example, we'll use the common small block WOT range of 4400-4800 rpms.
Two things you need to know about props: 1) diameter 2) pitch. The stamping in your prop hub may look like this: 14.50X21 Diameter is stated first.
Without going into the scientific equations and theories, I'll simplify...A 1" change in pitch will create about a 200 rpm change. (Going from a 21 pitch to a 23 pitch will drop the rpms by about 400 rpm...comes out of the hole slower, but has more top end speed, and vice-versa).
Changing from a three-blade to four-blade prop will decrease rpm about 50-100 rpm.
A stainless 21 prop will produce higher performance than the equivalent aluminum because stainless has thinner blades and is stronger with less flex. A bad strike with stainless will probably trash your lower gear set whereas an aluminum blade will sacrifice itself.
If you have Bravo Three props, diameter X pitch is a fixed equation and you have no options in the set. Pitch is determined by diameter and the pitch is incremental in even numbers (ie: 22, 24,26,28,30) This data applies to B3's only.
Back to our 4400-4800 example from above:
Example prop is an aluminum three-blade 14.50X21. Under good weather conditions - breeze with very light surface chop, 1/2 tank of fuel, functioning tach, and no passengers...you pull a couple of speed runs and determine that your current prop is turning at 4200 rpms. You're over-propped, and engine damage CAN occur because it's being overworked, or, "lugged".
Answer: find a wheel that's about a 18" pitch. The net change will be an increase in rpm of about 600...which means you're right near the top end...perfect.
Finding "the perfect size" may be almost impossible in some cases,and this is where a competent prop shop can fine tune your prop by adding cup, changing the rake, or balancing the blades.
A marine engine produces its maximum shaft horsepower at wide-open throttle, and that is why the object here is to find a propeller that will allow the engine to run nearest to the top end of the WOT recommendation.