PROPELLER SIZE SELECTION -
Why worry about propeller size?
While your prop pushes your boat, it also acts as a single speed transmission. Your boat is not like a car with many speeds" or "gears" that can be used while underway. The prop size (diameter and pitch) determines the "gear" you're in all the time- whether pulling up a skier, pleasure cruising, or going for all-out speed. Most boat manufacturers use a simple performance test that determines the correct prop size for almost all applications. It is explained below. We have also included a few definitions and hints.
If you have further questions, call us. We're open 8:30-5:00 central time. Good luck!
There is no "master chart" that shows what prop your boat needs.
You need to know three things:
The size propeller (diameter and pitch) currently on your boat.
Pitch is usually stamped on the prop somewhere. You can
sometimes determine this if you have the prop's model number
or some other identifying marks from the prop.
Your engine's recommended RPM limit (red line).
This information is in your engine owner's manual. Common RPM
(revolutions per minute) limits are; stern drives (I/O) 4200 to 4800
RPM; outboards 5000 to 5800 RPM.
Test your current prop. How much above/below are your engine's
RPMs vs. the red line?
This test should be performed at full throttle, maximum speed at full
trim and with a light load. Your engine should be in tune, your prop
should be in good shape. Don't count on what you remember from
last season! Perform this test before you buy any prop.
Interpreting the result.
If your propeller is the properly sized for your boat, your actual RPM during the above test should be within 100-200 RPM of your engine's RPM upper limit. You want your engine to be able to develop full power without exceeding the RPM limit. A correctly sized prop on your boat will let your engine develop full power, but will not allow it to go over the RPM limit.
A general rule of thumb is each inch of pitch change will result in a change of around 150-200 RPMs. If your test shows you need higher RPMs, select a prop with lesser pitch (shifting down). If your test shows you are over the RPM limit, choose a prop with higher pitch (shifting up).
Most applications are a compromise. Most boaters want good
acceleration and top speed.
Using a higher pitch won't always increase top speed. If you
"over pitch", your engine won't develop full power at the lower
RPM and you could lose speed while dangerously laboring your motor.
A 21 pitch prop turns 4200 RPM on your boat, but 4600 is the upper RPM limit. Try a 19" pitch prop to shift down" and increase your RPMs. If your outboard is turning 6100 RPMs and your red line is 5600, "shift up" and try a prop with 2" more pitch and test again.
Propellers are labeled diameter x pitch. (A 13x19 prop is 13" in diameter and is 19" pitch)
diameter - the distance (in inches) across the circle made by the blade tips as the propeller rotates.
pitch - the distance (in inches) a prop moves forward with one 360° revolution (assuming zero slip).
MARINE PROPELLERS, INC.