Think of a prop as screw. The pitch is the theoretical distance the prop should move through the water in 1 revolution. A 24" pitch prop would move 24 inches or 2 feet. The theoretical speed is figured by multiplying the pitch x engine rpm and dividing by (gear reduction x 12) = feet per minute. Divide by 5280 = Miles per minute, multiply x 60 = miles per hour. This can be shortened to (Pitch x RPM) / (Reduction x 1056) = MPH.
Slip is figured by comparing this theoretical speed to the actual measured speed. By the way, for these calculations, add 1" to the pitch if your prop is cupped. (Just about all hi-performance props are cupped)
For the purpose of this test, look for the absolute fastest boat speed. So trim the boat by the Speedo! Assuming you are in the ball park, and do not have to worry about over revving the engines, ignore the tachometer for the moment.
If this is your first test of a new package you do need to watch the RPM, and you might not be able to test at full throttle. DO NOT over rev. Check with your engine manufacturer for maximum RPM.
Without getting the boat so loose you lose control, open the throttle to full speed. Try increasing and lowering the trim until you find the fastest mph on your GPS or Speedo. Now look at the tachometer, and make notes of the trim position, rpm and speed. You can repeat this at any throttle setting or speed. In fact you may be more interested in your cruising speed than top end. The highest RPM is not necessarily the fastest speed !
Write your numbers down, maybe include notes as to weather, number of passengers, amount of fuel, etc. When you get back to the dock, pull out your calculator and do the following: Figure your theoretical speed from above, subtract you actual speed. Divide this difference by the actual speed. The number you get is your slip percentage.
Slip is never a negative number, and will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent.
Now that you know the slip, what does it mean? Less is not necessarily better. Prop slip is not the same as efficiency.
Prop Slip Calculator
Use this calculator to figure out just about anything in regard to propeller, ratio, RPM and speed. You can change one parameter and recalculate the rest. Use the CalcT button to figure the theoretical speed from the actual speed and slip. Be sure to take a look at the Prop Slip
(above) for an explanation of how to find your prop slip.
Boat Speed Predictions
The Square Root of (Total Shaft Horsepower / Weight ) x Constant = Speed
Optimum Slip Percent (For max top speed)
Heavy V bottom
Low drive height, cabin, side by side engines
10 - 13 %
Fast V bottom
High Drives, boxes, notched transom, good bottom
8 - 9 %
Pleasure type cat
6 - 8 %
Late Skater full race, etc
4 - 6%
Some of the newer cats require an even higher constant than those above.
The calculator below uses the above formula. Plug in your info, and click "Calculate" on the item you want to compute. By using known values for the weight, power and speed you can come up with a more accurate constant for your boat. You can then see what effect changes to weight or power will have on top speed. Using the calculator in this fashion will produce surprisingly accurate results.
This information can be used to pick a starting point when propping out a new boat or engine combination. You can plug these results into the formulas on the Prop Slip page or use our Prop Slip Calculator and come up with a propeller pitch. Just remember there is no substitute for accurate testing!