This type of testing requires an experienced driver, good water conditions and enough room to not be a hazard to others or yourself. Take the time to become familiar with the handling characteristics of your boat before attempting a full speed run.
Where do you start? First, it is important to have accurate speed and RPM information. This is critical for the best results! If you are serious about getting the most performance out of your boat, invest in accurate instruments. Today GPS is your best choice for measureing speed howeerver it is best to do 2 runs in oppoiste directions and take the average. We prefer digital tachometers, especially for testing.
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 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)
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.For the purpose of this test, we are looking for the absolute fastest boat speed. So trim the boat by the speedo! Assuming we are in the ball park, and do not have to worry about over revving the engines, ignore the tachs for the moment.
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 tach, 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 trusty (or rusty if you are like me) 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 less than 5% on a light cat to more than 25% on a heavy workboat. Most hi performance applications will be between 5 and 12 percent.
Mercury has a handy slide rule type calculator available to do the math. Contact us for the part number and availability if you are interested.
Now that you know the slip, what does it mean? Less is not necessarily better. Prop slip is not the same as efficiency. I will work on that explanation and post it on a separate page soon. Take a look at our boat speed calculator and our on-line slip calculator.