According to the RDP faithful, the panacea for all things related to boating problems is the propeller. “Have you tried a different prop” will assuredly make everything better is the comforting advice offered up daily by ‘helpful’ inmates of this popular boating forum.

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Recently, a new thread, “Skool Me on Prop Slip” prompted me to revisit this topic although we’ve talked about it several times in the past. Maybe a little different kind of presentation will help take some additional mystery about props and “slip” out of the equation. Let’s try a step-by-step approach.

  • Truth: A different prop is never a guarantee that it will solve a particular performance deficiency, but you never know for sure until you put it on the boat and try it.

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  • A quick recap – theoretical prop thrust is the distance a boat will move forward each time a propeller rotates one full revolution and it’s depemdent upon propeller pitch – i.e – if you have a 24” pitch, that amount of pitch will ‘theoretically’ move the boat forward 24 inches if it’s 100% efficient (no “slip”). Since it’s nearly impossible to have a 100% efficient propeller, the amount of “slip” is usually expressed as a percentage. For example, if that 24” pitch propeller only moves a boat forward 20 inches with each revolution it is 83.3% efficient, or has a ‘slip’ factor of 16.7%.

  • So, how do you calculate actual propeller efficiency and/or ‘slip’? If you have a desk calculator, it’s relatively easy to do, but it gets even easier for those of us who are not math majors. Several good “prop calculator apps” are available on line for free – one of the best and easiest to use is provided from Mercury Racing – just go to this link -- and fill in the missing information. It will do the math for you.

  • Most important, be sure you insert accurate data into the prop calculator. You know the old saying, “trash in, trash out.” Be sure you know the pitch of the propeller (it should be stamped somewhere on the hub). And, it your prop has been reworked or cupped by a prop shop, this could throw-off the calculations depending upon how much the pitch may have been altered. It’s also essential to have a reliable way to determine boat speed (miles an hour), Usually a good GPS speedometer is pretty accurate. You also need to be sure that your tachometer is reading correctly. And, you need to know what the gear ratio is in your drive unit.

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  • As I said earlier, the only way to determine if a different prop improves your performance is to try it on the boat with a water test. When you do this don’t do it on a family boating day with a bunch of people on board. If possible, just arrange to meet someone at the water so you (the driver) and passenger (the recorder) are in the boat with a modest (about a half tank full) of gas. Start your test day with your regular prop. Then once you’ve gathered all the data (writing it down – your recorder’s job), head back in and change props and perform the same battery of tests again. Once that’s done, compare the results, run the numbers through the ‘prop calculator’ and see the differences.

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  • Normally I like to run the following tests – start with acceleration, idle speed up to 30 mph. Measure this with a stop-watch or even just use the second hand on your wristwatch. How long does it take to go from off-plane idle speed to 30mph under a full throttle acceleration? Next, maintain a steady 3000rpm cruise speed (don’t forget to adjust the trim at each rpm level to achieve a comfortable and efficient trim angle) and record that speed. Then, increase your rpm in 500 rpm increments and record each speed. Finally make a wide-open throttle run (in both directions) and gather those top speed numbers as well as recording maximum rpm.

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  • Now you have a bunch of numbers, so here are some general rule of thumb conclusions to help you decide if you are making progress (these ‘slip’ levels apply to wide open throttle – generally at speeds slower than WOT, you would expect these percentage levels to be larger/higher numbers). If your ‘slip’ percentage is under 10% you are doing great. Under 10% is normally something only boats prepared specifically for racing may achieve. The 10-12% level is excellent for a non-race boat intended for general recreational use. 13 to 15% ‘slip is the level you’d like to achieve for most boats and tells you you’ve got a pretty efficient overall set-up. 16 to 18% is about average or slightly below average and your search for the right prop probably isn’t over yet. If you’re at 19% ‘slip’ or more, you’ve still got work to do and perhaps more factors are at work than just not the perfect prop.

  • It’s also important to keep the recorded data of your test day somewhere you can refer to it again later. You know, you’re probably going to be trying a different prop again sooner than you think.
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When doing these on-water prop evaluations, remember, always wear your life vest and do it when boat traffic is at a minimum.