The right prop

[sean-nós]

People looking to improve the performance of their boats should have a better understanding of what to look for in a new prop versus their old prop. ALL boat motors should be able to attain the minimum recommended RPM for the motor you have installed with your normal weight load, if you do not do this you can expect the motor to lose some or a lot of it's longevity, depending on how low the maximum RPM are and how bad you are Lugging the motor. When you cannot reach minimum RPM, then your motor is Lugging at ALL RPM levels, not just at WOT. Whereas, being able to attain maximum recommended WOT RPM will increase longevity and fuel efficiency as well.

1. Before you make any decisions about changing props, you need to know if your outboard motor is at the best height for maximum performance with any prop. Your motor normally should be at least even with the bottom of the transom of the boat, and preferably an inch or two higher for best performance. Your prop WILL gain RPM and speed by raising the motor higher, unless it starts ventilating. On inboard outboard boats you cannot change the height of the outdrive unless you have an old OM stringer drive, so the following does not apply. Normally the anti ventilation plate should be at least 1” or higher above the bottom of the boat for the best speeds and the least drag created by the motor. When the boat is running at higher cruising speeds you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.

Motor Transom Height
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2. Then you need to know how much prop slip you have in order to determine if you are using the wrong prop pitch and/or blade geometry for your boat and motor combination. Just getting your motor to turn the correct RPM at WOT does NOT mean your boat is running efficiently and you are attaining good fuel efficiency and maximum speed from your motor. If your Prop Slip is over 15%, using a GPS, then you should consider changing to a prop with a different blade geometry than the prop you now have, to improve your performance, speed and fuel efficiency at all RPM.
Checking the amount of prop slip you have, will give you a very good indication of how well the prop you are running is at converting Theoretical speed to Actual speed. Every time your prop slip is higher than 15% you should definitely consider changing props, but you need to make sure your speed numbers are correct, and that means using a GPS for speed and running the speed tests in 180 degree opposite directions to account for any wind or running water that influences the test results, add the two numbers together and divide by two for Actual speed results.

Prop Slip Formula

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3. Then if you expect to be able to determine the increase or decrease in RPM when changing pitches in a prop, and you are not well versed in ALL of the properties of prop blade geometry then you need to stay with the same Manufacturer and Model of prop that you have installed on the boat at this time, unless your Prop Slip is over 15% using a GPS. Changing from one Model of prop from the same Manufacturer and the same base material in the EXACT same pitch can cause a drop of 800 RPM.

Theoretical RPM change with Pitch change in the same Manufacturer and Model of Prop

The old way of guessing is not required as their is a mathematical formula that will be extremely close but not exact, because some manufacturers even change the rake between different pitches and may not have gotten the diameter change just right either. Prop pitch changes RPM dependent on the actual initial pitch of a prop, the RPM it is turning and the pitch change in the new prop from the same manufacturer and model of prop. The RPM change is also directly proportional to the RPM you are turning.
For instance:

Wide Open throttle RPM X (Old Pitch/New Pitch)

21" pitch to 20" pitch = 6,000 RPM X (21/20) = 6,300 300 RPM change per inch

21" pitch to 20" pitch = 4,600 RPM X (21/20) = 4,830 230 RPM change per inch

16" pitch to 15" pitch = 6,000 RPM X (16/15) = 6,400 400 RPM change per inch

16" pitch to 15" pitch = 4,600 RPM X (16/15) = 4,906 306 RPM change per inch

11" pitch to 10" pitch = 6,000 RPM X (11/10) = 6,600 600 RPM change per inch

..7" pitch to.. 6" pitch = 6,000 RPM X (7/6) .. = 7,000 1,000 RPM change per inch

4.. On most boats if your Prop Slip is over 15%, you should change prop pitch and/or geometry to lower your slip and increase performance. An easy change for performance if you are experiencing high prop slip you can to go to a 4 blade prop with more blade surface area. The things that need to be taken into account are how much difference there is in the blade geometry and how many RPM you will lose having the 4th blade versus the three blades. As a normal course of events if you have an aluminum 3 blade prop on your boat you can expect about a 200 to 300 RPM drop in RPM, changing pitch for pitch, if you buy a Solas Amita aluminum 4 blade prop. 4 Blade props can also have thinner blades as each one of them only has to carry ¼ of the HP per blade whereas 3 blade props have to carry 1/3 of the HP on each blade, with a 300 HP motor a 4 blade prop only has 75 HP on each blade and a 3 blade prop has to handle 100 HP on each blade. Theoretically this means the blades on a 4 blade prop can be cast 25% thinner than a comparable 3 blade propeller.
If you are running a stainless steel prop and still having high Prop Slip problems, then you would want to consult with a prop shop to get them to lower the pitch on the prop or that will let you exchange props if it doesn't cure the problem of high prop slip , as normally stainless props have much better blade geometry and don't have as high a Prop Slip as aluminum props do. But you can't just change Stainless props pitch for pitch the same way you can't just change all aluminum props pitch for pitch, because of blade geometry differences between models of props, even from the same manufacturers.


Most people with outboard or inboard/outboard motors use aluminum propellers to propel their boat. Most people also believe that aluminum props also flex a lot while running, and thus lose pitch and forward speed when propelling a boat at cruising speeds and above, versus a stainless steel prop, and that is because of the aluminum flexing.

This theory is just like the theory that all propellers lose about 200 RPM for every inch of pitch that you decrease in a prop as stated above.

The actual difference between most aluminum props and stainless props has nothing to do with flexing of the aluminum propeller, for speeds up to and above 45 MPH, that is why aluminum props are much thicker than stainless steel props. The differences are in the actual time and money spent in the blade geometry of the two props, and that is one of the reasons why stainless props cost much more than aluminum props. There is a lot more engineering designed into stainless props, partly because aluminum can't be cast with the tolerances required for the blade geometry design and partly because the stainless props can be designed much thinner, which helps lower drag. Although some of the better aluminum prop manufacturers these days are using squeeze casting technology which increases the strength of the prop and thus they are able to make the blades thinner.

Most people do not realize, the reason that stainless propellers perform much better than most aluminum props is because of these design enhancements and thinner blade sections that are NOT implemented into most aluminum props and it is NOT because of aluminum blade flex.

Having a motor turn the prop to the correct recommended RPM is Extremely important for motor longevity, but it does Not mean that your prop is efficiently using those RPM for maximum speed and fuel economy. checking the amount of prop slip you have, will give you a very good indication of how well the prop you are running is at converting Theoretical speed to Actual speed. Every time your prop slip is higher than 15% you should definitely consider changing props, but you need to make sure your speed numbers are correct, and that means using a GPS for speed and running the speed tests in 180 degree opposite directions to account for any wind or running water that influences the test results.
Heavy progressive parabolic rake, flat rake, progressive pitch, trailing edge cup, more blade surface area, thinner blade sections and blade tip cup are the things that can reduce prop slip and raise the boat for less wetted surface area, and thus increase speed when used in the right combinations for a particular hull design and motor combination.

Picking a very good prop for your particular boat and motor combination requires knowledge, but it is not an exact science. Some people use mathematical software and physics to get very close to what prop fits best in Pitch and Diameter for a certain boat and motor combination, but then is where the knowledge of prop geometry and experience is required to pick a better prop that will conform better to your uses and needs.

Aluminum props are very economical and can produce very good results for most boats, but for the maximum performance from your boat normally you would buy a stainless prop, although you will probably have to test a few of them to get the BEST one for your boat and uses if you are looking for Maximum speed.