Back in the late 90’s, we would spend our summers waterskiing at our favourite pond where my parents had a cabin. At the time, our good friend had a 50 hp Yamaha on his 17 foot boat that he had crafted a steering wheel and turned the boat into a bow rider. We thought we had the world but we did have a problem. As good as the 50 hp was, it struggled to get the heavier skiers out of the water. And this was where my first exposure to changing a prop started.
At the same time, my father and his good buddy both had bigger cruisers and they were playing with prop size, but for the purpose of fuel economy. Whereas, we wanted better torque and acceleration (towing). Keep in mind, this was a time well before Google and finding insights was not easy…as Dad and his buddy played the trial and error game to find the correct prop, we were able to observe from their learnings, that to solve our problem, we needed to go with a lower prop pitch to achieve better acceleration and torque. For us, top end speed wasn’t the objective, we just wanted to get the heavier skiers out of the water.
As I was putting this piece together…it was like a walk down memory lane to revisit some of the terms that folks looking to upgrade their propeller (prop) need to refresh on. Here are some of those terms to refresh you with…
Understand Prop Size (Pitch & Diameter)
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Diameter = how wide the circle is that the blades make.
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Pitch = how far the prop moves forward in one revolution.
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Lower pitch = better acceleration, higher RPM.
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Higher pitch = better top speed, lower RPM.
The pitch of a propeller for an outboard motor refers to the distance the prop would move forward in one complete revolution, assuming there is no slippage (like a screw in wood). It's usually measured in inches.
So how is pitch determined? Manufacturers design propellers with specific blade angles and curves that dictate how far the prop will "travel" with each rotation. For example, a 21-inch pitch prop theoretically moves the boat forward 21 inches per revolution.
What about pitch and performance? Higher pitch (e.g., 23"): More speed, less acceleration. Good for lighter boats or high-speed cruising. Lower pitch (e.g., 17"): More torque and acceleration, less top speed. Better for heavier boats or towing.
So…before you even start looking at Pitch options…you need to start with your current motor’s specs. You’ll need to confirm your Make, model and horsepower. Where to find…Check the manual or plate on the motor.
Now…how do you determine the correct pitch for your boat? Here are some pointers to help…
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Check your engine’s WOT (Wide Open Throttle) RPM range (e.g., 5000–5800 RPM).
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Run the boat at WOT and check RPMs with your current prop.
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If RPM is too low, decrease pitch.
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If RPM is too high, increase pitch.
General Rule:
Every 1" change in pitch = ~150–200 RPM change (opposite direction).
Out of curiosity, I just searched up the WOT range of the old 50 HP Yamaha we used…apparently it was 5800-6000 RPM.
At present, you’re going to be faced with the decision of how many blades you’ll want for your prop. Here’s some quick insights to help you make your decision.
Number of Blades
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3-blade: common, good all-around choice.
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4-blade: better handling, faster planing, more lift; often used for heavier boats or towing.
Okay…if you’re at the point now of making decisions…the choice of materials has to come into play.
Material: Aluminum vs. Stainless Steel
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Aluminum: Cheaper, good for casual/recreational use.
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Stainless steel: Stronger, more efficient, lasts longer, but more expensive.
How to impress your father-n-law or that old skipper…knowing whether you should “pitch up” or “pitch down”...and what the fig does that actually mean? Now, this will be the most technical part of this piece… Engine rpms and pitch are inversely related. Increasing the pitch will decrease engine rpms and decreasing the pitch will increase engine rpms. As a general rule, a two-inch increase in pitch will result in a reduction of 300 to 400 rpm. Conversely, a two-inch decrease in pitch will result in an increase of 300 to 400 rpm. Going back to the question of performance at WOT, if your engine is under revving, consider a propeller with less pitch. If your engine is over revving, consider a propeller with more pitch.
We’ll finish off with some general “Prop Talk”. Now, be prepared for filling your tool box with some new terms here for your Prop Talk vocab…you’ll be adding rake and cupping. But first, let’s recap…
Propeller size is expressed with two numbers, diameter and pitch, with diameter always stated first. Diameter is two times the distance from the center of the hub to the tip of any blade.
Smaller prop diameters generally go with smaller engines, or with fast high performing boats. Pitch is the theoretical forward distance, in inches, that a propeller travels during one revolution. There is always some “slip” between the propeller and the water (generally 10 to 15 percent) so the actual distance traveled is somewhat less than the theoretical value. Think of pitch as speed, or as the gear selection on a car’s transmission.
Rake is the degree that the blades slant forward or backward in relation to the hub. Rake can affect how water flows through the propeller, which can make a difference regarding boat performance. Aft rake helps to lift the boat’s bow, decreasing the hull’s wetted surface area and improving top end planing speed. Today’s aggressively raked propellers may require that you add a high performance trim tab to your shopping list. The blade tips of these new propellers may strike the older style trim tabs on your engine.
Cupping of the trailing edge of the propeller blade is common on many propellers. A downward curve of the lip of the blade (like a plane’s wing with the “flaps” down) allows a better hole shot, less slippage and ventilation, and helps the propeller get a better bite on the water. A cupped prop may allow the engine to be trimmed with the prop closer to the surface, and will also decrease rpm by 150 to 300.
Final points…
If your boat is underperforming or not reaching the recommended WOT RPM, the pitch may need to be adjusted. For example:
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If the engine is over-revving (RPM higher than the recommended range), you may need a prop with lower pitch.
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If the engine is under-revving (RPM lower than the recommended range), you may need a prop with higher pitch.
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Also, if your boat struggles with acceleration or handling under load, a different prop size or style may be necessary.
This simple little piece isn’t about turning anybody into a prop expert. However, it’s a great starting point if you’re looking to change out your prop or if you think you’re not getting the performance you desire. In our case, back in the late 90s…we dropped the prop pitch, gained some torque and those heavier skiers came out of the water easier.