Propeller Pitch

What is propeller pitch?

Propeller Pitch is the displacement a propeller makes in a complete spin of 360° degrees. Or in other words, the distance a boat prop will move through the water for each revolution. For example, a 21-pitch propeller would move forward 21 inches in one revolution.

The pitch concept is not exclusive to propellers. Other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn when hit by the screwdriver.

It is important to note that in real life boat propellers will experience "slip" or "slippage." Since a boat prop slides through water versus the friction of turning through a solid material (like wood), slippage is simply the difference between how far a boat propeller should have gone based on its pitch rating, and how far it actually did go. Slippage on boat propellers can range from as little as 10% for very efficient racing boats to as much as 40% for cruisers.

How does propeller pitch affect the performance of my boat?

In the example of a wood screw, think of screwing a screw into wood. If the threads are narrow, you’ll have to make a lot more turns to get the screw set, but it is easier to make each turn. If the threads are wider apart, the screw goes in further with each turn, but it takes more effort to make the turn. The same concept applies to boat propellers.

The pitch of a boat prop will influence the operating RPMs of the motor. A higher pitch boat prop will reduce the RPM’s you are turning at any given speed and will allow you to reach maximum top end speeds like a high gear, but you may experience slower acceleration for getting your boat up on plane or towing a skier. A lower pitch boat prop will allow you to reach higher RPM’s and improve your hole shot, giving you the quick start and power to get a wakeboarder up and running quickly, however, you may trade some top end speed for that.

As an example, when you drive your car, you start off in a low gear that lets you get going easily. When you are cruising down the freeway, you’ll be in a high gear that allows you to go at higher speeds. If you try to start off in 4th gear, you’ll have problems; conversely, if you try to run at 55 mph in 1st gear you’ll have a serious problem. With a boat prop, you have only the one "gear" so you need to decide which one will best meet your needs and boating style.

How do I know which propeller pitch is best for my boat?

Perhaps the most important criteria for selecting propeller pitch is your personal performance goals. You will always need to find a compromise between low-end torque or faster top speeds. You can't have both. The trick is to find a boat prop that delivers acceptable acceleration and top speed for your style of boating. If you load your boat with a lot of people or heavy diving or fishing gear, you might need a low pitch for better acceleration. If your boat doesn’t have any problem getting up on plane quickly but you need more speed, a higher pitch is the way to go.

If you're changing pitch on a recreational boat, the general rule of thumb is that each inch of pitch is worth about 200 RPMs. Lowering the pitch of your boat prop will increase RPMs; and conversely, increasing your propeller pitch will decrease engine RPMs. For example, going from a 23 pitch to a 21 pitch will increase your top end engine RPM’s by about 400 revolutions.

Prior to purchasing a new boat prop, you will need some preliminary information about your boat and how it performs with its current prop configuration.

Step 1: Verify what your engine manufacturer has published as the recommended operating range of RPM’s that will allow your engine to perform efficiently and safely, then find the diameter and pitch of your existing boat prop. Usually a boat prop will have the diameter and propeller pitch stamped on it, most likely somewhere on the hub. It will typically be displayed as "diameter X pitch."

Step 2: Run your boat at wide open throttle (WOT) with a typical load and trimmed for best speed and track your actual RPM’s. Your boat prop should allow you to run your boat with a wide open throttle and reach the mid to upper end of your recommended operating RPMs for your motor.

Step 3: Based on the results of your WOT test, determine if you need, or want, to make any changes to your propeller pitch to adjust the overall performance.

For more information on performing the Wide Open Throttle test and evaluating the results, read The Wide Open Throttle Test – an Important Step in Selecting the Right Boat Prop.

If you have a wide range of uses for your boat such as high speed cruising, wakeboarding with all your friends, fishing, etc., you may want to consider having more than one boat prop, each with a different propeller pitch, so you can optimize the power of your engine for your different applications.

Once you have determined the propeller pitch best suited to your boat and your style of boating, then all you need is your engine make, model and year, and you can use a simple Boat Propeller Finder like the one on Savvyboater.com to find the perfect aluminum or stainless steel boat prop for you.

Be prepared with an Extra Propeller

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