Boat Props – Run a Simple Wide Open Throttle Test Before Shopping For a New Boat Prop
By Loren Hadley
If you are considering buying a new boat prop to increase your boat’s performance, consider doing a Wide Open Throttle Test first. Start with your current prop if possible and get a performance baseline.
If your current prop allows you to operate in your engine manufacturer’s Recommended Operating Range for your engine, then you’ll have a good baseline to make prop configuration decisions.
If it doesn’t allow you to run in the optimal range, then you’ll have some data that will suggest prop changes that will improve your engine’s performance, efficiency and safety.
Do your Homework
The first step in performing a Wide Open Throttle Test is to identify the Recommended Operating Range for your engine. It will be a band of RPMs specified by the engines manufacturer.
It should be published in your Owner’s Manual. If you can’t find it there, your engine dealer, or manufacturer should be able to provide you with this information.
Hit The Water
The next step is the to take your boat out on the water where you can safely run at full throttle. Take your boat to full throttle watching your RPMs. If your engine doesn’t exceed the maximum recommended RPMs, make several test runs, adjusting the trim for your best performance. Note the Wide Open Throttle RPMs on each run.
If it looks like you will reach your maximum recommended RPMs before you reach full throttle, don’t exceed the maximum recommended RPMs. Estimate where you think the RPMs would get to if you did hit full throttle. Make a note of the RPMs.
Do the Math
Selecting your boat prop is sort of like choosing a single gear to run your car in. The size and pitch of your prop will have a direct impact on the number of RPMs your engine can turn. A prop with too little pitch can allow your boat to operate above the maximum recommended operating range and could lead to engine damage or unsafe conditions.>A prop with too much pitch may keep you from reaching the most efficient operating range for your motor, causing performance and fuel consumption to suffer. The results of your Wide Open Throttle test will help you narrow your prop selections to the best options for your boat and motor. Within that selection, you can choose the options that will give you better hole shot, or improved top end speed, depending on how you use your boat.
If your actual RPMs allowed you to reach the upper portion of the recommended operating range without exceeding the upper limit, then your current prop is a good option and you have a solid baseline to make performance oriented prop change decisions.
If you couldn’t reach the upper end of your recommended operating range, we will want to subtract the actual RPMs you reached from the maximum RPMs in the recommended operating range. For example, if the maximum recommended RPMS are 5000 and your actual wide open throttle RPMs were 4600, then 5000-4600=400.
We will want to try a prop that has less pitch, which will increase the RPMs your engine can turn. A good rule of thumb is that decreasing pitch by 1″ will increase RPMs by roughly 200. Given that we need to gain 400 RPMs, we will want to reduce our pitch by 2 inches. If we have a 16″ pitch prop now, let’s try a 14″ pitch prop.
The reverse is true if your engine can turn in excess of the recommended maximum RPMs. Say we estimated that at full throttle, our engine would have hit 5600 RPMs. Since our maximum RPMs are 5000, we want to reduce our RPMs by 5600-5000=600. Increasing pitch by 1″ will reduce decrease RPMs by roughly 200. With our 16″ pitch prop, we will want to add 3″ of pitch so we’ll consider a 19″ pitch prop.
Armed with the results of your Wide Open Throttle Test, you’ll have good information to select a new prop to improve your performance. If your dealer has test props or allows exchanges, you can take your new prop selection, install it and run another Wide Open Throttle Test. You’ll know if you are in the right place and if you are getting the performance enhancements you were seeking.
About the Author: Loren Hadley writes for SavvyBoater, an on-line retailer specializing in providing boaters with great deals on popular and unique marine supplies. http://www.SavvyBoater.com offers Solas Propeller’s extensive line of Boat Propellers for sports and recreational boats.
In addition to their handy Boat Prop Finder they offer a variety of expert articles to help you find the prop you’re looking for.
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