If you are thinking of purchasing a new propeller to increase your boat’s performance, consider doing a wide open throttle test first. Start with your current accessory if possible and get a performance baseline. If your current propeller allows you to operate in the engine manufacturer’s recommended operating range for your engine, then you will have a good baseline for making propeller configuration decisions. If it doesn’t allow you to run in the optimum range, you’ll have some data that will suggest changes to your propellers 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 an RPM band specified by the engine manufacturer. It should be posted 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 to get your boat out on the water where you can safely run at full speed. Bring your boat up to speed by watching your RPM. If your engine does not exceed the maximum recommended RPM, run several tests and adjust the trim for best performance. Note the wide open throttle RPM on each stroke.
If it looks like you will reach the maximum recommended RPM before reaching full throttle, do not exceed the maximum recommended RPM. Calculate where you think the RPM would go if you went full throttle. Take note of the RPM.
do the math
Selecting your boat’s propeller is somewhat like choosing a single gear to run your car. The size and pitch of your propeller will have a direct impact on the amount of RPM your motor can turn. A propeller 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 propeller with too much pitch can prevent you from achieving the most efficient operating range for your engine, causing performance and fuel consumption to suffer. The results of your wide open throttle test will help you narrow down your accessory selections to the best options for your boat and engine. Within that selection, you can choose the options that will give you a better holeshot or improved top speed, depending on how you use your boat.
If your actual RPM allowed you to reach the top of the recommended operating range without exceeding the upper limit, then your current prop is a good fit and you have a solid foundation for making performance-oriented prop changing decisions.
If you were unable to reach the top end of your recommended operating range, we will subtract the actual RPM you achieved from the maximum RPM in the recommended operating range. For example, if the maximum recommended RPMS is 5000 and the actual RPM at wide open throttle was 4600, then 5000-4600=400. We will want to test a prop that has less pitch, which will increase the RPM your motor can turn. A good rule of thumb is that decreasing the pitch by 1″ will increase the RPM by about 200. Since we need to gain 400 RPM, we’ll want to reduce our pitch by 2 inches. If we now have a 16″ pitch support, try a 14″ pitch.
The reverse is true if your engine can spin in excess of the maximum recommended RPM. Let’s say we estimated that at full throttle, our motor would have reached 5600 RPM. Since our max RPM is 5000, we want to reduce our RPM by 5600-5000 = 600. Increasing the pitch by 1″ will reduce the RPM by about 200. With our 16″ pitch strut, we’ll want to add 3″ of pitch, so we will consider a 19″ step strut.
Armed with the results of your wide open throttle test, you will have good information to select a new propeller to improve your performance. If your dealer has test fixtures or allows trade-ins, you can take your new selection of fixtures, install them, and run another fully open throttle test. You’ll know if you’re in the right place and getting the performance gains you’re looking for.