How to Perform a WOT RPM Test in 3 Steps
Because propeller pitch plays such a significant role in the performance of an outboard or sterndrive, it's necessary to run a Wide Open Throttle (WOT) test every time you fit a new propeller.
Whenever you purchase a new outboard or boat, it's wise to perform a WOT RPM test. Every outboard and sterndrive motor has a WOT rating, as established by the manufacturer. The WOT rating is the RPM range the motor should achieve when running at wide open throttle. If the propeller has too much pitch, the motor will not reach its optimum RPM at WOT, while a prop with too little pitch will exceed the recommended RPM limit at WOT.
The WOT Test
The WOT test is done in three simple steps:
1. Find out your outboard or sterndrive's WOT rating
2. Run your boat at WOT
3. Replace the propeller with a higher or lower pitch prop
To get the most accurate result from the WOT RPM test, there are a few things you need to do to ensure the data from the wide open throttle test is as precise as possible.
Step 1 - WOT Rating
First, you'll need to know your motor's WOT rating. This will be clearly stated in the owner's manual. If you don't have one, your local dealer should be able to help you out, or contact the manufacturer for the information.
Step 2 - Run at WOT
To establish your motor's RPM, you'll need a tachometer. If your boat isn't equipped with one, it's a good idea to fit one, although you can use a handheld non-contact tachometer instead.
Run your boat exactly as you would at any other time, so the WOT test results reflect your boat's typical performance. Load all the gear and passengers you would normally carry on board. Fill the fuel tank to about two-thirds to get an average weight, and find an open stretch of water. You'll need enough space to get on plane and run at WOT in a straight line for some time, so look for a sizable body of water. Pick a time when the water is calm and there aren't many other watercraft around to interfere with your WOT runs.
Get on plane, go to wide open throttle and trim out the motor until you reach top speed. Note the motor's RPM and bring the boat to a halt. Next, repeat the WOT run in the opposite direction to account for wind or tides, and take an average from the two RPM readings. If your RPM at wide open throttle isn't right in the middle of your WOT rating, you may need to change the propeller for one with a suitable pitch.
Remember, the WOT rating provides an RPM range, and your motor can run toward the upper or lower limit of this range. If you run your boat heavier or lighter, you could push your motor out of the optimum RPM window. It's always best to run a motor as close as possible to the middle of its WOT RPM range.
Step 3 - Replace the Propeller
If the test shows the motor exceeds the optimum RPM at WOT, the motor is "under propped."
To remedy an under-propped motor, change the propeller to one with a greater pitch. The higher pitch prop creates more resistance in the water, which slows the engine and brings the RPM down to optimum speed. If the WOT test shows the motor fails to reach the optimum RPM, the motor is "over propped." To remedy an over-propped motor, change the propeller to one with a lesser pitch. The lower-pitch prop creates less resistance in the water, which allows the engine's RPM to increase and achieve optimum speed.
Propeller pitch is measured in inches. Changing prop pitch by 1 inch will change the WOT RPM of a motor by about 200 RPM.
Prop pitch and RPM work inversely to one another, so decreasing prop pitch will increase the RPM, and increasing prop pitch will decrease the RPM.
For example, Yamaha states that the WOT rating for its 90HP outboard is 5,000-6,000 RPM with peak power at 5,500 RPM. If the WOT test showed the Yamaha 90 reaching just over 6,000 RPM, you would need to fit a prop with at least 2 inches more pitch.
Although it may be tempting to slightly over-prop your motor to get better hole shot and low-end performance, or under-prop your motor for high speed gains, remember that changes in weight or weather conditions could tip the motor out of the WOT RPM range. By keeping your motor in the middle of the RPM window, your motor will likely continue running within its WOT rating at all times.