Why the WOT RPM Test Matters
All outboard and sterndrive motors have a wide-open throttle (WOT) rating, as determined by the manufacturer. The WOT rating is the RPM range the motor should achieve when the throttle is set to maximum.
Ensuring that your outboard or sterndrive is operating within its RPM window is essential to the longevity of the motor. To determine whether your motor is running within the correct RPM range, it’s necessary to do a WOT RPM test. This test establishes whether the propeller is of a suitable pitch for your motor/boat combination. Anyone fitting a new propeller, installing a new outboard or buying a pre-owned boat should run the WOT test.
What Affects a Motor’s WOT RPM Range?
As a boat’s motor drives the propeller, it encounters a huge amount of resistance caused by the prop trying to rotate and drive itself through the water. This resistance slows the motor, making it work harder and use more power as it increase the prop’s rotation.
Because the prop pitch increases or decreases the resistance with the water, the motor will never achieve the correct RPM at WOT if the pitch is incompatible.
A motor that underperforms and fails to reach its optimal RPM or over-performs and exceeds it at WOT due to incorrect prop pitch will experience increased wear. However, a motor that operates within its RPM range when running at WOT will not be overly stressed and therefore last longer, provided it’s serviced and maintained.
Over-Propped and Under-Propped Motors
A motor that can’t reach its designated RPM range while at WOT is said to be “over-propped,” while exceeding it is considered “under-propped.”
Over-Propped
The motor fails to reach the correct RPM at WOT because the prop pitch is too high. The higher the prop pitch, the more resistance it encounters.
Under-Propped
The motor exceeds the correct RPM at WOT because the prop pitch is too low. The lower the prop pitch, the less resistance it encounters.
Running an Over-Propped or Under-Propped Motor
Even if your never run at wide open throttle, you should still perform the WOT-RPM test, since how an engine performs at WOT applies to all throttle positions.
In other words, an over-propped motor that lugs at WOT also lugs at quarter-throttle, just like an under-propped motor that revs too high at WOT does the same thing at quarter-throttle. Some boaters like to run an incorrect prop size to get a better hole-shot or top speed, but doing so places additional stresses on the motor. Running a prop size that takes your motor out of its WOT rating is not recommended.
Correcting Over-Propped and Under-Propped Motors
Prop pitch is measured in inches, and every inch of pitch affects the motor by about 200 RPM.
Lower pitch equals less water resistance, so a prop with a lower pitch allows the motor to generate higher RPM, while the opposite is true for a prop with a higher pitch. To correct an over-propped motor, fit a propeller with a lower pitch and vice-versa for correcting an under-propped motor.
For example, a Yamaha F150 has a WOT rating of 5,000-6,000 RPM, with 5,500 being the sweet spot. If a WOT test showed the motor was struggling to achieve 5,000 RPM, it would be necessary to fit a prop with a couple of inches less pitch. If the WOT test showed the motor was revving to 6,000 RPM or above, a prop with more pitch would be needed. Remember, you can fit a slightly lower pitch prop for increased hole shot and low end performance, or a slightly higher pitch prop for better top speed, but make sure the motor stays within its all-important WOT rating.
How to Check Your Motor’s RPM at WOT
The only way to determine whether your outboard or sterndrive is running within its correct RPM window at WOT is to do the test.
The three basic steps to the WOT RPM test are:
- Find out your motor’s WOT rating from the owner’s manual, or by contacting the manufacturer or your local dealer.
- Take your boat to wide open throttle at an open stretch of water, pushing the throttle all the way forward and noting the maximum RPM the motor reaches.
- Compare the maximum RPM to the WOT rating, then fit a lower pitch propeller if you don’t reach max RPM or a higher pitch propeller if you exceed it.
In most cases, it’ll only be necessary to fit a new propeller that’s one inch of pitch lower or higher, but if your motor was way out of its RPM window, you may have to install a prop that’s several inches higher or lower.
