Tips for Adjusting Propeller Pitch
A boat’s propeller converts the engine’s power into movement, and having the correct pitch maximizes performance while minimizing running costs.
You can spend tens of thousands of dollars on mighty outboard motors boasting hundreds of horsepower, but if the prop’s pitch isn't right, you won’t really get to experience maximum performance.
How Prop Pitch Affects a Motor
Propeller pitch determines how far in inches the prop will travel through water for each revolution. For example, a prop with 21-inch pitch will move forward 21 inches per revolution, whereas a prop with 17-inch pitch will only move forward 17 inches per revolution.
Propellers encounter resistance from the water as they turn, and this resistance will be greater or lesser depending upon the propeller’s pitch.
Lower pitch prop = less resistance
Higher pitch prop = more resistance
A prop that is easier to turn requires less effort from the motor, so the motor is able to reach higher RPM. If a motor is running too low on RPM, then the boat is under-propped, and is over-propped if the RPM is too high.
When a boat is under-propped, the propeller pitch is too low. The lower pitch encounters less resistance from the water, allowing the prop to revolve more freely, which places less strain on the motor and allows it to reach higher RPM.
When a boat is over-propped, the propeller pitch is too high. The higher pitch encounters more resistance from the water, preventing the prop from revolving as freely, which places more strain on the motor and will only allow it to reach lower RPM.
Finding the Perfect Pitch
Every boat motor has an ideal RPM range when run at wide-open throttle (WOT). This range is determined by the manufacturer, and can be found in the owner’s manual.
For example, a Yamaha F150 4-stroke engine has a range of 5,000 – 6,000 RPM at WOT, and an Evinrude E-TEC 2-stroke engine has a range of 5,300 – 6,000 RPM at WOT. To determine whether a prop’s pitch is correct or not, the boat is run at WOT, and the motor’s actual RPM is then compared to the manufacturer’s recommended RPM range.
Any discrepancy between the manufacturer’s RPM range and the actual RPM the motor is running at can be corrected by choosing a propeller with a different pitch. Adjusting the pitch by 1 inch adjusts the motor’s WOT RPM by 150-200 RPM.
If the motor’s RPM is at the top end of or above the manufacturer’s RPM range, the boat is under-propped. Increasing the pitch by an inch or two decreases the RPM accordingly, and puts you back in the RPM range sweet spot. If the motor’s RPM is below the manufacturer’s range, the boat is over-propped and decreasing the propeller pitch will increase the RPM to where it should be.
Adjusting Pitch for Performance
Propeller pitch also affects the high- and low-speed characteristics of a boat, which can help fine-tune the performance a boat for a particular use.
Low pitch = better acceleration and power at low speeds, but can’t achieve a higher top speed.
High pitch = higher top speed, but can’t accelerate well and provides less power at low speeds.
A boater who values outright top speed may want to fit a slightly higher-pitch propeller. However, boaters who require good low speed performance may prefer fitting a lower pitch prop that offers better hole shot and acceleration.
Regardless of your performance requirements, with the correct prop pitch, you won’t be subjecting your motor to the stresses of running over or under the rev range. This helps keep it in top condition, and reduces the risk of expensive failures. You’ll also notice improved fuel economy as the engine uses less energy to deliver optimal performance.