Marine Gear Ratio Calculator
Calculate the optimal gear ratio for your marine engine and propeller combination. Find the right reduction ratio to match engine RPM to propeller speed for best performance.
How to Use
- Enter your engine RPM at cruising speed or wide-open throttle (WOT)
- Enter your propeller pitch in inches (stamped on the prop)
- Enter your target boat speed in knots
- Adjust propeller slip percentage (10-20% is typical for most boats)
- Select your engine and drive type for tailored recommendations
- Click Calculate to find the ideal gear ratio
What is gear ratio? The gear ratio (or reduction ratio) is how much the gearbox slows the engine RPM before it reaches the propeller. A 2:1 ratio means the prop turns at half the engine RPM.
Why does it matter? The right gear ratio matches your engine's power band to the propeller's optimal RPM range. Too low a ratio overloads the engine; too high wastes power at excessive prop speed.
Higher ratios (3:1, 4:1) suit low-RPM diesel engines and displacement hulls. Lower ratios (1.5:1, 2:1) suit high-RPM petrol engines and planing boats.
Frequently Asked Questions
What gear ratio do I need for my boat?
It depends on your engine RPM and propeller. Petrol outboards typically use 1.6:1 to 2.3:1, sterndrives 2.0:1 to 2.5:1, and diesel inboards 2.0:1 to 4.0:1. Lower-RPM engines need higher gear ratios to bring prop speed into the optimal range.
What is a marine gear ratio?
The gear ratio (reduction ratio) describes how much the gearbox or lower unit reduces engine RPM before it reaches the propeller. A 2:1 ratio means the propeller turns at half the engine RPM. The right ratio matches engine power band to the propeller's efficient operating speed.
How does gear ratio affect boat performance?
A lower gear ratio (e.g. 1.5:1) gives higher prop RPM and suits high-speed planing boats. A higher gear ratio (e.g. 3:1 or 4:1) gives more torque at the prop and suits displacement hulls and heavy loads. Mismatched ratios cause poor acceleration, overheating, or the engine not reaching full RPM.