Calculate the optimal gear ratio for your boat's propulsion system
Typical performance outboard setup
Common diesel cruiser setup
Low-speed, high-torque application
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Download NowThe gearbox is the translator between an engine that loves to rev and a propeller that wants to turn slowly and bite hard, and getting that translation right is most of propulsion. Too tall a ratio and the engine over-revs with a small prop; too short and it lugs against a wheel it cannot spin up.
The ratio you need falls out of the speed, pitch and engine rpm you are working with, because they together fix the shaft speed the prop must turn at to drive the boat without excessive slip. That same ratio also sets propeller tip speed, where too much of it brings noise, erosion and wasted power.
Enter engine rpm, target speed, prop pitch and diameter, and the calculator recommends a gear ratio, the resulting prop rpm, the slip you can expect and tip speed so you can match the box to the wheel.
Prop RPM = (Speed × 1056) ÷ (Pitch × (1 − Slip)); Gear Ratio = Engine RPM ÷ Prop RPM; Tip Speed = (Prop RPM × Diameter × π) ÷ 12| Gear Ratio | Character | Best For |
|---|---|---|
| 1.5 - 2.0:1 | Higher prop rpm | Small fast planing boats |
| 2.0 - 2.5:1 | Balanced | Runabouts, medium cruisers |
| 2.5 - 3.0:1 | Lower prop rpm, bigger wheel | Heavy cruisers, trawlers |
| 3.0:1+ | Very low prop rpm | Trawlers, large displacement hulls |
Guidance on gear ratio by duty.
Formula verified against standard naval-architecture references · Updated October 2026
The ratio that lets the engine reach its rated rpm while turning the prop at the speed that drives your target boat speed on your chosen pitch. Work it out as engine rpm divided by the required prop rpm, where prop rpm comes from speed, pitch and slip. Enter those here and the calculator returns a ratio, then you pick the nearest available box and confirm with a tachometer on the water.
Indirectly but strongly. The wrong ratio either over-revs the engine with a prop that is too free or under-revs it with one that is too tight, and both cap speed and hurt fuel economy. The right ratio puts the engine and prop at their efficient point, which is usually what lets a boat reach its true potential rather than adding raw power.
Slip is the shortfall between the distance a prop should advance each turn and how far the boat actually moves, typically ten to twenty-five percent. Ignoring it predicts too high a shaft speed and the wrong ratio. Including a realistic slip gives a gear and prop combination that holds speed in the real, grippy-but-imperfect world rather than on paper.
As a working ceiling, keep propeller tips below roughly seven thousand feet per minute. Beyond that they begin to cavitate and ventilate, producing noise, vibration, lost thrust and pitted blade faces, especially on cheaper wheels. If tip speed is the problem, add pitch or diameter and use a taller gear ratio rather than simply spinning faster.
Raise it (a taller number) when the engine over-revs or tips are too fast, because a bigger, slower wheel is more efficient. Lower it when the boat cannot reach rated rpm on level trim and you want a freer prop. Change one thing at a time and measure, because ratio, pitch and diameter are coupled levers on the same outcome.
Yes, in the lower unit, expressed as transom-to-gear ratio such as 2.08:1. The principle is identical: it trades engine rpm for propeller torque and speed. Outboards are usually fixed-ratio, so you tune the package through prop pitch and diameter, while inboards give you a selectable gear case to match the drivetrain more precisely.
Run the boat in normal trim and load, and read the tachometer at wide-open throttle. If the engine reaches the top of its rated rpm band the ratio and prop are correct; if it falls short or over-revs, adjust pitch, diameter or gear. That on-water check is the final judge that no calculation can replace.