Boat Propeller Calculator
Propping a boat right is the cheapest way to gain speed and fuel economy. The correct propeller lets the engine reach its rated WOT RPM at the rated boat speed without lugging or over-revving. This boat propeller calculator works two ways: sizing mode takes a target boat speed and returns the pitch you should buy; prediction mode takes your current pitch and returns the boat speed you should see. Both modes use engine RPM, gear ratio, and propeller slip as inputs and handle outboard, sterndrive, inboard, pontoon, and heavy-cruiser drive types.
The boat propeller formula
Propeller pitch is the theoretical distance the prop advances in one revolution. Actual advance is less because the prop slips in the water. So:
- Theoretical speed (mph) = (prop RPM x pitch_in) / 1056 (1056 converts in/min to mph)
- Actual speed = theoretical speed x (1 - slip)
- Prop pitch (in) = target speed x 1056 / (prop RPM x (1 - slip)) in sizing mode
Prop RPM = engine RPM / gear ratio. For a 5,400 RPM outboard with 2.0 gear ratio, prop RPM = 2,700. At 21 inch pitch with 10 percent slip: theoretical = 53.7 mph, actual = 48.3 mph. This boat propeller sizing calculator math is unchanged whether you work in inches or millimeters.
Boat propeller size chart
A boat propeller size chart starts with target WOT speed and engine RPM and lists the pitch that lands in the target range. Common combinations:
| Boat type | Engine RPM at WOT | Gear ratio | Typical pitch | Typical speed |
|---|---|---|---|---|
| Bass boat (200 HP) | 5800 | 1.87 | 23-25 | 62-70 mph |
| Pontoon (115 HP) | 5500 | 2.33 | 11-13 | 18-25 mph |
| Runabout (150 HP) | 5500 | 2.07 | 17-19 | 38-45 mph |
| Inboard ski boat | 4200 | 1.50 | 14-16 | 32-40 mph |
| Offshore cruiser | 4200 | 1.65 | 19-21 | 32-38 mph |
Propeller slip by hull type
Slip is the percentage difference between theoretical and actual boat speed. Light boats with clean props slip less; heavy boats or those pushing a lot of water slip more.
- Planing outboard boat: 8-12 percent
- Sterndrive planing hull: 10-15 percent
- Inboard boat propeller calculator typical: 20-30 percent (shafted inboards have higher slip)
- Pontoon or barge: 15-25 percent
- Heavy displacement cruiser: 25-35 percent
- Model boat propeller size calculator: 15-25 percent for typical RC models
Using the sizing calculator
A boat propeller sizing calculator like this one starts with three numbers you pull from the engine spec sheet: rated WOT RPM range, gear ratio, and hull type. Pick a target speed from the cruise or WOT figure you want. The calculator returns a prop pitch. Subtract 1 inch for the next-smaller stock size if the math lands between whole inches. An inboard boat propeller calculator setup uses 25 percent slip instead of 10 percent, so a 21 inch pitch outboard would be a 16-17 inch pitch inboard for similar speed.
Boat propeller calculator app logic
A boat propeller calculator app usually needs three screens: engine specs, hull slip, and target speed. This one folds all three into a single form. The sizing output is a starting point. Real-world validation requires a WOT test after installing the prop. If engine RPM lands below the rated WOT range, the prop is over-pitched (too tall a pitch) and the engine is lugging. If it lands above, the prop is under-pitched and the engine is over-revving. Change pitch by one inch increments until the engine hits the middle of its WOT RPM band.