| Scenario | Speed | Engine RPM | Gear Ratio | Pitch | Slip | Expected Result |
|---|---|---|---|---|---|---|
| Find Pitch | 38.0 mph | 5600 | 1.86 | — | 12% | Pitch ≈ 18.1 in |
| Find Speed | — | 5200 | 2.00 | 21 in | 10% | Speed ≈ 46.7 mph |
| Find Slip | 32.5 mph | 5000 | 1.75 | 19 in | — | Slip ≈ 16.4% |
For pitch in inches and speed in miles per hour:
Rearranged forms used by the different modes:
- Pitch(in) = Speed(mph) × GearRatio × 1056 / (EngineRPM × (1 − Slip))
- Slip = 1 − Speed(mph) / TheoreticalSpeed(mph)
- TheoreticalSpeed(mph) = (EngineRPM × Pitch(in)) / (GearRatio × 1056)
- Select the mode that matches what you want to solve.
- Enter your known values, including gear ratio and units.
- Use a realistic slip value when computing pitch or speed.
- Press Calculate to show results above the form.
- Export CSV for logs, or PDF for quick sharing.
Pitch and advance per revolution
Propeller pitch is the ideal forward travel in one turn, measured in inches. A 19‑inch pitch would advance 19 inches in a perfect solid. Real water is not solid, so effective advance is lower. This calculator links pitch to speed by converting inches per minute into miles per hour using the 1056 constant. It helps compare pitch changes while holding engine RPM and gear ratio constant.
Gear ratio and prop shaft speed
Gear ratio expresses engine turns per prop turn. With a 1.86 ratio, 5600 engine RPM becomes about 3011 prop RPM. Because pitch is per prop revolution, the calculator always evaluates prop shaft speed internally before estimating advance. Common marine ratios range roughly from 1.50 to 2.50, while many small aircraft drives are direct. Enter the ratio carefully; a small error shifts speed and pitch noticeably.
Slip percent and real-water losses
Slip is the percentage difference between theoretical and measured speed. Planing hulls often show 8–18% at cruise, while heavily loaded craft or rough water can exceed 20%. The slip mode computes slip from your measured speed and theoretical speed at the same RPM and pitch. For pitch or speed modes, start with a conservative slip value, then refine it after sea trials.
Interpreting theoretical versus observed speed
Theoretical speed assumes zero slip, so it is an upper bound. If your observed speed is close to theoretical, verify tach accuracy and unit selection, because real systems rarely run near 0% slip. Large gaps can signal ventilation, cavitation, excess drag, or a mismatched prop. Use the theoretical figure to understand the maximum potential before efficiency losses are applied.
Using the calculator for setup decisions
Use “Find Pitch” when you know steady speed and RPM from a run, and want the implied pitch that matches your slip estimate. Use “Find Required RPM” to check whether a target speed is realistic without over‑revving. Log multiple runs, compare slip trends with load and trim, then export results to share with your prop shop or test team. A two‑inch pitch change often shifts top speed about three mph at the same RPM.
What does propeller pitch mean?
Pitch is the ideal forward distance a prop would move in one revolution, measured in inches. Higher pitch can increase speed potential but usually reduces acceleration and may lower achievable RPM.
Why is gear ratio required?
Gear ratio converts engine RPM to prop shaft RPM. Because pitch acts at the prop, an incorrect ratio will misstate theoretical speed and the pitch implied by your test run.
What slip value should I start with?
For many planing boats, begin around 10–15% at cruise and adjust using measured data. Workboats, rough water, or heavy loads may need higher assumed slip.
Why does the formula use 1056?
1056 converts inches per minute into miles per hour: 12 inches per foot, 5280 feet per mile, and 60 minutes per hour combine into 1056.
Can I use knots or km/h?
Yes. Enter speed in your preferred unit, and the calculator converts internally to mph for computation. Results are shown back in the selected unit for easier comparison.
How do I improve accuracy of my results?
Use steady-state readings, confirm tachometer and GPS accuracy, and record trim, load, and water conditions. Run multiple passes in opposite directions and average the speeds before calculating slip.