Estimate theoretical boat speed, propeller slip, and propulsion efficiency from RPM, pitch, diameter, and actual speed. Visualize performance curves and get expert recommendations for propeller selection.
This tool computes the theoretical boat speed based on propeller pitch and RPM, then compares it with your actual measured speed to determine propeller slip and propulsion efficiency. These metrics are essential for evaluating whether your propeller is correctly matched to your vessel, engine, and operating conditions.
Theoretical Speed (kn) = RPM × Pitch (in) × 60 / (1852 × 39.37)
Slip (%) = (Theoretical Speed − Actual Speed) / Theoretical Speed × 100
Efficiency (%) = Actual Speed / Theoretical Speed × 100
Where 1 nautical mile = 1852 m, 1 m = 39.37 in.
Propeller slip is the difference between the theoretical distance a propeller should advance per revolution (based on pitch) and the actual distance it advances through water. Slip is not a sign of inefficiency — it is a necessary phenomenon caused by the fluid nature of water. A certain amount of slip (typically 5–15% for well-matched installations) indicates that the propeller is generating thrust by "biting" into the water. Too little slip suggests over-pitching (engine struggles to reach rated RPM), while excessive slip indicates under-pitching (engine over-revs with poor thrust).
Understanding slip allows boat owners and marine engineers to select the correct propeller pitch for a given hull and engine combination. This directly impacts fuel economy, top speed, acceleration, and engine longevity.
The speed your boat would achieve if the propeller acted as a perfect screw moving through a solid medium. This is always higher than actual speed due to slip.
Expressed as a percentage, this indicates how much "slippage" occurs. 5–15% is typical for planing hulls; 20–40% for displacement hulls. Values above 30% suggest mismatched pitch or hull fouling.
The reciprocal of slip, this is the ratio of actual to theoretical speed. Higher efficiency means better use of engine power.
The actual distance the boat moves forward per propeller revolution. This is the effective pitch after accounting for slip.
A 28-foot express cruiser was repowered from a 250 HP to a 300 HP engine. The existing propeller had a 17" pitch. At 3200 RPM, the boat achieved 26 knots. Using this calculator, the theoretical speed was 32.5 knots, giving a slip of 20% — slightly high for a planing hull. After consulting the performance curves, the owner switched to a 19" pitch propeller. At the same RPM, the new theoretical speed was 36.4 knots, and the actual speed increased to 29.5 knots, reducing slip to 19% and improving fuel efficiency by 8%. The calculator helped identify the optimal pitch without trial-and-error.
Selecting the right propeller involves balancing several factors: engine power curve, hull design, intended use (cruising, fishing, watersports), and operating conditions. As a rule of thumb:
This calculator provides the quantitative data to make informed decisions. For critical applications, always consult with a certified marine propulsion specialist and consider performing sea trials with multiple propeller candidates.
| Vessel Type | Typical RPM Range | Pitch Range (in) | Typical Slip (%) | Efficiency (%) |
|---|---|---|---|---|
| High-speed planing (sport boats) | 4500–6000 | 17–26 | 5–12 | 88–95 |
| Family cruisers | 3000–4500 | 14–20 | 8–15 | 85–92 |
| Work boats / Trawlers | 1200–2400 | 22–36 | 15–30 | 70–85 |
| Displacement sail auxiliaries | 1500–3000 | 12–18 | 20–35 | 65–80 |
| Commercial fishing vessels | 1400–2200 | 26–40 | 18–28 | 72–82 |