Calculate propeller slip percentage from pitch speed and actual speed. Estimate efficiency and compare to typical slip ranges for aircraft, boats, and drones.
Propeller slip is the difference between the theoretical pitch speed and the actual forward speed, expressed as a percentage: Slip = (Pitch Speed − Actual Speed) / Pitch Speed × 100. Some slip is always present — a propeller works by accelerating a mass of fluid backward, which requires the blade to meet the fluid at an angle of attack.
This calculator computes propeller slip from three possible input combinations: direct pitch and actual speeds, RPM with pitch and actual speed, or RPM with pitch and a target slip percentage. It handles both MPH and knots for marine and aviation applications.
Typical slip ranges vary dramatically by application: 10-20% for aircraft in cruise, 15-30% for RC planes, 10-20% for planing boats, 30-50% for displacement hulls, and up to 60% for sailboat auxiliary propellers. The reference table helps you compare your calculated slip to expected values.
Understanding slip is essential for propeller selection, boat speed prediction, fuel efficiency analysis, and diagnosing prop/engine matching problems.
Measuring and comparing propeller slip is the simplest way to evaluate whether your prop/engine combination is well-matched to your vehicle.
For boat owners, tracking slip over time can detect problems like hull fouling, prop damage, or engine derating before they cause failures. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
Slip (%) = (V_pitch − V_actual) / V_pitch × 100. Pitch speed = Pitch × RPM (in consistent length/time units). Efficiency ≈ (1 − slip) × η_blade (typically 0.90-0.95).
Result: Slip = 15.4%, lost speed = 20 mph, efficiency ≈ 80%
Slip = (130 − 110)/130 × 100 = 15.4%. This 20 mph difference is the energy going into accelerating the slipstream rather than advancing the vehicle.
Use consistent units, verify assumptions, and document conversion standards for repeatable outcomes.
Most mistakes come from mixed standards, rounding too early, or misread labels. Recheck final values before use. ## Practical Notes
Use concise notes to keep each section focused on outcomes. ## Practical Notes
Check assumptions and units before interpreting the number. ## Practical Notes
Capture practical pitfalls by scenario before sharing the result. ## Practical Notes
Use one example per section to avoid misapplying the same formula. ## Practical Notes
Document rounding and precision choices before you finalize outputs. ## Practical Notes
Flag unusual inputs, especially values outside expected ranges. ## Practical Notes
Apply this as a quality checkpoint for repeatable calculations.
No. A propeller needs an angle of attack to produce thrust, which requires the blade to "screw" through the fluid faster than the vehicle moves. Zero slip would mean zero thrust.
Overloaded boat (too heavy, too much drag), wrong prop pitch (too high for the power available), damaged blades, growth/fouling, or operating in rough water. Use this as a practical reminder before finalizing the result.
Lower slip generally means better fuel economy at a given speed. However, very low slip can mean the prop is under-pitched and the engine is working harder than necessary.
For planing boats: 10-15% at cruise speed is optimal. Higher slip at low speed (hole shot) is normal. Displacement boats have naturally higher slip (30-50%).
More blades can slightly reduce slip by distributing the thrust over a larger blade area, but the main benefit is smoother operation. Too many blades increase interference and drag.
Slip is highest at zero speed (bollard pull / static thrust) and decreases as speed increases toward the theoretical pitch speed. At very high speed, slip approaches zero but thrust also vanishes.