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info Overview

Estimate a boat's theoretical and real speed from propeller pitch in inches, engine RPM, and a 0-50% slip allowance, shown in mph, km/h, and knots.

📘 How to Use

  1. Enter the nominal propeller pitch in inches stamped on the prop
  2. Enter the propeller shaft RPM (divide engine RPM by the gear ratio for geared drives)
  3. Set the slip percentage with the slider and read theoretical and actual speed in three units

Propeller Pitch Speed Calculator

in

Nominal pitch stamped on the prop (1-50 in)

rpm

Propeller shaft speed (500-8000)

15 %
0 50

Real-world prop loss; 15-30% is typical

speed Results

Actual speed (with slip)

— mph
— km/h • — knots

Theoretical speed (no slip)

— mph
— km/h • — knots

Applied slip

— %

※ 1056 = 63360 in/mi / 60 min/h (theoretical-speed constant)

※ Slip represents real-world propeller thrust loss

※ For geared outboards/sterndrives, enter propeller shaft RPM (engine RPM divided by the gear ratio), not crankshaft RPM

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Article

Propeller Pitch Speed Calculator | Estimate Boat Speed from Pitch, RPM and Slip

Estimate a boat's theoretical and actual speed from propeller pitch, shaft RPM, and slip, shown side by side in mph, km/h, and knots. A quick way to sanity-check a prop swap or compare pitch options before you buy.

💡 Tool Overview

Propeller pitch is the theoretical distance the prop advances in one full revolution, in inches. A 19-inch prop is designed to move 19 inches forward per turn. Theoretical speed is pitch × RPM ÷ 1056, where the constant 1056 converts inches-per-minute to miles-per-hour (63,360 inches per mile ÷ 60 minutes per hour).

Because water is not a solid medium, the prop never advances its full geometric pitch. The gap between theoretical and actual advance is called slip, expressed as a percentage. For most boats slip falls between 5% and 25%, with 15–30% typical depending on hull and load. Actual speed is the theoretical speed reduced by the slip allowance.

One input trips people up: the RPM field expects propeller shaft speed, not crankshaft speed. On geared outboards and sterndrives the engine spins faster than the prop, so divide engine RPM by the gear ratio before entering it here.

🧐 FAQ

How do I find my actual slip percentage?

Run the boat at a known RPM, record GPS speed, and compare it to the theoretical speed this tool reports. Slip % = (theoretical − actual) ÷ theoretical × 100. GPS is more reliable than a paddlewheel speedometer for this.

What slip number should I assume if I have no data?

For a planing hull at speed, 10–20% is a reasonable starting point. High-performance setups often see 7–12%, while heavy displacement hulls can exceed 40%. Start near 15% and refine once you have on-water numbers.

Why does a bigger pitch not always mean a faster boat?

A higher pitch raises theoretical speed at the same RPM, but it loads the engine harder. If the boat is heavy or underpowered, the engine may not reach its rated RPM, so real speed can drop instead of rising.

Why show knots as well as mph?

Charts and marine navigation use knots (1 knot = 1.852 km/h). Displaying mph, km/h, and knots together lets road-speed intuition and marine convention sit side by side.

📚 Why Some Slip Is a Good Thing

It is tempting to treat slip as pure loss, but a propeller needs slip to generate thrust at all — zero slip would mean zero forward force. The blades work by accelerating water backward, and that requires the prop to advance less than its geometric pitch.

Performance boaters often use slip as a diagnostic. A slip number well above the expected range can point to a damaged or ventilating prop, the wrong pitch, or a hull that is not getting on plane, while an unusually low number may mean the prop is loading the engine below its powerband. Reading slip alongside RPM tells you whether the engine is actually turning in its recommended range.