Skip to main content
Marine Free Pro Features Available

Propeller Pitch & Slip Calculator

Theoretical Speed, Actual Speed, and Slip Percentage for Inboard, Sterndrive, and Outboard Marine Propellers

Free propeller pitch and slip calculator for marine mechanics, boat owners, and propeller shops who need to evaluate whether a propeller is correctly matched to the engine and hull. Enter the propeller pitch (inches), gear ratio, engine RPM at wide-open throttle (WOT), and actual boat speed. The calculator returns the theoretical speed (what the boat would do with zero slip), the actual slip percentage, and a diagnostic assessment of whether the slip is within normal range for your application.

Propeller slip is the difference between the theoretical speed (pitch times RPM) and the actual speed through the water. Typical slip ranges from 10-15% for planing hulls, 15-25% for semi-displacement hulls, and 25-45% for displacement hulls and sailboats under power. Slip that is too low usually means the engine is under-loaded (over-revving past the rated WOT RPM range), which causes premature wear and wastes fuel. Slip that is too high means the engine is lugging (unable to reach rated WOT RPM), which causes overheating, carbon buildup, and poor performance.

The calculator also works in reverse: given a target speed, gear ratio, and acceptable slip range, it recommends the optimal pitch that will load the engine into its rated WOT RPM band. This is the calculation every propeller shop does when re-propping a boat after an engine repower, hull modification, or performance complaint.

Pro Tip: As a rule of thumb, each inch of pitch change moves the WOT RPM by approximately 150-200 RPM. If the engine is 400 RPM below the manufacturer's rated WOT range, try dropping 2 inches of pitch. If the engine over-revs by 300 RPM, go up 2 inches. Always verify with a GPS speed test after the prop change.

Size the DC wiring from the battery to your engine systems

ABYC DC Wire Sizing Calculator →

Calculate the battery bank capacity for your vessel

Marine Battery Bank Sizing Calculator →

Determine sacrificial anode sizing for hull and running gear protection

Sacrificial Anode Calculator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Propeller Pitch & Slip Calculator
Pop Out

How It Works

  1. Enter Propeller Specifications

    Input the propeller diameter and pitch in inches. These are stamped on the prop hub or listed on the manufacturer's data sheet. A "13x19" prop means 13-inch diameter and 19-inch pitch.

  2. Enter Gear Ratio

    Input the transmission or lower unit gear ratio. Common ratios are 1.5:1 to 2.5:1 for inboards, 1.6:1 to 2.0:1 for sterndrives, and 1.75:1 to 2.33:1 for outboards. Check the engine or drive manufacturer's specifications.

  3. Enter Engine RPM and Actual Speed

    Input the engine RPM at wide-open throttle and the actual boat speed from a GPS. Use a GPS reading, not the speedometer, because paddlewheel and pitot-tube speedometers are often 5-15% inaccurate on planing boats.

  4. Review Slip and Recommendations

    Check the theoretical speed, actual slip percentage, and whether the slip falls within the normal range for your hull type. If slip is abnormal, the calculator suggests pitch adjustments or identifies possible causes.

Built For

  • Marine mechanics verifying that a propeller is loading the engine into the correct WOT RPM range after a repower
  • Propeller shops recommending the optimal pitch when a customer reports sluggish performance or over-revving
  • Boat owners evaluating prop performance after adding a generator, davit crane, or other weight that changed the hull loading
  • Sailboat owners checking whether their folding or feathering prop is providing adequate motoring performance

Features & Capabilities

Slip Percentage Calculation

Calculates slip as the percentage difference between theoretical speed (pitch x shaft RPM / 1056) and actual GPS speed. Displays the result with reference ranges for planing, semi-displacement, and displacement hulls.

Reverse Pitch Recommendation

Given a target speed, gear ratio, and acceptable slip range, calculates the optimal propeller pitch to load the engine within its rated WOT RPM band. Useful when re-propping after a repower or hull modification.

Hull Type Classification

Provides expected slip ranges based on hull type: 10-15% for planing hulls at speed, 15-25% for semi-displacement, and 25-45% for full-displacement and sailboats. Flags results that fall outside the normal range.

RPM Impact Estimator

Estimates the RPM change per inch of pitch change (approximately 150-200 RPM/inch for most applications) so you can predict the effect of a pitch change before buying a new propeller.

Frequently Asked Questions

Propeller slip is the difference between the theoretical distance the prop should move through the water in one revolution (the pitch) and the actual distance it travels. A 19-inch pitch prop theoretically advances 19 inches per revolution, but because water is not a solid medium, the actual advance is less. The difference, expressed as a percentage, is the slip. Think of it like a screw turning in soft wood versus hard wood — the soft wood allows the screw to spin without advancing as far.
For planing hulls running on plane, 10-15% slip is normal. Semi-displacement hulls typically show 15-25% slip. Full-displacement hulls and sailboats under power commonly show 25-45% slip because their hull speed is limited by waterline length. Slip below 8% on a planing hull usually means the engine is over-revving past its rated WOT range. Slip above 20% on a planing hull suggests the engine is over-loaded and cannot reach rated WOT RPM.
Increasing pitch loads the engine more, reducing WOT RPM. Decreasing pitch unloads the engine, allowing higher WOT RPM. As a general rule, each inch of pitch change shifts WOT RPM by 150-200 RPM, though this varies with hull type, engine torque curve, and propeller design. The engine manufacturer specifies a WOT RPM range (for example, 5000-5600 RPM for a typical outboard), and the propeller pitch should be selected to put the engine in the upper half of that range at full throttle with a normal load.
Always use GPS speed for slip calculations. Paddlewheel speedometers are affected by hull growth, turbulence, and mounting position, and are typically 5-15% inaccurate on planing boats. Pitot-tube speedometers are better but still less accurate than GPS. A handheld GPS or phone GPS app gives speed accurate to within 1-2%, which is sufficient for propeller slip analysis.

Learn More

Marine

Propeller Pitch, Slip & Performance

How to calculate propeller slip, diagnose cavitation, and select the right pitch for your boat based on RPM, gear ratio, and GPS speed.

Read Guide →

Related Tools

Marine Live

ABYC DC Wire Sizing & Voltage Drop Calculator

Size marine DC wiring per ABYC E-11. Calculates minimum AWG for voltage drop and ampacity with engine room derating.

Launch Tool →
Marine Live

Sacrificial Anode Weight Calculator

Size zinc, aluminum, or magnesium anodes for hull, shaft, and propeller corrosion protection by water type and surface area.

Launch Tool →
Marine Live

Marine Battery Bank Sizing Calculator

Size house and starting battery banks for boats. Calculates CCA, Ah capacity with DoD limits, temperature derating, and chemistry comparison.

Launch Tool →