Propeller Pitch Calculator: Speed, Pitch, RPM + Over/Under-Propped

Propeller Pitch Calculator

Speed · pitch · slip · RPM — with the correct constant for your units

Units ★

Drivetrain

Reduction ratio, e.g. 1.86 means the engine turns 1.86 times for each turn of the propeller shaft. Use 1 for a direct drive.
Only used for the pitch-to-diameter ratio sanity check. Clear it to skip.

Speed Data

Leave blank in Speed mode if you only want the theoretical figure without a slip comparison.
The target speed is a real-world speed, so a slip allowance is needed to work back to the theoretical figure.

Hull & Engine Context (optional)

From the engine manual. Used to diagnose over-propping and under-propping — clear either field to skip.
Common workshop rule of thumb is roughly 150–200 RPM per inch, but it varies with engine, gearbox and hull — adjust to whatever your own experience gives.

Result

Pick what you want to solve for, choose your units, and enter what you know — the tool derives the correct conversion constant for that unit pair, works the theoretical speed and slip, checks the pitch-to-diameter ratio against your hull type, and tells you whether the engine is reaching its rated throttle range

The theoretical speed here treats the propeller as a screw advancing one full pitch per revolution through a solid medium, which water is not — so theoretical speed is always higher than reality and slip is the normal, expected difference rather than a fault. Because everything is derived from that idealisation, this is a selection and diagnosis aid, not a performance prediction: it takes no account of hull resistance, displacement, trim, blade count, blade area or cup, engine torque curve, transom height, ventilation or the state of the bottom, all of which move real speed substantially. Slip figures are only as good as the speed measurement behind them, so use GPS speed over ground taken on a calm day and, where current is significant, average reciprocal runs in both directions — a paddlewheel log or a single downstream run will produce misleading numbers, and a negative slip result almost always means a measurement problem rather than a remarkable propeller. Pitch-to-diameter reference bands are broad generalisations across hull families and a well-matched propeller can sit outside them. The pitch-change rule of thumb assumes everything else stays constant and gets less reliable the further you move from the current propeller. Confirm any final propeller selection against the engine and gearbox manufacturer's recommendations and, where possible, by testing on the water.

Propeller Pitch Calculator: Speed, Pitch, Slip and RPM, With the Right Constant for Your Units

Nearly every free prop calculator out there uses the number 1056 no matter what units you pick and that number is only actually correct for pitch in inches and speed in miles per hour. Switch to knots or millimetres without changing the constant and every single answer quietly comes out wrong, this prop slip calculator don’t make that mistake. It works out the correct divisor for whatever unit pair you actually select, inches to knots, millimetres to km/h, whatever and shows that constant right on screen so you can check the arithmetic yourself. As a full boat speed calculator, it solves in four directions, speed, pitch, slip or RPM, whichever one you leave out, it works out from the other three. Slip is where the real diagnosis happens, some slip is normal since a propeller in water isn’t a screw in wood, but too little on a heavy boat usually means your speed reading is off, and too much points at a fouled bottom, an overloaded hull, damaged blades or just the wrong prop. Add your engine’s rated wide-open-throttle range and it’ll tell you if your over propped under propped, and roughly how much pitch change would fix it.

How to Use

This tool has four tabs, pick whichever one matches what you actually want to find out: Speed, Pitch, Slip % and RPM.

Step 1: Set your units first
  • Select your Speed Unit: mph, km/h, or knots.
  • Select your Pitch / Diameter Unit: inches or millimetres.
  • Look at the small note that appears below, this shows you the exact conversion constant the tool is using for your specific unit combination, so you can double check it yourself rather then just trusting a black box.
Step 2: Enter your drivetrain numbers
  • Enter your Engine RPM.
  • Enter your Gear Ratio, for example 1.86 means the engine spins 1.86 times for every one turn of the prop shaft. Just use 1 if you’ve got a direct drive with no reduction.
  • Enter your Propeller Pitch, and optionally the Propeller Diameter too, the diameter is only used for a pitch-to-diameter sanity check, leave it blank if you don’t want that.
Step 3: Fill in your speed data
  • In Speed mode, enter your Actual Measured Speed (GPS) if you have one, this gives you a slip comparison alongside the theoretical figure or leave it blank if you just want the theoretical number on it’s own.
  • In Pitch mode, you’ll instead see a Target Speed field and an Assumed Slip (%) field, since working backward to a pitch from a real-world target speed needs a slip allowance built in.
  • In Slip % and RPM modes, fill in whichever fields are showing, the tool only asks for what it actually needs to solve for that particular unknown.
Step 4: Add hull and engine context, if you have it
  • Select your Hull Type: Planing, Semi-displacement, Displacement or Heavy workboat/tug. This gives you a reference band to compare your pitch-to-diameter ratio against.
  • If you know it, enter your engine’s Rated WOT RPM, both the low and high end of the range, straight from the engine manual. With this filled in, the tool tells you whether your engine is actually reaching it’s designed rev range or not.
  • Check the RPM Change per Inch of Pitch field, a common shop rule of thumb is roughly 150 to 200 RPM per inch, but adjust it if your own experience with this engine and hull gives a different number.
Step 5: Calculate and read your result
  • Tap Calculate. You’ll get the value you were solving for, plus slip percentage where relevant, the pitch-to-diameter ratio checked against your hull type and, if you filled in the WOT range, a clear over-propped or under-propped call with an estimated pitch change to bring the engine back into it’s proper rev range.
Step 6: Export
  • Use Print / PDF for a clean printable copy or Copy to paste the numbers into your workshop notes.

Key Features

  • 4-way solver, speed, pitch, slip or RPM, give it any three and it finds the fourth
  • Unit-specific conversion constant, derived fresh for whatever speed and pitch units you actually pick, shown on screen so you can verify it, not a hardcoded 1056 reused for every unit blindly
  • Slip percentage compared against real GPS speed, the actual useful diagnostic number in all of this
  • Pitch-to-diameter ratio checked against reference bands for planing, semi-displacement, displacement, and workboat hulls
  • Over-propped / under-propped diagnosis, from your engine’s rated wide-open-throttle RPM range
  • Editable pitch-change guidance, using an RPM-per-inch factor you can tune to your own engine and hull
  • Export as PDF or copy the result.

Formula / Logic Used

Theoretical Speed

Speedtheoretical=Pitch×Engine RPMGear Ratio×KSpeed_{theoretical} = \frac{Pitch \times Engine\ RPM}{Gear\ Ratio \times K}

Where KK is a constant that’s derived fresh from whichever units you’ve actually selected, not a fixed number, for inches and mph it works out to 1056, but for inches and knots it’s 1215.22 and for millimetres and km/h it’s a completely different figure again, 16666.67.

Slip Percentage

Slip%=Speedtheoretical−SpeedactualSpeedtheoretical×100Slip\% = \frac{Speed_{theoretical} – Speed_{actual}}{Speed_{theoretical}} \times 100

Pitch, Slip or RPM Solved from the Other Three

The same core relationship gets rearranged depending on what your solving for, given any three of pitch, engine RPM, gear ratio and speed, whichever fourth value is missing gets worked out directly from that rearranged formula.

pitch = (theoretical_speed * gear_ratio * K) / engine_rpm
engine_rpm = (pitch * gear_ratio * K) / theoretical_speed

Pitch-to-Diameter Ratio

P/D=PitchDiameterP/D = \frac{Pitch}{Diameter}

Over/Under-Propped Diagnosis

Your engine RPM at wide-open-throttle is compared against the manufacturer’s rated range, if it falls below the low end, the boat is over-propped, too much pitch for the engine to spin up into it’s proper rev band, and if it’s above the high end, it’s under-propped. The pitch change needed is then estimated using your RPM-per-inch factor:

if actual_WOT_RPM < rated_low: over-propped, needs less pitch
if actual_WOT_RPM > rated_high: under-propped, needs more pitch
estimated_pitch_change = RPM_difference / RPM_change_per_inch

Who Should Use This Tool

Boat owners and marine mechanics diagnosing whether there current propeller is right for there hull and engine, or picking a replacement pitch. Also useful for marine engineering students learning propeller slip, pitch-to-diameter ratio and the over/under-propped diagnosis for there marine propulsion coursework.

Frequently Asked Questions (FAQs)

1. Why is 1056 wrong for calculating boat speed in knots?

1056 is only correct for pitch in inches converted directly into miles per hour, it’s derived from that exact unit pair and nothing else. Switch to knots or millimetres without changing this number and the answer comes out wrong every time, which is exactly why this tool works out the correct constant fresh for whatever units you’ve actually selected and shows it on screen.

2. What does a negative slip percentage mean?

It almost always means there’s a measurement problem, not a genuinely remarkable propeller. Usually this comes from a bad speed reading, like a paddlewheel log instead of GPS, or from not accounting for current on a one-way run, so check your speed source before assuming anything’s actually wrong with the prop itself.

3. How much propeller slip is normal for my boat?

It depends heavily on hull type, a fast planing hull typically runs somewhere around 10 to 20% slip, while a heavy displacement trawler often see less then that. Too little slip on a heavy boat usually signals a bad speed reading rather then unusually efficient propulsion, since some slip is a normal and expected part of how a prop works in water.

4. How do I know if my boat is over-propped or under-propped?

Compare your engine’s actual RPM at full throttle against it’s rated wide-open-throttle range from the manual, if it can’t reach the low end of that range, it’s over-propped, too much pitch and if it exceeds the high end, it’s under-propped. This tool does that comparison automatically once you fill in both the rated range and your actual reading.

5. Can this tool actually predict my boat’s real top speed?

No, and it says so directly, theoretical speed treats the prop like a screw in a solid material, which water genuinely isn’t, so real speed always comes out lower then the idealised number this tool calculates. It ignores hull resistance, weight, trim, blade design and a bunch of other real factors, it’s meant as a selection and diagnosis aid, not a performance prediction, always confirm any prop change with actual on-water testing.

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