A propeller pitch calculator answers one narrow question: if a propeller behaved like a screw threading through a solid, how fast would the boat go? Pitch is the distance that ideal screw would advance in one turn. Multiply it by the number of turns per minute at the propeller shaft and you have a theoretical speed. Everything the boat fails to achieve against that figure is slip.
Arb Digital builds free calculators that stay inside what their formula can honestly support. This is a marine page throughout — the pitch and slip conventions here are the ones used for boat propellers. Aircraft propellers use a different framework, and the last thing you want is to carry a marine slip percentage into an aviation problem. The boat speed calculator approaches speed from the other direction, estimating it from engine power, displacement and hull type, and it explicitly assumes a generic propeller. This page is the propeller-specific half of that picture.
What This Propeller Pitch Calculator Does
It takes the pitch stamped on your propeller, the engine speed you actually reach and the gear reduction between the two. From those it computes the propeller shaft speed, then the theoretical distance the boat would cover per minute if there were no slip at all, and converts that into mph, knots or km/h.
If you also enter a measured speed, it computes two more things. Slip is the percentage shortfall between theoretical and actual speed. Effective pitch is the distance the boat really advances per revolution, expressed in inches so it can be compared directly against the pitch on the prop. A 21-inch prop showing 17.7 inches of effective pitch is telling you the same story as a 15.8 per cent slip figure, in units you can match against a catalogue.
What it does not do is tell you which propeller to buy. That decision is set by the RPM your engine reaches at full throttle against the manufacturer's recommended wide-open-throttle band, by how the boat is loaded, and by what the hull is asked to do. Those are the propeller dealer's questions, not a formula's.
How to Use It
- Enter the pitch from the propeller. It is the second number in a size like 14 × 21, or is stamped inside the hub.
- Use your observed wide-open-throttle RPM. The rated maximum is what the engine can do; what it actually turns with this prop on this boat is what matters.
- Enter the gear ratio as a single number. A 1.86:1 lower unit is entered as 1.86. It divides engine RPM to give propeller RPM.
- Add a GPS speed if you have one. Slip and effective pitch only appear when there is a real speed to compare against.
- Read slip as a diagnostic, not a score. Different hulls have different healthy slip ranges, and a low number is not automatically better.
The Formula: How Propeller Pitch and Slip Are Calculated
Propeller shaft speed is engine RPM divided by the gear ratio. Theoretical speed in miles per hour is then pitch (inches) × propeller RPM × 60 ÷ 63,360, where 63,360 is the number of inches in a mile. In knots, multiply the mph figure by 0.868976; in km/h, by 1.609344. Those conversion factors follow from the exact definitions in NIST Special Publication 811, the NIST Guide to the SI, which fixes the international mile at 1.609344 km and the nautical mile at 1.852 km exactly.
Slip is (theoretical − actual) ÷ theoretical, expressed as a percentage. Effective pitch is actual speed converted back into inches of advance per revolution: measured mph × 63,360 ÷ (60 × propeller RPM).
None of this predicts thrust, and that limitation is fundamental rather than a shortcut. Thrust comes from accelerating a mass of water rearwards. As NASA Glenn Research Center's propeller thrust page sets out, thrust equals mass flow rate through the disc multiplied by the velocity change across it, and the page notes that a propeller is really a rotating wing, which makes the detailed analysis genuinely complex. Pitch is one geometric parameter of that wing. It is not the whole blade.
Work the defaults by hand. Engine at 5,000 RPM through a 1.86:1 gear gives a propeller speed of 5,000 ÷ 1.86 = 2,688.2 RPM. With 21 inches of pitch, theoretical advance is 21 × 2,688.2 = 56,451.6 inches per minute, or 3,387,097 inches per hour, which divided by 63,360 gives 53.46 mph — 46.45 knots. Against a measured 45 mph, slip is (53.46 − 45) ÷ 53.46 = 15.8 per cent, and effective pitch is 45 × 63,360 ÷ (60 × 2,688.2) = 17.68 inches.
Slip Is Not Waste
The word suggests inefficiency, and that reading is wrong. A propeller generates thrust exactly by pushing water backwards, and pushing water backwards is what makes the boat advance less than the geometric pitch per revolution. Zero slip would mean zero thrust. A propeller in a solid nut has perfect pitch efficiency and produces no useful force at all.
Healthy slip therefore varies enormously with the job. A light, fast planing hull with a well-matched prop often runs in the region of 10 to 15 per cent. A heavy displacement hull, a workboat, or anything pushing a large wake runs far higher, and that is correct for what it is doing. A sailboat under auxiliary power is higher still.
What slip is genuinely useful for is comparison against itself. Measure it on a known day with a known load, then measure again after a prop change or a hull cleaning. A slip figure that has climbed sharply with no other change points at something physical: fouling, a damaged blade, a slipping hub, or a lot more weight aboard than last time.
Marine and Aircraft Propellers Do Not Share Conventions
The first line of this page says marine deliberately. Marine practice quotes pitch in inches, quotes size as diameter × pitch, and expresses the shortfall against geometric pitch as a slip percentage. That is the framework used above.
Aircraft practice is built differently. Propellers are described by geometric pitch and effective pitch, and the difference between them is called slip in some texts and glide in others. Performance is normally handled through the advance ratio, J = V ÷ (nD), where V is true airspeed, n is revolutions per second and D is the diameter — a dimensionless group that appears nowhere in the marine calculation. Blade angle varies along the span by design, so a single pitch number is a nominal value taken at a reference station, usually 75 per cent of the radius.
Constant-speed propellers complicate it further, because the governor changes blade angle continuously to hold RPM, so pitch is not a fixed property of the propeller at all. None of that maps onto the marine arithmetic here, so do not carry a number across.
Why Two Boats With the Same Prop Go Different Speeds
Pitch is one dimension of a three-dimensional object. Diameter sets how much water the blade sweeps, and a larger diameter absorbs more torque at the same pitch. Blade area ratio changes how heavily loaded each square inch is, which is what decides whether the prop ventilates or cavitates when the boat leans into a turn. Rake affects how the prop lifts the stern. Cup — a small trailing-edge curl — behaves like extra pitch, typically an inch or two of it, without changing the stamped number at all.
Then there is what happens behind the hull. The propeller works in water the hull has already disturbed, so wake fraction and thrust deduction both matter, and both depend on hull form and running trim. Mounting height on the transom changes how much of the blade is in clean water. Loading changes running attitude, which changes everything else.
This is why a formula gets you an expectation and a test run gets you the truth. Check the shaft speed side of the problem with the gear ratio calculator, the power side with the horsepower calculator and the engine displacement calculator, and convert between speed units with the speed converter.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using engine RPM as propeller RPM — on almost every marine drive there is a reduction gear, and forgetting it inflates the theoretical speed by that ratio.
- Comparing a speedometer reading to the result — paddlewheel and pitot speedometers are frequently several miles per hour out. Use GPS, and average two opposite runs.
- Treating low slip as good — slip is the mechanism by which thrust is produced. What matters is whether it is normal for that hull and load.
- Assuming pitch alone predicts speed — diameter, blade area, rake and cup all change the outcome, and cup in particular acts like extra pitch that is not stamped anywhere.
- Applying marine slip to an aircraft propeller — aviation uses advance ratio and a blade angle that varies along the span, and the conventions do not transfer.
Related Free Tools From Arb Digital
The boat speed calculator estimates top speed from power, weight and hull type, which is the complementary approach to this one. Use the gear ratio calculator for the drive reduction, the horsepower calculator and the engine displacement calculator on the engine side, and the speed converter for units. For the aerodynamic cousin of this problem, the drone motor thrust calculator works an air propeller, and the drag force calculator handles the resistance the thrust has to overcome. Everything Arb Digital publishes is listed on the free online tools hub.
Frequently Asked Questions
Pitch is the distance a propeller would advance in one full revolution if it were turning through a solid rather than water. A 21-inch pitch prop would move the boat 21 inches per turn with no slip at all.
It depends entirely on the hull. A well-matched propeller on a light planing boat often sits somewhere around 10 to 15 per cent, while heavy displacement hulls and workboats run considerably higher and are perfectly healthy doing so.
No. Thrust is produced by accelerating water backwards, and that is precisely what creates slip. A propeller with zero slip would produce no thrust. Slip is most useful as a figure you compare against your own earlier measurements.
Yes. As a rule of thumb in the marine trade, each inch of pitch shifts wide-open-throttle engine speed by roughly 150 to 200 RPM in the opposite direction. More pitch loads the engine more and lowers the RPM it can reach.
No. Aircraft propeller performance is normally described through the advance ratio, with a blade angle that varies along the span and, on constant-speed units, changes continuously in flight. The conventions and the numbers do not transfer between marine and aviation practice.
Because the calculation is geometric. It does not know your hull's drag, your load, your trim or the condition of the bottom. It tells you what the propeller geometry allows, not what the boat will achieve.
It is the distance the boat actually advances per propeller revolution, worked backwards from your measured speed and expressed in inches. Comparing it to the stamped pitch is the same information as the slip percentage, in units you can match to a catalogue.
This tool is provided for educational and study use. It evaluates a geometric relation between pitch, shaft speed and advance, and does not model thrust, blade design, cavitation, hull drag or loading. Propeller selection should follow the engine and boat manufacturer's recommended wide-open-throttle range and the advice of a propeller specialist.
