Propeller Size Calculators

Horsepower = torque (lb-ft) × RPM ÷ 5,252. Enter each engine's horsepower; adding its peak torque lets the calculator work out how much power the engine really makes when it runs below rated RPM, capped at the rated horsepower. Without torque, a gas engine follows a typical power curve that peaks at the top of its rated RPM range (about 3% below peak power at 5,200 of 6,000 RPM, falling faster below that), and a diesel makes power in proportion to RPM up to its rated RPM. Outboard horsepower is normally rated at the prop shaft, so none is taken off. Inboard and sterndrive power is reduced 5% for drivetrain losses; if your engine is rated at the crankshaft (older outboards, for example), enter the loss under More details. Choosing Inboard sets rake to 0 and shaft angle to 10° as starting points, since inboard props usually have no rake and run on an angled shaft; change them to match your boat. With more than one engine, top speed and hull factor use the combined power, while blade loading and diameter use one engine's power. Gas engines are usually propped into the upper half of their rated range; diesels are propped to reach their rated full-load RPM, and falling short means the engine is lugging.

Prop RPM = engine RPM ÷ gear ratio. Theoretical speed (mph) = pitch × prop RPM ÷ 1056. Slip = 1 − actual speed ÷ theoretical speed. With a shaft angle, the prop advances along the shaft, so the boat's speed is multiplied by cos(shaft angle) before comparing it with pitch.

The thrust and torque of the new prop come from the Wageningen B-series polynomials (Oosterveld and van Oossanen, 1975), which give thrust and torque coefficients from advance coefficient J, pitch ÷ diameter, blade area ratio and blade count. Cup is counted as extra pitch. The series is scaled to your boat with your sea trial: your engine's power at the trial RPM fixes a torque scale, your assumed prop efficiency fixes a thrust scale, and the boat's resistance is taken as proportional to speed, which is the same assumption behind Crouch's formula. For any candidate prop the calculator then finds the RPM where the power the prop absorbs equals the power the engine makes, or the RPM limit if that comes first. It is valid for pitch ÷ diameter 0.5–1.45, area ratio 0.30–1.05 and J up to about 1.7; outside that it falls back to the rules of thumb. The series was measured on ship-type props, so cupped, raked outboard props will differ; expect RPM within roughly 150 and speed within about a mph, and confirm with a test prop.

With the series match above, the recommended pitch is the one whose predicted WOT RPM equals your target, rounded to the nearest half inch. The older rule-of-thumb figure (new pitch ≈ current pitch plus cup × current WOT RPM ÷ target RPM, less anything the new prop adds in cup, diameter or blade area, holding slip, rounded to the half inch) is shown for comparison when it differs by an inch or more. Each inch of pitch usually moves WOT RPM about 150–250. If a prop is too light, the engine hits its RPM limit and speed falls short of what it could do. Always confirm with a sea trial at normal load.

Each of these loads the engine like extra pitch. Cup counts as about 0.5 in (light), 1 in (medium) or 1.5 in (heavy). Each inch of diameter loads the engine about as much as 2 in of pitch. Extra blade area counts as 2.6 × (1 − (current ratio ÷ new ratio)²) inches, which with typical ratios of 0.55, 0.70 and 0.85 gives the usual rule of about 1 in less pitch from 3 to 4 blades and 1.5 in less from 3 to 5. When diameter or blade area changes, top speed is shown as a range: the low end holds today's slip, and the high end assumes the new prop's lower slip lets the engine's power carry through, but never better than 5% slip. These are rules of thumb; real results vary by prop model.

Diameter (in) = 632.7 × shaft hp^0.2 ÷ prop RPM^0.6, using one engine's power at its rated maximum RPM (Gerr). That is for a 3-blade prop; 4-blade props run about 6% smaller and 5-blade about 10% smaller. The result is rounded to the nearest whole inch and kept at or under the max diameter if you enter one. On outboards and sterndrives, the gearcase and prop line mostly set which diameters are available.

Thrust (lb) = 375 × shaft hp × efficiency ÷ mph. Projected blade area = π × D² ÷ 4 × area ratio × (1.067 − 0.229 × pitch ÷ diameter) × cos(rake), after Dave Gerr's Propeller Handbook. Makers publish the area ratio as DAR (developed), EAR (expanded) or PAR (projected); DAR and EAR are close enough to use the same way, and a PAR is already projected. With no published figure, a typical ratio for the blade count is used. Thrust along a tilted shaft is divided by cos(shaft angle), and the shaft angle swing is the change in the angle the water meets the blade at 70% radius over each turn. Under 8 psi is comfortable; 8–10 psi is the usual limit for slower boats, and fast planing boats commonly run higher. With the series match, thrust comes from the matched model. The cavitation row compares thrust loading with Burrill's 5% back-cavitation line (loading coefficient T ÷ (½ρ·projected area·(Va² + (0.7πnD)²)) against cavitation number at 0.7 radius, using curve fits to the Burrill chart). That line is a design limit for ship propellers; outboard props run well above it, so watch how the number changes between props rather than its absolute value.

Hull factor = knots × √(weight ÷ shaft hp), worked out from your sea trial using the power the engines make at that RPM. Top speed at full power then comes from Crouch's formula: knots = hull factor ÷ √(weight ÷ shaft hp). Typical hull factors: heavy cruiser 130–150, average runabout or cruiser 150–170, fast runabout or bass boat 180–200, performance boat 200–220.

These are estimates for comparing options, not a substitute for an in-person prop evaluation.