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 is assumed to make its full rating anywhere in its rated range and a diesel at its rated RPM, scaled down in proportion below that. Inboard and sterndrive power is reduced 5% for drivetrain losses. 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.

New pitch ≈ current effective pitch × current WOT RPM ÷ target RPM, less anything the new prop adds in cup, diameter or blade area, holding the current slip, rounded to the nearest half inch. Each inch of pitch usually moves WOT RPM about 150–200. 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.

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.