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Enter your boat and engine once, then use the tabs to size a prop from a sea trial, work out speed and slip, check blade loading, find your hull factor, or estimate top speed and diameter. The example numbers are a 200 hp outboard.
Enter what the current prop did at wide-open throttle. This finds its slip and the pitch that puts the engine at your target RPM.
Pitch shown is the number stamped on the prop, at the new diameter and cup. Assumes the same prop model, blade count and rake, holding the current slip.
| Pitch | WOT RPM | Top speed | In range |
|---|
More blade area loads the engine like extra pitch. Enter each prop's area ratio from the maker, or keep the typical values. The ratio for your current blade count should be your current prop's. Uses the new diameter and cup above, and the rake of the current prop on the Blade loading tab.
| Blades | Area ratio | Pitch for target RPM | WOT RPM at same pitch | Blade loading | Typical trade-offs |
|---|
Work out how fast a pitch and RPM should push the boat, or enter an actual GPS speed to find slip.
Prop shaft turning RPM
Blade loading is the thrust each square inch of blade carries. Heavily loaded props are more prone to cavitation, pitting and blowout. Compare your current prop with one you're considering.
The hull factor describes how efficiently a hull turns power into speed. Work it out once from a real sea trial, and the other tabs use it to predict top speed, expected slip and blade loading.
| Boat type | Factor |
|---|---|
| Heavy cruiser, trawler-style planing hull | 130–150 |
| Average runabout or cruiser | 150–170 |
| Fast runabout, bass boat | 180–200 |
| Performance boat | 200–220 |
Predict top speed from weight, power and hull factor, see what a horsepower change would do, and check the prop diameter the engine can swing.
Talk to our shop in Port Huron about prop sizing, cavitation, blowout or damage. Call (586) 960-5829.
Power, engines and fuel. 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. The hull factor uses the power at the sea-trial RPM, and blade loading uses the power at the target RPM. Inboard and sterndrive power is reduced 5% for drivetrain losses. With more than one engine, top speed and hull factor use the combined power, while each prop's blade loading uses 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.
Speed and slip. 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.
Cup. Cup is the curl on the trailing edge of the blades. It loads the prop like extra pitch, so it's counted as effective pitch: about 0.5 in for light cup, 1 in for medium and 1.5 in for heavy. These are rules of thumb; how much cup a prop really adds varies by model.
Diameter. A bigger diameter grips more water and pulls RPM down, much like more pitch. As a rule of thumb, each inch of diameter loads the engine about as much as 2 in of pitch, so going up an inch in diameter usually means coming down about 2 in of pitch to hold RPM. When the diameter changes, top speed is shown as a range: the low end holds today's slip, and the high end assumes the bigger prop's lower slip lets the engine's power carry through, but never better than 5% slip. Pitch ÷ diameter is shown for reference; most outboard and sterndrive props fall between about 1.0 and 1.5.
Blade count and area. Extra blade area loads the engine like extra pitch. The comparison works out the pitch-equivalent of the area difference from your current prop as 2.6 × (1 − (current ratio ÷ new ratio)²) inches. With typical ratios of 0.55, 0.70 and 0.85 that gives the usual rule of thumb: about 1 in less pitch going from 3 to 4 blades and 1.5 in less from 3 to 5. A higher-area prop with the same blade count shows a smaller version of the same effect. Real results depend on the prop model, so treat the comparison as a starting point.
Prop sizing. New pitch ≈ current effective pitch × current WOT RPM ÷ target RPM, minus the new prop's cup, assuming the new prop is the same style and absorbs the same power. Each inch of pitch usually moves WOT RPM about 150–200. Always confirm with a sea trial at normal load.
Blade loading. Thrust (lb) = 375 × shaft hp × efficiency ÷ mph. Projected blade area = π × D² ÷ 4 × area ratio × (1.067 − 0.229 × pitch ÷ diameter), 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; a PAR is already projected, so it skips the pitch correction. With no published figure, a typical ratio for the blade count is used. Rake tips the blades aft, so projected area is multiplied by cos(rake), and thrust along a tilted shaft is divided by cos(shaft angle). 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, top speed and diameter. Hull factor = knots × √(weight ÷ shaft hp), from a real sea trial. Top speed then comes from Crouch's formula: knots = hull factor ÷ √(weight ÷ shaft hp). Diameter (in) = 632.7 × shaft hp0.2 ÷ prop RPM0.6 at rated max RPM (Gerr). On outboards and sterndrives, the gearcase and prop model mostly set diameter.
These are estimates for comparing options, not a substitute for an in-person prop evaluation.
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