Propeller sizing explained: what we need and why

Sizing a propeller isn't mysterious — it's three numbers and two constraints. But each number has to be right, because they multiply through the calculation. Here's what we ask for and what happens to it.

Engine power and max RPM

Rated power (from the engine plate, not memory) and the RPM at which it's delivered anchor everything. Together they describe how much work the engine can do and how fast it wants to spin while doing it.

Gearbox ratio — the one owners guess wrong

The gearbox turns crankshaft speed into shaft speed: a 3,000 rpm engine behind a 2.5:1 box spins the shaft at 1,200 rpm. Optimum diameter follows shaft speed, so a wrong ratio sizes the wrong propeller — and ratios vary between gearbox variants that look identical. It's stamped on the gearbox plate. Genuinely unsure? Leave it blank and photograph the plate; we'd rather compute from facts than guesses.

From shaft speed to diameter

Power and shaft speed give an optimum diameter — the size that converts your engine's work into thrust most efficiently. Practical sizing works in a band from about 90% to 100% of that optimum, adjusted for the sizes actually manufactured and your clearance ceiling.

The two constraints

• Tip clearance: the gap between blade tip and hull should be roughly 10–15% of diameter. Less and you get noise and pressure pulses; it caps the maximum diameter regardless of what the maths wants.
• Shaft or saildrive fitting: shaft diameter and taper — or saildrive spline — determine which hub fits. A 30 mm shaft and a 1¼-inch shaft are not the same thing, despite the rumours.

Why we verify anyway

Basic sizing is quick once power, RPM and ratio are correct. But water flow around a heavy deadwood, a worn cutlass bearing, an optimistic engine-hours meter — these are the things a calculator can't see and an engineer can. Every recommendation from the Builder is checked by a human before anything is supplied. We won't sell a propeller we don't believe in.