Propeller basics: diameter, pitch, slip and blades

Every propeller conversation comes down to four ideas: diameter, pitch, blade area and slip. Understand them and every quote, forum thread and brochure suddenly makes sense.

Diameter — the lever arm

Diameter is the circle the blade tips draw. Bigger diameter grips more water at lower shaft speed, which is why displacement boats — always shaft-speed-limited — want the largest diameter their aperture and tip clearance allow (roughly 10–15% of diameter between tip and hull). A large, slow-turning propeller beats a small fast one every time; Isambard Kingdom Brunel got this right on the SS Great Britain in 1845, and the physics hasn't moved since. Diameter is set by engine power and shaft RPM — which is why the first questions we ask are power, max RPM and gearbox ratio. See how sizing works.

Pitch — the gearing

Pitch is how far the propeller would screw forward in one revolution if water were solid — it's the boat's gearing. Too much pitch and the engine lugs like a car starting uphill in fourth; too little and it screams without pushing. A fixed propeller has exactly one pitch, correct at exactly one combination of boat speed, load and weather. Every other moment it's a compromise — the compromise a self-pitching Autoprop removes by re-pitching continuously.

Slip — the useful loss

The prop never advances its full pitch; the difference is slip. Some slip is necessary — it's how blades generate thrust — but excessive slip is power turned into churned water instead of motion. Symptoms: high revs, modest speed, black smoke under load, a wake that looks angrier than your progress. Bruntons' engineers prefer to talk about efficiency rather than slip, and the reframe matters: slip sounds like a defect to eliminate, efficiency is a budget to spend well — and it's the whole-propeller number that actually predicts your fuel bill.

Blades — the trade nobody explains

• More blades = more area = smoother thrust and more grip per revolution, but more drag under sail and more surface to foul.
• Fewer blades = less drag and often better peak efficiency, but each blade carries more load.
• The right count is application-specific: racers fold two blades away; cruisers balance three; heavy or high-power boats spread load across four.

None of these numbers stands alone — diameter, pitch, blade area and RPM interact, constrained by cavitation and your aperture. Which is why the honest answer to 'what prop do I need?' is always a calculation plus a human check, never a chart lookup.