A propeller is sized by four decisions — diameter, pitch, blade count, and blade-area ratio. Here’s how we make each one.

The four size decisions

Sizing a propeller is picking four numbers that work together: diameter, pitch, blade count, and blade-area ratio. Get them right, and the engine will spin at rated RPM at wide-open throttle (WOT) and burn as little fuel as possible at cruise. Get them wrong, and the engine will either labour or over-rev, the boat will drag, and your fuel bill will climb.

Start with the engine, not the boat

The engine data sheet gives you the two most important inputs:

• Rated RPM at WOT — the propeller must let the engine reach this. Not 50 RPM short, and not 300 RPM over.
• Rated continuous RPM at full load — your cruise point should sit here.

From rated RPM, we divide by the gearbox reduction to get propeller shaft RPM. This is the number everything else is designed around.

Diameter — the biggest single lever

Bigger diameter = more thrust for the same RPM, up to the point where blade-tip speed goes supersonic (practically ~45 m/s). Diameter is limited by:

• Hull aperture clearance (typically 15% hull-to-tip clearance).
• Shaft-to-waterline depth.
• Rudder clearance.

Within those limits, pick as big a propeller as the hull allows. A one-inch increase in diameter is roughly equivalent to a two-inch pitch increase in loading effect.

Pitch — matching engine RPM

Pitch is the fine-tune. Once diameter is fixed by the hull, pitch is picked to make the engine land at rated WOT RPM. Rule of thumb: 2 inches of pitch ≈ 450 RPM. Over-propped? Reduce pitch. Under-propped (over-revving)? Increase pitch.

Blade count

More blades = smoother, less vibration, more thrust at a given diameter — but slightly less efficient. Common choices:

• 3 blades: planing hulls, pleasure craft, maximum efficiency.
• 4 blades: displacement cruisers, workboats, smoother at speed.
• 5 blades: large commercial, trawlers, maximum thrust and smoothness.

Blade-area ratio (BAR)

BAR is the total blade area divided by the disc area of the propeller. A heavily-loaded (high thrust-per-area) propeller needs more blade area to avoid cavitation. Typical BAR:

• Pleasure craft: 0.5–0.6
• Fast workboats: 0.6–0.8
• Heavy displacement / tugs: 0.8–1.1

Worked example — 42 ft prawn trawler

Engine: 300 hp @ 1,800 RPM, gearbox 4:1 reduction. Hull aperture allows 40″ diameter. We’d typically specify a 40″ × 28″ four-blade propeller in manganese bronze, BAR 0.75. At WOT, the engine should reach 1,800 RPM at tow with the net streamed. Cruise free-running at 1,500 RPM.

Conclusion

Propeller sizing is engineering, not guesswork — and the engine data sheet is the starting point. If you’d like a propeller sized to your boat, send us the engine and hull data and we’ll run the numbers.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.

Add a blade and you add thrust and smoothness — but lose a half-knot. Here’s the real trade-off.

The simple rule

Fewer blades = more efficient (higher top end). More blades = smoother, more thrust at a given diameter. That’s the whole trade-off, and the right answer depends on what you want your boat to do.

Three blades

Classic planing and semi-planing choice. Best fuel economy at WOT. Simplest and cheapest to repair. Can be rougher at idle, more cavitation-prone when heavily loaded. Good for sports fishers, runabouts, light cruisers.

Four blades

Smoother at low RPM. More thrust per diameter — useful when the hull aperture is restrictive. Slight loss of top-end efficiency. Good for cruisers, displacement sail under power, and semi-displacement workboats.

Five blades

Maximum thrust, smoothest running. Common on large commercial vessels, trawlers, and premium motor-yachts where quietness is prized. Most expensive to make and repair.

Workshop lore

If you can run 3 blades and still make rated RPM at WOT, do it. If you can’t — hull aperture too tight, too much vibration — add a blade. If still not enough thrust, go to five. Rarely do we recommend starting at five without a reason.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.

The right winch is sized to the chain and anchor, not to the brochure photo. Here’s how to pick it.

The line-pull number

Every anchor winch is rated for a maximum line pull — the force it can continuously apply to the chain. A conservative rule: pick a winch rated at three times the combined weight of anchor plus chain. That gives margin for breakout and for pulling the vessel against wind.

Horizontal vs vertical

Horizontal winches sit on deck, chain feeding over the roller. Pros: easy install, easy service. Cons: deck space, exposed motor.

Vertical winches have the capstan on deck but the motor and gearbox below deck. Pros: tidy deck, protected motor. Cons: chain-locker depth needed.

12V vs 24V

Any winch over about 1,000 kg line pull should be 24V — the current draw at 12V becomes unreasonable (300A+). 24V halves the current and lets you use thinner cable.

Battery and cable

Winch draws large but short peaks. A dedicated 100 Ah AGM battery close to the winch, cable sized for less than 10% voltage drop at peak amps, breaker in the circuit near the battery. The battery is kept charged by the engine when running.

Deck reinforcement

A winch pulls up and backwards hard. The deck under the mounting must be reinforced — a steel or aluminium backing plate, bolted through with 10mm bolts minimum. Don’t rely on self-tappers.

Chain stopper

A separate chain stopper takes anchor load when you’re at anchor. The winch isn’t designed to hold ground tackle under sailing-anchor loads — it’s for raising and lowering.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.