Propeller

Converting power into thrust.

The propeller is the final link in the drivetrain — converting rotational power from the engine and shaft into the thrust that drives your vessel through the water. It is one of the most precisely engineered components on board, and its specification has a direct impact on speed, fuel consumption, engine loading, and manoeuvrability.

A propeller that is incorrectly sized, damaged, or poorly matched to the engine will either overload or under-load the drivetrain — both of which reduce performance and accelerate wear. Accurate specification and regular inspection are essential.

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Key specifications

Every propeller is defined by a set of interdependent specifications. Changing one — for example increasing diameter — affects engine loading, cavitation behaviour, and top speed. These are the critical dimensions tracked per propeller:

Property	Description
Diameter (D)	The full circle swept by the blade tips — the single most important dimension, determining thrust capacity and engine loading
Radius (R)	Half the diameter — used in engineering calculations for blade section analysis
Pitch (P)	The theoretical distance the propeller would travel through a solid medium in one full revolution — determines speed versus thrust at a given RPM
Blade count	Number of blades — typically 2, 3, or 4. More blades provide smoother thrust and reduced vibration at the cost of slightly lower efficiency
Disc Area Ratio (DAR)	The ratio of total blade area to the area of the propeller disc — higher DAR reduces cavitation risk on high-powered installations
Rotation direction	Right-hand (clockwise) or left-hand (anticlockwise) as viewed from astern — critical for twin-engine installations where counter-rotating propellers cancel torque
Material	Bronze (manganese or nickel-aluminium), stainless steel, or composite — each with different strength, corrosion resistance, and repairability characteristics
Surface finish	Polished, painted, or coated — surface condition directly affects hydrodynamic efficiency and fuel consumption

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Diameter and pitch

Diameter and pitch are always specified together — a propeller is described as "D x P" (e.g. 18" x 14"). Diameter controls how much water the propeller can move; pitch controls how far it advances per revolution.

A larger diameter generates more thrust at lower RPM but requires more torque from the engine. A higher pitch increases theoretical speed but loads the engine more heavily. The correct combination allows the engine to reach its rated RPM at full throttle under normal load — if the engine over-revs, the pitch is too low; if it can't reach rated RPM, the pitch is too high.

These specifications must be recorded accurately because a replacement propeller with even slightly different dimensions will change the engine's operating characteristics. A 1" change in pitch can shift full-throttle RPM by 150-200 RPM.

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Material selection

Propeller material affects performance, durability, and corrosion behaviour. The most common materials for yacht propellers are:

• Manganese bronze — the most common yacht propeller material. Good strength, reasonable cost, and repairable by welding. Susceptible to dezincification in some seawater conditions.

• Nickel-aluminium bronze (NAB) — superior corrosion resistance and strength. The preferred material for high-performance and long-service applications. More expensive but significantly more durable.

• Stainless steel — highest strength-to-weight ratio, thinner blade sections possible, best performance. However, susceptible to crevice corrosion and not easily repaired.

• Composite — lightweight and immune to galvanic corrosion. Used primarily on smaller craft. Not repairable — replacement on damage.

Material choice also affects the vessel's cathodic protection requirements. A bronze propeller on a stainless steel shaft creates a galvanic couple that must be managed with correctly sized sacrificial anodes.

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Inspection and maintenance

Propeller condition should be assessed at every haulout. Key inspection points:

• Blade damage — nicks, bends, or missing material from grounding or debris impact. Even minor damage creates vibration and reduces efficiency.

• Erosion and pitting — cavitation erosion on blade faces indicates incorrect pitch, overloading, or poor blade geometry.

• Marine growth — fouling on blade surfaces significantly reduces performance. Antifouling coatings or regular cleaning are essential.

• Shaft seal — check the cutlass bearing and shaft seal for wear at the same time as propeller inspection.

• Anode condition — shaft anodes and propeller nut anodes protect against galvanic corrosion. Replace when depleted beyond 50%.

A damaged or fouled propeller can increase fuel consumption by 10-20%. Recording propeller specifications, inspection dates, and any repairs in YachtPrep ensures you always have the correct replacement dimensions and a complete service history.