Diameter, pitch, number of blades, cavitation… These parameters define the actual efficiency of your propulsion. Understanding the propeller means understanding why the right choice can radically change your range and performance.
How a propeller generates thrust
A boat propeller is a rotating mechanical component that converts rotational energy into thrust by accelerating a mass of water. Its geometry determines how much water it "grabs" with each turn, how quickly it can do so, and with what efficiency. The engine provides power; the propeller translates this power into thrust by sucking in and pushing out water. If the translation is poor, no matter the quality of the engine: you lose watts through turbulence, cavitation, or running at too low a speed.
For a boat owner, a shipyard, a river operator, or any user looking to optimise propulsion, the right choice of propeller directly affects speed, fuel or energy consumption, and engine durability. It enables a boat to reach its nominal engine speed at full load, to transition to planing without struggle, to manoeuvre cleanly in port, and to consume what it should consume. A poor choice is the complete opposite – and often, in the long run, it leads to damage to the engine.
The technical parameters to know
A propeller reads like a technical identity card. On its hub, there are usually two or three numbers (for example 14 × 19 or 3 × 13 × 17) which respectively designate the number of blades, the diameter and the pitch, both expressed in inches.
The diameter
It is the imaginary circle described by the tips of the rotating blades. The larger it is, the more water the propeller moves per turn, which increases the thrust available at low speed. Conversely, it puts more strain on the engine. Heavy or displacement boats benefit from large diameters; planing and lightweight hulls prefer smaller diameters to reach their top speed.
The pitch (pitch)
The pitch is the theoretical distance travelled by the propeller in one complete turn, like a screw in solid material. It is the parameter that has the most influence on top speed. A long pitch favours speed, a short pitch improves acceleration and thrust at low speed, ideal for towing or loaded boats. Generally speaking, an additional inch of pitch reduces the engine speed by 200 to 300 rpm.
The number of blades
Here, we often summarise it like this:
Two blades are the choice for sailing boats and some small applications. It is very efficient in pure performance. Fewer blades mean less wetted surface, thus less drag when the propeller is stationary. However, it vibrates more and the thrust is less linear.
The three-blade propellers are the most common in leisure boating: they represent a common compromise between speed and efficiency. Four-blade propellers provide more thrust at low speeds, less vibration, and better handling in reverse, at the slight expense of maximum speed. Five-blade propellers are reserved for very specific applications, where reducing vibrations takes precedence over any other criteria.
PITCH ADJUSTMENT — POINT OF CAUTION
A propeller with an unsuitable pitch leads to either overspeed (risk of engine damage) or underspeed (premature wear and excessive energy consumption). Research indicates that poor adjustment can cost 10 to 15% of range on an electric system.
What electric propulsion changes
The transition to electric propulsion reshuffles the deck of propeller sizing. Internal combustion engines have an ascending power curve: they only deliver their maximum torque at high RPM. Electric motors, on the other hand, provide maximum torque from the very first turns.
With an electric motor, torque is available immediately, at any RPM. This fundamental difference alters the optimal relationship between the motor and its propeller.
However, it would be erroneous to believe that low RPM torque is a decisive advantage for the propeller: the power absorbed by a propeller varies according to the cube of its rotational speed. At low RPM, even with plenty of torque available, the effective power is modest. The real advantage of the electric motor lies elsewhere: in the finesse of electronic control of rotational speed, which allows for continuous optimisation of the propeller's operating point.
⚡ WEENAV ANGLE
The motors KRONOS and ARION de Weenav intègrent un variateur de vitesse et un contrôleur électronique qui pilotent le régime avec une précision impossible sur un moteur thermique. L'hélice travaille constamment dans sa plage de rendement optimale, ce qui se traduit directement par une meilleure autonomie et une réduction de la cavitation.
Specifically, for an inboard electric motor like the ARION, the propeller can often be sized with a slightly longer pitch than a thermal equivalent of the same power. This sizing must remain consistent with the function of the boat and its actual use, taking advantage of the instant torque available at startup to operate the propeller in a more efficient range during cruising.
The major families of helices
Non-fixed: Blades solidly attached to the hub. The simplest and most common solution.
= ✓ Perfectly suited, ideal for leisure and common professional uses
Not variable: The angle of the blades can be adjusted during navigation to optimise performance according to speed and load.
= ✓✓ Excellent coupling with the electric system: the inverter and the variable blades optimise together
Folding propeller: Blades that fold in line when stationary to reduce drag (used in motor sailing).
= ✓ Relevant for electric motorised sailboats: reduces drag in sail mode
Counter-rotative: Two propellers rotating in opposite directions on the same axis, cancelling out rotational losses and improving efficiency.
✓ Notable efficiency gains, relevant for maximising long-distance autonomy
Toroidal : Curved blades forming a continuous ring, limiting the vortices at the blade tip and the noise.
= ✓✓ Ideal for silent electric propulsion: less turbulence, less noise
Aluminium, stainless steel or bronze?
The choice of material affects durability, performance and maintenance budget. It must also take into account the nature of the hull and the overall electrical installation.
Aluminium dominates up to about 40 horsepower. Cheaper (3 to 4 times less than an equivalent stainless steel) and lighter, it has the advantage of acting as a mechanical fuse: in the event of an impact with an obstacle, it deforms or breaks before the damage travels up to the gearbox or the engine. This is a significant asset in busy navigation areas.
Stainless steel is essential beyond 150 HP. Its higher mechanical resistance allows for thinner, therefore lighter and more efficient blades. A stainless steel propeller is 5 times more resistant than aluminium and is particularly suited for saltwater. Some special alloys exhibit a resistance 30 to 40% higher than standard stainless steel. The downside: a cost three to four times higher, and in the event of an impact, it is more often the base that suffers the damage.
Bronze remains the traditional material for sailing boat propellers and some inboards. Very resistant to marine corrosion, it is however heavy and raises questions of galvanic compatibility depending on the nature of the hull.
IMPORTANT GALVANIC NOTE
On an aluminium boat, the risk of electrolysis must be taken very seriously. A propeller made of bronze or stainless steel coupled with an aluminium hull, without rigorous electrical insulation, can lead to accelerated corrosion. The aluminium-magnesium alloy remains the safest choice for an aluminium hull. This point is all the more critical as electrical systems carry high voltages (up to 689 V on the KRONOS), which require careful installation.
Cavitation and ventilation: do not confuse them
Cavitation
Cavitation arises from a disturbance in the water flow upstream of the propeller. When a low-pressure area forms on the surface of a blade, the pressure can drop below the vapour tension of the water: it vaporises locally and forms microbubbles. In other words, cavitation occurs when vapour bubbles form on the blades of the propeller. These bubbles then collapse with considerable energy, creating shock waves that erode the surface of the propeller, generate vibrations, and degrade propulsion efficiency.
Manufacturers carefully calculate the relationships between the shape, surface, and angle of attack of the blades to push this phenomenon to the maximum. However, a damaged, undersized, or out-of-range propeller can trigger cavitation even on a flawless installation.
ELECTRIC ADVANTAGE
The precise electronic regulation of Weenav motors reduces the risk of cavitation by avoiding sudden and uncontrolled increases in speed. The gradual acceleration controlled by the inverter allows the propeller to always operate under stable flow conditions.
Ventilation: a common misuse of language
It is often heard from boaters that their propeller "cavitates" in tight turns or when the trim is too high. In reality, this is ventilation: the propeller sucks in air from the surface, suddenly loses its grip in the water, and the engine races. The anti-ventilation plate, positioned above the propeller on most drives, exists precisely to contain this phenomenon. Cavitation and ventilation are two distinct mechanisms, even if their symptoms (drop in thrust, engine racing, vibrations) may seem similar.
Choosing the right propeller for an electric motor
Sizing a propeller for an electric motor is not just about finding a mechanically compatible part. It is an exercise in overall optimisation that incorporates the type of hull, usage profile, motor power, and, for an electric system, the desired range.
The rigorous approach consists of first defining the primary use: top speed, traction, economical navigation, or vibrational comfort. For a fishing boat, a robust and economical solution is often sought. In contrast, for water skiing or wakeboarding, a propeller that favours acceleration is more likely to be preferred. These priorities determine the right compromise between diameter, pitch, and number of blades. A pitch that is too short will cause the engine to run unnecessarily fast, increasing consumption and wasting electrical energy. A pitch that is too long will prevent it from reaching its optimal operating range.
On a well-sized electric motor, the propeller is the last lever of optimisation — and often the most accessible. A change in pitch can measurably alter the boat's range.
For Weenav systems, the design office supports each motorisation project to define the propeller geometry that is most consistent with the KRONOS or ARION propulsion group, the characteristics of the hull, and the client's navigation profile. This is one of the key steps in the four design phases that the technical team manages from A to Z.
⚡ CHECK-LIST PROPELLER & ELECTRIC MOTORISATION
Diameter : suitable for the power and weight of the hull, do not overestimate.
Pitch : check that the motor reaches its nominal speed range at full load.
Blades : 3 blades for speed, 4 for thrust and vibrational comfort.
Material : consistent with the nature of the hull and the electrical installation.
Maintenance : regularly inspect the leading edges, an early sign of cavitation.
Your electric motorisation project, from A to Z.
Our design office defines with you the entire system (motor, batteries, propeller) suitable for your boat and your use.
Published on 22/04/2026
Writer: Sophie Castelain