The Propeller

A propeller is described by four basic dimensions.

---

The Number of Blades

Note that fewer blades are more efficient but also produce more vibration. Most boat propellers have three blades, since this offers a good compromise between efficiency, vibration, and manufacturing cost.

---

The Diameter

Propellers are usually marked with two figures, e.g. 270 x 235 (mm). The first figure gives the diameter, the second the pitch.

---

The Pitch

This figure describes the horizontal distance a propeller would travel in one full revolution. If the pitch is too high, the engine cannot reach full revolutions at full throttle. If the pitch is too low, the engine over-revs. Both reduce performance, accelerate engine wear, increase consumption, and can also cause gearbox damage.

---

The Direction of Rotation

A propeller that turns clockwise as seen from astern is right-handed. One that turns anti-clockwise is left-handed. This must not be confused with right- or left-rotating, since for example a right-handed propeller turns left when going astern and is therefore left-rotating in that moment.

Motorboats are almost exclusively fitted with right-handed propellers. This must also be considered when steering the boat, since the direction of rotation changes the boat's behaviour. This shows itself in the so-called paddle-wheel effect.

The paddle-wheel effect causes the propeller to produce not only thrust along the boat but also a sideways pull. As if the propeller were rolling like a wheel on the ground. A right-handed propeller therefore pulls the stern of the boat to the right when going forward and to the left when going astern. The effect is felt particularly when going astern and must be taken into account when manoeuvring. On yachts with two counter-rotating propellers the effect cancels out completely.