Sailboat Propeller Terminology
If you are new to boating or are considering buying a boat that is unfamiliar to you sooner or later you will need to think about propellers. Understanding the basic parts of the propeller can be useful when researching propeller upgrades or solving problems when your boat is not performing under power.
Hopefully this article will provide you with some of the basic terminology used when discussing sailboat propeller design and functionality.
Sometime known as ventilation – aeration occurs when air comes in to contact with the propeller blades. This means that the blades don’t make direct contact with the water and they can end up spinning freely.
On recreational sailing boats this can often feel like a slipping transmission. It generally happens when a boat is turned hard or when you trim the drive up too far.
The side of the blade that faces the bow – also known as the suction side (low pressure).
The side of the blade that faces the stern – the pressure side or pitch side (high pressure).
The number of blades on the propeller.
The part of the propeller where the blades attach to the hub.
The blade tip is the maximum outer edge of the blade – and furthest away from the centre of the hub. The tip separates the leading edge from the trailing edge of the propeller.
Cavitation is often mistaken for aeration or ventilation and occurs when there is a disturbance of the water flow over the face of the propeller blade(s). It occurs when there is a large reduction in pressure on the back of the propeller blade, which can result in tiny bubbles forming on the blade. In extreme cases excessive cavitation can cause enough heat to physically damage the face of the blade.
If you have ever come across a propeller blade that has lots of little holes/pits on the face of the blade- then this could well have been the result of cavitation.
Cavitation can be caused by a number of things including incorrect pitch, bent blades or other physical damage to the blade edges.
The cup is a small radius of curvature located on the trailing edge of the blade. This curved lip on the propeller blade helps the blade “bite” and increases efficiency in high speed/high RPM applications where the propeller is cavitating.
Diameter is measured from tip to tip of opposite blades – or twice the radius (distance from the centre of the hub to the tip of a blade). If you are looking at a three-blade propeller it is the theoretical circle the prop creates when it rotates.
The solid cylindrical section at the centre of a propeller that slips on to the shaft. During the manufacturing process the blades are cast as part of the hub. The normally accommodates a taper or spline.
The key is usually a long rectangular section of material that fits into a milled slot in your shaft. The propeller hub will also have a milled section to the correct width accommodating the top half of the key material. Its job is to transmit torque to the propeller from the shaft.
The leading edge of a propeller is the edge that leads into the flow when providing forward thrust. When viewing the propeller from astern, this edge is furthest away.
The trailing edge of the propeller is the edge that trails the flow when providing forward thrust. When viewed astern, this edge is closest to you.
Pitch is defined as the theoretical forward movement of a propeller during one revolution — assuming there is no “slippage” between the propeller blade and the water.
The distance from the axis of rotation to the blade tip. The radius multiplied by two is equal to the diameter.
The rake of a propeller is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller and has implications on a boat’s performance.
When viewing a propeller from the stern (facing forward): Right-hand propellers rotate clockwise to provide forward thrust and left-hand propellers rotate counter-clockwise to provide forward thrust.
Ventilation occurs where surface air or exhaust gases are drawn into the propeller blades. When this happens, boat speed is lost and engine RPM climbs rapidly. This can be caused by excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.