One of the single-most important items on any boat is the propeller. Choosing the right prop can make a big difference to your boat’s performance – and your hip pocket.
We’ve put together a few technical terms to help you navigate the jargon and understand the different components and features of propellers. Armed with the right information, you’ll be able to make an informed decision when it comes to choosing the right prop for your boat.
Pitch – the theoretical forward travel distance that a propeller would transit in one unrestricted rotation, measured in inches
Diameter – the measurement of the total width of the propeller measured across the center of the prop, usually in inches
Pitch and diameter have a major effect on engine rpm. As a general rule, on a planing hull one inch variance in pitch will produce a variation of around 200rpm but, while it may not necessarily change the overall speed of the craft, it will change the performance characteristics.
Blade size and diameter can change the attitude of a boat, and can affect the trim angle and engine height.
Smaller-pitch props generally have larger diameters, particularly for deep-water applications, while surface-piercing or chopper props may be very large in diameter to equalise the aeration.
Thrust – the transfer of engine horsepower to the water in order to produce motion
Rake – the angle of the blades measured in degrees perpendicular against the hub or shaft. The rake angle can affect the boat’s trim and has a major impact on centrifugal force, which affects overall efficiency.
Cupping – a spoon or curl shape just under the leading edge, which changes the performance characteristics.
Cupping can improve grip, but can cost a few top-end rpm.
Rotation – refers to the direction of turning. Right-hand propellers (the most common type) turn clockwise when viewed from the rear of the vessel, left-hand props turns anticlockwise (often used to balance the forces in multiple-rig or multi-propeller applications).
Revolutions – the speed at which the propeller turns relative to drive speed
Cavitation – the formation of air bubbles generally near the leading edge of the blades. Often caused by transmitting too much power or speed through the propeller
Ventilation – the formation of air bubbles, usually near the leading edge or front side of the blades. Mostly due to incorrect running height and trim angle (propellers too high in the water can draw in air across the blades)
Cavitation and ventilation can be caused by the hull, engine height, trim, speed or overpowering. These forces create uneven pressures on the blade surfaces, causing inefficiencies in power and performance, fuel economy, excessive vibration through uneven balance, heightened wear and even damage to the propeller.
Excessive cavitation or ventilation can actually produce substantial heat, which can damage or warp the propeller under load.