Club Marine eNews

Proper propping

Propellers

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.
Propellers
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.
Propellers
Aluminium propellers

  • The lightest and cheapest option
  • Generally do a mighty fine job on outboard engines up to about 115hp
  • Will often run smoother than an equivalent stainless item at low revs on lower horsepower engines
  • Softer that stainless – can be more prone to corrosion, damage and warping
  • Easier for specialist marine repairers to fix

Stainless steel propellers
  • Much stronger, heavier and less prone to warping under load – a popular choice for higher horsepower and performance boating
  • More impact-resistant, greater longevity and retain the shape longer, particularly on leading edges
  • There’s some advantage to the centrifugal force on a heavier propeller, offering higher momentum and torque

Bronze or copper alloy propellers
  • Generally used for shaft- and V-drive applications
  • Copper alloys are widely used in marine applications because of their resistance to corrosion and ease of manufacture
  • Bronze alloys are the preferred metal for casting a large range of shaft-driven propellers, including for large ships

Plastic composite propellers
  • A cost-efficient, lightweight alternative; good choice for a spare propeller
  • Can be prone to warping under load and to impact damage
  • Usually made thicker to compensate for warping, making them a little less efficient
  • Excellent corrosion resistance
Propellers
Contra-rotating – (pictured) used by sterndrive legs and some outboard drives, where both propellers are in line yet rotate in opposing directions on a single drive. This arrangement delivers terrific torque and grip in both forward and reverse gear.
Feathering/folding propellers – commonly used by yachts, these props have blades that fold together to minimise drag when under sail and unfold under centrifugal force when the engine is running
Chopper props and surface-piercing drives use their vented design to improve speed. Very limited drive at lower speeds, and perhaps even less drive in reverse, but are extremely efficient in high-speed racing boats.
Three-blade props (the most common props for recreational boats) are generally lower cost, give terrific all-round performance, have excellent acceleration and good bite in reverse gear.
Four-blade props (popular in performance and fishing applications) are usually made from stainless steel, have terrific inherent strength, provide great pushing and holding power with added grip, tend to push heavier packages more effectively, and will generally be more fuel efficient than three-blade designs. Generally don’t provide good grip in reverse, which reduces low-speed manoeuvrability.
Twin-engine set-ups generally use counter-rotating props, whereby the starboard prop is usually right-hand and the port side is left-hand.

Always refer to your trusted and trained marine technician when considering what the ideal propeller is for your needs.

Taken from a story in Vol 33.1 Club Marine Magazine.