It’s time to take a critical look at the propellers on your boat to find out if they are doing their job properly. Leisure Boating looks at the importance of propeller maintenance to ensure your boat is running efficiently.
SHAFT CARE AND REPLACEMENT
If the prop shaft is bent, there will be a vibration felt in the boat. On inboard motors the bearings need to be checked to see if they are damaged or worn, If the prop shaft is bent, your servicing dealer will need to do the straightening or replacement. Your dealer should also check seals, couplers and bearings that might have been compromised by the continual vibration. If you are technically minded enough to replace the prop shaft yourself, don’t forget to use liberal amounts of marine grease when refitting.
Keep an eye on older sterndrive motors with rubber hubs, as these hubs can become brittle over time and start cracking. Replace any rubber hubs and the housing if they are showing signs of wear.
Whether your boat has an inboard motor, sterndrive or outboard motor, you still need to carefully check the blades for damage. If there are visible dings and chunks missing from the blades then a repair is the order of the day. In some instances it is very difficult to determine if the blades are bent, and this will cause an irregularity in consumption and boat speed. Bent blades will often result in vibrations when underway. If you look carefully at the propeller from the side, it is sometimes possible to determine if the blade is bent. The other option is to measure the distance from the outer edge of the blade to a fixed object or point, such as a spot on the hull to determine whether the blades are bent.
Inspect the seals regularly for discarded fishing line that can get tangled and cause damage. If there is damage to the seals, get your marine dealer to replace them. Ensure that your dealer replaces the gear case lube, and make sure that they have pressure tested the gear case.
Pitch is the distance in inches a propeller will theoretically travel in a full revolution. The lower the pitch, the better the hole shot or pushing power it has. This greater pushing power is achieved at a lower top speed. A propeller with a higher pitch will have less acceleration but a higher top speed. The correct propeller pitch is achieved when the motor reaches its wide open throttle and doesn’t exceed it, as rated by the manufacturer with a normal to heavy load. Each increase or decrease in pitch by an inch affects the motor by approximately 150 RPM.
Blade diameter is the total width of the circle at the blade tips while the propeller spins. Larger diameter blades mean that more water is pushed and the blade reaches further into the water. Propellers with smaller diameters are usually used on smaller boats with lighter weights.
Ventilation is when air forms around the propeller blades, causing a gain in RPM and a loss of speed because the blades are not biting into the water. Varying amounts of reduced forward movement is the outcome of blade ventilation.
Cavitation is when pressure on the water across the blade’s surface is reduced to the point that it forms bubbles. When the bubbles burst, they have the ability to cause cavitation burn which can deteriorate the surface of the propeller.
Propeller care is important to keep your boat running smoothly. Whenever you have the opportunity, give your propeller the TLC it deserves.