Why is my boat propeller making unusual noise?

Unusual propeller noise usually comes from rope or weed, damaged or fouled blades, disturbed water flow, or wear in the shaft bearing or seal. Check whether it follows engine revs, shaft rotation or boat speed, then inspect the propeller before applying more power; stop if you hear a new knock or feel strong vibration.

Pinpoint when the noise occurs

Start at the helm. Note engine revs, boat speed, whether you are turning and whether the noise occurs in forward, reverse or both. Avoid repeatedly reproducing severe vibration.

• Also present in neutral: check the engine, mounts, exhaust and gearbox input side first, because the propeller normally is not turning.
• Only when in gear: suspect the propeller, shaft, coupling, bearings or drivetrain under load.
• Still present sailing with the engine stopped: check whether the shaft is freewheeling. A rotating shaft can still make a bearing or seal squeak.
• Only within a narrow rev range: resonance or propeller singing is possible, but first rule out loose parts and blade damage.

Stop immediately for heavy knocking, sudden loss of thrust, severe vibration or water entering around the shaft seal. Switch off, remove the key and isolate starting power before anyone touches the propeller or shaft.

Check blades, fastenings and anode

Check for rope, fishing line and weed first, particularly if the noise appeared suddenly. Line can wind tightly behind the propeller hub and damage a seal without being visible from outside. Arrange a lift or underwater inspection if necessary.

With the boat securely ashore, inspect each blade for bent tips, missing metal, cracks and uneven barnacle growth. One heavily fouled blade can upset the balance. Look for fresh contact marks on the hull, rudder or rope cutter.

• Check the propeller nut and its specified locking arrangement, including any split pin, tab washer or locking screw.
• Check for movement between the propeller hub and shaft. There should be no looseness at that joint.
• Inspect the shaft anode for looseness and contact marks. Replace it if more than 50% consumed or covered by a hard shell.
• On folding or feathering propellers, check that every blade moves freely and reaches its proper position.

Inspect the propeller at least every 12 months, more often with heavy use. Grease movable blades at least once each season with the maker's specified grease. Servicing requirements differ between Gori, Flexofold and other designs, so follow the instructions for the exact model, including nut torque.

Cavitation, ventilation and propeller singing

A gravelly hiss or crackle under load can indicate cavitation, where vapour bubbles form and collapse around the blades. Damaged edges, heavy fouling, unsuitable propeller loading or disturbed inflow can contribute. Persistent cavitation may leave pitting on the blades.