Shaft, Propeller, Gearbox and Propulsion System Service and Repair

Mechanical face-type shaft seals are highly reliable, but the rubber bellows age and harden over time. Following the manufacturer’s recommendation, the bellows should generally be replaced every 6 years as a precaution, even if no damage is visible. The carbon and stainless steel sealing faces and the bellows clamps should be checked at every haul-out.

The most common causes of vibration under way are an unbalanced or damaged propeller, a bent shaft, a worn shaft bearing, poor engine-to-shaft alignment and worn engine mounts. If ignored, it causes lasting damage to gearbox bearings, seals and engine mounts. The RPM at which the vibration starts is an important clue to its source.

When the boat is ashore it rests on blocks and stands; once launched, the hull is supported differently and flexes slightly. Alignment done ashore can therefore be lost when the boat goes back in the water. The correct procedure is a preliminary alignment ashore and the final alignment after the boat has been launched and the hull has settled.

With traditional packing, a small amount of dripping while the shaft is turning is normal and cools the packing; when the shaft is stationary there should be little or no dripping. Overtightening causes heat and shaft wear, while a loose gland lets too much water into the bilge. Correct adjustment and timely repacking prevent both problems.

Shaft bearings should be checked at every haul-out. If there is noticeable play when the shaft is moved up and down by hand, if there is vibration or knocking under way, or if the rubber bearing surface is visibly worn, the bearing needs replacing. A worn bearing also quickly wears the shaft and the seal.

Bent blades, edge nicks and moderate impact damage can usually be repaired; with straightening, weld build-up, profile checks and balancing, the propeller can be returned to service. If a blade is broken off, the hub is cracked or there is heavy material loss, a new propeller may be the better choice. After inspection we present both options.

An unbalanced propeller applies an uneven force to the shaft with every revolution. This causes vibration, noise, wear of the cutlass bearing and shaft seal, and in the long run damage to the gearbox bearings. A repaired propeller should not go back on the boat without being balanced.

Cavitation is the gradual pitting of the blade surface caused by the collapse of vapour bubbles that form when pressure on the blade drops too low. Blade damage, the wrong propeller choice or a distorted blade profile increase cavitation. If sponge-like pitting appears on the blade surface, the propeller should be inspected.

These symptoms are usually caused by low gearbox oil level, low operating pressure, worn clutch plates or a misadjusted shift cable. The control adjustment and oil level are checked first; if the problem persists, the operating pressure is measured to assess the internal condition of the gearbox.

Milky gearbox oil means water has mixed with the oil. The most common cause is an internal leak in the seawater-cooled gearbox oil cooler. The boat should not be used in this condition, as water-contaminated oil quickly destroys bearings and gears. The cooler must be repaired or replaced and the oil system flushed.

The shaft anode should be checked every season and replaced when it is about half worn. The anode protects the shaft and propeller from galvanic corrosion by corroding in their place. If the anode wears much faster than expected, it may indicate stray current or a bonding problem on the boat.

Both. Shaft seal checks, shaft alignment, vibration diagnosis and much of the gearbox service can be done with the boat afloat. Shaft and bearing replacement, propeller removal and some seal replacements require the boat to be hauled out. We provide on-site service in marinas and boatyards across Turkey.

Increased water ingress through the shaft seal or stuffing box requires urgent attention. Make sure the bilge pumps are working, do not leave the boat unattended and contact us immediately. We respond as quickly as possible to locate the leak, make it safe and carry out a permanent repair.