An account of the failure of a small vessel’s propeller shaft seal. It is not a ‘fit and forget’ piece of equipment.
What the reporter told us
The yacht had sheltered in xxx harbour the previous evening – prior to resuming passage towards yyy. Owner started the engine and opened the floor hatch for visual inspection, but discovered about 1 metre of seawater in the engine space – rising above prop-shaft, gearbox, and accessories.
The engine-driven bilge pump failed to start (submerged electrics?). An initial swift inspection of hoses and clips revealed no failures. The owner started to rig a small, spare electrical bilge pump. I singled up mooring lines ready to slip and run aground alongside a nearby pier. VHF radio calls to the harbour Master went unanswered, but a VHF call to the nearby MRCC requesting local assistance resulted in the swift arrival of an RNLI inshore rescue boat with 3 crew, who were exercising nearby and heard my call to MRCC. They swiftly produced a salvage pump which reduced the inflow sufficiently, then arranged a tow to a nearby boat-lift, which promptly lifted the boat out onto the hard.
Inspection by an engineer, confirmed by the insurers’ surveyor, revealed failure of the propeller shaft seal. The ‘rotor’ segment had become unsecured from the prop shaft and moved sufficiently to break the effective surface seal against water ingress.
That ‘rotor’ is secured by two pairs of grub screws, crucially dependent on proper torque for security. Queries to several senior yard engineers and an online search indicated this is a not-infrequent problem. Several working boats – including one licensed for passengers – were known to have suffered similar failures.
- Boats’ watertight integrity depends crucially on the proper fitting and maintenance of small grub screws. There is no effective means of inspection of proper fitting and function, other than disassembly and refitting with new parts.
- Such shaft seals may be provided with bespoke locking collars (as provided by manufacturer xxx on their larger commercial shaft seals), or by fitting a pair of stainless steel jubilee clips onto the prop shaft, preventing movement of the ‘rotor’.
- Flexible rubber bellows form part of the ‘stator’ structure and ought to be inspected for wear and/or replaced at intervals. Failures have been reported, with resultant down flooding. Few, if any, boat owners inspect these seals and fewer have a means of effecting temporary ‘get-you-home’ strapping.
- The units (thousands in service) are marketed as ‘maintenance-free’. Only engineers who sail/live aboard confess to routine inspection. Other owners don’t. The manufacturer’s website recommends both inspection AND replacement at intervals.
This type of seal is a precision bit of engineering and as such needs to be installed carefully and maintained. They are not ‘fit and forget’ pieces of equipment but will work well if the manufacturer’s instructions are followed. All boats that have an inboard engine coupled to a propeller by a propeller shaft will have a shaft seal of some sort and most will drip a little. The cheaper rubber hose types have a lip seal that relies on water for lubrication. If they dry out because they have not been ‘burped’ (letting trapped air out) they will fail pretty quickly!
A useful piece of advice for the small craft audience is to look after any through-hull fitting which might allow water to pass in and out of the hull – the list includes seacocks, breathers, some ventilation openings and stern glands. Safe practice includes knowing where they are, how they work and what maintenance they require. ALL require some degree of user maintenance if they are to work as expected.