Propeller Shaft Seals: Which Is Best?

Although the dripless shaftseal has been around for many years a lot of boats still have the old stuffing box.

The stuffing box is an old solution to the problem of allowing the prop shaft to turn and exit the hull without letting to much water in.

The advantages of this system are that it is cheap and reliable. Maintenance is easily performed with basic tools and skill level. The packing is cheap and at a pinch it can be replaced on smaller craft whilst the boat is in the water. Though obviously it easier to do this whilst on the slip.

The down side of this system is that whilst underway the seal must be allowed to drip to keep the seal lubricated and cool whilst in use. Obviously this allows a small but steady trickle of salt water into the hull which then must collected and disposed of. Also as the packing material is compressed against the expensive shaft it wears a groove into your expensive prop shaft overtime. Eventually the shaft diameter is reduced so much that seal cannot be maintained and the shaft must repaired or replaced.

To overcome the disadvantages of the stuffing box two types of dripless shaft seal have been developed.

The simplest is a rubber lip seal packed with grease and cooled by seawater. This system is very tolerant of vibrations and alignment. The seal is stationary with shaft rotating inside it which still causes some shaft wear and the shaft must be very smooth where it contacts the seal.

The other type are face seals. The basic principle is that two rings with highly polished surfaces are pressed together, one stationary and one rotating, which forms a watertight seal.
This seems to be the most popular type of dripless seal and also the most expensive. Generally speaking they seem very reliable if properly maintained. There have been  reports of serious leaks when they do fail. In some instances this has been attributed to sand or other contaminants getting into the seal. The rubber bellows should checked regularly for deterioration, both against leakage and to ensure it is producing enough pressure to make a good seal at the rotor.

Which is the best option?

To my mind boats that see a lot of regular use and are subject to a regular maintenance schedule would opt for a dripless seal. This ensures a dry bilge and no or low wear on the prop shaft. The seal should be inspected carefully before and after every use and parts subject to wear or deteriation replaced at the recommended intervals.

Boats that are used infrequently would probably be better off sticking with the old stuffing box. The maintenance is very low, problems occurring through lack of use are minimal. Wear on the packing is gradual so a sudden failure and ingress of water is extremely unlikely.