What to know about Dripless Shaft Seals

The connection between the engine and propeller requires a big hole in the hull under the waterline. A dripless shaft seal or similar solution is required.

 

A sailboat’s meant to sail, but there are plenty of situations and maneuvers where it’s great to have an engine on the boat. Unfortunately the connection between this engine and its propeller requires a big hole in the hull under the waterline with a rotating shaft going through. Inventors and technicians have found different solutions to cope with this challenge over many years. For a long time stuffing boxes were the state-of-the-art technology—simple and safe devices, but wet bilges were unavoidable. The compression of the shaft packing inside the stuffing box (or stern gland) must be just enough to accomplish a watertight seal when the shaft is stationary so an unattended boat will not sink and to drip slightly when the shaft is turning to lubricate and cool the shaft as well as the packing.

A wet bilge was acceptable on old boats where basically everything from the galley sink to the fridge drain went into the (smelly) bilge, which would have to be pumped regularly anyway.

Standards have changed nowadays and many boat owners want dripless seals so they won’t have to deal with such a smelly mess anymore. On a metal boat damp areas are prone to corrosion, so it’s even more important than on other boats to keep them dry. Pitufa’s aluminum bilge is dry and we want to keep it this way—especially as we use the space for storage. 

Hurrah! Our new ProfiSeal
Hurrah! Our new ProfiSeal

What is a Dripless shaft seal?

Dripless shaft seals are based on one of two different principles: lip seals or face seals. Among other manufacturers, Volvo Penta and Vetus offer shaft seals based on lip seals. Usually these have no moving parts and the seal is accomplished by one or more elastic lips around the propeller shaft (also called radial shaft seals or Simmerrings). The lips require regular greasing, which might be a cumbersome and splashy job while the boat is in the water. 

Face seals on the other hand are supposed to be maintenance-free.

The principle of a face seal is a rotating ring (attached tightly around the shaft) which is pressed against a stationary ring (attached to the stern tube). One ring is made of graphite or carbon (which has a self-lubricating property), the other one from high-quality stainless steel. A very popular shaft seal based on this principle is the PSS from PYI Inc. Here a rubber bellow that is attached to the stern tube acts as a spring to push the stationary carbon ring against the rotating stainless steel collar fixed on the propeller shaft in front. 

Of course, rubber fatigues and can become brittle with age, especially in an environment where it is exposed to heat, vibration, saltwater and accidental fuel spills. However, the price of a PSS is rather moderate, so exchanging it regularly won’t kill the cruising kitty.

Pitufa’s previous owner was German, so she came with many gadgets ‘made in Germany’. One of those is our shaft seal: it’s called ProfiSeal (www.profiseal.com) and its design is based on a face seal. Instead of a rather flimsy rubber harmonica, it features a solid metal housing that is flange-mounted to the stern tube. As Pitufa is an aluminum boat, the flange and housing are also made of aluminum—the standard version, however, is made of bronze. Inside the housing a rotary unit (fixed to the shaft) consists of a massive stainless steel spring and a graphite ring, which is pressed against the forward end of the housing where a stainless steel ring sits. Like the PSS it has a hose barb for the intake of cooling water. It is very compact, only 11 cm (4.3”) long for our 35 mm shaft, making it probably the only face-type option when space is limited.

Our ProfiSeal has kept the boat dry for incredible 23 years (all year cruising in the tropics, mainly under sail) and its endurance against the constant galvanic attacks from a much nobler close-by neighborhood is extraordinary. The standard version, which is made of bronze, lasts probably even longer. We only changed the rotating unit and the stainless counter ring after we had to pull the shaft for repairs six years ago. We contacted the company and were astounded by the swift customer support. Spare parts were available without delay and came with professional advice which helped with the re-installation. By now the aluminum housing shows some corrosion, so we have decided to replace it when we haul out the boat next time—of course we’ll install a ProfiSeal again. High quality comes at a slightly higher price, but you definitely get what you pay for and you don’t have to worry about maintenance or replacements for a very long time.

Birgit Hackl
Birgit Hackl, Christian Feldbauer and ship’s cat Leeloo have been exploring the world on their yacht Pitufa since 2011. Visit their blog: www.pitufa.at