POSTED May 5, 2021
The 3 Most Common Shaft Seals Explained
Stern gland, stern seal, gland package, gland seal, shaft seal, stuﬃng box, log seal, PSS, tides… you may have heard all of these terms and wonder what do they all mean? In this article we will go over the most common styles of shaft seals, the pros and cons to each type, their serviceability and the life expectancy of each seal.
Stuﬃng Box / Gland Packing Shaft Seal
They’re leaking, you say! Well, they are supposed to… a little bit. These seals are the old and reliable option for a shaft seal. Imagine inside the gland a few strips of a waxy rope around the shaft. As they are compressed longitudinally, they seal onto the shaft and into the body of the gland, stopping most of the water from getting past.
Stuffing box seals are supposed to drip every 30 seconds or so and correct tension is extremely important. Too loose and they will stream water in, too tight they wont drip, however the internal packing will overheat when underway and burn out. This in turn will cause the packing to lose its sealing ability and drip and leak more frequently than it should.
You can do a quick test on these style shaft seals by touching the body of the gland when underway. If it is warm to touch it’s okay, and if they are too hot to touch then they are too tight and you risk burning them out. An even better option is to use an infrared temperature gun to check their temperature – less than 40°C is ideal.
Some may have an option to grease and others may have a cooling water feed from the engine. These are generally not required for cruisers and semi-planing boats.
There are two common styles, one tensions up on studs and the other tensions by the follower being a large nut with locking nut. These are standard threads so extra care must be taken when adjusting on a clockwise rotating shaft, as I’ve seen these nuts completely spin oﬀ. The option you choose may come down to accessibility and the amount of space you have between the stern tube and gearbox ﬂange to accomodate gland and coupling.
These shaft seals are serviceable and it is possible (although not preferred) to repack the seal without hauling the boat out of the water. Don’t forget about the rubber bellows, as they can crack and wear out too (and this requires the vessel out of water). Expect to have these glands removed and overhauled every 7-10 years, although adjustment and repacking will be required over the years depending on how much use they are subject to.
Packingless Shaft Seals (PSS)
Packingless Shaft Seals (PSS) are the predecessor to the stuffing box. These shaft seals are dripless, and rely on the principles of a mechanical face seal. A thin ﬁlm of water sits between the stationary carbon ﬂange and the stainless-steel rotor which is attached to the shaft. The seal attaches to the stern tube by a compressible bellow which houses the carbon ﬂange.
When fitting or adjusting the bellow compression on these seals only soapy water should be used, because getting grease or oil on those sealing faces is a big “no no” as contaminates on the sealing surface will prevent the ability to seal. Remember that the rotor has a total of 4 grub-screws to secure it to the shaft. You’ll ﬁnd 2 per hole, one bites the shaft, the other acts as a locking screw behind it. Bellow alignment is key to a smooth running seal, if misaligned the carbon ﬂange can bounce on the rotor preventing sealing and risking damage to the carbon ﬂange causing the seal to leak.
After slipping your boat for annual maintenance or repairs, on return to water remember to ‘burp’ your PSS before engine startup to vent any trapped air from the stern tube and bellow. This helps initial sealing and to prevent running the seal dry and burning the carbon ﬂange. When burping, be gentle around the water supply fittings as they can break easily.
These seals require an engine water feed. On twin engine installations you should also have a bridging hose between seals, this ensures either seal will still receive the required water for sealing if you happen to be motoring on just one engine due to breakdown (the second shaft will still rotate due to the prop walking) or have a restricted water supply from one engine to the seal.
If you notice your bellow swelling, check where the cooling water supply has been plumbed into the engine – ideally it should be near the end of the cooling system. If plumbed close to the raw water pump, any restriction or blockage of a cooler can cause water to divert direct to the gland and your bellows will swell, which will aﬀect the integrity of the bellow and shorten its serviceable lifespan.
To replace any components of the seal, it is a requirement for the boat to be out of the water and to split the coupling from the shaft. In some cases, you may even need to remove rudders and/or props to achieve the access required.
PSS shaft seals are best suited to vessels that get used a lot. You can expect to achieve around 5 years before a bellow replacement is required. The biggest demise of these seals is sitting without being used, so get out there and enjoy your boat.
Manecraft is another brand/style of dripless seal that functions on the same principle as a “PSS” seal.
Lip Style Shaft Seals
Tides seal / Volvo Stuﬃng Box Tides seals and the genuine Volvo stuﬃng box, all seal to the shaft by means of a lip seal. Being a lip seal they do wear a groove into the shaft and at some point you may ﬁnd you’ll need to vary the length of the stern tube hose to allow a new sealing surface on the shaft.
Tides also have the beneﬁt of being able to ﬁt a spare seal to the shaft in case of a leaking or damaged original seal. The replacement seal can be ﬁtted in the water, with no need to split the coupling from the shaft and also saving money on hauling the boat from the water.
The same as with the PSS seals, a bridging hose should be ﬁtted between shaft seals in a twin engine installation to maintain cooling water and lubrication to the second seal during single motor operation in a breakdown situation.
The Volvo rubber stuffing box is a small and compact simple design with minimal maintenance required – just apply some grease to the seal annually. These also rely on water to lubricate and cool, so when relaunching it is necessary to vent the trapped air from the seal.
These lip seal type shaft seals are more of a set and forget, in terms of leaking, however retaining clamps and hoses should all be periodically inspected as with all styles of shaft seals.
In conclusion, whichever shaft seal you have or opt for in the future, be sure to maintain periodic inspections of all components. Regular inspection and maintenance to hose clamps, hoses, bellows, fittings and shafts will ensure trouble free boating. If in doubt, have it professionally inspected and/or replaced. It doesn’t take much to sink a boat, and the last thing you want to worry about is water coming in.
About the Author – Brendan Sutton
Brendan is a dual certified Marine Surveyor & Engineer, who completed his trade at well respected marine engineering company based on the Hawkesbury River. After relocating his family to the Gold Coast, he joined the BoatBuy team in QLD. Liked this article? We would love to hear from you. Feel free to email Brendan with any boating related questions you might have here.