Shaft seals are essential to the reliability and environmental compliance of a vessels primary shafting.
They sit either on the forward end or both the aft and forward ends of a Stern tube on the conventional shafting of a vessel.
They can also be found on Thruster or podded type propulsion units. These seals are very similar to the aft seals found on conventional Stern tube arrangements
ATZ with our partner Aegir Marine have the parts, service and experience to overhaul most shaft seal types.

Shafts Seals

For the purpose of this section we will discuss shafts seals as found on Stern tubes as the principles can be applied to seals in other propulsion arrangements

 Two main types of Sterntube configuration exist, which are:

  • Oil lubricated stern tube – fitted to most ships and filled with oil to provide white metal bearings with lubrication and cooling, and has a seal fitted at either end (fwd and aft)
  • Water lubricated – fitted to newer ships and those operating within sensitive marine areas as well as military vessels requiring resistance to underwater explosions. These usually have only a single seal on the inboard side. Lubrication and cooling of composite bearings in the tube are provided by seawater.

Oil lubricated seals are:

Usually a 2-lip seal arrangement at the forward end of the Sterntube
  • The aft-most lip seal, seals against the Sterntube oil that lubricates the Sterntube bearings and sees the Sterntube pressure (dictated by the height of the Sterntube gravity tank) on its pressure side
  • The forward-most lip seal seals a separate volume of oil whose pressure is dictated by a header tank lower than the gravity tank. Its purpose is to reduce the differential pressure across the aft most lip seal – aiding its longevity – but also to provide a double barrier solution, should the aft-most lip seal fail.
Usually a 3 or 4 lip seal arrangement at the aft end of the Sterntube
  • The forward-most lip seal, seals against the Sterntube oil that lubricates the Sterntube bearings and sees the Sterntube pressure (dictated by the height of the Sterntube gravity tank) on its pressure side. In a 4-lip arrangement a standby seal with its own header tank sits alongside the forward-most lip seal
  • The two aft most lip seals are positioned to seal against the seawater draft pressure

Water lubricated seals are either a:

  • Forward mounted mechanical shaft seal that is essentially a rotating ring sealing axially, and in compression with, a static ring.
  • Forward mounted rotating lip seal sealing axially, and in compression with, a static ring.

In both case they are normally supported by an emergency inflatable seal as should failure occur, this is only a single barrier system

Reliable Operation

The importance of reliable operation of the shaft seals cannot be underestimated. Through failure comes the possibility of:

  • Seawater flooding of machinery spaces
  • Loss of use of a shaft line
  • Damage to the Sterntube bearings and tail shaft
  • Leakage of oil and associated environmental impact

Routine Maintenance

Routine maintenance is carried out at 5 yearly intervals and usually consists of:

Oil lubricated
  • Replacement of all lip seals. If the tail shaft is not being removed, then the seals can be supplied split and hot bonded around the shaft
  • Replacement of O Rings
  • Replacement of fasteners
  • Inspection of the liner on which the lip seals operate. The liner can be set to a new operating positioned or machined to renew the surface, if required.
  • Dependent on material choice the main seal housings can last the life of the vesse.
Water lubricated
  • Replacement of the non-metallic axial seal component
  • Refurbishment of metallic axial seal components
  • If the tail shaft is not being removed, the above components can be supplied split 
  • Replacement of O Rings
  • Replacement of fasteners
  • Dependent on material choice the main seal housings can last the life of the vessel

Extensive Experience

ATZ with our partner Aegir Marine have extensive experience with most shaft seals types are in position to meet your parts and service needs for stern tubes, thrusters, pods and more. We use specialist dive teams for underwater fault finding and repair of outboard seals.


FAQ’s

What is the best cause of action if my water lubricated shaft seal is leaking excessively?

If within the limits of the maker, plan for overhaul at the next opportunity. If leakage is beyond these limits are starting to test the limits of the bilge pumps then the shaft should be stopped, locked in position and the emergency inflatable seal activated. Make plans for immediate repair and investigation.

My lip seals need replacing but I do not want to pull the tail shaft. Can this be done?

Yes. Make sure that your new lip seals are supplied split and that the specialist tooling and experience for bonding is brought in to carry out the job.

The oil level in Sterntube gravity tank is dropping. What is the issue?

If the seal header tank oil levels are rising, then the lip seals either side of the Sterntube volume are leaking. This should be monitored, and plans made for repair.

If the seal header tank oil levels are not rising, then either there is a system pipework leak (trace and rectify) or oil is leaking outboard of the vessel. This should be investigated immediately with a visual check at the eft end of the vessel and a dive inspection if required. 

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