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As their name implies, dripless shaft seals are designed to completely stop water from entering a boat’s hull via the stern tube. There are two main types of dripless seals known as face seals and lip seals which many boat manufacturers now fit to production boats. They can also be fitted as replacements to boats with traditional stuffing boxes.

Face seals

A face seal has a flexible bellows attached to the stern tube that presses a carbon or graphite flange against a stainless steel rotor, or sealing ring, which is attached to the spinning propeller shaft, making a seal between the spinning rotor and the flange. Once installed correctly they should not need adjustment or maintenance, aside from a check every now and again of the hoses, clamps and sealing faces.

These are popular as they are considered maintenance free and keep the water out even if the shaft is slightly out of alignment. However, while they are known to last for more than five years with no problems, if either the setscrews that hold the rotor in place or the bellows fail, then water can quickly flow into the bilge and serious problems will arise. This might occur because of fatigue of the bellows material or improper installation. If your boat is fitted with a face seal, you need to be aware that it is definitely not maintenance free.

Here are some maintenance checks:

  • The bellows should be inspected every six months and checked for signs of cracks, splits, tears or brittleness.
  • If leaks are noticed where the bellows attach to the shaft log or flange, check the hose clamps for looseness or damage.
  • Be aware that if the seal or bellows are attached incorrectly the seal faces will be subjected to uneven pressure which will cause uneven wear.
  • A common cause of minor leaking is where foreign material gets between the rotor and flange – a grain of sand can cause the seal to be broken. The remedy for this is to clean the two surfaces, which is easier said than done with the boat in the water but more easily done in the boatyard.
  • The bellows can lose tension over time due to slippage, which can result in leaks as the pressure on the face seal reduces. A remedy that can prevent this from happening is to clamp a propeller shaft anode right alongside the face seal as a backup to prevent the bellows from slipping along the shaft.
  • A further action to be aware of is that face seals are lubricated by water and that if air is trapped in the bellows, for example after the boat has been out of the water, then the seal will no longer be lubricated. Some systems have a water injection tube to deal with this but others need to be manually squeezed or “burped” once back in the water so that the air can escape. If in doubt your yard should be able to assist with this.

Some manufacturers advise the bellows should be replaced every six years or less, even if they appear to be in good condition. As always, it is advisable to check through your manufacturer’s instructions and include the seal inspection as part of your winter maintenance schedule.

Lip seals

Lip seals also use a flexible rubber type sleeve to keep the water out but unlike face seals which use a stainless steel rotor, the seal is made from nitrile rubber and bears directly against the propeller shaft. The shaft rotates inside the seal and no water escapes into the boat. Most lip seals are lubricated by water that comes from the engine’s raw water system, while some are oil lubricated. There is no dripping of water and the seal does not need to be tightened.

boat engine lip seal

Things to be aware of about lip seals include:

  • Lip seals need a smooth, pristine shaft surface. If the shaft is not in an unblemished and polished condition it will leak.
  • They rely on a water supply for lubrication which should be checked periodically.
  • Oil lubricated lip seals are supplied by a small header tank of oil which needs to be kept topped up. If this runs dry the seal will heat up and fail.
  • Correct installation is vital so it would be advisable to have this done professionally to avoid any leakage.

The only maintenance required is as follows:

  • Lip seals should be greased with manufacturer’s recommended waterproof grease every 200 hours of operation or once a year.
  • Replacement of the seals is usually advised every 5 years or after 500 hours of operation.
  • As with face seals, lip seals need to have any trapped air removed from them after launching. This is done by squeezing the sleeve and pushing it back towards the stern.

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