How Does A Marine Propeller Shaft Seal Work?

Face type "Dripless" propeller shaft seals are a newer type of propeller seal as compared to the original seal method, the Stuffing Box.

See PM&I Stuffing Box Catalog here: Marine Stuffing Box Catalog

A good propeller shaft seal that is properly installed will provide 2 benefits compared to a Stuffing Box:

1.  There should be no water leaking into the bilge

2.  The system will not wear grooves into the shaft

Face type propeller shaft seals are similar to mechanical seals.  There is a stationary face and a rotating face and between them is the sealing surface.

In the image above the green Stationary Friction Ring is the end of the stationary part of the seal.  The stainless Rotating Seal Ring rotates with the shaft.  Between the two material "faces" is where the water is sealed.

Because this seal does not rely on pushing packing up against the propeller shaft there is no possibility of wear on the shaft which can cause costly shaft replacement or installation of shaft liners. 

The seal faces are pushed together with relatively small force by a spring that is located under the yellow Covered Spring Chamber.

If properly adjusted for pressure, if sealing water is present (water hose barb), if sealing faces are aligned (flat against each other) and if the seal faces are clean and not damaged there should be no water leaking out of this propeller shaft seal and into the vessel bilge.

Compared to oil lubricated shaft seals this seal is water lubricated and the danger of an oil spill is removed.  Some face type propeller shaft seals are oil lubricated.

For larger shafts an Emergency Air Seal can be included.  If the shaft is damaged and bent and water is leaking into the vessel the shaft can be secured (stopped) and the air seal inflated to stop water from entering the vessel.  The shaft can not be turned with the emergency air seal inflated.   It will melt quickly from the friction.

The Emergency Air Seal takes the place of the Versatile Adaptor Ring in the image above and it includes an air fitting for inflation with hand pump.  It takes a very small amount of air to inflate and should NOT be attached to the vessel air supply system which can burst the small inner tube.

The Emergency Air Seal will be bolted to the stern tube flange.  If you are not using the Emergency Air Seal then 4 different types of adaptor can be attached to the Versatile Adaptor Ring: Flanged Adaptor, Thru Hull Adapter, Hose Adaptor or Clamp Adaptor depending on the set up of your vessel.

See Propeller Shaft Seal Adaptors

The danger of this type shaft seal system is heat build up that can damage the green Stationary Frication Ring.  Water must be present in the shaft seal internal areas to remove excess heat even when the vessel is backing down.  A small hose is attached to the water hose barb.  Not a lot of water is needed.  A thru hull scoop can be used.  If the vessel does have a cooling water system or other water source this type water can be supplied.