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Avoid the dangers of poorly installed shaft seals

2015 September 18

 Dear Boat Doctor,

I recently had a scary experience on my boat when my PSS dripless prop shaft seal began to leak in a rather dramatic fashion.  The water rose above the floorboards. I was able to get it under control and later consulted with the boatyard, determining that the shaft collar was not correctly installed. The stainless ring on the prop shaft slipped and allowed water to enter the boat.

I want to caution my fellow boaters to check their shaft seals, these are great devices, but it can scare you if they fail.

Jim Fuhry

Port Washington, Wisconsin

Dear Jim,

Water rapidly filling your bilge does get your attention. Like any mechanical device, a dripless shaft seal can fail but with proper attention and maintenance they perform well. Unfortunately, improper installation is not uncommon.

The PSS shaft seal consists of a rubber bellows that attaches to the shaft log, with a carbon interface inserted into the forward end of the bellows and pressed against a stainless steel ring that is attached to the shaft with set screws. The compression of the rubber bellows holds the two pieces tightly together.

There are several factors critical to the installation of a PSS shaft seal. The stainless steel ring needs to be firmly attached to the prop shaft and needs to be in the correct position on the shaft. Longer term, the rubber bellows needs to be inspected and ultimately replaced as a maintenance item.

The stainless ring is held in place with set screws installed with thread locker on the shaft. Secondary set screws are installed right over the top of the first set. Thread locker helps prevent water intrusion and it is also important to make sure the ring is in a position that puts appropriate pressure on the rubber bellows, to ensure a good seal.

The stainless ring can be backed up with a shaft zinc installed immediately forward of it. There is also a shaft retention collar that can be used in place of the shaft zinc. The idea is to install a secondary item on the shaft to prevent the ring from slipping. 

It is also important to monitor the condition of the rubber bellows which can get brittle over time. The manufacturer recommends replacement of the bellows periodically. Be aware that replacement requires that the shaft be uncoupled from the shaft coupler and partially removed from the boat.

The PSS is a very nice device but it requires proper installation and maintenance, much like a traditional stuffing box. I think a dry bilge is worth this additional responsibility, and I’ve been happy with the one I have on my boat.