A mechanical seal is created by the contact of a turning surface against a stationary surface. In the case of the PSS Shaft Seal, the stationary surface is a carbon flange held in contact against a stainless steel rotor which turns with the shaft. The carbon flange is attached to the boat by a nitrile bellow which, with the help of water pressure, produces a constant contact between the carbon and the stainless steel ring. These type of seals are unaffected by engine motion or vibrations. The result is a 100% watertight and totally maintenance-free seal.
All type of mechanical seals will wear out in time. Water conditions (clean, salt fresh, temp), type of material the faces are made of, o-ring material, all affect the life of the seal. The debate with me is how the seal will fail, more than when. Catastrophic failure can cause big leaks, packing can usually be tightened down some when it starts to leak to much. Love the idea of a dry bilge though.
i prefer a packing gland with GFO packing. you would never have a catastrophic failure with it unless the hose let go. the slight drip is no problem and a lot less expensive to just use this hi tech packing.
Mechanical seals like this started in pumps. Almost no one uses a packed pump any more. For good reason, the mechanicals are PROVEN to greatly reduce maintenance.
I just replaced a mechanical seal on a sludge pump on Tuesday. It had starting leaking about a 6 months ago, but only leaked when the pump shaft turned, one minute of run time each hour. The seal was really worn, the carbon face was nearly gone, about 1/8" of wear. Yet it only leaked a little, and wouldn't have sunk your boat in any reasonable amount of time.
I don't think I'd worry any more about a catastrophic failure of a mechanical seal any more than I would worry about a packing gland failing.
Of course I own an Etap, and it can't sink that way in any event, so I may be biased!
there are two types of mechanical seals - the "carbon ring" and the rubber ring types.
carbon rings presumably serve longer but are sensitive to alignment. the rubber (well, some sort of flexible plastic essentially) ring wears out sooner, supposedly, but is not as sensitive to alignment.
As a case in point, PO of my boat installed rubber type seal in 1995 and it served for a total of 800 engine hours until I bought the boat, without leaking. I had it replaced with a similar seal.
Mine started leaking last year, the boat is an '01, presumably this was the original as I am not the original owner of the boat. Its a Benny 361 and the seal is made by Volvo. The instructions included with the replacement recommend replacing every 5 years.
the instructions that came with our pss seal syst. say you have to get the mating surfaces wet before you run the gear otherwise you will get premature failure of the unit. all you have to do is slide the bellows down a little to let water in then let go. also if you don't use boat for a while you should do same again as the water on the mating surfaces will have dried out. the seal does not leak the water is just a lube to slow/prevent wear
I think PSS states that if the bellows is adjacent to battery storage the bellows will fail quicker and need replacing. Looks like you would disconnect shaft slide it aft remove the old one and install the new one. I'll be doing it in the Fall since mine is 10 years old and near my batteries.