After replacing a shaft in 1988 and again in 1992, we replaced out Sabre 28's stuffing box with a PSS shaft seal. When we sold the boat in 2005, it was still going strong.
When we bought Victoria
, I replaced the stuffing box in 2006 and have had no issues. It's maintenance free and easy to install. I check it each spring before launching and periodically throughout the season.
IMO, generally speaking, things just don't "give out", there are usually warning signs. Stuffing boxes have sunk their share of boats, so they are by no means "safer" than anything else. But to me, ANY drip is bad.... water belongs outside the boat.
What Sabreman said!!! I was a professional marine engineer for 30 years (licensed Chief Engineer), and I can tell you that, properly installed, a mechanical seal is the way to go (PSS). I installed mine in 2007, and will be overhauling the seal before splash day this year (the overhaul kit includes the elastomer bellows, set screws and o-rings). Note that the manufacturer recommends not exceeding a 6 year interval between overhauls. This is probably over-cautious, but better safe than sorry.
Granted, you have to break the coupling and shift the prop shaft to install or maintain one of these. I installed a small water supply line teed off my raw water system to feed flushing water through the seal. This prevents "dry running" - which can burn out a mechanical seal in short order - and it constantly flushes clean water across the seal faces. This seal works well, and I can highly recommend it. Some items to watch for:
a) Make sure that the shaft is polished clean before installation, and there are no sharp edges on the shaft which might cut the O-rings.
b) Lightly lubricate the shaft before installation. (for the o-rings)
c) DO NOT touch or lubricate the seal faces before or during installation.
d) ALWAYS use new SS set screws when assembling or re-assembling the seal - they are only good for one use!