In most all shaft logs
, chemical and biological conditions and processes lead to encrustations (both from chemical processes and living ones) on the shaft.
When you rotate the shaft, those lime encrustations are wiped off the shaft by the rubber staves of your shaft bearing. With consequent wear of your Cutless (TM) bearing or other shaft bearing.
And if you've a dripless shaft seal
, the lime encrustation will cause some stiction between the rotor and the stator. So when the shaft turns, you're effectively testing the torsional strength of the rubber bellows.
The situation is environment
dependant. Some locations are much more prone to precipitation of lime and biological processes than others.
A month or three back one CF member
wrote about their intention to leave their cruiser in a wet berth in Indonesian waters for several months. I shuddered, knowing too well from experience that three months later they're going to find a good imitation of a chunk of coral
in their shaft log. And no Cutless bearing survives that. Cruisers who have left boats in wet berths in Singapore
have reported exactly that (good imitations of coral
that have grown in their shaft logs
, causing rapid and massive wear to their shaft bearings).
temperature, the local sea water
chemistry, the ease with which a shaft bearing and the whole shaft log are flushed with water versus the tendency for the water in the shaft bearing to be stagnant are all factors.
Prevention is the big one.
Design a shaft log that is easily and readily flushed, that does not harbour stagnant water with biologicals, and is not friendly to encrusting life forms. No one seems to have achieved all of those contradictory goals, although some are better than others.
Everything else is just remediation.
The easy one is to turn your prop shaft once a week. Manually works. An engine
run that turns and forces (such as if you have forced water injection into a dripless shaft seal) water lubrication and flushing
is better. In other words, use the boat.
We had one shaft bearing/Cutless (TM) bearing last just over 10 years. That was when we were cruising non-stop. With less regular use, and in certain waters, shaft bearings might last only three years even in low turbidity water.
With a dripless shaft seal
, the idea is to break the stiction manually (by burping the shaft seal) rather than spin the shaft to test whether the torsional strength of the rubber bellows is greater or less than the adhesion of the lime and biological encrustation.
There are shaft seals
with newer, different rubber compounds making up the staves. At least one of those was hyped (or at least had media hype with generic hand gestures) as suggesting it resisted being cut by shaft encrustations. Remember that the original idea behind the Cutless (TM) brand name was that the stave bearing cut the shaft less - pointing to the contradiction between a hard seal (not cut by grit but then liable to wear the shaft) and a soft seal (cut by grit but not wearing the shaft much). In the long run, it may be better to replace shaft bearings than to replace prop shafts.