> My cutless bearing problems
> come from crust-like build-up on the prop shaft ,
> due to in water, inactive boat
> storage during long winter months . This crust
> cuts the rubber away and scores the shaft .
No one else has responded, so I'll have a try at this. Of course it comes with the usual warning that I'm not an expert corrosion engineer/electrochemist/mechanical engineer/polymer engineer, just an interested observer.
I think two or three things are involved:
1. Precipitation of CaCO3 Calcium Carbonate on the shaft
A search of the Net using a phrase such as "precipitation of CaCO3 in seawater" finds a lump of links to pages such as this one from corrosiondoctors.com: [www.corrosion-doctors.org
Reading that suggests that tiny changes in the pH of water, perhaps associated with the ss shaft being a cathode (and a lump of zinc acting as an anode) might be involved.
That it happens inside the shaft bearing and not on the exposed shaft suggests to me that the stagnancy of the water trapped around the shaft by the shaft bearing might also be a factor.
I try to run the engine and spin the shaft at least once a month. I met cruisers in SE Asia who were more religious: the one that impressed me most argued that the engine and prop shaft have to be turned over at least once a week (that cruiser, when at dock in Penang, ran every engine and motor (outboard, inboard auxiliary, water pumps, etc) every Monday. Monday for him was engine and pump day).
Zygote has a PSS dripless shaft seal. I've not seen it documented anywhere, but I reckon that the rotor and stator stick together (by some chemical or biological process) after about 7 days without shaft rotation. I reckon that breaking that stiction (which is tiny after one week but bigger after one month) stresses the rubber bellows. So I try to make a regular routine of entering the engine room to burp the bellows before starting the engine and rotating the shaft.
In summary, I think regularly rotating the shaft is needed. And perhaps the precipitation of the Calcium carbonate is a sign that you have good anodic protecting of the shaft/prop?
2. The nature of shaft bearings aka cutless bearings.
I figure that the engineers who designed improved cutless bearings - which means the Borg Warner engineers who came up with the shape and polymer inside Morse Marine's Bloater and the Vesco engineers who came up with the shape and polymer inside the Vesconite thingies - all focused on motor vessels running their shafts in gritty water. So they are trying to balance wear of the shaft and wear of the polymer.
I prefer them to prioritise not wearing the shaft (i.e. a softer polymer and channels that keep grit away from the shaft) and prefer to replace the shaft bearing than to replace the shaft (a shaft made skinny and weak by abrasion would be a problem, no?)
But that's different from your hassle: a chunk of limestone that has formed on the shaft inside the bearing. An engineer looking at the problem might say we have a spectrum of choices with two extremes if we just focus on the cutless bearing:
(a) let the limestone/coral rip the rubber polymer apart; or
(b) make the polymer strong enough and shaped like a scraper so it can rip the limestone off the shaft.
You can see the contradiction. A polymer strong enough to rip the limestone off the shaft will likely be hard enough to abrade the shaft. A polymer shaped to act as a scraper (in both reverse and forward rotation) might be expensive to make (and not do a good job of handling grit in water).
Of course that engineer might also say that the problem is easily solved by just running the engine and spinning the prop more frequently.
No independent authority seems to have done a comparison of the Vesconite Hilube bearings and the Morse Marine bearings. I cannot imagine that Practical Sailor or anyone else would tackle the job.
And if you look at our position in the political economic system, i.e. end users with no pricing power (we only buy and install a new shaft bearing every few years) in an imperfect market (because we don't have enough information and do not know how to balance the cost of installing a new shaft bearing against the unknown promises that any extra price of Vesconite Hilube will be compensated by a longer lived shaft bearing that is even gentler on our shaft), then we're simply stuck in the dark.
My recommendation: when in a wet berth, do everything you can to spin the prop at least every month if not every week.
BCC 116 Zygote,
Scarborough Marina, Moreton Bay, Queensland, Australia
Edited 1 times. Last edit at 03/06/17 01:17PM by Bil.