Shafted!

Sorry but i dont agree with much of what is written above!!!
if your prop shaft is worn, pitted or what ever in 4 years there is a reason why! rather than just replace or move ther shaft, try and find out the cause. otherwise in 3-4 years we will all be here answering the same questions your posting now.
If you suspect wear due to missalignment them look at the gland or bearings and see if they are worn! if you think its bad cathodic protection use a meter and check out bonding etc.
provention is better than cure and you cant cure a situation untill you know the situation!
 
While I agree with your sentiments, and I assumed there's be some exaination and correction of the cause when I posted about misalignment myself, he still has to do something about the shaft (i.e. replace or move it).
 
Just a thought - is there any clearance between the P bracket and the prop? If the prop is very close to the bracket you may not be getting the necessary flow of water through the cutless bearing. This would definitely cause premature failure of the cutless and could possibly give rise to crevice corrosion due to trapped de-oxygenated water.
 
It sounds like crevice corrosion. Post a photo and we'll have a look. My propshaft has been in for over 30 years and is fine. The fact that it's magnetic is a big clue, I think.
 
Despite the advice the original poster received and the same comment from a number of posters, the fact that stainless steel is magnetic DOES NOT mean that it is not of marine quality. Some of the best high performance propellor shaft stainless steels ARE VERY DEFINITELY MAGNETIC (my own included).

John
 
Very true. It is very common to see 300 series stainless steel used for yacht propeller shafts. 316 is more common but no doubt there are some 304 ones out there. However, 300 series steels suffer from the drawback that they are not hardenable, so if strength is of great importance it is normal to specify a 400 series martensitic stainless instead. These typically have 13% chromium but low nickel and rather more carbon. They are hardenable by heat treatment to almost the same extent that carbon steels are, e.g. it would be easy to double or treble the strength of the original metal. There is a slight reduction in corrosion resistance but this is not terribly critical. For example, 400 series stainless steels are widely used in refinery processes, where chloride corrosion is a far greater problem than it is in seawater.

If very high strength is required it is possible to go to a precipitation hardening stainless steel such as 17/4 PH. This perhaps best known in yachting circles as the alloy used by Wichard for some of their superb fittings. They combine high corrosion resistance with high strength but are almost certainly magnetic.

Finally, duplex stainless steels are a relatively recent newcomer to the field, combining high strength with good corrosion resistance. I doubt that much has filtered through to the yachting market but they are also magnetic.
 
As already indicated if a magnet is attracted then it is IMHO unlikely to be the right grade of stainless so replacement is a good idea.
If you wish to go on using that shaft then you can easily buy a rubber and carbon seal that is water lubricated. I bought a "Stopelo" 25 mm. one in a Chandlers in Dunkirk for less than £100. It will fit in a different position and the seal is between a graphite ring in a rubber bellows and a stainless ring on the prop shaft. Shaft condition is then less important.
Do remember to flood the seal whenever the boat goes in the water.
 
my bene 351 has the same set up as you, i changed the cutlass bearing 2 years ago, £17 is what it costs from ancasta, simple job but need a puller to get the prop off, 2 plastic screws is what holds it in, i changed it agan last week, 1/2 mill play in it. but £17 hey whos bothered, the shaft had a bit of corrosion on it from crevis stuff bujt not serious, polished it with some emery, you must have newby tattoed on your forehead and they are being a bit like a certain tyre and exhaust chain that scare people that dont know by showing them bits that look horrible but are quite normal
 
some one trying it on perhaps, beneteau didnt get where they are by providing the wrong thing and as for the posters shooting from the hip!!
as aways a superb definitive post.
stu
 
beneteau provide the "volvo" seal, one of the most simple reliable seals around, the seal is nothing to do with the cutlass bearing.
stu
 
As already indicated if a magnet is attracted then it is IMHO unlikely to be the right grade of stainless so replacement is a good idea.

Might pay you to actually read the posts just above your own from people who actually know what they are talking about before making such claims.

Thanks for your explanation Vyv, you expanded my knowledge.

For the original poster - without seeing the damage to the shaft (and you don't, I think, actually say where the damage is) I could not comment on the actual cause of the problem but if the shaft is not run frequently I suspect the cause may be crevice corrosion in the stagnant water area between the shaft and the cutless seal (or even the stagnant water in the shaft log).

Also, if the shaft is not rotated regularly marine life and debris collects in the bearing spaces of cutless seals and damages it when the shaft is next run shortening the bearing life dramatically. A couple of years only life is not unusual. As others have mentioned operating in dirty water also shortens the bearing life (I actually know of one boat that operated commercially over a shallow and sandy bottom and the ingress of sand eroded through the shaft log tube itself and nearly sank the boat).

On my own boat I don't use cutless bearings but have a bearing machined up out of Thordon plastic (a plastic suited to water lubricated bearings) - after 800 hours of running it is still as tight as when new.

John
 
Thanks again everybody for the comments. Slightly sad to see that just because I am not an engineer someone has posted 'you must have newby tattoed on your forehead'. That's a pretty unkind comment. Just for the record I have been sailing and around boats 40 years plus, but I'm by training a lawyer, not a scientist or engineer, and also I work and so cannot get to the boat at the time when people like marine engineers doing work on it can.

I try and do everything I can myself, but I am also experienced enough to know when something is beyond me. I don't have a prop puller and even if I did I probably wouldn't attempt this task as I am not confident as to how I would do it.

If you take the view, as one poster above does, that everyone in the marine trade is a thief and a crook, and trying to do down the 'innocent mug' who comes along, then perhaps I am a mug and ripe for the fleecing. But that's a pretty sad view of the world and not really one I subscribe to. In my experience, there's not that many 'theives and crooks' in the boat business, simply because there are not that many fortunes to be made in it. Much easier to do something like used cars, property development, etc etc, that makes serious money. Most people who are in the boat business are in it because it makes a living and is a nice way to spend your time.

Finally is the salaried engineer in the employ of a big marine engineering company really going to be in league with the owners to rip off the customer and split the take? It seems a shade unlikely to me.

Great to read the learned discourses on crevice corrosion and so on, and I'm learning a lot, but can we hold off on the personal abuse and conspiracy theories, which don't really help - thanks.

"Lord Finchley tried to mend the electric light. It struck him dead, and serve him right. It is the business of the wealthy man, to give employment to the artisan".

Cheers.
 
I think that you are dead right and your approach sound.

I am perhaps a little different to yourself as I do alot of work in the marine industry (professional, not spanner wielding) and that is in boat/shipyards and for owners in a number of countries (I am in NZ myself). But I still find it easier and better to get a yard to do many jobs on my own boat which they either have the tools on hand (who the hell with only one boat wants to own a prop puller, for example /forums/images/graemlins/smile.gif) or the ability to get the job over and done with rapidly - after all they will usually have done the same or similar job many times before. Usually it works out cheaper in the long run and even more so if I value my own time as being higher than what I pay the yard.

It is interesting in that some of those on this forum who are ready to criticise the yards, etc as "crooks", etc are the ones who seem to do the most foolish and amateurish things to their own boats or give unsound advice to other forumites.

John
 
In my business of engineering failure diagnosis I come across many examples of incorrect material supply, not for any dubious or criminal reason but simply because it is not easy to differentiate between these materials. It is perfectly possible for a shaft to be made from the wrong stuff because of a cock-up at the supplier's warehouse, completely out of the control of Beneteau or any other builder.

I think that this is a case where an expert needs to be consulted. As an intermediate step it would help if you could post a photograph here, or if you want to e-mail me one send me a PM.

Vyv
 
Crevice corrosion is simply what it says - a crevice. Nobody said how big the crevice had to be! When stainless shroud wire tarnishes you are seeing crevice corrosion on a small scale. Polishing is the standard way of removing it and preventing recurrence. Bolted joints commonly corrode due to the crevice created beneath the head of the bolt, the nut and/or the washer. Polishing the corrosion product away is clearly the answer in this case.
 
no offence meant!! but if you are happy to pay for a new shaft where, IMHO and a few others who know what they are talking about, none is needed you carry on son!
stu
 
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