Shafted!

T

TimfromMersea

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Boat at West Mersea, Essex. Live in Wivenhoe, Esse
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Good morning Forumites,

As a 'new poster' I wonder if I can seek Forum members's advice please? My Beneteau Oceanis is 4 seasons old now and I have owned her from new. The engine has done about 350 hours. I recently had to have the cutless bearing changed, and the marine engineering company (local Volvo agent) tells me that this is not unusual in silty, muddy East Coast water.

But what is concerning me is that they say I have to have a new Prop Shaft as well, as the old one has pitted and eroded badly. They say that it is not the right grade of stainless steel as it is magnetic?

A new shaft has been ordered (not from B'teau) at £215 plus VAT, and they have sent me some pictures of the damage. I have sent these across to the B'eau agent from whom the boat was purchased, but they have kept quiet so far.

My question is - how long should a prop shaft last? I had in mind that these were something that lasted almost as long as the boat! Is four years only unusual?

The Volvo agents tell me that the only way that the grade of stainless steel can be confirmed is by a metallurgist. I have phoned this morning and one will be pleased to help me, for £1,250 plus VAT! Is there a 'Metallurgist Lite' service anywhere as it can hardly be worth spending £1,250 for the sake of a report on a prop shaft that costs £215 plus VAT.

Maybe it's just something I should accept and pay up and smile - but I must confess I do feel shafted, hence the title of my post.

Any ideas out there?

Many thanks!
 
IMHO the cause of the tale of woe is probably

1. Engine out of alignment.(premature wear on the cutlass)
2. Anode not fitted/wiring fault. (pitting)
 
If the shaft is magnetic it is not marine grade. My cutless bearing on a Medway mud berth lasts ten years.
A 316 grade SS shaft should last at least 20 years on a pleasure boat unless it is the wrong grade and corrodes away.
See what happens when you Euro stuff!
 
I'm still on the origianl shaft (stainless) from 1964! Pitting of stainless is crevice corrosion caused by lack of oxygen (stainless steel requires oxygen to remain stainless). I don't think its an anode thing. 4 years seems very very short life.

Probably worth reading 'Metal Corrosion in Boats by Nigel Warren'. Has some 'nice' pictures of crevice corrosion in stainless prop shafts. I have a copy here on Mersea.

What seems a mystery is why it happens on one baot and not annother.

Warren says "Scientists now make a distinction between 'crevice corrosion' and 'pitting'. Crevice Corrision takes place when a definite shielding of the oxide film occurs, thus starving the metal of oxygen (.......eg under a barnacle). Pitting on the other hand can take place on the exposed clear surface. But it takes a more corrosive environment, like a higher salt content to cause pitting rather than crevice corrosion."
 
Get your local machinist to turn you up a gearbox spacer to move the shaft back so the bearing surface is on new metal. If tyou have the space at the propeller end to let your shaft protrude another X millimetres. That will be a lot cheaper.
 
Only a thought , but most large scrapyards use an analyser to identify metals. It's a portable device and very accurate. Large chemical complexes also use them especially to identify various grades of stainless.
 
Try either Bodycote or Amtac for a reasonably priced compositional analysis. Amtac will quote on-line. In the past I have found them very reasonable for a spectrographic analysis - well under £100 last time I used them. You only need three elements: Chromium, nickel and molybdenum to determine your material.

Extensive pitting may either be 304 instead of 316, or electrolytic corrosion. What bonding/anodes do you have in place?
 
Given that the you have had the boat from new and she is a production boat from a well known builders I think it unlikely (but not impossable) that the propshaft is the wrong grade.

it is more likely some other problem. My prop shaft is 14 years old , has survived, Ireland, east coast, south coast, Atlantic and MED but lots og other things hav happened - it even fell out once but I got it back!

welcome to BOATS - things just keep on breaking and you keep on fixing them....
 
There should be no wear on the shaft in four years, unless there is significant misalignment between the engine and the shaft.

If the wear is only in the region of the cutless bearing, and there is no sign of corrosion anywhere else on the shaft, it's probably not a composition problem. I agree with the suggesiton about having a spacer made up, I did that myself after re-engining my boat 'cos I was too tight to buy a new shaft, although I have a flexible coupling at each side of the spacer. Don't forget to connect them both electrically (it's an anti-corrosion thing).

You must ensure that the cutless bearing runs on good metal so you may find it necessary to have a new taper and keyway cut in the shaft (more money) if the resultant prop position fouls the boat, but if you save the waste, a scrape analysis (filings taken with a new file and collected in a poly bag) shouldn't cost more than a couple of hundred. Your metallurgist's quote is taking the piss.

I don't know your boat, but mine had a cutless bearing on both ends of the prop tube (no P-brackes on Long keeled boats) and the softer engine moutings on my new engine caused the shaft to whip and the outer bearing to wear out very quickly. Removing the inner cutless bearing stoped the problem. If you have a P-bracket, this won't be an option.

I've just been speaking to a pal on the phone....he said he once had a tight bearing on his boat that caused wear on the shaft (25mm bearing on a 1" shaft)......I'm a bit sceptical regarding that being the only problem, the bearing material is sacrificial after all, but it may be something else to consider.
 
I have the 'egg' anode she came with, and this appears to be working (I've replaced it once). It also appears to be connected electrically properly, although I'm no expert on this. I certainly haven't altered anything that Beneteau fitted to start with.

She has a Bruntons Autoprop that I had fitted and this carries a 'cone anode' which is screwed to the front of the prop.

It's a great idea to make up a spacer to alter the shaft position! Wish someone had thought of it before I ordered a new shaft................

The problem is that I'm not an engineer, you see, and so I have to have these jobs done by 'friendly neighbourhood Volvo Penta dealer'.

Thanks for all the replies, I do appreciate all the input and it's most interesting to hear everyone's ideas.
 
Any chance you could post the pictures of the damage on here?

If you don't know how, any post by DogWatch has a link to an excellent set of instructions.

Might be possible to get an opinion if we can see the damaged shaft.
 
If the old propshaft hasn't rusted (pitting from galvanic action isn't rust) and is magnetic, then it is probably EN 1.4462 stainless steel. This is a mixture of ferrite and austenitic steel. It has the disadvantage over 316 SS steel that it has lower resistance to galvanic action ( so, you must always have an anode on the shaft) but it is a proper steel for a propshaft. It has much higher torsion resistance than 316 SS. Even when pitted, it will have much less tendency to break.
 
Umm, if you've got a badly pitted and corroded prop shaft, you surely don't want to keep it do you? The point is they break! The spacer idea will work of course but you'll need to recut the taper and shorten the length of the shaft. Sounds like the new shaft isn't that expensive by the time you've done that because by the time you've paid for the recutting and the spacer, you've still got a pitted and eroded shaft. But what's the point of a spacer. You haven't said the shaft is worn. So what's the point of moving the pitted area? and that's different to worn. But check with V - whoops sorry, the East Coast agents to ensure they are talking about pitting and corrosion as opposed to wear. The problem isn't with your anodes. An anode cannot cope with galvanic action in this area. You've lost the oxide film on the surface and the metal exposed is now less noble than the rest of the shaft. and in the words of Mr Warren - that's death - in a very short space of time. No anode can cope with that - its the small bit of the shaft that's lost it's oxide film fighting with the rest of the shaft that hasn't and consequently is more noble. The only issue you have, I think, with the manufacturers is that if its magnetic its not regarded as a 'seaging marine' stainless steel. But it still might be fit for purpose in the eyes of the manufacturer
 
Yes, no problem.

I'm off out tonight to the last Yacht Club Friday Night Winter Carvery Supper, but I'll call up one of Dogwatch's posts tomorrow night probably, and post the pics on here after reading his instructions.

I'll be interested in all your opinions.

The power of the Web eh! It's not all cheap flights and porn.

Thanks again everyone.
 
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