No problems here with being disagreed with. It's a healthy discussion, which is one of this forum's, and its members great strengths :-) The crevice corrosion has to start somehow, due to a weakness in the oxide layer. Inclusions, or grain structure might be one of those initial causes.
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Ross365
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Three people have suggesting pickling (one with passivation also). You've not commented on that - could you say why not? You are not really overwhelmed with practical options for attempting to minimise the problem. I know nothing at all about boats, but these shafts being tubes (?) it would be interesting to know how deep the pits/crevices are relative to the thickness if the tube wall, i.e. what is the loss of section?
bert
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I've got nothing against pickling at all, if that's what will stop or at least slow the corrosion I'll get some and do it. 
I just want to make sure the shafts aren't knackered before spending the cash.
The shafts are solid and are about 40mm in diameter. The pits are 1 to 1.5mm deep
I just want to make sure the shafts aren't knackered before spending the cash.
The shafts are solid and are about 40mm in diameter. The pits are 1 to 1.5mm deep
Ross365
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I think it is likely that below the pits, there may be oxidation going down the grain boundaries, so the depth affected may be greater than you can observe from the surface. When I was involved in laboratory testing, we had tube test specimens which we would cut in to sections, polish them and then examine them in cross-section under a microscope to reveal the true picture (we were concerned about pitting leading to leaks). In your situation, I don't know whether there are non-destructive test methods (like eddy-current tests or ultrasound) which would tell you anything.
While you've had some "best intention" advice on the thread, we don't seem to have a 'prop-shaft professional' among us, who could make comments based on experience; I wonder how much it would cost to do that? What are the worst case outcomes here - can it break and damage the tube that it's in?
Newlands
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What sort of cutless bearings do you have? And do they have a spiral groove machined in them to allow water to get into the tube? I’m a bit surprised that you have three of them, but if the shaft is 12’ long and 40mm dia then it probably needs something to stop it whipping.
I always machined up my own cutless bearings from Vesconite with a decent water groove in them - have never liked the usual hard rubber/brass shell type as they seem to wear the shaft out and only last a season or two.
I always machined up my own cutless bearings from Vesconite with a decent water groove in them - have never liked the usual hard rubber/brass shell type as they seem to wear the shaft out and only last a season or two.
Newlands
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Depends where the break occurs and how well the shaft is supported. At the inboard end there is usually some sort of gland which doesn’t really support the shaft, just prevents too much water water seeping out into the boat. Because the engine is on flexible mounts, the shaft moves around on most boats and is only supported by a single cutless bearing at the extreme outer end. But in this case it seems there are two intermediate bearings as well.
usually the worst that happens is the the broken end of the prop shaft just slips backwards and jams up the rudder, which is very annoying. If it breaks at the very end where the prop is then it falls away without jamming the prop - also very annoying.
if it was me I would run it for another 6-12 months and then pull it again to see how much worse it had got.
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I doubt it, I'm pretty sure a grain boundary is far smaller than what eddy current testing could pick up, but even if it could I think the pitting at the surface would swamp any tiny cracks from being detected. Wouldn't be cheap either.
bert
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The rear cutlass bearings are being replaced at the moment as they were worn out with the port engine having a vibration in gear but I don't know what grooves they have.
This pic was before I cleaned the with scotchbrite
The Boat has deep sea seals fitted keeping the water out of the boat.
Most of the pitting is up at that end in the water that would be changed the least.
This pic was before I cleaned the with scotchbrite
The Boat has deep sea seals fitted keeping the water out of the boat.
Most of the pitting is up at that end in the water that would be changed the least.
Hood
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Does the deep sea seal have water injected?
bert
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I'm not 100% sure but there is some pipework going to them. It's not a very clear picture.
That pipework wouldn't be anything else apart from water injection could it?
If so then the water in the shaft tubes should be changed with the engines running meaning the corrosion has happened within the last 2 year's.
That pipework wouldn't be anything else apart from water injection could it?
If so then the water in the shaft tubes should be changed with the engines running meaning the corrosion has happened within the last 2 year's.
Newlands
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It might be worth heading to the YBW forum to ask their advice - there are one or two marine engineers there who are really clued up on corrosion in marine environments as well as having experience of your deep sea seal. I know that some seals can run dry when a boat is relaunched after haul-out unless the pipe work is bled properly - Volvo seals particularly, which are a bit similar to deepsea seals.
