Corrosion and damage to propshaft - how serious

[djs]

Hi
We are hauled out and have drawn the shaft as part of the trouble shooting on a vibration we have experienced.

The cause of the vibration has been identified and we can solve it easily. Before we worked out the cause we arranged for a marine engineer in to look at the shaft as one possible cause was a bent shaft and he was going to spin it up and check it in the workshop.

However, on coming this morning to look at it he expressed concern over the crevice corrosion we have in a couple of spots. His view is that the shaft should be replaced. Although he did say get a second opinion as it’s a big decision.

The groove visible in one of the shots was caused by the loose component that was causing the vibration. As mentioned we are confident that this will,be resolved so won’t get any worse.

On the side of leaving it be...
The crevice corrosion isn’t any worse (visibly) now than when we last had the shaft out 18 months ago.
It’s a sailboat, and when we motor we aren’t running at high revs or under high load.
We have a limited budget and if we don’t have to replace we will be saving ourselves a grand or so.

On the side of replacing it...
We won’t be out of the water again for a couple of years.

I’ll attach 4 photos and I’d be interested in opinions and experience of this kind of damage. It’s a 1.5 inch shaft, 13 foot long on a 17tonne Hans Christian. We are in the US.

Cheers

Dave

 

[pcatterall]

I guess that you are satisfied that you have eliminated the possibility of a bent shaft ?
So it is now just the corrosion you are concerned with?
What kind of seal is fitted?
If its a conventional stuffing box then a clean of the shaft ensuring no sharp or high spots may do the trick.
Have you priced a replacement shaft? when I did mine the cost was less than I thought and you have got it out so you can be sure of getting an exact replica and you have time ( I guess)?

 

[Milgrip]

Not familiar with your boat but have you drawn the shaft right out? If that corrosion or groove is anywhere close to or in contact with a gland or oil seal then it should be replaced otherwise if it has not got worse since you last examined it, check it again next time it's out of the water. But if in any doubt, do get a second opinion

 

[jneale]

Not familiar with your boat but have you drawn the shaft right out? If that corrosion or groove is anywhere close to or in contact with a gland or oil seal then it should be replaced otherwise if it has not got worse since you last examined it, check it again next time it's out of the water. But if in any doubt, do get a second opinion

I would replace it. The groove looks pretty deep and the pitting was caused by electrolysis.

 

[vyv_cox]

You will see several similar examples on my website under Metallurgy. Removal of the original cause of the crevice will slow the corrosion but once started the pits created will themselves be crevices, so that some continuing corrosion may be inevitable. However, if you have monitored the progress for 18 months without further degradation it seems likely that it will not advance significantly in the next two years.

The problem with some pitting, the circumferential example in your third photo being one, is that it can act as a stress raiser and induce fatigue cracking. In this case the severity depends entirely on its location. If between a P-bracket and heavy prop it might be problematic but in a relatively unstressed part of the shaft it would not be of concern.

 

[Fr J Hackett]

A grand for a prop shaft? What are the dimensions? It should be somewhere between £150 and £200 tops.

 

[pcatterall]

A grand for a prop shaft? What are the dimensions? It should be somewhere between £150 and £200 tops.

+ 1 ( I missed the OPs 'grand cost!) Our experience some years ago now was more like £200 for a 1 1/2" diameter 7 foot shaft.

 

[Spyro]

A grand for a prop shaft? What are the dimensions? It should be somewhere between £150 and £200 tops.

I'd say 1.5 inch 13ft long. Probably a good bit more than £200

 

[Deleted member 36384]

Perhaps his price, the "grand", is in US Dollars, about £800, as the boat is USA based.

 

[pvb]

I'd say 1.5 inch 13ft long. Probably a good bit more than £200

T Norris would want around £700 plus delivery, based on their 2018 price list.

 

[djs]

Hi folks, thanks for taking the time to reply.
Yes we have three quotes for AQ22 and they are all in the same ballpark of 1100 US dollars.
Vivvid, thanks for the comments re location being part of the consideration. The worst crevice corrosion is towards the prop end of the longest unsupported run between stern gland, which is a PSS, and the cutless bearing. Distance of approx 6 feet. The groove in the third photo is at the sterngland rotor position. It was a loose rotor that we think caused the damage.

The damage around the stern gland area is a concern. If the o rings in the rotor do end up near the scoring it would be a problem.

We have put the same question out to a number of experienced cruising friends and their opinions are split. Needless to say the guys who had their shaft crack on them 100 miles north of Bermuda say get a new one!

On a general note about costs in the USA. Whilst it seems that you can get a lot of boat for your money here, marinas, marine services and kit is expensive. Obviously the exchange rate at the mo makes things seem worse but even so.

Cheers
Dave

Www.graceoflongstone.com - just another sailing blog, with hardly anything in it about sailing.

 

[john_morris_uk]

I’ve replaced three prop shafts. I’ve always gone to the steel stockists and bought ground s/s bar of the correct diameter and taken it to a friendly machine shop. It’s not a fair comparison.because the last one was machined by the apprentice under supervision of a friend in his lunch break. It cost me two packets of chocolate hob nobs. Normally the machining is a few pounds because it’s such a easy Job.

Go to a jobbing machine shop with the s/s bar in your hand and the old one as the example. Pay cash. It won’t cost you $1100 dollars methinks.

Ps I used three different machine shops for each of the above shafts. There are plenty around in the backstreets if you start looking.

 

[pvb]

Go to a jobbing machine shop with the s/s bar in your hand and the old one as the example.

Wandering around with two 4 metre lumps of stainless steel isn't perhaps as easy as you make out. To start with, they'd weigh over 70kg.

 

[Fr J Hackett]

Wandering around with two 4 metre lumps of stainless steel isn't perhaps as easy as you make out. To start with, they'd weigh over 70kg.

But it can be done but perhaps not in the States, I did what John did which is why I said a grand was extortionate and I am guilty of not fully reading the post didn't realise it was in the US.

 

[rogerthebodger]

But it can be done but perhaps not in the States, I did what John did which is why I said a grand was extortionate and I am guilty of not fully reading the post didn't realise it was in the US.

Carring around a 4 meter length of anything in the states would be easy, I don't mean walking but using one of their big pickup trucks are not so common in the UK but is is other places in the world.