keel stud stainless grade?

mogmog2

mogmog2

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I'm contemplating replacing my Centaur keel studs for longer ones, to accommodate a thicker laminate repair/reinforcement job and much thicker plate washers.

I asked on the pretty reliable mig welding forum for suggestions of sources of 1"UNC SS studs, and also received a suggestion that 2207 or 2507 would be a more appropriate grade than 316.

Having looked into this, that would indeed appear to be the case, but I've seen reference on here to duplex being 'eye-wateringly expensive'.

My question is, whilst it may be better, is it overkill? On his site, Vyv states that SS keel stud failure is rare in GRP boats and mine have no evidence of crevice corrosion after 45 years.

A secondary question is that the original studs are engineer's studs with an unthreaded section that passes through the hull and I've asked for quotes on this (for 316 but I presumably need to specify 316L?).
Obviously, off the shelf, all threaded stud would be less expensive but I have slight concerns that any movement would saw at the GRP and also the threads would provide a path for water ingress.
Thoughts?
Thanks.
 
mogmog2 said:
A secondary question is that the original studs are engineer's studs with an unthreaded section that passes through the hull and I've asked for quotes on this (for 316 but I presumably need to specify 316L?).
Obviously, off the shelf, all threaded stud would be less expensive but I have slight concerns that any movement would saw at the GRP and also the threads would provide a path for water ingress.
Thoughts? Thanks.
Click to expand...

I've never heard of stud failure in a Centaur and it's highly unlikely they can move enough in the hole to cause a problem. The usual point of failure is in the stub around the perimeter of the keel caused by flexing, not around the studs. The only failed (as opposed to wobbly) keel I've personally seen had the complete bottom of the stub still attached , exactly the same shape as the keel top and larger plates under the stud nuts wouldn't have helped.
 
There are two basic alternatives for keel bolts on older boats. Either larger diameter 316 stainless steel or smaller section, stronger (8.8) carbon steel ones. Beneteau and Moody, to name but two, have used the latter but probably the vast majority, especially nowadays, use large 316. Going to duplex, as in your suggestion, would certainly be stronger and not horrendously expensive, but definitely would be overkill. Provided the stainless ones are well sealed and torqued up to the correct figure they will last indefinitely. I see no real reason for selecting 316L, which is an alloy chiefly intended for welding. In your circumstances I would buy lengths of threaded 316 bar and cut to length.
 
Graham376 said:
I've never heard of stud failure in a Centaur and it's highly unlikely they can move enough in the hole to cause a problem. The usual point of failure is in the stub around the perimeter of the keel caused by flexing, not around the studs. The only failed (as opposed to wobbly) keel I've personally seen had the complete bottom of the stub still attached , exactly the same shape as the keel top and larger plates under the stud nuts wouldn't have helped.
Click to expand...

Agree that stud failure is not a problem, really asking was it worth spending extra on better studs & I think not as confirmed now by Vyv (thanks).

Agree that flexing of the stub/hull is one of the key issues. It has been recorded that overtightening of the nuts has crushed the laminate - bigger plate washers would help there a bit. Furthermore, my plate washers have deformed which suggests to me that they could be beefier.

You can see why they fail round the edges - on the second pic, between the outer gelcoat and inner green layer, at the interface between the white there are dark moisture lines. Assuming this is at least partially representative of the whole stub, there will be extents that are now not bonded together, as well as the compromised pigmented layer not performing.

I would like to strategically make the laminate thicker - that would help reduce flexing, but it is not possible to add much as the original studs are only long enough for the original thickness - there is no spare thread on most of them. This also makes adding reinforcing webs suboptimal IMO as you can't lay too much down on the stub floor. So if I'm replacing the studs to allow more laminate, it makes sense to allow for larger and thicker plate washers as well.

FWIW, this is what I'm dealing with ATM: longitudinal and radial cracks, plus a layer of pigmented laminate which is full of osmotic cells. I'm going to take it back to the (apparently ATM) sound yellow layer & rebuild. Hours of entertainment ahead!
 

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Looks like you're having fun:( I was lucky with our Centaur, just needed the keels rebedding but, had to reinforce the Berwick's as stress cracks from having been bounced through 180 degrees by incoming tide on a drying mooring. I would go along with Viv's suggestion and just use threaded bar.
 
mogmog2 said:
I'm contemplating replacing my Centaur keel studs for longer ones, to accommodate a thicker laminate repair/reinforcement job and much thicker plate washers.

I asked on the pretty reliable mig welding forum for suggestions of sources of 1"UNC SS studs, and also received a suggestion that 2207 or 2507 would be a more appropriate grade than 316.
Click to expand...
I'd be surprised if your Westerly has UNC studs. Unified is much more common with American influence. Much more likely to be Whitworth on a '70s UK boat. At 1" dia. both threads are 8tpi.
I can't see any benefit in a duplex steel for your application.
 
earlybird said:
I'd be surprised if your Westerly has UNC studs. Unified is much more common with American influence. Much more likely to be Whitworth on a '70s UK boat. At 1" dia. both threads are 8tpi.
I can't see any benefit in a duplex steel for your application.
Click to expand...
Thanks.
I don't know much about threads etc, especially old ones. I concluded Whitworth socket sizes were nearest the measurements of the largest nuts, but the measurements I took didn't seem to properly match any of the systems.
I had seen UNC mentioned somewhere and went with that. I've since seen that at some level they are interchangeable, but that may not be desirable in this application?
 
The difference between UNC and Whitworth is the the former has an included angle of 60 degrees and the latter of 55 degrees, plus one has a radiussed peak and the other flat, but I cannot remember which at the moment. Google is your friend.
I agree with Earlybird about likelihood of Whitworth but suggest taking one of your studs to an old school fastener supplier who has both types and compare the two. I wouldn't bother to use duplex ( or stainless at all really) but would want to get the thread right. UNC and Whitworth are only interchangeable in the same way as 13mm and 1/2" drill bits are, ie not in proper jobs.
 
Rum Run, After Earlybirds response, I had looked up the technical differences but to the uninitiated, the practical implications aren't obvious other than "it fits but it's not quite right" - I like your 13mm 1/2" analogy.
PS Google is no-one's friend. ;)
 
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In puritanical terms, mixing UNC and Whitworth might result in a small loss of ultimate thread strength and perhaps a slightly greater tendency to slacken due to vibration. I wouldn't think that such issues will really degrade Westerly keel-bolts.
In practice, tightening up the nuts will cause micro thread deformation to a closer fit.
 
Graham376 said:
I wouldn't risk unmatched threads, local machine shop may turn some studs for you at reasonable cost - for cash.
Click to expand...

I asked on the mig welding forum as there are people of that persuasion there, but got no takers. Instead I had the question of Duplex SS raised. Lol. That's the internet for you.
 
Marine grade 316 stainless steel is similar to Grade 304 with one notable exception. As well as chromium and nickel, Marine grade 316 also contains 2% to 3% of molybdenum, a hard, silver white metallic element used to toughen steels and increase corrosion-resistance in nickel alloys.
 
I’d use a bronze. Will last forever. Nickel aluminium bronze maybe as it is pretty strong. Not expensive either.
 
In the tradition of drifting the thread for personal benefit; if the current studs are ok in mild steel but the nuts are rusty, would there be any benefit or problems in having new nuts in bronze? Obviously not cheap but maybe less rust chipping and painting.
 
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