Centre board pivot.

C

Casey

Well-Known Member
Joined
16 Jan 2003
Messages
292
Location
New Forest, Hampshire, England
Visit site
I have just withdrawn the bolt which acts as a pivot on my centre board. Thanks to it being put in properly many years ago, before I had the boat, it came out fairly easily. Not being a metal worker (nor anything much else come to that !) I don't know what to replace it with.

The bolt is ten inches long by about 5/8" in diameter and corroded in the middle where the centreboard fits. The question is what metal do I have the replacement bolt made from?



<hr width=100% size=1>
 
Stainless steel, 316 grade, is your best bet. It may take some tracking down in that size, and you should look for companies supplying nuts and bolts to industry. Because all fastenings for the food industry need to be stainless steel, it has become much more available than it was in the past.
Peter.

<hr width=100% size=1>
 
Sorry don't agree with Peter Duck. Avoid using stainless steel below the waterline as its subject to crevice corrosion. It needs oxygen to retain its stainless qualities. I guess the centreboard plate is mild steel, use the same for the bolt. Yes it will suffer from some corrosion but it won't be fighting the potential of the centreboard and since it comes out easily, look to check it every so often. It won't be very dear and should be easy to get hold of.

<hr width=100% size=1>
 
Thanks for your advice people and I must agree with you Tillergirl. The pivot goes through at least nine inches of cast iron and about half an inch of mild steel. To introduce stainless steel would tend to encourage a lot of electrolytic action. Perhaps I should have mentioned this in my first post.

Would wrought iron be better than a mild steel bolt?

<hr width=100% size=1>
 
Please do not think that I didn't know about crevice corrosion when I advocated stainless steel. In the early days of my ownership of my boat I pulled a stainless bolt [put there by a previous owner] out and sure enough, there was unmistakeable crevice corrosion. Over the space of 5 years, it had developed pits about 0.010" deep. I replaced it with a copper bolt, as the rest of the hull is copper-fastened. Now a mild steel pin will expand about 8 times its volume as it corrodes, and it will corrode quickly, believe me! This means that the centreboard may jam on its pivot bolt if it is not exercised frequently. Also, you will need to replace the pin at least annually. After you have tried to extract the first pin, I doubt that you will use mild steel a second time. I would expect that drilling it out would be the only successful method. It seems to me that the loss of a small amount of stainless bolt [which won't expand as it corrodes, and will be removable] is a lesser evil than the espoused alternatives. You might also investigate the use of Delrin rod. This is a surprisingly hard plastic which is good at abrasion resistance, and, of course, doesn't corrode at all.
Peter.

<hr width=100% size=1>
 
Well I do think that this comes down to weighing the evidence you like, ignoring that you don't and making an informed choice confident that you are going to monitor it. Nigel Warren's book (sorry I attributed it to Nigel Calder) quotes mild steel as corroding in its unprotected state at a rate of 5 mils per year. Yes if you left the boat for quite a while it would be difficult to pivot. but with a boat in fairly regular use, it isn't going to be a problem. If the boat dries out, the rate of corrosion will increase (1mm per year is quoted). The crucial issue for me would be that mild steel gives a warning of failure: the pivot will be difficult to operate after leaving the boat for a while and then once coaxed back into life will have excessive play. Stainless need oxygen to retain its qualities and crucially for me gives no warming of imniment failure. It's also more expensive. I think a centreboard pivot has got to be regarded as a serviceable item to be regularly checked and replaced from time to time. I replaced mine every five years and there was no significant corrosion and that was on a drying mooring. Why did I replace it? Because having drawn it, the cost of a new one was not significant in the scheme of things and peace of mind was worth it.

I think I am also influenced by the fact that the bolt was relatively easily withdrawn now which suggests to me that there is a degree of harmony in the metals that are being used now. We know that stainless has a different potential to mild steel and therefore I would argue that we don't know what reaction there is likely to be. There may be none of course. On TG, I had a stainless rod which went through the galvanised rudder and transom fittings as the pivot. Two winters ago (in my usual mud berth) the stainless and galvanised fitting cold welded themselves. Now I have a mild steel rod cold galvanised which is corroding but has not cold welded itself. It is my intention to replace it with a galvanised rod and treat this as a serviceable item to be checked annually. Again the cost is not great - cheaper for example on adding any new electronic gismo which I don't need anyway.

And finally wrought iron. Nigel Warren says that "there is no direct technical evidence from controlled experiments that wrought iron is significantly better than mild steel in seawater"

So my view for what its worth is I would go for mild steel for all the above reasons. I agree there are different views of course and I respect them.

On Tiller Girl I had a stainless r

<hr width=100% size=1>
 
You must log in or register to reply here.

Share this page

Top