Bolts for timber

[rednecker]

Hi all any advice appreciated, need to know what type of bolts too use as I
Have to replace 51 yr old bolts through chain plates and galvanised deck shelf brackets
and large gal angles at the end of bilge stringers/ transom,
Any of the bolts that were originaly above the water line seem to have been galvanised iron bolts and nuts with cast bronze heads, they looked sound from either side but on removal were almost coroded thru where they have passed through the mahogany planking, any that were below the waterline were bronze, but these have seemed to react with the mahogany quite badly too the bolts are fine but the timber is in bad shape in that arangment

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[ccscott49]

Do not use bronze bolts, with galvanised chain plates. Use hot dipped galvanised bolts and nuts, above and below the waterline. Might be an idea to have the chain plates and any other fittings, shot blasted and re-galvanised aswell.

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[dickh]

Agree with csscott - only use Hot Dipped Galvanised Bolts - Don't be fobbed off with Zinc Plated Bolts - often described as galvanised. There was a company called The Bermondsey Galvanised Nut & Bolt Co many years ao that I used but don't know wether they are still around. - try a search on Google.
An alternative could be Sheradised Bolts - they are steel bolts which are heated in a zinc rich mixture and coated with zinc - and you can paint the straight away. I used them on a boat I built many years ago and they seemed to be OK then and supposedly better than Hot Dipped, but they would say that wouldn't they?


<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[Mirelle]

Sorry, disagree - in part

The late Peter Brown, designer and surveyor of Woodbridge, recommended bronze bolts in just this position and consequently Mirelle has galvanised chain plates secured by bronze bolts passing through teak planking oak frames and galvanised hanging knees.

The old galvanised bolts were a bit iffy when pulled at the age of 53, pulled a bronze one a couple of years ago at age 64 and it and the timber and the ironwork were all fine. So far so good.

NB Peter did not recommend this below the waterline; "iron bolts through iron floors and replace regularly" was his prescription there.

Agree about the shot blasting and regalvanising.

<hr width=100% size=1>Que scais-je?<P ID="edit"><FONT SIZE=-1>Edited by Mirelle on 20/02/2004 21:31 (server time).</FONT></P>

 

[ccscott49]

Re: Sorry, disagree - in part

Why bother with bronze bolts above the waterline and galvanised below? Why not just use galvanised all through? Just dont understand why you would bother.

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[Mirelle]

Re: Sorry, disagree - in part

Rust streaking on topsides.

<hr width=100% size=1>Que scais-je?

 

[ccscott49]

Re: Sorry, disagree - in part

I see! So the chainplates dont rust then? OK enough, use bronze if you want. I used stainless for mine, but the chainplates are stainless aswell.

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[roly_voya]

Ithink the problem you are having is caused by galvanic currents affecting the wood which can result in rot in timber and or corrosion in metals. This is well known in the case of steels in oak (galvanised or stainless) and I have seen a galvanised bolt which had worked fin attached to a galvanised chain for 80years disintigrate in 10 and rot a large hole in the stem peace when the chain was replaced with stainless wire. Personally I would always try for all bronze as it seams the most inert particularly when passing through highly acid woods such as Oak.

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[ccscott49]

Bronze+galvanised (zinc)+immersed in salt water+wood=disaster.

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[Mirelle]

Re: Sorry, disagree - in part

Just so, in practice.

The galvanising on a bolt is mechanically abraded as the bolt is driven; the galvansing on the chainplate is not, it is pulled up slowly against bedding compound.

A bolt will start weeping long before a chainplate.

If I win the Lottery I will replace the chain plates with bronze!

<hr width=100% size=1>Que scais-je?

 

[ccscott49]

Re: Sorry, disagree - in part

Bronze chainplates!! Now you`re talking!!! Bronze floors, bronze hanging knees!!! Perfect world!!

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[DMGibson]

Re: Sorry, disagree - in part

Unless your planking fastenings are iron, in which case they will all fall out. The bronze bits are more noble than the iron, so the fastenings will be leached away in preference to the bronze fittings.

I agree it is less of a problem above the waterline, but why take the risk?



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[DMGibson]

Agreed

The chemicals put out by the reaction between the bronze and the wood actually erode the wood, so you end up with lovely bronze bits with no boat attached.

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[ccscott49]

Re: Sorry, disagree - in part

I fully agree! My boat is luckily all copper and bronze fittings and fastenings, even the shafts, p brackets and props.in fact everthing below the waterline is copper or bronze! Great!

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[rednecker]

So Bronze is ok above water, Gal Is ok below, What about Stainless bolts with gal fittings above water line? seems strange the builder basicaly had it back to front?
No wonder when i posted on here several months back no one knew of them...
they must have gone broke fairly quick


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[Mirelle]

Basically, should be OK, but I would never use below w/l.

<hr width=100% size=1>Que scais-je?

 

[Dirk]

Im sure this has been discussed at length before but the golden rule is simply to use similar metals in all of the boat. Easier said than done I know but the fact is nobody has the perfect answer!

Ask the question a thousand times and you will get a thousand different answers!

I used Silicone bronze coach screws in a stainless stem band assumining the bronze was more noble and the stainless was more likely to corode. After one season some of the bronze hex heads had completely eroded!

Black art? I think so!

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