charred mahogany

[EASLOOP]

I have got what could become a serious problem with the steel floor bolts used in my boat. Starting from the outside of the hull (toward the bow) I have mahogany planks on oak frames several of which are overlaid with steel floors on the inside. Hot dipped galvanized bolts clamp the plank, frame and floors from the outside in. Unfortunately the mahogany in the vicinity of the bolts is turning black leading to replacement of two planks so far. I am now going to remove all of the bolts and replace them with ?????????

There seems to be some sort of reaction going on between the mahogany and the bolts. The oak frames are completely untouched. The floors look fine, but the planks? well.

Can someone advise me on what sort of bolts I need. I have read some articles about silicon bronze but I may be failing to understand the matter.

I look forward to your response.

 

[johnlilley]

Not an unusual problem but unfortunately, no easy answer.
The mahogany planking being strained by the ferrous bolts need not be of any serious concern unless of course there is localised electrolytic decay surrounding the bolt heads which, is usual problem on the SCOD's and other similar build. The majority of Buchanan designed vessels originally had steel floors fitted, in some cases these have been removed and replaced with cast bronze floors at incredible expense. Most likely what has happened is that the galvanised coating has eroded away on the outer head of the bolt exposing the underlying steel which causes both electrolytic damage and staining to the mahogany. The staining, as I just said is of no concern, the electrolytic damage possibly is as the bolt hole drilling becomes enlarged in the mahogany leading to leakage when the plank is strained such as when beating windward.
Oak also suffers the staining as any ferrous passes through it however, it is likely the galvanising is acting as a barrier through the oak at this time however, the tannin in oak reacts with the bare ferrous bolts causing both corrosion in the steel and softening in the oak.
It is generally unwise to use bronze bolts holding steel floors as, there will be a reaction with the copper content of the bolt against the steel, this will cause corrosion and again, subsequent electrolytic damage internally. Bronze bolts with bronze floors generally will not give any problem however, this obviously requires the replacement of the floors with bronze which is prohibitively expensive.

Probably, the only economic answer is to replace the floor bolts periodically with similar galvanised bolts as soon as corrosion is starting to develop on the external bolt head. It can sometimes be possible to insulate the bolts from the structure with sleeves etc. Perhaps tuphnol and the like but, this does gets complicated and slightly risky in that all holes have to be further enlarged.

Unfortunately this is a problem created at original build that the majority of owners have to live with and be aware of, the cost of new galvanised bolts is relatively low and, provided access is reasonable, the easiest thing is just to replace them regularly. The difficulty is that softening and electrolytic damage occurs beneath the steel floors unseen. Some builders had originally fitted the floors direct to the planking, other builders who were aware of the problems fitted the floors on tarred canvas to form a barrier.

The only other relatively permanent solution which, is not as strong as the original installation of steel floors but is far longer lived and therefore stronger after a couple of decades is the installation of substantial timber floors, these can be fastened in which ever way is possible including bronze screws, copper rivets, bronze bolts etc. And, once there, will most likely remain intact for the life of the vessel if correctly fitted.

Be interested to see alternative ideas, but I hope this suggestion is helpful.

Regards John Lilley

 

[sarabande]

some details on corrosion in fittings used in wood, here.

www.timber.org.au/resources/ManualNo6-EmbeddedCorrosion.pdf

There's a table showing the pH value of various woods: mahogany comes out at 3.0 to 3.5 - which is very surprising.

 

[EASLOOP]

charred

John,
Thank you for your comprensive assessment of my problem. i will now look at the wooden floor solution removing what remains of the old galvanized floors. two of the floors actually go under the hog so I will have to treat the exposed edge after hacksawing out the steel.

My boat is a great pleasure to me and I would hate to see her broken up. If I can get another 30 years out of her that will no doubt see me out - might even give pleasure to my children after me.
Thanks again

 

[EASLOOP]

charred

p.s. She is an East Anglian Sloop out of the Buchanan stable.