Monday, 15 February 2010

Which wood would be good?

I'm very lucky in that Chertsey has a lot left about it that's original, and more that, while not original, is pretty old and traceable to its working history. Even if this wasn't the case, of course I'd want to restore it sympathetically and make it look right; the fact that it is makes me feel even more keenly the responsibility to make sure, as far as practically possible, that it not only looks right, but is right.

So, the gunnels will be wood. At present it doesn't have any, but the hull sides are dead straight and there's no structural need to put steel ones on, so from the start I was determined that they would be made of wood.

The question is, what sort of wood. Originally they would have been oak. Everyone seems to want to warn me off using oak though. Mainly this seems to be on the grounds that the tannin in oak reacts with the steel mounting (what is the proper name for that?) and the bolts. What I haven't established yet is whether this reaction merely causes discolouration (not really a problem as the whole lot will be tarred or otherwise blacked) or whether it actually contributes to the corrosion of the steel. Or maybe it makes some compound that attacks the wood. Even if the latter, I'm wondering can they not be isolated from each other? The wooden gunnel will in any case be bedded into some suitable waterproof compound, and it must be possible to sheathe the bolts (or use stainless ones, except Jim already has a hundred otherwise suitable bolts left over from a job where the building inspector changed his mind, as they so often do).
Bath with its kerouing gunnels, fitted in 1982

Many people seemed to have successfully used tropical hardwoods - kerouing is one that frequently pops up in conversation, and apparently goes by other names according to where it's grown. I know this is what Dave and Izzy used on Bath and their gunnels still look fine after more than twenty five years, but... Firstly... tropical hardwood... no matter how sustainably grown (and we haven't yet tracked down a source of sustainable kerouing) still has to travel a long way; and I would always have doubts about claims to sustainablity anyway; it's such a complex area. And secondly, my heart tells me it should be oak, for authenticity, unless there's a really good reason for it not to be.

Is it cost perhaps? Oak certainly isn't cheap. Jim was researching this at the weekend and good quality English or European oak is going to work out at around £600 for the quantity we'd need. Now, to be honest, that doesn't sound too bad to me, in the scheme of things. Maybe the tropical stuff would work out cheaper? But I don't think I want to know. Is it durability? How long would oak, properly maintained, last?

So, if you know something about oak that I don't, a really good reason for avoiding it, please do let me know....

8 comments:

  1. According to the likes of George Wain in the working days the cappings didn't last that long, so the the effects of Tanic acid didn't really come into it. They were looking for durabilty and Oak was widely available. I think for a restored boat the cappings could last anything like 10 to 15 years and tannin would be a problem.
    On Dove we went for Iroko, cheaper, just as durable and no Tannin.
    Andrew

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  2. Paul NB Capricorn15 February 2010 at 20:27

    Brinklow Boats use iroko partly I think because they can't get long enough lengths of oak - you want as few joints as possible.

    Keruing is durable but horrible wood - leaks loads of resin for the first couple of years and splinters turn septic.

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  3. Oh, I forgot, if you want to see the effect that Oak has on Iron have look at my Picasaweb (restoration of Dove "The Front")You can see where the Cants have been.

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  4. Thanks, that's really useful info.

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  5. Andrew - is that specifically oak rather than water corrosion - does it have a particular quality? And it seems it didn't stop you using oak again for the cants?

    Is it possible to isolate the wood from the metal, with the gunnels?

    Right, definitely not keruing then at any rate. Sounds horrible!

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  6. Sarah, It's the Tannin and damp that forms Tannic Acid and that rots the iron. The reason I went for Oak cants is, Iroko does't bend easy, it just splits, Oak is bit more plyable but you have to watch the grain.
    I also used "gutter seal" between the iron and the wood.
    There is another wood called Idigbo, very simular to Oak and cheaper but also contains Tannin.

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  7. Sarah personally I don't know what all the fuss is about for while I agree that Oak contains Tannin acid and as we know acid attacks metal all I would say is to look at the present condition of your 3" x 2.5" Gunwale angles.(I am going by your photo's) Chertsey is now nearly 75 years old and has probably had Oak gunwales for most of it's life so I think you can safely say that if you fit Oak gunwales properly then it should last at least another 75 years perhaps. When I replaced some of the gunwales on Bingley I used oak and laid it on top of a layer of tarred felt. The felt was the hairy sandy coloured type you used to buy from plumbers/builders merchants which was 3" wide and on a roll. This was stuck to the gunwale angle with a good coat of hot pitch and tar mix(osmukanta) then the top surface of the felt coated in the same manner then the pre drilled oak gunwale quickly positioned on top and the tarred felt then sandwiched between both by fitting and tightening all the bolts. I totally agree with any comments on Keruing. I built Bingley's back cabin using this timber and all I can say is I was continually sharpening saws and chisels and every single splinter (and it makes loads) went septic. It oozed a resinous sap and the tongue and grooves all opened up as it shrank!

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  8. Thanks Blossom, that's the lines I was thinking along, of using a barrier. I'm not sure how long Chertsey has been totally gunnel-less but I'm guessing that's probably contributed to the good condition that the angle is in.

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