I’d a rather alarmed client talk to me about my most recently completed project, a cabin table for the yacht he’s building.
Knowing atmospheric moisture would be a significant factor, I built the table from bookmatched quarter sawn boards, paying particular attention to the stability of the stock during the construction. The boards chosen had been air dried for 2 years, kiln dried to 9% RH and had been in my shop over a year before I started working on them. There was trace evidence of case hardening when I first started planing the boards, but as I equalised the stock removal from each face, any distortion also equalised.
The design called for “fiddles” to be fitted around the table edges. Concerned about the effects of extreme humidity, I made the table blank deliberately over-sized, using the offcuts from cutting to final size to form the fidddles in an attempt to get as close to a perfect grain match as possible. This resulted in cross grain mouldings employed when going across the grain.
At the client’s request, the finish was kept to 3 wipe on coats of Danish Oil… He was willing to trade an easily damaged finish for one that was quickly and easily repaired. I tried to pay close attention to ensuring that both the table top and underside received equal treatment, deliverately flooding the end grain to ensure thorough penetration by the finish…
I kept the finished table in the shop for a week to check for unequal distortion before delivery.
Subsequent to installation, the table top has warped across the grain, aparently curling upwards by about as much as half an inch over its 3ft width. When asked, he admitted he’d applied more finish after installation, but apparently only to the table top.
Am I right in thinking that the distortion will need to be allowed to continue until the table reaches equilibrium with the RH of its new location before trying to equalise wood movement by applying more oil to the underside? I tried to explain that this was the most likely cause, but he didn’t seem at all convinced despite a history of prety extreme wood movement whenever solid timber has been employed in fitting out the living spaces in the yacht.
Mike Wallace
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Replies
Did he say what kind of finish he applied to the top? If the wood was indeed quartersawn, it's hard to imagine what else could have caused that much movement in such a short time.
If it were me, I would try to remove at least the top from the "marine" environment and back into a reasonably dry location for a couple of weeks or so to let it flatten out, and then apply a more robust finish all around.
I'm not sure what you mean by "fiddles." Are you referring to what are otherwise known as "breadboard" ends?
-Steve
The finish used was just Danish Oil... from the same can that I'd originally used. Fiddles are raised edges running around the outer edges of the table to stop stuff sliding off as the boat pitches and rolls...Mike Wallace
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Species?
Can't tell for sure without knowing your location, but 9% hardwood would fail around here simply because it's too dry for the application.
EMC in a moored yacht interior depends on the size of the yacht, time of year and whether or not it is heated, but locally will almost always be above 12% for hardwoods, and often above 18% for the denser species, rarely drying to below 10% in our dry season.
You can canvas your local yards and shops to find out what your typical EMC swings look like and how that affects their stock decisions. You may have to redo this table using wood acclimated to the application.
Edited 12/17/2007 12:23 am by BobSmalser
Bob... 9% KD is what all the yards supply around here. Being stacked in my shop for as long as it was, I doubt it would have absorbed very much more; my shop has central heating.
The boat's still as far removed from a marine environment as it gets right now, about 20 miles inland inside an old barn; although it's subject to atmospheric temperatures, the m/c inside the boat is ambient RH + condensation... This time of year everything inside the boat feels slightly damp to touch apparently. The boat's a 50 footer with cabin space for 8 berths.Mike Wallace
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This time of year in a barn in the Mearns area or Aberdeen, which is where I believe you are, you'll get average RH values of about 70% to 80%, probably nearer 80%. That equates to wood MC eventually reaching approximately 15% to 16%.
A bit of a slather of danish oil won't stop the wood absorbing moisture--- the stuff is after all nowt but thin varnish. The likely moisture gain of your wood from 9% MC to about 16% MC could be a significant cause of your problems. Slainte.Richard Jones Furniture
I fully expected the board to move some with the change in MC; even with central heating I canna hermetically seal the workshop, and over the lifetime of the project it's bound to have seen changes; that's what's throwing me with the cupping... up until the finish was applied and the table handed over, all the movement was linear...
I'm hanging onto the hope that as equilibrium is reached, the cupping will relax some... Delusional..?? Mike Wallace
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"I'm hanging onto the hope that as equilibrium is reached, the cupping will relax some... Delusional..??"
It's hard to say Mike. It might straighten out, but if it does that would suggest there is more heat and/or moisture one side than the other, and that caused the concavity in the first place due to uneven moisture absorption. That's possible as I've seen even a light bulb on on one side of a panel cause problems, daft as that may seem.
I just don't know all the circumstances and the conditions that the piece is subjected to, but if there is something like uneven heat that could do it. I've seen a laptop computer that was left on standby much of the time destroy a solid wood table top.
Give it a couple or three weeks and see how it alters and then go from there. Slainte.Richard Jones Furniture
Give it a couple or three weeks and see how it alters and then go from there
Yupp... that's about what I'd figured, though Bob's talk of mould has caused a few more hairs to bypass gray and go straight to white... Thanks Bob..!! ;P~
Wait n see but prep for the worst...
I did get a dig in with the client... asked when he'd made damn near every other fixture n fitting in the boat out of either aluminium, stainless steel or beech block, why the hell didn't he do that with this table too...?? He just laughed...Mike Wallace
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I'm with Bob on the suitability, or not, of sycamore (Acer pseudoplatanus) for use in a boat. Like other maples, eg, Acer saccharum or rubrum, which are very similar in their characteristics, none of these timbers are particularly durable in permanently semi-damp environments, especially if there is little or no air movement. Slainte.Richard Jones Furniture
Like Bob, I would like to know the species. Also, where there any attachments to the bottom of the table? What were they and how were they attached?
The shop I was involved with made lots of wood items for some custom yacht builders. We used primarily teak and mahogany but other woods were used occasionally. In most cases, we would acclimate the wood and the built item at the yacht builder's yard which was at a higher humidity than our shop.
Species is Scots Sycamore; I believe it behaves similarly to your Maple...
Construction by agreed design was as spartan as it gets... 3 edge jointed fairly wide boards... Only hardware involved is where the table top joins with the supporting column; column is tubular aluminium with a square load spreading pad welded to the top... I can't give sizes or a better description of the column because it was installed in the boat long before I'd started the table. All the fixing holes in the pad have been countersunk and slotted to allow generous wood movement. That's all the detail I have I'm afraid.Mike Wallace
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"The likely moisture gain of your wood from 9% MC to about 16% MC could be a significant cause of your problems. "Ditto. Sounds like here in the rain forest. High ambient humidity in a confined space atop a bilge that's probably still damp with no heat.The usual solution in marine joinery is to use a stable, durable wood like VG Honduras Mahogany, but I'm afraid even there you would have problems.A 30"-wide table top of VG Sycamore will grow a full 7/16" across the grain as it swells from 9% to 16% MC, but even VG Hondo would have swelled 3/16", and would benefit from acclimatizing to the application. If the wood you are using is what we call Sycamore Maple (Acer pseudoplatanus) it's stability is a bit worse than our Sycamore. Another concern is using a non-durable wood. I don't have any experience with Sycamore, but in maple even the heartwood can easily grow mold in marine applications. All the finish does is slow things down a bit...epoxy saturation being more effective than a couple coats of oil. If I were gonna try a light-colored, non-durable wood in a yacht interior, I'd saturate it with CPES (a thin epoxy sealer) and apply 6 coats or so of spar varnish top and bottom. The CPES will keep the MC lower and will help prevent mold, and provide a more stable base for the varnish (and resulting longer life) than any other primer. In marine work such pieces are often installed, removed for finishing, and reinstalled using bedding compound at all points of hardware contact to inhibit the moisture intrusion that causes rot and shortens the life of finishes.
Edited 12/17/2007 10:16 pm by BobSmalser
Thanks for the heads up re mould Bob... movement I expected, but spalting never occurred to me... I'm not too proud to pin the rookie badge back on again... ;)
I guess I'd best make provision for a remake using a marine tolerent species huh..?
Pity... the table has some awesome fiddleback figure to it.. Ye live n learn eh...??Mike Wallace
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I understand.
I just made a set of French doors for the retirement home I'm building, and when I add up all the costs of tempered glass, door hardware, and the premium VG wood I could have sold, I could have bought Andersen Door and Window Company's best unit. ;)
Mike,
The 3 foot wide table only has three boards in it, and they're all vertical grain quarter sawn? It isn't impossible I suppose, but I'm getting skeptical. Are you sure there weren't some, or even a fair amount, of the rings running off of vertical towards one edge or the other of the boards?
Even perfectly vertical grain quarter boards a foot wide might develop a little bit of a cup. If the grain was less than perfect then cupping was probably inevitable. With no structure under the table, like aprons or stretchers, to resist the cupping the table was bound to distort.
John W.
Edited 12/19/2007 5:34 pm ET by JohnWW
The slightly unequal finish on the two faces of the table isn't likely to be the primary cause of the problem. Oil isn't an especially good moisture sealant and a coat or two more or less on one face wouldn't make much of a difference.
Quarter sawn shows less cupping than flat sawn wood, but anything less than perfectly quarter sawn, where the grain is exactly perpendicular to the board face, or wood from close to the heart of the tree will still cup some, especially if it went from a very dry shop to a damp marine environment. It would only take the slight cupping of one board in the middle of the table to create a kink that would be a half inch deep across a three foot width.
Some woods are much more stable than others as they gain or lose moisture, mahogany is a favorite for boat work because it is especially stable. You didn't say what the wood was used for the table top, it might be contributing to the problem.
You can hope that equalizing the finish will correct the problem, do it right away, there is nothing gained by waiting, but it is unlikely to make the table flatten out. The wood has taken on a new shape now that it has gained moisture and it will want to stay that way until it loses the moisture, which probably won't happen on a boat.
If you bring the table back into your shop it will most likely flatten out again in a few days. Short of encapsulating the top in a shell of epoxy there is no finish that will stop moisture gain and loss and make the top absolutely stable.
John White
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