Can green timber expand?

[MikeG.]

I am involved in a conversation on the Tally Ho Youtube channel about the design of a scarf joint. One of the other contributors there has claimed that the design of scarfs in a marine environment has to allow for expansion of the timber.......but the timber in question is green (unseasoned). Does anyone know (yes, Richard, I mean you!! ) if any research has been done on this? Or whether there is empirical evidence of green timbers expanding beyond their original size?

 

[Suffolkboy]

Even green timber has been cut. If it was cut when the tree was dormant then made into something, it can take up water if the external environment is more humid/wet than the wood.

Similarly I bet there is very little green timber that is cut and then instantly made into something. There will always be a degree of drying, so, when you make something out of it, potentially it could swell.

Maybe.

 

[SammyQ]

Withdrawn, superceded by more informed comment.

Sam

 

[Droogs]

it has been known for green wood to expand in an explosive manner in extreme cold.

 

[SammyQ]

Water, 1.0ml at 4°C, 1.09ml at -2°C. Get enough cells full of water, and the cumulative effect will be much greater than 9%, hence fracturing.

Sam

 

[user22161]

I bet there is very little green timber that is cut and then instantly made into something.

I wondered about this. Watching a TV program in the last couple of days which in part showed an old fashioned craft workshop making various items including a couple of spindle back chairs, from green wood. The presenter commentated that the piece he was playing with on a lathe felt damp.

Surely, these products will be reduced to bones when they dry out?

 

[Trevanion]

Surely, these products will be reduced to bones when they dry out?

One of the things I was taught about Windsor chairs when I was in college is that the seats (which were often Elm) were put on the chair when it was fairly green so that the timber close around the tenons of the legs and spindles. The ends of the tenons were sometimes shaped in a bulbous nature and a spoon bit in a bit and brace was used in the mortices to create a cauldron shaped hole, so when the seat shrunk it effectively locked them in for life. That's also why so many seats on these chairs are warped slightly because of the drying out process. I think the rest of the components were shaped green by a bodger and then left to dry, the legs, back supports and arms are usually made from beech whilst the spindles are usually assorted fruitwoods, which woodworm absolutely loves to eat away.

I've taken apart quite a lot of these chairs now and it's always the legs that usually the hardest to get off because of the seat locking around the bulbous tenons, the rest of it is usually pretty easy since they were glued with hide glue.

 

[user22161]

Thanks Trevanion I must be a bit thick but I'm still having problems getting my head around dimensional effects of shrinkage on joints. What would happen to dovetails drying out as the pins and tails shrink? Would they stay tight or become loose?

 

[Sheffield Tony]

If you're turning wood by leg power on a pole lathe, you want it to be green. Much easier. It will shrink maybe 5-10%, which you can allow for, or use - by assembling dried stretchers etc into green legs - to allow glue-free assembly.

My not quite a set of green ash chairs [1 2 3 4] are all held together by drying shrinkage, no glue. The four of them are still in daily use at our dining table, and have not come apart yet

 

[Sgian Dubh]

I am involved in a conversation on the Tally Ho Youtube channel about the design of a scarf joint. One of the other contributors there has claimed that the design of scarfs in a marine environment has to allow for expansion of the timber.......but the timber in question is green (unseasoned). Does anyone know (yes, Richard, I mean you!! ) if any research has been done on this? Or whether there is empirical evidence of green timbers expanding beyond their original size?

I think you might be referring to the comment by Thomas Westgard, where he says, "The shape of cut Leo is using allows the joint to stay clamped tight without cracking, regardless whether it shrinks or expands as the water content changes." I think he has a point because the wood in use in that Tally Ho video is said to be "green", and during the construction of those scarf joints the wood appears to work as if it's wet enough to work nicely with a mix of hand and power tools , but it's not so wet that I could notice evidence of free water either gently exuding or even bursting out of cut cells as the work took place. I've worked sopping wet wood (rather reluctantly in the past) and experienced that phenomenon, and it's a bit of bother keeping steels tools from developing lots of rust, plus, really wet wood doesn't really seem to plane particularly well as I recall it, and he did get his planes working nicely on those scarf joints.

Green sets a bit of a moving target with regard to wood's moisture content. Basically, green means the tree has been felled and the boards derived from the sawlog haven't been seasoned. All felled living trees will have throughout them wood that is at least at fibre saturation point (FSP), and much of it a great deal wetter. All FSP means, of course, is that all the the cell walls in the wood are fully saturated. All of the cell lumen within that log, or board, or any other 'lump' of wood may, or may not have free water within them. It's normal for almost any example of wood to have within it a moisture gradient, either wetter at the core than at the shell, or the opposite, being drier at the core and wetter at the shell, and the same applies to all wood, even a freshly felled living tree, or a board sawn from that tree and subsequently seasoned. The exception to that being wood that is kept in unvarying conditions of dryness or wetness for a long enough period for the wood to reach equilibrium moisture content (EMC), e.g., pilings kept permanently in water, or some sort of controlled laboratory conditions or storage facilities where relative humidity and temperature are strictly controlled and unvaried.

So, going back to to what Thomas said in that thread, I read that he recognises the wood used to make those scarf joints is pretty wet, and I'd guess it's somewhere near or just under FSP (the lack of free water oozing from the wood as it's worked is my evidence, although I may have missed seeing that phenomenon). And those beam things are being placed somewhere up in the hull of the boat so therefore not likely to be permanently wet, and most likely will become somewhat drier, and get wetter as they are affected by the conditions they experience. So, it seems likely to me that there will be some expansion and contraction of those beams in service as they adapt to their environment. I imaging, being a boat, there will be times when everything gets pretty wet, and other times when it's pretty dry. In a sense, that's similar to other exterior artifacts, e.g., the oak table sitting on my patio. Right now, it's pretty much sopping because it seems to have rained almost incessantly for several weeks, but back in the summer it felt nice and dry with the sun beating down on it and little or no rain for long periods.

Incidentally, I could point you to Chapters 6 and 7 of a book I think is half decent on the subject of timber technology, and in particular, section 6.2, but I won't as that might be a bit naughty of me, ha, ha. Slainte.

 

[MikeG.]

Thanks Richard for that fulsome response.

My scepticism of the concept of green wood expanding starts with the assumption that the wood is already at its maximum size (and maximum "greenness") at the moment it is worked. I accept that if this is long after it was felled then this won't be the case, but assuming it was felled in the previous few weeks I guess my question is can it expand past the size it was as a newly felled log. I accept that it can shrink and then expand again, but wonder if it can expand before it shrinks. If the former, then the joint doesn't need to be designed for expansion. If the latter, then it does.

 

[AndyT]

Looking back, the shipping container of timber from Suriname arrived in video 55, posted on 7 September, so the timber in question must have had three months or more to lose some of its moisture and presumably shrink a bit.

 

[Sgian Dubh]

Thanks Richard for that fulsome response.

My scepticism of the concept of green wood expanding starts with the assumption that the wood is already at its maximum size (and maximum "greenness") at the moment it is worked.

Maximum "greenness", Mike, could suggest the condition of the wood is not only at FSP, but it also is as fully wet as possible, i.e., all the cell lumen and all the voids in the wood are also full of water. In natural growing conditions that's something unlikely to be the case. So, in my opinion, I'd assume green means all the cell walls are fully saturated, i.e., at FSP, and there is some liquid water unevenly distributed throughout the wood's volume that is, in part, bound to the cell walls, and within the lumen and other voids, e.g., the tube like vascular tissue within the angiosperms: the gymnosperm's vascular tissue is not tube like, being tracheidal, i.e., elongated cells that transfer liquid from one cell to the next via osmosis.

But, basically, if a piece of wood is definitely at FSP throughout its length, width, and thickness, and therefore probably with at least some free water within in the wood's voids, it's as big as it's going to get. Making the wood wetter still is not going to make it gain size.

Going back to my earlier post, I don't think that is what Thomas was suggesting. I didn't see anyone actually stating that adding yet more water to wood that was already either green, or at FSP and above would cause it to swell further. Maybe someone did say that, and I missed it, although I did rather quickly skim through the video (couldn't be arsed watching basically the same thing for 30 odd minutes), but I do think I carefully read the various comments in that bit of thread where I'd spotted you'd jumped in with your point about scarf joints, and using green wood in old houses and so on. Slainte.

 

[MikeG.]

....... I don't think that is what Thomas was suggesting. I didn't see anyone actually stating that adding yet more water to wood that was already either green, or at FSP and above would cause it to swell further. .......

He said that the joint need to be designed to allow for expansion. That's where my eyebrows were metaphorically raised. If it were seasoned wood going into an outside environment then obviously allowance for expansion is fundamental. But green wood?

As an aside, it's my belief that an under-squinted scarf wouldn't suffer the issues that he described, because the two pieces of timber would expand across their width fairly equally, and not along their length. There would have been no additional pressure on the joint even if the timbers expanded.

 

[MusicMan]

That has to be right, MikeG. Shrinkage even from wet is very low in the grain direction (of order 0.1%). And if the beams were cut at the same time, they should contract equally across the grain.

Keith

 

[Sgian Dubh]

....... I don't think that is what Thomas was suggesting. I didn't see anyone actually stating that adding yet more water to wood that was already either green, or at FSP and above would cause it to swell further. .......

He said that the joint need to be designed to allow for expansion. That's where my eyebrows were metaphorically raised. If it were seasoned wood going into an outside environment then obviously allowance for expansion is fundamental. But green wood?

As an aside, it's my belief that an under-squinted scarf wouldn't suffer the issues that he described, because the two pieces of timber would expand across their width fairly equally, and not along their length. There would have been no additional pressure on the joint even if the timbers expanded.

I now see the point you're trying to make - sorry for being a bit dense on that front. I agree, (as MusicMan states) that longitudinal movement factors are generally essentially inconsequential in wood , i.e., ~1.0% or about 3 mm of shortening in a 3m long board as it dries from FSP (~30% MC) to oven dry or 0% MC. And bear in mind that very little wood in service experiences such a wide range of moisture content change: residential furniture, for example, here in the UK tends to very between about 6% - 13% MC: I've no real idea what MC range a lump of wood inside the hull of a boat or ship might experience, but it's sure to be only a part of that FSP (~30%) to 0% MC range. The exception to this typical longitudinal shrinkage factor will sometimes be found in juvenile wood (near the pith) and reaction wood, where the longitudinal shrinkage factor can be up to 20 times greater, i.e., up to 2%, which usually results in distortion leading to rejection of the piece of wood because it warps severely.

In other words, I think you're correct to say that an "under-squinted" scarf would be just as good from that point of view. However, and here I have to reiterate that I skipped through the video fast and may have missed the point, but isn't the idea to drop one beam in place vertically, and then drop the other half vertically on top to bring the scarfing together, followed by fixing with vertically inserted bolts to lock the joint? If the method of assembly must be that way for some technical reason I'm not aware of because the two pieces can't be assembled by sliding them together horizontally, then the "over-squinted" ends of the scarf become a necessity. In fact, if that's the only method of assembly, then that form of scarf joint is probably more elaborate than it needs to be, but it does make a nice visual statement of considered manufacture, if you get my drift. Simpler to execute (and perhaps visually cruder) cuts parallel with the long grain and perpendicular to the faces and edges would suffice just as well, wouldn't they? Slainte.

 

[MikeG.]

Indeed, Richard, that was my supposition (and suggestion). However Leo said that Thomas Westgard had it right, so he too is thinking that longitudinal forces are what drives the design of that joint. I can't see the need for the step in the middle at all. That is only ever there when the joint is wedged (as the location of the wedges). The squints at the top and bottom of the joint might be useful for initial location purposes only, but serve no structural function, and no other function I can think of, as far as I can tell. This looks to me like a classic case of making a joint because they're always made that way, without fully understanding what is going on in that joint.....unless, of course, there are forces I am unaware of in which case I readily admit my ignorance in the subject of marine joinery.

 

[Sgian Dubh]

Indeed, Richard, that was my supposition (and suggestion). However Leo said that Thomas Westgard had it right, so he too is thinking that longitudinal forces are what drives the design of that joint.

I don't see it either.

Incidentally, I've no desire to nip over to that Tally Ho blog thing and weigh in with my comments. You're more than welcome to though, if you think anything I've said here adds validity to the points you've been trying to make, ha, ha. Slainte.

 

[MikeG.]

Thanks Richard, but I won't. I'll watch and learn, because there may be more to this than meets the eye simply because of the movements inherent in a wooden boat. I have no idea.

 

[AndyT]

Leo did demonstrate how the whole beam will need to be bent, in two planes, to conform to the shape needed.

I think this is different from most scarfed pieces in timber framed house construction, where the forces are a bit simpler.

I'd imagine that the forces change quite a lot in heavy seas, as the whole thing twists and heaves. Add that thought to the difficulty of getting both big heavy bits into position and there could be good reason for the details.

Apart from that, he is trying to rebuild the boat as it was and I think has mentioned elsewhere that he's not put in improvements that weren't in the original design.
(That's all having to be pragmatic of course, I assume it will have a modern engine, power supply etc.)