James – your reply, with your pictures here and in your earlier post
(
http://forums.wcha.org/showthread.php?t=6677 ) exemplify what I say below.
Robert – the expansion and contraction of wood can be complex – what follows is not the whole story, but if comprehensible, is a good part of the story.
Wood – all wood -- expands and contracts depending on its moisture content. This is commonly experienced in old houses with doors that swell and stick in the summer, returning to smooth operation in the dryer winter air of a heated house.
Canvas-covered wooden boats and canoes are generally built with seasoned lumber – dry wood. After construction, even when oiled and/or varnished and/or painted, the wood will get wet when the canoe is used, often very wet – and even if the canoe does not get tipped over and swamped, dripped into from paddles, splashed into when people get in and out, or rained into, they operate in a high humidity environment. Any and all of which leads to the planks on a canoe swelling, becoming wider, and squeezing each other as they are held in place by the ribs. (Wood expands much more across the grain than along the grain – so the ribs, which do not expand across the width of the canoe, hold the planks in place as they expand against each other.) Then, when the canoe is stored and dries out in the winter, the wood shrinks and the planks stop squeezing each other, returning to their as-built condition. However, if the planks were built tight, and if the swelling is great, and when the cycle repeats itself many times over the years, the squeezed wood will take a permanent compression set, and when it dries out, it will stay shrunk and small cracks will open between the planks. Often the cracks will close again when the wood is re-wet, but sometimes the compression set is great enough that small cracks will remain even after a re-wetting.
This is all of no moment in a canvas-covered canoe – it is the canvas, and not the wood, that keeps the water out. The strength and structure of the canoe are basically unaffected.
However, if cracks are filled while the boat is dry, one of two things will happen. Sometimes whatever was used to fill the cracks is simply squeezed out, if it is soft and flexible. Much more often, the filling material cannot be squeezed out, so the swelling wood, pressing against itself and the fill, simply undergoes more compressive setting, and when the wood dries out again, a wider crack is left, leaving the filling material loose.
Ordinary finishes may slow down the expansion and shrinking of wood from moisture, but they will not prevent it, even if only from air-borne water vapor (as with the old house’s swelling and shrinking doors). It is sometimes possible to encapsulate wood with epoxy and some other plastic-type materials, but doing so generally adds substantially to the weight of a canoe, and encapsulating works only as long as there is no break or crack or wear in the encapsulating material – not likely in a well-used canoe – and encapsulation is virtually impossible to do after a canoe is built.
Small cracks will not show through canvas at all, if the hull has been properly faired. Material getting between the canvas and the planking will show as lumps, and sometimes a crack between planks can be an avenue for sand, etc. to get where it ought not to be. That is why many w/c canoeists are a bit fussy about getting dirt in the canoe.
Attached are some pictures of my 1931 Old Town 50 pounder as I bought it, with (I believe) its original canvas. There are some cracks between a few of the planks, although not as severe as in James’s canoe or many other canoes, and there are also a few holes in the planks, but as you can see, the cracks between planks and even the holes were not visible on the outside, even with the old paint, which was crackled and scratched, even gouged in a couple of places, and with various bumps showing. It looks as though the interior may have had a new coat or two of varnish at some time in the past when the planking was tight, and you can see that the varnish has cracked at the plank joints. Newly painted, the bumps show on the outside, but again, the cracks between planks do not. This canoe has served me well the last two seasons in the condition shown, with a couple of coats of new paint.
