Ripping planks

So I have this nice ten-foot 2x6 that I want to rip into a couple of sticks 2 1/5 wide, then glue them up to give me a blank 2 1/2 by 3. (I'm trying different ways of making oars). The original plank is a nice, straight specimen of construction-grade spruce from the local lumberyard (NOT a borg). As I rip, the kerf begins to close, so I wedge it open and continue. After the cut is complete, I have one more or less straight chunk, and one that curves a good four inches over its ten foot length. This is not uncommon, but my question is: how come does it do that? Is the stress release due to drying, or is it locked into the tree as it grows? Are there clues to look for in the planks in the lumberyard pile that would reduce the chances of this? (I'm thinking of grain orientation, or where in the log the plank came from). Any good references? Thanks.
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This is due to drying stress, but is related to the properties of the piece also. Look for pieces that seem softer, using your finger nail to test hardness, and for pieces that are uniform hardness on all four sides, the uniform, softer pieces will be less likely to have case hardened during drying.
Case hardening is drying stress from an uneven drying condition, case hardening is relieved in the better hardwood grades when it is kiln dried by the application of wet steam conditioning of the lumber while it is still hot. Softwoods are rarely conditioned at the end of the drying cycle.
Another defect to look for is compression wood, this will show up as a wider than normal growth ring that is slightly discoloured. Compression wood will only be on one side of the lumber or tree, causing uneven stress in the piece. It is caused by a tree that was leaning and tried to straighten as it grow. there is a lot of it here in hurricane and tornado country.
Look for straight grained pieces, the grain of the wood is the line it would have spit down if chopped with an ax, this is independant of the growth rings. The straighter the grain the less stresses built up during kiln drying the piece.
Basilisk
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Bruce Hoadley is your man, and this book should be in every serious woodworker's shop:
(Amazon.com product link shortened)- Hoadley/dp/B000UQH160/ref=sr_1_3?ie=UTF8&s=hi&qid28223788&sr=8-3
Scott
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Drying stress, more often than not. They can curl after jointing and thickness planing also. Try to plane both sides equally. Pick a QS or rift sawn board if you can find one.
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"Jim Willemin" wrote:

The operative word is "Spruce".
It will be kiln dried which make it squirrelly when ripped.
Bottom line............................
You can't get there from here with spruce.
Lew
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On Tue, 02 Dec 2008 06:50:55 -0600, Jim Willemin

Internal wood stress is very common and may be a cause for kickback. The internal stress could be from the tree, how the wood is dried, and the uniformity of the stock. Your best bet would be to look for quarter-sawn wood, costs more but more stable. Or, you can use narrower pieces, laminate, and glue to a straight edge.
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Re-glue it however you want it, flipping the growth rings in alternating directions, then work the completed piece on jointer, saw, planer, etc. It will then be more stable than any single board ever was or will be. If we ad to toss every bowed piece of cutoff wood, it would be ridiculous. That is why you glue up blocks in the first place.
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