On Tue, 02 Dec 2008 06:50:55 -0600, Jim Willemin
wrote:

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.

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.