Originally Posted by MT4Runner
Also flipped the strips end-for-end or upside down to make sure that any grain runout was canceled by straight grain or opposite runout in its mating piece. Grain runout is when the grain runs off the side of the board instead off the end of the board (which is ideal straight grain).
If you have a grain line that runs all the way across the board, it's going to break there. If the grain runs across the board in less than a foot, it's going to break right away. If it runs across the board in 4' or more, there's probably enough strength in the board to not ever break. So, if we had some grain that ran from upper left to lower right, we'd mate it with straight grain wood, or a board that that had runout going to lower left to upper right--counteracting the stress on the runout and minimizing the chance the oar could break from wood splitting.
I thought some more about this--figured a sketch would be helpful.
Let's talk about grain runout. Ideally, we want straight grain, where the grain in the wood runs out each end. If your grain is very tight and straight, you can have a few grain lines that run off the side of the board, but in general, the grain should be very parallel to the length of the board. (See first two examples). If the wood for your oar shaft looks like this, you probably don't even have to laminate it--you can use it solid.
Then we see really bad wood (second two examples). The first one is wavy and will break where the grain runs convex off the board. It also has enough wave that it might break across the board. The second one has really bad runout across the board and a knot that might be above your blade or under your oarlock--REALLY BAD high stress locations. You can use this stuff for blades (if you fiberglass them) or for firewood.
Then we have "maybe" wood (3rd pair). If you have a straight board with tight grain, but some runout that crosses the entire board maybe over the course of 4-5', you can laminate it to another board and mostly have their runout cancel each other. Knots are also not as big of a concern if they are near the end...they may not look as good aesthetically, but you can bury them lower in the blade where there is less stress, or up in the handle--both areas also get a LOT of grain removed, so they may get cut away entirely.
To laminate this wood, let's look at two examples using the "maybe" boards from the 3rd pair. You do not want the grain runout parallel--it can break just like the bad runout wood. You want the grain runout to cross with the other board so the fracture lines cancel each other. This makes a very strong shaft.
So...we had a couple of planks that had runout over a span of 7-8', so really strong wood, but we still wanted to hedge our bets and make sure we had "lifetime" oars and not something we'd worry about breaking the second time out.