: I thought it was cedar, with a fair amount of grain run-out.

That's likely to be the problem. I'm really picky about the grain in my paddle blanks and I've yet to have a paddle break, despite using them to push off from the shore or the bottom frequently (sorry Greg). If you cannot find a 2x4 with straight vertical grain, buy a 4x4 and resaw it to get the proper grain orientation. Based on the wood I've sorted through, it seems that 4x4's are cut from a better part of the tree than most 2x4's. One other advantage to this method is that the blanks will be slighly thicker, giving you more options in loom size.