Of course there are stagnation points in 3D flows. Otherwise there couldn't be a Kutta condition on an airplane wing.
I agree that if you freeze frame a paddle in the water, there will be a spill line, which you can call the stagnation line. But that line will constantly shift along the face of the blade, and may be crooked, as the paddle moves through the water, unless the paddle is pulled perfectly flush to the water at all points along stagnation/spillage line, which seems literally impossible.
Maybe "stagnation point" or "stagnation area" is the wrong term to describe how a cupped paddle "holds" water better than a flat paddle, but there are experiments that show that. The longer the water is "held" before spilling, the more power the blade has.
My experience is mainly with canoe paddles, but I have paddled with dozens of kayak paddles including GP's. Many paddles do have a mildly raised dihedral line down the center of the blade; others have a more rounded camber. Many argue this helps helps avoid flutter; others say it does so at the penalty of reducing power (the "held" water). Completely contrary, however, is that there are many flat-faced bent shaft canoe racing blades that don't have any undue flutter.
However, I have never seen any paddle in all my life that has the very high and sharply defined ridge of the Tuktu Aleutian paddle. That's not meant to be some sort of statement about my paddle knowledge. Rather, it's supposed to reflect the complete absence of this supposed performance feature in the minds and designs of competitive paddle makers over the last 60 years.
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