I think the most critical parts of paddle design (Euro blade) is the shape of the blade curve. I like my blade to curve to a depth 1/4" short of reaching the opposite side of the shaft to eliminate flutter and coming back 1/8" short of reaching the center of the shaft at the tip to provide more power at the beginning of the forward stroke. To keep the blade area of the shaft strong but still light, I keep it 1/2" thick at the deepest part of the curve, which is halfway down the length of the blade.

I provide some basic info on one my webpages:

http://www.rollordrown.com/kayak/paddle.html