I have been looking at Simon's page
, and I think that he has got it all wrong. To my mind, you need to consider the angle the arm moves through as viewed from above. If I stick my right arm out to the side in the normal paddling position it is perhaps at about 10 degrees from the forward position (it doesn't make much difference if my trunk rotates or not, since I am measuring the angle from the line of forward travel to my arm), when I move it into the position for a vertical stroke on the left side, it is maybe at about 50 degrees. If I want my right wrist to remain neutral, then I will need to have a 60 degree feather on my paddle on the basis of my guessed figures. I am a slalom paddler, and therefore I have a higher stroke than most, but it is definately not vertical, my top hand doesn't actually pass very far over the centerline of the boat when I am paddling forwards, in which case my right arm only goes to about 30 degrees from the direction of forward travel, so actually I need a 40 degree feather. Of course, these angles change as the blade is moved throught the water, so nothing is exact, and some wrist roll is necesary. Playboaters are notorious for a having very poor forward stroke, :-), and it is quite possible that the arm maintains approximately the same angle to the forward direction when viewed from above, in which case they can handle a zero degree feather. I know that when I tried some zero feather paddles it felt dreadful, my wrist was being forced to turn the wrong way to get the purchase on the water.
So finally, I would suggest that if you find that zero feather works for you, it might be worth considering a bit of work on your forward stroke!