[Joky]

The movement of paddling has a natural rotation of the shaft when you raised the fixed hand for a stroke on the other side, it's quite straightforward to figure out sitting and mimicing the movement.

During this movement if the blade aren't feathered at all you have to compensate with some bending of the wrist. The amount of rotation of the shaft induced depends on how much you raise the hand/elbow, and so is fairly dependent on your style of stroke. This is the main way I think should be approached feathering: how much vertical do you intend to paddle? From there the angle should follow to optimize for the least amount of wrist twisting.

In general paddling very vertical will come with more angle in between the blades. I practice slalom and use to have 70-80 degrees crossing, but I tend to paddle less vertically now (aging? Lack of training?) and I'm down to 60 degrees comfortably now.

[amluto]

I have never believed this argument. In particular, I, and my bones, joints, etc, are approximately symmetric under left-right reflection. This means that I am not chiral, and I would expect an optimized paddle that I hold, symmetrically, in two hands, to be similarly non-chiral. (It’s a paddle, not a oar!)

More concretely, if some biomechanical factor made it a good idea to rotate the top of the left paddle forward θ degrees at the start of the left-side stroke, I would want to rotate the top of the right paddle forward θ degrees at the start of the right-side stroke. But with a feathered paddle, one of those thetas is positive and one is negative.

This is wrong in a way that used to bother me deeply whenever I kayaked. I would unfeather any unfeatherable paddle I used to restore proper symmetry.

I suppose what’s really happening is that people feather to reduce wind resistance or because that’s how they learned, and once they’ve learned it, it feels natural.

I would be willing to believe that feathering either direction is somehow biomechanically superior to not feathering at all and that the symmetry is broken arbitrarily, but I would want to see evidence :)

[colanderman]

The asymmetry is compensated thusly: when you feather your paddle, you should grip in only with your dominant hand. It should rotate freely in your non-dominant hand.

Your wrist movements can then be symmetric -- your non-dominant wrist is free to move as you please.

And because you are free to set the feather as desired -- you can set it in such a way to minimize the rotation required of your dominant hand (and, per the above, non-dominant hand) to transition from one half of the stroke to the other.