This post contains a link to an article by Brian Day

He presents his personal view on this subject.

In summary, he concludes ..

There isn’t one answer to the feathered versus unfeathered argument—there are two. If you want to avoid repetitive use injury, you should try to keep your wrists straight during your forward stroke, and there are a couple of ways to do it.

Which feather angle is best? Both. If you prefer a short paddle for maximum power use a feather angle between 30 and 45 degrees. If you would rather use a long paddle allowing your hands to be held low, an unfeathered paddle will keep your wrists straight. That’s the answer. Keep your wrists straight and keep paddling.