The feather on the paddle for white water is normally very small. often
flat it would likely be 10 perhaps 30 degrees. a sea paddle will be
longer 220 CM for me at 5'8" and my feathere angle is about 60 or 70
degrees for the sea. When in white water I try to borrow a shorter
paddle with little or no feather.

Can you clarify?

Amongst the group that we paddle with all the paddles are 90 degree through
to 45 degree, one or two 30 degree, but I can't think of any who paddle with
flat paddles - ie with no feather at all - there are some at the pool we
use, but three of the kayak groups I know who use the same pool all bring
their own paddles with either 45 or 90 degree blades.

Not looking for an argument, just not clear as to what you are saying
because it doesn't match what I see locally.

Ewan Scott