But
would not the true wind blow stronger on the flag than the relative wind?
I shall ask an ancient mariner to explain. With pictures.
(flexi, Fri 10 Oct 2003, 16:00, archived)

not necessarily
the SLOWEST a boat moves (generally speaking) is with the wind. a wind blowing say 90-degrees from either side would move the boat along MUCH faster than on a downwind leg.
(berlinvirus, Fri 10 Oct 2003, 16:25, archived)

The light
of revelation shines brightly. Thanks - that has bugged me for years.
(flexi, Sat 11 Oct 2003, 11:38, archived)