Do a Google search. I have seen at least one do-it-yourself article on
building a wind vane. (they are bloody expensive to purchase). One
thing you probably already know, they don;t work when motoring.
'
Actually they do work when motoring, they don't work without wind, and
they don't work so well if you are motoring hard due to cavitation.
Also make sure you have a light air vane, else they won't work below
10 knts.
Tom
ps above applies to the Monitor wind vane from personal experience,
other
vanes may be different.