On Sun, 26 Jul 2009 11:58:01 -0700, RichH wrote:

Probably not a rig tuning issue but rather a hydrodynamic anomaly with
your paddlewheel.

If the paddlewheel is mounted off of the boat's centerline it will
rotate differently on on tack than the other and 1. differing depths (on
one tack the paddlewheel is deeper than on the opposite tack. 2. the
amount of leeways 'slip' the boat is experiencing versus #1 above.

Unless the paddlewheel is accurately mounted ON the boats centerline it
will virtually ALWAYS read differently when SAILING on different tacks.
Its a problem of hydrodynamics versus different 'flow regimes'.

Thanks Rich,
I'm happy to have had so many quality responses to this thread. Lots of
informed opinion and much to think about.
In fact the paddlewheel is mounted off-centre to port by about 10 inches
or so. I'll be back up to the old girl in a day or two and the bug-
hunting is going to start with verifying your proposition. I know I do
have some looseness in the rig that must be dealt with but nonetheless,
your suggestion seems to satisfy Ockham's razor.
We'll see.
Cheers