b393capt wrote:
1) Both my Raymarine ST60 Wind Machine and E-80 Chartplotter have the
ability to display a value "VMG wind". What is this ? Why is it
negative sometimes ?

2) What does the TACK button do on the Wind Machine. Does it simply
make my heading display on the wind-machine ?


It's the speed at which you are moving into (positive value) or from
(negative value) the direction of the wind. It is calculated as
cos(heading-wind)*speed.

So if you are sailing due East with a Northerly wind, your VMG is 0.
When you sail due North, it will be identical to your speed.

It is used when you want to sail to a target (almost) directly upwind.
You should then sail at maximum VMG. In other words it tells you whether
you will get there faster when sail at a different angle to the wind.

The Tack button will tell you what your course will be when you tack and
sail at the same wind angle.

Example: you sail close hauled on port tack, and there's a cliff you
need to clear when you tack. You wait with tacking until the bearing to
the cliff is "past" the tack heading the system predicts you will be
able to attain.

--
Kees