I also have the Navman 3100 and a Raymarine ST4000+ autopilot and
Garmin 276C GPS which I have tried to connect to get the autopilot to
follow the wind, but so far, I have not been able to get it to work.
Otherwise, the system is fully operational, all the navigation data I
need is visible on the 3100 Repeater (after Garmin fixed the BWC
sentence error).

I have taken the NMEA-out from the 3100 Wind instrument and led it
directly to the NMEA-in on the autopilot, as when I connected the wind
NMEA output onto the NMEA bus, the GPS data was affected, I assume
from having 2 NMEA 'talkers' on the same bus.

The direct connection from the 3100 wind to the autopilot has not
worked, the autopilot complains that there is no wind information.
The 3100 manual states that it outputs MVW and VPW, which should be
decoded by the autopilot.

I have purchased the Brookhouse NMEA multiplexer, but so far have only
played with it, I just havent had time to fully connect it and get the
laptop attached to see exactly what NMEA sentences are sent by the
3100 wind. Since 99% of the time, the 'steer to waypoint' function
is sufficient, I have put off figuring out the 'steer to wind' on the
back burner and have focused on sailing and racing the boat (J/35)
rather than messing around with the electronics.

As for the difficulty in hooking up the units, the Navman manuals are
pretty clear on the hookup. The white wire out of the 3100 Wind is
the NMEA out. The ST4000 manual is not as clear, as it has a Red and
a Blue which are NMEA +ve (Red) and -ve (Blue). I've hooked the
NMEA-out to both the red and blue and hooked up the NMEA (-) to each
also, but so far have not had any luck in getting it to work. Maybe
someone more gizmo savvy can explain +ve and -ve...

Good luck, I plan on spending some serious time working on the boat
soon, and maybe I will have time to get the Brookhouse multiplexer to
work.