This ball is inside a watertight enclosure (kinda) and the ball runs in
rubberized shaft for each axis.. On the end of the shaft is a slotted wheel
that is runs through opical sensors.

The only way I know this is because the housing is a clear plastic.. I
assume the interior is weatherized since the water could (did) get in around
the ball. I did notice a drain hole in the bottom of this housing.

The 1721 radar with the track ball came out in the earily '90s and the MK II
has retained the same track ball technology.

I had the 1721, early version, on my last boat and really appreciated the
bright clear CRT screen.. I've looked at the LCD displays and for this
upgrade, I opted for a used MK II for it's display quality..

Steve
s/v Good Intentions