"MWB" wrote in message
...
I forget what these are called.


It is known as a "Clear View Screen"

The round device uses centrifugal force to remove water droplets.

The center part is the motor. The round transparent disc is attached to the
shaft of the center motor and spins when the motor turns. The spinning
action removes the water from the disc and so navigators are able to get a
clear view of the outside by looking through the spinning disc. Of course
the edges of the spinning disc are constructed in such a way that water will
not be able to enter the bridge.

There are several makers. This is one:
http://www.frenchmarine.com/Product....ID=826&CID=150



Regards,

Tuppence