It's a 'Kent Clear-view Screen' which is fitted to the outside of the windscreen and has a spinning outer disk that throws water off because of centrifugal force. This is much more effective that a conventional wiper blade where there is a large volume of water to clear as you would find in heavy sea conditions and heavy rain.
The disk is driven by a motor and belt, or by direct drive in modern units, on the inside if the windscreen.
There's a bit on WikiPedia here: https://en.wikipedia.org/wiki/Clear_view_screen
I hope that's helpful.