An eco friendly boat motor?

I spotted this in a museum in Perast on the Bay of Kotor in Montenegro. Looks like pretty hard work to me. I wouldn't have thought it offered many advantages over oars.

Barrie

Looks like it may give good prop speed if the master beat his slave.
I'd say the man sweat from operating this device would pollute with sweat and other nasties  but they didn't consider such things back when this was made.