These guys are correct: that is the prop for a towed water powered generator. The wing shaped gizmo is to help keep the prop from leaping out of the water at higher speeds. Look around the boat
for something that looks like an alternator
or electric motor
. It will have some sort of trunion or swivel mount that is placed at or near the stern rail or transom.
These are very useful devices whilst voyaging... will usually suply more consistent power than a wind generator