From The Story of Dartmouth, by John P. Martin:

This is the 70-foot ferry Sherbrooke launched at Dartmouth in 1816. Inside the housing on the deck, a team of eight horses harnessed to iron stanchions traveled around a cog-wheel which turned a crank. The crank then moved the single propeller located under the middle of the boat. As auxillary power, sails were hoisted whenever the wind was favorable. This type of ferry was common at the time.