If I had to guess this looks like an effect boaters would've noticed and made use of before these ferries were a thing. Something like people leaving their rudders tied to one side while at anchor in the river, to make sure the boat stays close to land. Then when it's time to go, they'd switch the rudder over to get into the middle of the water. Then the ferrie people just improved on it a bit. Surely an engineer wouldn't have come up with this :D