I noticed a few photos on the wall near the canteen on the MV Holiday Island ferry yesterday that showed the boat in dry dock and pointed out its propellor, which is probably not what you think: it’s outfitted with a Voith Schneider Propeller, in which:

“…a rotor casing which ends flush with the ship’s bottom is fitted with a number of axially parallel blades and rotates about a vertical axis. To generate thrust, each of the propeller blades performs an oscillating motion about its own axis. This is superimposed on the uniform rotary motion.”

Who knew?