But simple paddle wheels of constant diameter operating in a cylindrical casing, entirely above the bottom of the boat, would just churn up a lot of water but not produce much propulsive force. The key to the Hotchkiss cone must be that water is drawn in at the small end, and the paddle wheel (which I see has slightly curved blades) accelerates the water both along the cone towards the biģ end and out to the larger diameter. Thus water is drawn in at the small end and out at the large end, but using a single hull opening rather than separate intake and outlet openings, as with other water jet propulsion systems. I guess they were normally installed as handed pairs to cancel out any lateral forces.
Simple devices and suitable for shallow water, but I wonder how the overall efficiency compares with a screw propeller outside the hull.