Horizontal Darrieus Rotor Variation
If the rotor has a small diameter and is long along its longitudinal dimension, then it should spin relatively fast. Hopefully this will allow it to drive the boat propeller directly (without going through a gearbox). Any type of turbine that is derived from a “vertical axis” wind turbine design should work for this application. For example, a Savonius Rotor or a Gorlov Helical Turbine may be used.
Articulated Hydrofoils Variation
In this variation, the hydrofoils are allowed to rotate about the red dots, but this rotation is limited by stops.
Contemplating the diagram above, you can’t help wondering what would happen if the hydrofoils were not allowed to articulate, having instead constant zero pitch (chord exactly horizontal). In this case, if the boat were started moving by some means other than the hydrofoils, then maybe it would subsequently continue moving forward due to the forces generated by the hydrofoils. This behavior is, of course, reminiscent of that of the Darrieus Wind Turbine.
Articulated Slats Variation