On a flat calm misty day I once tried powering Aztec with my dingy and 2 hp outboard. After what seemed a long time it was actually moving the boat at nearly 1.5 kn. I think you would need a very long shaft outboard of at least 4hp on a large bracket mounted to the transom of the boat to seriously consider this as an alternative form of propulsion in a calm sea. Maybe for re-entering your berth, but then you may have problems controlling the motor for the final approach.
When I tried this on Aztec I had the motor on the dinghy and the dinghy was tied alongside aft of the mid ship part of the boat. This works in flat calm water and enables a crew member to control the outboard. Steering was done using the boats rudder with occasional help from turning the outboard.
Mike
Mike Edwards
Seal 28 "Aztec"
Mike Edwards
Seal 28 "Aztec"