Hull speed is 6 knots. Anything past 6 hp is weight in the wrong end of a boat that already sits stern-down with two people in the cockpit.
Shaft length
Long shaft, every time. The transom is high and the boat pitches; a short shaft ventilates the moment there is any sea.
Weight where it matters
Two kilos on the bracket feels like nothing ashore and like a lot at anchor. Compare dry weights before power.
Mounting
Spread the load with a proper backing plate inside. The transom is a sandwich, and a bracket bolted through it without one crushes the core.
