When I used to do a lot of solo sibbing, I had a boat something over 3 metres (around 3.4?) with a 3 hp 2 stroke and it went pretty briskly. On mirror smooth water it would all but plane and on the sea it would get there eventually. However, the speed of an immersed hull (rather than a planing hull) is dictated by its waterline length so even though your boat is smaller and lighter, it may not go quite so fast. If you don't have to fight strong currents, tides or head winds, then the small engine you have in mind should be adequate. Thing is, a bigger engine always has that bit in reserve for the unexpected, and it isn't always whining away at full revs. If you can afford to go bigger (say up to 6?) then it may be wise.
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