I have a lift keeler in Jersey. I started with a Mercury 3.3hp which was enough for local stuff but wasn’t so good for long distance, I managed to wear out the piston which is pretty unusual as usually they don’t do enough work in their life span for this to happen. It took roughly 400nm. So have reluctantly moved to a 26kg Mercury 5hp sail drive with an external tank, which (touch wood) is yet to miss a beat and with it's larger propeller, our speed is much less affected by chop. My view is if you’re cruising or as we are, racing offshore under IRC Cat 3 get something that can handle chop, tied, heavy crew etc… If you’re just locally racing or coastal cruising a 2.5hp will probably be ok, but don’t expect it to last forever.