220 amp hours is 73.3 amps for 3 hours or 3520 watts. That's 4.72 HP before the gearbox, probably closer to 4-4.5 HP real world shaft horsepower. Take your 4 cycle outboard or inboard, and calculate how much power it takes to move the boat at 3.5 kts based on fuel consumption per hour. 1 gallon per hour is very close to 12 hp. You should be able to cruise at 3.5 kts on .29 gallons per hour if their numbers are correct. Typical 4 cycle gas engine efficiency with a fairly efficient engine is .5 pounds of fuel per horsepower hour, and gas weighs 6 pounds per gallon. Two cycles can burn anything from 50% more upward, often as much as double. There is no accurate way to calculate horsepower based on fuel consumption on a two cycle, though direct injection engines will tend to burn about 20% more than good 4 cycle... about .6 gallons per hp per hour. That would be something like the Etech or Optimax.

H.W.