my 73 18 ss is rated for and has a 140 hp mercury that flys across the water but drinks the fuel, i also my a 77 18 ss that is rated for a 150 hp, it depends on the year
whats the max HP on your boat?
Go with the max if you want to stay legal.
Make sure your transom can hold it back.
You don't want your motor next to you up at the dash.
GLG
CBG, what waters are you going to be on? Rivers and lakes are calmer and you may go fast and for a day out, may have local marinas to re-fuel at. If your going on big water, rule out speed (waves) and go with less hp and get more mpg. Not sure why but my 18' Offshore is only rated at 125hp.