Short or long shaft depending on how tall your transom is. For that boat a 9.9 should be perfect. A smaller motor would work but having a backup that will actually push the boat is a real benefit.
I have a Merc 9.9 4 stroke on my 20ft and its perfect. Slow troll about .8-1.0mph or almost plane the boat with just me in it. top end about 10mph.
I have a Merc 9.9 4 stroke on my 20ft and its perfect. Slow troll about .8-1.0mph or almost plane the boat with just me in it. top end about 10mph.