i had the merc 15hp 4stroke..then switched to a honda 8...youre prolly gonna want a long shaft..the merc was nice but i sold it with my other boat and boat an 8 hp honda 4stroke with an extra long shaft...the honda is WAY quiter than the merc if thats a concern..i have a 21 ft bayliner and the 8 is more than enough..i use a garelick mount(15 in travel)and a ez steer hooked to the outdrive...could probably give a little better mounting info if you happen to have a pic of the back of the boat