I have a 6hp for my 18 footer, and really want something bigger. I had a gear blow in my old lower unit while shrimpin' on Hood Canal. An 11 mile run going 5 mph into the wind was as good as it got. Talk about a long run. I am sure that I could have at least pushed 10mph with my father in-laws 9.9. If you are spending $5-20 grand for a boat, get a bit more motor, you can always throttle back on the 9.9.

One other piece of advice would be to have a connector that will allow your big motor to run off of your little motor's gas tank, and to have a 2-5 gallon container of gas sealed and stored. Gas tanks can get fouled in rough weather, and clean gas can save the day.

These practices are in place on my boat at all times, cause it only takes once for me to learn a lesson.

Andy