Well, a couple questions and a few comments.
Is it a 2 or 4 stroke? (Different weights).
Where are you going to use it? What kind of water conditions?
A comment. Since the engine is not enough to get your boat up on plane, then the larger you go, (hp) the more resistance you will encounter. In effect, your hull will act like a displacement hull, and your bow wave (and resistance) will increase with ever knot of speed you try to increase. So a larger engine will reach a point where the resistance of the bow wave is the same as the extra hp from the engine. In other words you'll see no or very little increase in speed. A bigger engine is not necessarily the proper choice. That's why I asked about what kind of water conditions you'll be using it in. Still water like a lake is much different than a swift current in a river or wave action in the ocean. In the last two, a larger engine will help out because of the increased external pressures placed upon your boat by the waves, wind, current, etc.
Personally, I use a 9.9 hp engine. It's perfect for moving the boat thru the water for trolling, and it's still large enough to use as a get home engine should the main crap out.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
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