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That's sound thinking. Personally, I wouldn't hang a 15 off it after what you describe. Rigid hull is the way to go.

Is it a solid bottom RIB? They go on the plane much easier built like that. I still recommend the 15hp as you'll have power to spare and that's a great thing.

The weight of the average 9.9 hp is 39kg and a 15 hp is 41kg. Two kg or 5hp isn't going to haul your transom down.

A Chinese knock off of the Yamaha. We used them off Cairns for years and never had a problem.

Mine is a 2.7m fiberglass solid bottom and planes well with 2 people and an Evinrude 8hp. Your 3.1m would do well with a 15hp if lugging it around isn't a problem.