RIB outboard

That's pretty unusual for a RIB. Which one do you have?

Most 8ft RIBs take a 6 or 8hp, with as much as 10 max.

Why would you believe what some strangers on an internet forum tell you, when you WON'T believe what the people who designed and manufactured the boat tell you? You really think that WE know more about your boat than THEY do?

Why not test it yourself! borrow a couple motors to try. then you will truly know!..Dale

There rating it by various formulas somehow connected to some CG rule as well as how much torque it will take to break the transom !

And 50% more is quite a bit more

What kind of RIB is it?

RIB is a Highfield Ultralite 240. I'll call the Dealer and ask them why.

I had to look up what that was. At 59 lbs, that's pretty light. I suspect they reduced structural strength to get there, as it would be about half what I would expect a typical 8ft RIB to weigh.

I would think the transom would be the primary issue, so maybe there is a way to strengthen it. You would, however, be the test pilot. Be sure the water is warm.

Back when (ah, those were the good old days, weren't they?) I used to over power my inflatables by at least 5 hp and often 10, I had some pretty speedy little dinks. I also had some pretty dangerous little dinks and they rarely lasted more than a year or so.
I doubt the water LEO's will ticket you, but chances are you will be creating a monster. Certainly nothing you'd want to let a kid drive.

I have seen one of these. Planing one up with a 5hp?