It depends on a lot of factors but the two main ones will be the pitch of prop you have and whether or not there is any tidal flow or water movement to try and overcome.
You might find that the prop starts cavitating pretty quick due to poor flow over it.
I take it the tank would be full?
To give you an idea, in my line of work, we use small launch for very light towing work. It is about 8meters long and has a 2.3tonne bollard pull yet it is only powered by a Mermain 140hp turbo diesel. That Mermaid does however drive a 30" prop and its only spinning at 2300rpm at peak power.
I would guess that, as long as there is no other resistance to overcome, the rib will probably do it but your going to have to coax the outboard up through the revs very gently until its starts moving.
C'est pas l'homme qui prend la mer, c'est la mer qui prend l'homme....