I have a 15HP Yamaha 2 stroke on a 310 RIB. The motor is about 39Kg (88Lbs). I have fitted an extra purchase
on the davit to make lifting easier. The extra HP is great when cruising. Note that a 9.9 and a 15 are essentially the same motor and weigh the same!!!
You can take the davit rope
to one of the winches if necessary. I fitted a block to each davit so that I could pull at an angle and the rope
still feeds cleanly into the clutch
Recently I bought the lightest motor I could find (a 3.3 mercury
2 stroke @ 11Kg). When at home I take the 15Hp off and use the 3.3Hp. We typically have a lot of short runs at home and this saves a lot of lifting!
Note that if you buy a larger RIB the width is critical. I had to get a 2nd one as the first was too wide to lift
cleanly under the davits.