Hey i apologise to you Daniel.
I think your second explanation was better though. As for my friends boat i think the prop will have alot to do with the problems. Maybe he should experiment with different props.
Definatly though a 45 on a 5m boat would be criminal especially if you want to carry around divers. Im currently selling a Valiant v-490. Matched up to a yamaha 60 it is perfect however being a valiant (PVC tubes) it doesnt weigh much.
On somthing like a ribcraft you would want more power.
I always think that 3/4 of the manufacturers maximum is about right unless you are going to be lugging around lots of divers and their equipment.