Re: Help settle an argument?
Don't think in terms of pulling a skier. Simplify the issue and think of in terms of load being placed on the engine.
A light 18' hull that is typically operated with very little "payload," would do just fine with 90hp on the back. Conversely, a heavy 18' hull, typically loaded down with lots of people and "stuff," might not do so well.
For both these boats, however, prop selection is the key issue. Keep the engine in its targeted WOT range, no matter what it is that you use it for, and you'll have a happy motor. Use a prop that is too steep in pitch and you'll have an unhappy motor that will probably not live up to its longevity potential. Remember, also, that if the motor truly is too small for the boat, you may get to a point where installation of successively smaller pitch numbers don't help anymore, because you are either still lugging the engine or just can't get the boat to go fast enough to make skiing much fun.
As for your FIL's boat, if he is running about a 19" pitch prop on it, my guess is that he could drop down in pitch to a point where he could pull a skier without hurting anything. If he is running something on the order of a 15" pitch prop now, I would have to agree that he doesn't have much room to pull a skier. I say that merely because dropping down to a 13" pitch prop is only going to push his boat at about 20 mph and that's most likely without a skier.