Re: What size boat with 90HP O/B?
The 90 hp engines I am aware of have a 2.3:1 (thereabouts) gearbox which runs at a lower rpm than the higher hp enignes. This allows you to turn a larger prop with less pitch, both elements of pushing heavy loads.
The hull design is your biggest concern. Flat boats plane out with big loads easily but pound you to death in any kind of sea. Deep V hulls are great for rough water but there is minimal planing surface so you have to push them thru the water.....rather than them riding on the surface sort of thing.
The semi-V style attempts to give you the best of both worlds: sharp wave entry and flat planing area.
Obviously weight, and weight distribution are key elements. For stern heavy boats trim tabs help to keep a stern up with low hp.
Wave action is also part of the equation. Much easier to plane out on smooth water than rough.
How fast you really want to go is paramount. If you are happy running at WOT just on plane that's one thing. If you want white foam that's another.
Trim or not is also a big player as is "setup" of the engine to the boat interface.
There is no textbook answer to your question. It all depends, including which is the right prop. The only way you really know is to try it.
In short, with a good efficient semi-V hull of moderate weight and loading, you should be able to go to 20 ft and run 25 to 30 mph on calm water with a moderate load which is not bad.
Go to www old-omc.de and look at some of those boats and what's pushing them. You can get an idea as to how fast they are running by looking at the size of the water droplets in the spray....the smaller/finer the spray the higher the speed. Also they are probably empty, other than the passengers you see so if you loaded the boats up for a good sized fishing trip, performance would surely be different.
Also for those pulling skiiers, notice they are usually young women on large double skis.....my wife weighed just over 100# when I married her....I weigh 250 and like(d) to slalom. Makes a difference what pulls each.
HTH
Mark
