The recent searunner thread and comments on powering boats gave me an idea for a stand alone thread. A couple of things to consider when you're looking to power, or re-power a boat. The boat manufacturers recomend for planing hulls to have a ratio of weight per hp of ideally 25# per hp, and a max of 40# per hp. This is gross weight, ie hull, engine, gear, fuel, crew, etc.
So let's say you have a 1000# boat, 300# engine, 200# fuel (6#/gallon), 500# of crew and 200# of gear (ice, tackle, fish) for a gross of #2200, dividing by 25, that means ideally you want an 88hp motor, rounded up a 90, and with that 90, you can max it out with a gross weight of 3600#.
What's important to remember is that it is very easy to add quite a bit of weight with gear, ice, fish etc. Also over time, boats get heavier as you leave more and more gear on them, and engines loose some power over time.
Be very honest with yourself as to what loads you'll normally carry. Two folks will generally load a bot much more lighter than a family of 5, and your kids get heavier over the years. So while a 115 may be perfect for one owner, a 150 will be more appropriate for another.
Finally, if youre normally operating at 40#/hp, your engine will be working alot harder, and generally get worse mileage than a larger engine opperating around 24#/hp.
Jet or Prop?
Are these ratings at the impeller or prop? If you have a 90hp motor with jet you loose 25% at the jet, and that would leave 65hp.
I have always followed the rule of buying bigger then running it easier. My big block 496 ci, 375 hp gets almost the exact same mileage of the standard 350ci 280 hp. I can run at lower rpms at a higher speed. And when I need the extra power all I need to do is hit the gas.
This is for a prop, with a jet pump, you should assume you loose 30% efficiency, ie if your calcs show you need a 70 prop unit, you need a 100 with a jet pump.