Beau, I don't mean to discourage you but there's usually a reason why things are done a certain way--and not advisable in the ways they aren't done.
"I do not see why storing petrol on a boat is any different from Diesel "
Well, start with diesel having about twice the energy density of gasoline. For every gallon of gasoline you store, you can motor for 2x-4x more time with the same amount of diesel powering an inboard. If I need to carry 20 gallons of diesel, you'll need to carry 50 gallons of gasoline to make the same distance. Or more.
Then there's outboards on a sailboat. The ocean is not flat, and every time you move a pound of weight from the center of the boat to the far end--you make the boat pitch
and rock way way worse. Which also pulls your outboard prop out of the water, further reducing your range and increasing your fuel consumption
. That's why offshore sailboats simply DON'T do it this way. Can you do it? Sure. Do it well? No. Competitively? No. Safely or economically? Arguably, again, No.
If conventional logic and use was so wrong, and a better answer was so easy...it would probably be out there already. One Yamaha factory-sponsored show boat making one run under ideal conditions with factory support teams does NOT mean outboards are good for offshore sailboats.
Diesel motors used to be much heavier than petrol. That's changed in the last 20 years. And that's still ignoring the inboard/outboard question. If you don't like diesel, by all means put an Atomic4 inboard. Just remember, you're still going to need a big fuel tank
"The reason for the 65 hp is that this new boat will achieve 20knots "
On a motor sailor? Yes, I'm a skeptic. If you have a radical new design that you KNOW will work...you wouldn't be asking these questions, you'd have the engineering all done. So, you have a motorboat of some kind, with plans for auxiliary sailing. That's very different from a sailboat, which is probably going to make six knots with any conventional displacement hull
. There's a reason for conventions: Often, because they're what has worked.
Just what kind of superboat are you trying to build? A multihull