I am finishing a 31 foot Searunner
with a Saab 18 hp diesel
and since I am a power boat
person I plan to use it as a power cruiser.
To really learn this subject you should read "Voyaging Under Power" by Robert Beebe. It has a detailed technical section on fuel usage and the math of Range under power. Buellers "the Troller Yacht" covers this ground too, but Beebe's work is really the bible of this subject for small boat voyaging.
The case for power is pretty good comparing mono hull
sail vessels against mono hull
power vessels. Researching multi hull power vessel performance is a little more difficult.
I don't intend to make the case here for power in a multi hull, and my reasons for going the power route
are mine alone, but the one thing to keep in mind is that the weight of the vessel is the number one factor that will alter the range under power.
The conversion of a sailing mono hull to power cruising is handicapped by the ballast a mono hull is burdened with, a multi hull conversion does not suffer this burden.
A steadying sail is a good device on power mono hulls and should be considered on a power multi hull too. For obvious reasons flopper stoppers and other stabilizing gadgets found in mono hulls would not be part of a cruising multi hulls kit.
My goal is to burn .6 gallons per hour at 6 knots or 10 nmpg so 100 gallons would give me a 1000 mile range. There is room in my hull for over 250 gallons, and slowing down even to 5.5 Knts would boost my range - but I do not intend to pack around that much fuel on coastal cruises. For long hops additional 55 gallon drums would obviously add over 500 miles of range each. There is space inside for half a dozen of them, but it would be an ugly situation.
Six knots gives a consistent 1000 miles a week, which most mono hull sailors would kill for. I really can't picture anything short of a CAT 5 Hurricane
capsizing a power multi hull boat. Even then it wouldn't be a capsize
so much as the wind flinging the poor little boat through the air.
I would give you more accurate figures, but at the lower end of the scale fuel burn is more difficult to approximate than it is with heavier vessels, and because I am unsure what my actual water
line length will be when the boat is floating.
Most multi hulls strive to keep weight to a minimum and so fuel capacity is much less for even a large multi hull than it is for a power vessel. Yet when you look at those people who responded to your question as asked, you see that fuel economy is very good at displacement
speeds, and modest amounts of fuel can give you a substantial range.