how can I get fuel consumption rates per boat model

There are several boat testing and review sites that often have that information. I'd suggest just basically searching for it online.

I agree about doing a search but if you’re really wondering about fuel economy between a single diesel displacement hull Vs a twin gas cruiser...

Never mind.

Boat owners manuals very likely will have no info about fuel economy. I've never seen that.

Sometimes you can find magazine articles and on-line sites where someone has done a model review, and that might have info. And there's some kind of website out there that purports to give fuel consumption figures for various engines (can't remember where, dunno whether accurate or not).

But if you can find the fuel curves for a given engine, sometimes (often?) available from the engine manufacturer, that's generally the same consumption at a given RPM level no matter what boat the engines are installed in. X GPH @ Y RPM. What varies after that is that boat's resulting speed at that RPM level, so NMPG might be a bit elusive.

There's room for some easy common sense here, too. A small boat with a single diesel could be "economical" whereas a large boat with twin gas engines, not so much. Imagine a number line between those points, interpolate a bit...

-Chris

It is indeed often the case that fuel is not the major annual budget item.

Berthing and insurance are other larger -- somewhat predictable from year to year, once you know the numbers -- annual expenses.

"Maintenance" (all aspects of periodic, annual, preventive, repair, etc. service) can be another, but this one "category" can be all over the map in a given year. High this year, low the next, etc.

-Chris

And it depends on how much you can/want to do yourself.

"It is indeed often the case that fuel is not the major annual budget item."

This. If you're not buying fuel, you're not using the boat and if you're not using the boat, all of the other expense is wasted.