Any sailors or marine engineers or even mathematicians here? Fuel efficency question.

I have been told by an associate to expect a speed of 6kt and a fuel efficiency at 80% power to be a gallon per hour .
So in modern money thats 2.4 liters per km? isn’t this a bit high ?

OP: First, you want to go 75 nautical miles, not 75 knots. A knot refers to rate of speed (NM/hr), not distance.

I know people who have motored similarly sized yachts a comparable distance (from South Haven, MI, to Chicago) without refueling, but they had inboard motors with bigger fuel tanks.

I recall being told in a USCG Auxiliary class that the way to figure this out was to test drive it at various speeds and see which speed setting was most efficient. Based on my hazy memories of the examples we worked out in class, Odysseus’s associate’s estimate of 1 gal./hr at 6kts at 80% efficiency (6 nautical miles / gallon) is not unreasonable; I doubt you’ll get better mileage than that. You should plan on making a refueling stop and/or get your sails in working order before you make this trip.

Before you get into the fuel calculation, which is important enough, there are some other things to consider:

Are the pumps in working order and the cockpit drain free-flowing?
Suitable life jackets on board for all crew?
Proper ground tackle (anchor and sufficient rode for likely anchorages) and the cleat to tie if off to?
Has the boat been surveyed so that you know that it is currently seaworthy?
Is your route clear of overhead obstructions? A lot of drawbridges that are manned 24/7 during ‘normal’ pleasure boating months have different timetables and more sporadic manning at other times of the year.

Aside from that, @submariner‘s advice is good: You should motor around a bit on your own to get your own feel for the fuel consumption, optimum rate of speed (I expect that you’ll reach hull speed with that boat before you get to ‘75% power’ on the outboard) and a feel for how she responds and handles – as well as how the motor itself performs.

Then file a float plan with someone on shore who will look for you to report in from time to time – and call the Coast Guard if you don’t, and plan to take longer than you expected against worse conditions.

Since I haven’t motored since 1982, and I don’t recall my fuel consumption at that time (in a Catalina 27), I can’t offer the advice you really want. Have fun, and good luck.