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

I just bought my first pocket yacht and unfortunately I may have to attempt to travel 75 knots (86miles) of coastal waters on the outboard motor to get it home
It is a 23ft bilge keeler similar to this (roughly 190 CU ft of displacement I think), with one of these Mercury 15HP four stroke outboards.
If say I get a weekend of low swell & low winds and travel at 75% power, what speed should I expect with such a setup and more importantly how much fuel do you calculate me to need ?

I know the math is pretty complex and depends on a whole load of factors (weather; water displacement; engine efficiency etc) But since you know my speed my ship and motor would anyone care to make an experienced guesstimate please ?