Re: Back-up motors
This is a non starter... Geoff's on the money with his comments. There's no need, really.... further, island hopping in the Caribbean is rarely a 'light wind' situation. Much more likely you should be reefing and turning the engine off rather than trying to squeeze some debatable speed increase from an outboard that likely won't even have the prop in the water half the time.
The engine will likely get the boat up to hull speed.. there's no further gain to be realized with another few horsepower poorly applied.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"