Assuming it's propped correctly and can get to 5000 RPM, but not more than 5500 RPM at WOT, it will burn about 9 GPH at WOT. Throttle back to 4000 RPM, and it should drop to about 4.5 GPH.
That engine makes peak HP at around 5000 RPM and peak torque at around 4000 RPM. Ideally, your boat/engine/prop should be setup to to cruise nicely at the engine's torque peak; that's where you'll attain the very best on-plane fuel economy when trimmed correctly and loaded so that it's running level, side to side.
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