Trying to figure out GPH on my boat & engine

[thomas3120]

Hello,
I have a 1989 17' Boston Whaler Super Sport Limited which has the same exact hull as the Montauk but instead of the center console it has a "drop-in" gunwale to gunwale console with a large rear hatch. Probably slightly heavier than a Montauk.
The engine is a 1989 Johnson GT 100 (max. rated for my boat). It's a V4 with 2 carbs.

If I remember correctly on a good day, going with tide and wind and also trimmed up I got 48mph @ around 5200-5300rpm. WOT is 5000 (trimmed all the way down).

Wondering what GPH I can expect on various rpms, or at 3000rpm on plane?

Any info appreciated.

thomas

[Aussiejay]

Fuel consumption

Suggested is 10% of horsepower e.g. 70hp = 7 GPH at WOT. This a basic figure and only accounts for the engine and not the boat type, weight etc, this is based on 4 stroke from memory. But it's a good starting point. The other method is to take a log of how many gallons you put in fuel then, work out your rpm's for a trip vs fuel used etc....