Ok - so I read the article and found the section on fuel consumption. I fell for the old kt vs mph problem. The figures you gave were for 7.7 mph or 6.7 kt. At that speed my tables show that I burn 2.5 gph - exactly the same as you quote for the twins in the KK58. This makes more sense. A general rule of thumb is that going to twins doesn't significantly increase your fuel consumption at a given speed. Neither does going with a larger engine.
So back to your question - I'm not sure why the big difference. In general it takes less power to move less weight. Hull shape can make a big difference though - the KK58 is definitely shaped for fuel consumption. Larger engines should not make that much difference.
Richard
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