I have a 2007 Marlin 300, which should be similar to the hull on the 306, with twin yamaha F250 four strokes. I cruise in 2-3 foot seas at 4100 RPMs, gives me 24.8 knots and a total fuel burn of 20.8 gallons an hour. That is loaded with 5 guys, fishing equip, full load of fuel, ice and water, plus a full live well. If it is flat calm I get slightly better fuel burn of 20.5. I have found that trimming the engines down in chop or using the trim tabs raises the fuel burn to 22. I do not run my boat at top speed......too expensive in fuel. If I bump up the throttles to 4200 RPM fuel burn goes up to 22.8 in chop, 21.8 flat calm seas. I think engine trim has a lot of effect on fuel burn.
Saturday I took the my mob to see the airshow 10 adults, 2 kids, for a total of 12 on board, boat ran 23 knots at 4100 RPM and fuel burn of 22.5.
Hope this helps.
