One other way of looking at this fuel burn is the ratio of speed to Length waterline. It is generally accepted that the most effecient speeds are at or below the Sq root of Length Waterline. The length waterline of the Sea Horse is listed at 47.26 feet, and the sq root of this is 6.87 --or a fuel burn of about 2.1 gallons an hour or 3.3 NM/gal when running at this speed in knots. At 1.2 x sq root L W L= 8.2 a recorded fuel burn of 5.6 g/h or 1.5 NM/gal,(for a speed increase of 1.3 knots, the range is cut in half!) At max displacement speed 1.34 x sq rt LWL= 9.2 a calculated fuel burn of 7.6 miles a gallon or about 1.2 miles a gallon, about 1/3 of the low speed range. This shows that the engine is very well matched to the boat (assuming that it does get up to slightly over 9 knots) and that the most effecient cruise speed is at or below the sq rt LWL (where the fuel effeciency is 3.5 N miles a gallon or slightly better). The best way to see if there is any current is to have a calibrated speed log which gives speed thru the water, and compare this to the GPS speed over ground. If you are doing speed vs flow curves you need to run at least a mile in opposite directions on a closed course in smooth water. (to equate any difference in wind or current) Although most calibrated Flow Scans are quite accurate, a very good way of determining fuel burn is to keep a constant RPM and measure fuel used between fill ups on a long passage. The Flow Scans will normally show only main engine flow, and not generator consumption. With 1500 gallons, this should give the boat a range of at least 4500 miles. But this may not be real world conditions--with wind and seas against the boat--and the pitch, drag of paravanes and stabalizers etc will make drastic changes in "mileage". Bob Austin