The Manta that was tested for Passagemaker had a pair of the new Yanmar 250 hp lightweight aluminum diesels, the fuel consumption was read off the flow meter which we know is inaccurate, we have found this to be the case with the Volvo fuel flow meters too, by doing timed runs and measuring fuel. For a given horsepower we have found the Volvo's to be more fuel efficient as well. Having said that 9 knots is not an efficient speed for the Manta since it is beyond the true hull speed wall and into the semi displacement mode. Based on calculated fuel rate and tank level measuring at 8 knots the Manta has been consuming about 2-3 gals per hour depending on conditions, this has been verified over 15 hulls. Of course this is in relatively flat water and low wind. Actually you cant generalize about total wetted surface when it comes to power cats for a given overall length since this is totally dependent on hull fineness, the Manta has less wetted surface than some monohulls in its size range hence the frictional drag theory would not apply. Another variable of course is displacement, lighter displacement takes less horsepower in most conditions. 9-13 knots in the Manta is not a the most fuel efficient range which is the semi-displacement pre-planning speed. Once on top the Manta is burning the same at 15 knots that it is at 10 knots.