Bandama Hi, my experience with the F115 - 2008 is in fresh water in the Paraná River Delta and the Rio de la Plata, the biggest waves here are 2.5mts nothing like the sea, but the current is sometimes higher 10 km / h.
I have the 3 gauge instruments 6y8, consumption plan depends heavily on the propeller and the weight, after many tests I stick with the 13x19 prop where consumer got the best value - RPM - end-reaction speed throttle - load.
A 3700 rpm consumption is 15 lts. / hour and the yield is 2.5km/lts.
A 4200 rpm consumption is 20 lts. / hour and the yield is 2.2km/lts.
Obviously in each boat varied and do not know if there are differences by having 2 motors, also in salt water resistance movement is different, but at 5700 RPM and 37 knots consumes nearly 35 liters per hour and the yield is 1.8 km liters approx.
I hope you serve the data.
My boat is 21 feet