I agree with Symphony, but note, once you go over about 2300RPM the fuel consumption increases dramatically with only a small additional increase in speed. Waves and a headwind could easily double your consumption.
My Volvo Penta D1-30 in a Bav 38, 3 bladed folding prop uses 1.6 litres per hour on 2100 RPM doing 5.3 knots in calm seas. I have used nearly 3 litres per hour doing 2500 rpm into 20 knots and 1.5m seas, averaging only 4.5 kn. dropping to under 4 knots when pounding into waves. I may be a little out in my estimates for rough weather motoring as I avoid it as much as possible.
Be aware, the fuel gauges are typically not calibrated correctly. I have 1/3 tank left ( about 50 litres) when gauge reads half full.
regards
Craig
"Shirley Valentine"
Gold Coast
Australia