I have filled the tank with diesel until the fuel was into the feed pipe a couple of inches from the top. I did the same 102 and 68 engine hours ago. This gives a usage of 1.18 Litres per hour at my now normal, more restrained, cruising speed of 5.4 knots with a reasonably clean hull, albeit 6 months in the water.
This compares with an historic average of 1.54 Litres / hour when I often cruised at c 6 knots and to 1.6 - 1.7 litres an hour for much of last season when I had a dirty hull after wintering on the Hamble (but scrubbing at the start of the season) and several foulings from weed and plastic bags on the shaft or propeller.
With a static waterline length of 25 feet the generally accepted economic cruise speed is 5 knots, that appears to be pretty close.
The theoretical maximum hull speed is 6.81 knots. With Sancerre's big engine I have done over 7.2 knots with a clean hull but I have no idea how much extra fuel that would cost as I have only done it briefly.
It also validates the fuel gauge readings. The theoretical best accuracy with the pillar sensor in this tank is, iirc, +/- 2 Litres. I record fuel level and hours at every stop in Excel and the biggest deviation has been 3.9 litres and the vast majority have been within 2 litres with the gauge showing less than the calculation.
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