The best way to check fuel consumption is by filling the tank. Ideally this is done before and after a passage so just the underway hours are used in the calculations. When you are in coastal mode, as we have been, some interpolation is going to be required. As we have just topped off with diesel in Gibraltar we thought you might be interested in the figures.

To begin with, we used 1076 US gallons/4072 liters during the past three months. There are 88 hours on the genset during this period, five weeks of using the Kabola boiler for heating, and another eight weeks of using the boiler for domestic hot water. The engines have 181 hours on them during the three months.

We know the genset averages about half a gallon/1.9L per hour, so this adds up to 44 gallons/167L. The Kabola boiler requires an educated guess. We think during this period it used 50 gallons/188L. Deducting these two consumers from the total taken on board leaves 978 gallons/3701L used by our two propulsion engines. Dividing this by 181 hours gives 5.4 gallons/20.45L per hour. We spent 142 hours underway to cover 1509 nautical miiles. Roughly 50% of our time under power was at 10 knots, a quarter at 10.5 and a quarter at 11. The balance was at idle or very slow speed.

Complicating the analysis is the fact that we have been sitting so much the props have become fouled, which is a huge impact on efficiency (they are clean now). Also, we have been carrying large quantities of fresh water to use at anchor since we have not wanted to run the watermaker in most of the anchorages.

While the figures are not representative of what we expect on a passage, they are real world for this style of cruising.