Dear John and Anne, Apart from the fact that you can never carry enough fuel...we have transatalnticed ( 17 days) when we only motored for 7 hours but, more to the point, have motored long distances on the S American west coast where, Panama to Arica and even further south, light airs are the order of the day.We monitor fuel consumption prety carefully ( the standard 78 hp Volvo of old, with about 1000 hours on it) and in calmish seas with a fully laden boat we get 6.75 knots at 1900 revs and about 4.3 lph. Considerably less consumption if you are content to go slower, much greater for that extra half knot and huge increase if you cruise above 2300 or so. We carry 6 jerry cans giving about 130 more litres. Doesnt do a lot for trim when stowed in the lazarette , but comforting to have. I think I can honestly say that we have never had to rely on the extra fuel, but that is really a choice of route. Also the jerries have been invaluable on this coast where often the only fuel available is from a garage.( though yesterday a truck came to the marina near Valparaiso and delivered via the longest hose you can imagine 300l of City diesel--a first for us. I gather is gives 5% less power but less pollution. 27p per litre....)
Needless to say, after all the warnings posted on this site, we open up the engine to full revs every 8 hours or so for about ten minutes to decoke the turbo.
I think an SM should really have a fuel tank of 1000l to do justice to its long legs, but I think also that you would be unlucky to be caught short as it is.
Incidentally, do you find that the last 10l on your dipstick are somewhat optimistic? I keep overflowing when I add much more than 5 litres above 590. Ian and Judy