Hi BP ..looks like there is not much difference in consumption between the 25HP mariner two stroke that I have and the 15HP the guys have as our figures are roughly similar
For the longer multi day trips I personally carry 100 litres even if not using it all .. and don’t plan longer trips as 100 litres is enough to carry on an inflatable ..it means I can go almost 180 miles without having to think about refuelling. I don’t trust garages in remote Scotland to be open when I may want them.. few are 24 hours open.
I sit on top of 65 litres so it is not taking up space that I can use ( I don’t smoke )
Another 35 litres is kept under the front bench.
I carry a bottle of 2st oil and mix only when I fill the 25 litre tanks. I also carry 20 litres of water .. 8-16 tins of Guinness and a flask of whisky for emergency medical use .. so there is a lot of liquid sloshing about the inflatable on an average Gurnard multi day adventure.
Kaman and HH with their 20 HP four strokes .. carry half the fuel I do ..think HH said he used 33 litres which is half my usage. My advice .. always carry plenty liquids.. its better than drying out... you can get storm bound and use more than expected.