Most boats use between 1.0 and 1.5 litres per hour, my own Beta 43 habitually around the 1.2 mark. I suggest were in the area of diminishing returns here, even at well over £1 per litre, certainly in trying to cut costs further by changing the power plant. However, 3 mph cruising is probably why I see 1.2lph. With a displacement narrowboat hull, particularly in the shallow water on canals, every incremental increase in speed has a disproportionately ruinous effect on fuel consumption. Add that to the effect of the wake on the environment and the number of poorly moored boaters shouting at you to slow down, being more cognisant of cruising speed is a win-win-win situation.