TONY BROOKS’ VIEW
This is an interesting piece but the figures are likely to be subject to a fair degree of variation in real life.
Other web-based sources give a lower energy density, one at just over 32MJ/litre. I have also seen a typical narrowboat power requirement at canal speed as around 3hp.
I am also cautious about the thermal efficiency figure used. One web source claims around 55%, my text book says 35 to 40%. Then we have a mixture of direct and indirect injected engines. The direct injected ones will be a few percentage points more efficient.
Then we need to consider exactly what the engine was driving when the efficiency measurements were taken. For example, the coolant water pump takes some energy and that is necessary to allow the boat to move. So is some alternator output to supply any instruments and ensure the batteries are charged enough to work the glowplugs (if fitted) and start the engine next time.
Then there is the question of power losses in the gearbox, shaft and the propeller. We are back to the acknowledged difficulty in getting realistic figures for the horsepower required to drive a boat. In any case, consumption is likely to increase to the square of the speed.
The more usual way of quoting typical canal fuel consumptions is in litres per hour, typically between one and two litres per hour. How far you travel in that hour must depend upon water depth and lock usage. I would not like to try to convince an HMRC officer that fuel spent in lock working is not a necessary part of moving the boat.
That said, the general gist suggested by Simon is spot on.
‘All we need to know is how far we have travelled in miles and divide this by three, this gives us the fuel used for propulsion in litres’