TONY BROOKS’ VIEW

Canal Boat - - Technical -

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’

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