How many horse­power?

Canal Boat - - Back Cabin: Experts -

QHow many horse­power is my 1991 BMC 1.5 en­gine? I can’t find this in­for­ma­tion in the man­ual.

A CLARKE, via email

ATONY REPLIES… It isn’t straight­for­ward. What turns the pro­pel­ler is torque (twist­ing force), not horse­power. HP is ba­si­cally a com­bi­na­tion of torque and RPM.

The torque curve of a diesel is a sort of shal­low ‘hillock’: the torque climbs as the revs in­crease but then falls off with more speed. The HP curve keeps ris­ing be­cause the ex­tra revs make up for the re­duc­ing torque. Dis­place­ment boats tend to be propped for max torque: that is usu­ally close to the most eco­nom­i­cal speed, but means the max­i­mum revs the en­gine can pro­duce are less than quoted in the HP rat­ing. This quoted fig­ure might have been tested to var­i­ous stan­dards – with or with­out the wa­ter pump and al­ter­na­tor; max­i­mum (with a time limit) or a lower ‘con­tin­u­ous’ value.

From mem­ory, a BMC 1.5 revs to about 4,400rpm, but no way would you run one at that speed in a canal boat: 3,000 to 3,500 is more likely. So I would say that a ‘work­ing’ fig­ure will be be­tween 20 and 30hp, but in a man­ual it would prob­a­bly be quoted at more like 35hp.

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