How many horsepower?
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.