The origin of horsepower as a measure of engine strength

One horsepower is the force needed to move 33,000 pounds a foot in a minute. The Scottish inventor, mechanical engineer, and chemist, who decided this was James Watt. He wanted to make the power of steam engines more understandable. He lived (1736-1819) in a time when the car was yet to be invented but there were steam engines – new-fangled devices. He improved on Thomas Newcomen’s 1712 Newcomen steam engine with his Watt steam engine in 1776.

James Watt and his creation of the unit of measurement called hosepower
James Watt and his creation of the unit of measurement called hosepower. Image: MikeB from images in the public domain.

Horses were selected for the obvious reason that they were commonly used for transportation and people could relate to their strength. Watt visited London breweries which used strong dray horses and carried out some measurements and came up with the figure above.

Dr Morris states that he ‘decided to multiply it [the true horsepower] by 1.5 to produce his official figure for the power of one horse’. The reason for the 1.5 multiplication? It was ‘in order to rate his steam engine conservatively in terms of horsepower’. A 10 horsepower (hp) engine has the power of 15 dray horses. THe average horsepower today for a typical family car might be 120 HP which represents on my calculation the equivalent of 180 dray horses.

It seems to me that the overrating my James Watt was a bad decision.

Morris believes that he did this to make the power of his steam engines surprisingly good. Watt’s unit of horsepower was criticised as ‘a new and shockingly unscientific unit…insensibly coming into use’. It was a popular unit of measurement, nonetheless. Motor cars which came later appeared to have great power. And it is still used today over 200 years later.

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Post Category: Horses