THE NOMINAL HORSE POWER OF ENGLISH TRACTION ENGINES

intended or expected that you should print my letter, but
nevertheless it seems to have caused some interest among your folk
for I already have letters here to answer from your readers. This
is most encouraging.

Now Mr. Pratt of Capel St. Mary, who is a very good friend of
mine, tells me that you have asked him about our rating of engines
by their NHP and he asked me to answer you on that point. You will
understand that I am in no way specially qualified to do so, except
that I have handled engines of a variety of different sizes, and
discussed the point with men of lifelong experience. All the
figures quoted are entirely estimates from my experience, and not
out of books, so some people may tend to query some of them. As one
of your readers has written me on the same point I have done my
comments on a separate sheet so that I can send a carbon copy to
him without typing it all out twice, so please find this
enclosed.

May I wish you every success for the ALBUM and yourselves in the
New Year. Yours truly.

The method of indicating the size of traction engines by the
‘nominal horse power’ is as old as the engines themselves
and its origins are lost in the mists of time. It is not based on
any accurate scientific method, or mathematical calculation, and
serves merely to convey an approximate idea of the size and
capability of the engine relative to other similar engines.

Prior to the general adoption of traction engines a large number
of portable engines were sold for use on farms for driving
threshing and other machinery, most of which had previously been
turned by horses walking round and round pulling the spokes of a
‘horse gear’ which was connected to the machinery by
gearing and a drive shaft with primitive universal couplings. Thus,
when buying a ‘portable’ a farmer of those days was
interested in knowing how many horses working a horsegear the
engine would replace. Thus, portables came to be known as ‘4
horse’ etc., and when portables were modified so as to be able
to move themselves, and so from the earliest traction engines the
system was naturally retained. Thus we have engines ranging from
‘3 horse’ to about ’20 horse’ in common use.
Generally speaking, the three’s and four’s were
‘tractors’, that is, small haulage engines built to weigh
under 5 tons in order to gain certain concessions under early
highway legislation. Threshing engines were mostly five, six or
seven, exceptionally eight horse. Road locos. of five, six, seven,
eight or ten horse were usual, while ploughing engines (cable
system) were commonly ten, twelve, fourteen, or sixteen horse and
exceptionally twenty.

Three horse (also referred to as 3 ton) tractors were found to
be too small to be economic, and were few in number, although a
very few have survived and Mr. Pratt’s engine of this type
appeared in the ALBUM some time back. Hence it will be seen that
one and two horse engines are not to be expected, but one never
heard mention of a nine, eleven, thirteen or other odd number above
these, in spite of fives and sevens being very common.

While boiler and cylinder sizes increased with the horse power
there does not appear to have been any fixed or universally
accepted relation between them, or between the cylinder sizes of
singles and compounds of the same N.H.P. Likewise boiler pressure,
which is bound to influence the power actually produced is rarely
taken much into consideration, although this is to some extent
offset by the comparatively small range of boiler pressures found
in any given class of engine. Thus it became usual to accept the
makers ratings at the N.H.P. of an engine as a guide to its size.
Nevertheless confusion has arisen, and doubtless will do so again,
while I know that my own 7 NHP ‘Queen Mary’ was at one time
described in print in all good faith as an eight.

It is readily appreciated that all this is little help to you to
visualize the sizes of our engines so I append a few notes with the
object of giving some idea of the various types and sizes commonly
found here.

Three NHP tractor, weight 3 tons (tons of 2240 pounds) would
pull about 5 tons on the road. To look at, was about the size of an
‘Oliver 90’ gas tractor.

Four NHP tractor, the commonest tractor, weighed 5 tons (or 6
with show fittings, as my Peter Pan in the last ALBUM) would pull
about 12 tons on the road at 5-10 mph. Were usually compound. Stood
on as much ground as one of your automobiles.

Seven NHP Road Loco would weigh about 13 tons (16 if Showmans)
and pull about 30 tons. About 20 feet long and 7 feet wide. A
popular size.

Seven NHP threshing engine would be a good deal lighter, say 11
tons and would drive the biggest of our threshers with heavy baler
behind it.

Ten NHP Road Loco, would weigh 16-18 tons and pull 50. A big
engine for the specialist heavy haulage contractor or the biggest
Showmen.

Sixteen NHP ploughing engine would weigh perhaps 20 tons or more
and pull eight deep furrows in the heaviest land.

When ‘driving light’ on fairgrounds the load is
absolutely continuous, sometimes for 8 hours on end, and to ensure
adequate steam some Show engines were built with a ‘7 horse
engine on an 8 horse boiler’, but otherwise it was assumed that
the engine was so designed that the boiler could supply adequate
steam for normal work (according to the type of engine) if properly
fired on reasonable coal. Straw or wood burning traction engines
were not built for work in this country.