On 24 Jun 2006 at 10:37, Allan Jones wrote:
> <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> > All I can remember from school about Horsepower was that one was
equal to
> > 550 somethings-or-other per hour (but then a 40 year gap may have
confused
> > my
> > recollections). On the other hand my dad says horsepower has no
absolute
> > relation to engine power it is an arbitrary taxation calculation which
was
> > based
> > on either engine cylinder bore or stroke, but illogically not both.
>
> You're both right. They may (or may not) have been the same
originally, in
> the early 20th century, but soon became completely different as
engines
> became more efficient. In effect you can think of them as two different
> things (an engine power output, and an arbitrary tax calculation) that
> happen to be called by the same name.
>
> One horsepower is actually about 750 watts (0.75 kilowatts).
>
> Note also that a horse can produce considerably more than one
horsepower for
> a short time - IIRC about 8 hp for a while - as the original unit related to
> the average power a horse could produce over a whole day (including
time off
> for rests, mealbreaks, etc).
>
> > Clearly my 25 HP (or is it BHP?) is very different from the 20 ish HP
from
> > an SR or JP2 or whatever.
>
> No it's exactly the same. The difference is that the old engine
produced it
> at lower engine revs than your modern engine does. The modern
engine is
> usually geared down so that the prop shaft revolves slowly; if it was
geared
> down to the same speed as the old engine, then it could swing the
same big
> prop if there was room to fit it under the boat, but more commonly on a
> modern boat you use a smaller prop and don't gear the engine down
quite so
> much.
Hmm, not entirely as suggested above. I took the following from a
website about steam engine manufacturers but it is equally applicable to
diesels. The Bolinder with a 15hp rating is in fact 15 nominal
horsepower, which is the equivalent of about 90 brake horsepower. So it
all depends on what you are measuring.
This page contains references to brake horsepower (BHP), nominal horsepower
(NHP), and indicated horsepower (IHP).
BHP is a measure of the rate at which an engine does work measured by
resistance to an applied brake (i.e. the power available from the engine to do
useful work, discounting friction and energy losses within the engine itself).
One
horsepower = 550 foot lbs per second or 745.7 watt (in the USA 1 horsepower =
746 watt).
NHP, as used in steam engines ratings during the 19th and early 20th centuries,
was a commercial unit used by engine manufacturers and purchasers. It was
adopted by the Royal Agricultural Society in the 1840s to enable farmers to
compare the power of a steam engine with that of a horse. NHP was calculated
by reference to cylinder bore size and piston speed and, unlike BHP, was not a
measure of an engine's actual power output. In the case of single cylinder
steam
traction engines, one NHP is broadly equivalent to between 6 and 7 BHP, but
generally closer to 6 BHP. For a compound engine the figure may be closer to 7
BHP. On this basis a single cylinder 5 NHP traction engine might be expected to
produce a little over 30 BHP. Stationary engines were much more conservatively
rated. Information in early 20th century Paxman catalogues shows that for the
Company's horizontal stationary engines one NHP was equivalent to only around
2 to 2½ BHP. The actual output of an engine depended not only on cylinder size
but also on working steam pressure and engine speed. (On another page we have
more about NHP.)
IHP is a measure of the power output of a piston engine calculated from the
mean effective pressure as derived from an indicator diagram and the speed of
the engine in rpm.
--
David Kitching
http://www.brocross.com
fearrmeox adlaþ brægen
[Non-text portions of this message have been removed]
------------------------ Yahoo! Groups Sponsor --------------------~-->
Great things are happening at Yahoo! Groups. See the new email design.
http://us.click.yahoo.com/TISQkA/hOaOAA/yQLSAA/ygtolB/TM
--------------------------------------------------------------------~->
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/canals-list/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
