Paddleducks
Other Marine Models => Live steam => Topic started by: Roderick Smith on October 19, 2006, 12:10:53 PM
-
I am looking at full size rather than model, but thought it better to place this query here rather than in research.
I am trying to ascertain the formula by which a paddlesteamer's engine horsepower derived.
I have just had two helpful replies from Ronnie, of
www.rappahannockboatworks.com
and www.tinypower.com
His current engines are rated according to their actual power output measured on a dynamometer.
This would not have been true a century ago.
In the world of automotive engineering, internal combustion engines were rated on a formula based on bore and number of cylinders, but the formula ignored stroke. At the time, the formula accorded with actual power, but soon the power output was much greater than the rating. The rating survived, as registration fees were based on it.
Similarly in railway engineering, the tractive effort of a steam loco is based on a formula involving boiler pressure, piston area, stroke, wheel diameter (and the number of cylinders). The result is force, not power.
Ronnie suggested that the rating of early paddlesteamer engines was simply the cylinder diameter in inches. This would be very crude, and assumes that all engines are running on comparable pressure.
I don't have many figures available to support/refute this formula.
In my copy of Parsons 'Ships of the inland rivers', PS Adelaide is shown as having two cylinders of 14 in bore and 36 in stroke, and is rated as 36 hp. The diameter formula would rate it as 28 hp. Perhaps boiler pressure does get included? Parsons doesn't give that figure. IIRC 120 psi saturated is typical of Australian paddlesteamers; PS Melbourne has 150 psi, and a compound engine.
The calculation becomes more complex for compound engines.
In coming up with a theoretical figure, one has to know typical cut-off figures. Railway steam locos have variable cutoff, but I don't recall seeing any such control on the paddlesteamers which I have ridden. Ronnie's range does have variable cutoff.
There is an important control to get the balance right in triple compound engines. Is this a cutoff control?
Regards,
Roderick B Smith
Rail News Victoria Editor
-
regardless of size 1 cubic foot of water evaporated per hour equals one nominal HP
HP = PLAN
33,000
P=POUNDS PER SQUARE INCH
L=STROKE LENGTH IN FEET
A=AREA OF PISTON IN SQUARE INCHES
N=NUMBER OF STROKES PER MINUTE
...( 33,000 FEET PER MIN OR 550 FEET PER SECOND...= 1 HP)
P is Mean effective pressure eg at the valve chest or boiler it depends on distance and valve cut off
subtract 6% to 20% for friction to get the net effective hp
this is the formuls I use hope it helps
cheers Anthony
area of piston will be diameter squared x .7854 :D
]