The Steamboating Forum

TYPICAL STEAM LAUNCH ENGINES - OUTPUT POWER of the Engine

Output Horsepower, "Brake Horsepower", BHP = P L A N / 33,000

P is defined as the Brake Mean Effective Pressure (BMEP), in Pounds per Square Inch.

This number could theoretically be as high as the main steam pressure minus the engine exhaust pressure, but using this quantity, while giving the maximum theoretical power of the engine, would give a very uneconomical engine.

Typically for small launches the BMEP is about 40% to 50% of the main steam pressure, the lower fraction tending to the more efficient engines.

L is the length of steam piston stroke, in Feet

A is the Area of the cylinder bore, in Square Inches, equal to (3.14 / 4) x Cylinder Bore Squared.

N is the number of power strokes per minute, generally equal to the engine RPM x Number of Cylinders x 1 (for a single acting engine) or 2 (double acting engine)

For example, taking a 2 x 2 Double Acting Single Cylinder Engine, with 100 PSIG Steam, run at 300 RPM, with reasonable economy:

P = BMEP = 50 PSI
L = Stroke in Feet = 2 inches x 1 ft/12 in = 0.167 Feet
A = Bore Area = (3.14 / 4) x 2 x 2 = 3.14 square inches
N = Power Strokes per Minute = 300 x 2 = 600 Power strokes per minute

BHP = PLAN/33,000 = 0.5 Horsepower

Another example, a Tiny Power 3 inch bore x 4 inch stroke Single Cylinder Double Acting Engine, 350 RPM, with 150 PSIG steam, and Vacuum exhaust: PLAN/33,000 = 87 * 0.333 * 7.06 * 700 / 33,000 = 4.0 BHP

Another example, a 4 inch bore x 3 inch stroke Single Cylinder Double Acting Engine, 350 RPM, with 150 PSIG steam, and Vacuum exhaust: PLAN/33,000 = 87* 0.25 * 12.56 * 700 / 33,000 = 5.7 BHP

Another example, a 3 inch bore x 3 inch stroke Twin Cylinder Double Acting Simple Engine, 400 RPM, with 150 PSIG steam, and Atmospheric exhaust: PLAN/33,000 = 70 * 0.25 * 7.06 * 1600 / 33,000 = 6.0 BHP

Another example, a 3 inch bore x 2.99 inch stroke Three Cylinder Single Acting Simple Engine, 1000 RPM, with 200 PSIG steam, and Atmospheric exhaust: PLAN/33,000 = 94 * 0.249 * 7.06 * 3000 / 33,000 = 15 BHP

Thanks for posting these Fred.

Is that a Blackstaff-Wood compound I spy?

Not sure what type of engine here, just a nice looking compound found in my photo archives.

Of course, finding the output of a compound is somewhat more difficult than a simple engine, as the HP cylinder works between Main Steam pressure and intermediate pressure (usually aboit 1/3 HP inlet), and the LP cylinder works between the intermediate pressure and exhaust pressure. There is a good discussion about output of compound engines in the "engines and boilers" section of this forum. One method for this evaluation involves calculations for two "engines", the first is the HP cylinder, the second is the LP cylinder, then the two outputs are added for the total power.