Horsepower Estimation 
The Rating of Power
by E. W. Graeff
A word to the manufacturers is in order just now, on the subject of rating the power of their engines, for there certainly is something wrong when one manufacturer sells a 4x4 engine as 1½ h.p., another sells one of the same dimensions as 2 h.p., another as 2½ h.p. and another as 3 h.p., each claiming that he under rates rather than over rates the power of his engines. These conditions hold good in other sizes, and show very conclusively that the manufacturers never really test the exact power their engine delivers at the number of revolutions that it really attains when driving the propeller sold with it and installed in an ordinary type of boat.
Many manufacturers believe that they do test their engines, propellers, etc., by erecting a small tank, filling it with water, and turning up a propeller in it to the given number of turns. This means of testing is extremely crude, as the same conditions do not exist in the tank as when the propeller is in solid water under the stern of the boat and driving the boat ahead, as the water in the tank is immediately churned up and a sort of cavitation forms behind the propeller and entirely destroys the value of testing in this way.
Others test by a brake band on the flywheel, which, as you all know, is a crude way, unless the greatest care is taken in every detail. This care is not given as a usual thing, but the engine is put on the testing block, and run in, as they call it; then the brake band is put on and the engine is run for a few minutes until it pulls the desired number of pounds on the scale attached to the brake. The engine is then considered thoroughly tested; but when the buyer gets it, it may not develop the power claimed.
The surest and most accurate way of testing an engine is to attach it to a dynamo and require it to keep the right number of lights to their proper brilliancy for at least a half hour, or by other means, require it to come up to a standard electrical test for that length of time, for if an engine will carry a load for half an hour it is reasonable to believe that it will carry it indefinitely.
Guessing at power and then selling the engine at a guessed rating is hardly fair to the purchaser, and certainly is no credit to the manufacturer, even though they usually do guess to under-rate the engine and give more power than claimed.
The only fair way to rate the h.p. of an engine is to name the exact power it develops when placed in an ordinary type of hull, using the standard propeller furnished with the outfit, and I sincerely hope that the manufacturers will give the matter attention and get down to a way of rating their engines which will be fair, not only to the buyer, but to the manufacturer himself.
(Transcribed from The Rudder, April, 1903, p. 241. )
[Thanks to Greg Calkins for help in preparing this page. — LF]