The unit or measure of a horse power is the amount of power required to lift 33,000 lbs. one foot high in a minute.[62]
[62] See [page 407], Vol. II.
Nominal horse power is a term used to represent the commercial rating or power of an engine, and is usually based upon the area of the piston. It gives no measure of the engine power, however, because it does not take the piston speed into account.[63]
[63] See [page 374], Vol. II.
In a surface condensing engine the duty of the air pump is to merely pump the condensed steam and vapor from the condenser to the hot well, whereas in a jet condensing engine it has to also take the condensing water from the condenser, hence an air pump for a surface condenser may be made smaller than that for a jet condenser. As the air pump works against the pressure of the atmosphere, therefore the smaller it is the less of the engine power is absorbed in working it.
The injection cocks are regulated for opening by rods having handles attached. If the injection cocks are not open wide enough, the condenser will get hot and impair the vacuum, while if opened too wide, the water in the hot well will be cold and the boiler feed will be cold. These cocks should be so regulated as to keep the temperature in the hot well at about 100° Fahrenheit.
The parts of a marine engine that are exposed to danger in a cold climate are all pipes through which cold water circulates, and are liable to freeze.
The precautions necessary to prevent freezing in cold climates are to cover all pipes liable to freeze, to keep the water circulating through them, or to let it out of them if necessary, as in the case of the engine standing.
A marine engine may fail to start, or may be prevented from starting by the following causes:
1st. The H. P. slide valve may be off, or away from its seat, thus admitting the steam to both sides of the piston at the same time.