Improvements in economy.-In consequence of the improvements effected, the consumption of coal with the most recent engines is less than one-third that required for the engines generally used before 1860, and the effect on warships of this great reduction of coal expenditure has been twofold:

a. The increased distance ships are able to steam without exhausting their coal supply has rendered seagoing mastless armour-clad ships possible.

b. The reduced quantity of coal necessary to be carried for the same radius of action has enabled space and weight which would formerly have been required for coals to be devoted to other objects in order to increase their offensive or defensive powers.

Corresponding benefits have also been derived by the mercantile

marine.

Forced draught. The conditions of service of ships in the Royal Navy render it necessary to provide for the development of high power and speed on special occasions, such as the events of action, chasing, &c., although the greater portion of the work of the ship has to be performed at comparatively low powers. It is therefore desirable in warships to provide special means of forcing the boilers when the full speed is required. Formerly a steam jet in the chimney was used for this purpose, but this wastes a lot of fresh water. In 1882 the system of forming the stokeholds into closed compartments, and keeping them under air-pressure by means of blowing fans was adopted and continued to the present day, with results that are satisfactory, provided only a moderate pressure of air be used, and by this means the steam generating powers of the boilers have been largely increased. Details of the fittings required for this purpose are given in Chapter V. The introduction of forced draught has enabled the weight of machinery to be considerably reduced, and the average weight of machinery for the latest modern warships fitted with circular boilers is about 1 cwt. per I.H.P. developed with moderate forced draught. A lesser weight than this, viz., 1 cwt., was at one period allowed, but this is not now recommended.

From this brief sketch a general idea may be formed of the progress that has been made in marine engineering. The machinery of the Salamander,' built in 1832, weighed 275 tons, developed 400 I.H.P., and consumed 7 to 8 lbs. of coal per horse-power. In modern warships, machinery of the same weight would, under moderate forced draught, be capable of developing satisfactorily at least 3,000 I.H.P., with about one-fourth the consumption of coal per horse-power, and the space occupied would be considerably less. Another important feature is the great increase in the total power now available for the propulsion of vessels at high speeds. For example, in H.M.S. "Terrible,' which in 1845 represented the finest type of steam warship of the day, the I.H.P. was less than 2,000, and her speed about 10 knots, while in the present H.M.S. 'Terrible,' a first-class cruiser, the horse-power is 25,600, and the speed 22.8 knots. In a later series of cruisers, the Drake' class, the power is still greater, viz. 30,000 I.H.P. Future progress.-Quite recently water-tube boilers, of various types, have been adopted in the Navy with steam pressure in boilers of 300 lbs. and engines working at 250 lbs. per square inch. A great

C

impetus will probably be given to the use of higher steam pressures by the more extended use of this type of boiler, since there is then, within moderate limits, but little increase of boiler weights involved by higher pressures, the only increase of importance being in the engine. Probably the near future will see a general advance of steam pressure coupled with the use of the water-tube boiler, and, especially in the mercantile marine, the development of quadruple expansion engines.

Considerable progress is still possible as regards the boiler in the reduction of the great waste of heat which now takes place, due either to incomplete combustion, or the inability of the heat-absorbing surfaces, as now arranged, to prevent a serious loss of heat in the escaping gases. Further reductions in coal expenditure may be expected in the future under each of these heads.

More attention seems necessary also as regards the mechanical efficiency of the engines used in large vessels. Careful tests in this direction would probably point out many ways in which improvement would result.

CHAPTER II.

WORK AND EFFICIENCY.

Force, work, and energy.-Force is that which acts in producing or resisting motion in a body, and may be represented by a pressure or a pull. The British unit of force is the weight of one pound avoirdupois, and forces are therefore expressed generally as being equal to so many pounds weight.

A force is said to perform work when by its action resistance is overcome and motion produced. This union of force and motion is essential to the conception of work. However great the pressure applied, unless the body acted on be moved, no work is done. Energy is the term used to signify the capacity of a body for doing work. For example, if a force acts through a certain distance it is said to exert energy, while the resistance overcome through a certain distance by means of this exertion is the work done.

Measurement of work and energy:-The amount of work done is measured by multiplying the magnitude of the resistance—or, in other words, the force opposing the motion-by the distance through which the resistance is overcome, estimated in the direction of the resistance. Energy is measured in a similar manner.

The British unit of work is the foot-pound, which is a very convenient term, implying the combination of force and motion, which is the essential condition for the performance of work. One footpound represents the amount of work done in raising a weight of one pound through a distance of one foot, or more generally the exertion of a pressure of one pound through the distance of one foot. If 20 pounds be raised 50 ft., the amount of work performed is represented by 20 × 50 =1,000 foot-pounds.

Sometimes for convenience other units of work are used, but they are all formed on the same basis and expressed in a similar manner. For example, the work performed in raising one ton one inch is sometimes called one inch-ton, and it is equal to 2,240 inch-pounds or 2240 foot-pounds. The work of lifting one ton one foot is one foot-ton, and so on It will be seen that the different terms used are selfexplanatory and are convertible one to another. The foot-pound is, however, the general unit, the others only being employed for convenience in special cases.

12

Power, horse-power. In the conception of work and energy no question of time enters. When, however, we consider also the time taken to perform so much work, we are considering power. Just as the term work necessarily involves distance, so does the term power

involve time as well as distance. Power may be defined as the rate at which work is done. The natural unit of power would be the power of doing work at the rate of one foot-pound per minute, but it is too small to be convenient in engineering. The unit of power adopted is the power of doing work at the rate of 33,000 foot-pounds per minute. This unit of power is termed a horse-power.

Efficiency. In every machine there are always certain causes acting that produce waste of work, so that the whole work done by the machine is not usefully employed, some of it being exerted in overcoming the friction of the mechanism, and some wasted in various other ways. The fraction representing the ratio that the useful work done bears to the total energy exerted on the machine is called the Efficiency of the machine; or

In the propelling apparatus of a vessel, in which the useful work is measured by its effect in giving speed to the ship, there are four successive stages, in each of which a portion of the initial energy wasted, and these four stages all tend to decrease the total efficiency.

is

In the first place, only a portion of the heat of combustion of the coal is communicated to the water in the boiler, the remainder being wasted in various ways. The fraction of the total heat that is transmitted to the water in the boiler is, in ordinary cases, not more than 6 7

from to This fraction is called the Efficiency of the boiler. 10 10'

Secondly. The steam, after leaving the boiler, exerts energy on the piston of the engine; but, in consequence of the narrow limits of temperature between which the engine is worked, this energy represents only a small fraction of the total heat contained in the steam. The fraction varies very considerably, depending on the type of engine, 1 1 its steam pressure, rate of expansion, &c.—say from to In large 5 20' 1 1 5 9°

modern marine engines it may be taken as from to This fraction,

representing the ratio of the energy exerted by the steam to the total amount of heat expended on it, is called the Efficiency of the steam.

Thirdly. In the engine itself, a part of the energy exerted by the steam on the pistons is wasted in overcoming the friction of the working parts of the machinery, and in working the pumps, &c. The remainder appears as useful work in driving the propeller. The fraction, representing the ratio that this useful work bears to the total energy exerted by the pistons, is called the Efficiency of the mechanism 8 8

Its value is from to the former being more general,

10 10'

Fourthly. The propeller, in addition to driving the ship, expends some of the energy transmitted to it in agitating and churning the water in which it acts, and the work thus performed is wasted; the only useful work being that employed in overcoming the resistance of the ship and driving her through the water. The ratio of this useful work to the total energy expended by the propeller is called

the Efficiency of the propeller. It may be taken as averaging from

5 6 to

10 10°

The resultant Efficiency of the marine steam-engine or the whole propelling apparatus is made up of the four efficiencies just stated, and is given by the product of the four factors representing respectively the efficiencies of the boiler, the steam, the mechanism, and the propeller. Any improvement in the efficiency of the marine steam-engine, and, consequently, in the economy of its performance, is therefore due to an increase in one or more of these elements, and we shall deal with these several points, and in each case describe the efforts that have been made to increase the efficiency.

The efficiency of the marine steam-engine will be seen to be very low. Taking the best case indicated by our figures, viz. that of an engine which has the maximum efficiency in each of the four components of the resultant efficiency, the efficiency would be :

The highest efficiency now attainable is, therefore, a little over 7 per cent. with the marine steam-engine, and is generally less-say nearer 5 or 6 per cent.

Further information respecting this is given under the respective headings in greater detail.