The marine steam engine ..

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Contents

INTBODUOTOBY.

PAOB

I. HiSTOBY AND PbOGRESS OF MaBINE ENGINBERINa . 1

Appendix

PA6B

{A) A^Ucation of the Indicator Diagram to Determine the

Stresses on Crank-Shafts. Curves of Twisting Moments 483

{S) Effect of the Inertia of the Beciprocating Parts of the

Engines 485

(O) Extracts from the Board of Trade Bules relating to

Machinery and Boilers 491

(D) Lloyd's Bules for Boilers and Machinery .... 502

IND£X 509 r

THE

MAEINE STEAM-ENGINE,

Chapter I. . Bistort And Progress

The earliest steam eDgines were simply reciprocating engines, and for many purposes such engines are still used even at the present day. Until, however, a suitable method of turning reciprocating into rotative motion had been discovered and utilised not any progress was made in adapting the steam-engine to the propulsion of vessels. The adoption of the crank effected this desirable object, enabled the power of the engine to be transmitted to the propeller smoothly and without shock, and was an indispensable step in the progress of steam navigation.

The marine steam-engine may justly be considered as a production of the present century. In the latter part of the eighteenth century several attempts were made to adapt the steam-engine for the propulsion of boats, but none of them were quite successful. The first practical steamboat was built on the Clyde, in 1801, by William Symington, for Lord Dundas. She was called the ^ Charlotte Dundas,' and was worked for some time with success as a tug on the Forth and Clyde Canal, but was withdrawn from this service in consequence of an apprehension that the banks of the canal would suffer from the wash of the propeller. This boat was fitted with a single paddle-wheel placed near the stem, driven by a horizontal direct-acting engine, with connecting-rod and crank, and the general arrangement of her machinery would be considered creditable even at the present day.

The first recorded instance of steam navi<gration proving commercially successful was in America, where, in 1807, Robert Fulton built a s':eam vessel called the ' Clermont,' propelled by paddles driven by a Boulton (fe Watt engine. In 1812 Henry Bell built a vessel called the ' Cornet^' which was successfully worked on the Clyde as a passenger steamer between Glasgow and Greenock. The ' Comet ' was propelled by two pairs of paddles, each paddle having four floats or blades, somewhat resembling a pair of canoe paddles, crossed at right angles. The paddles were driven by an engine of somewhat peculiar design, which, however, approximated to the side-lever engine of a later day. This small boat was the first passenger steamer in Europe.

B

From this date the success of steam navigation may be said to have been secured, and the advancement that has been made since has not consisted so much in the discovery of new principles as in the extension of old ones, and the introduction and development of improved mechanism and workmanship, with consequent economy of fuel. The result has been a progressive increase in the size, power, and speed of steamships and in the extent of their voyages ; so that at the present day we have ships displacing 19,500 tons, and capable of being driven at a speed of 22 knots by engines developing more than 30,000 indicated horse-power, while even larger vessels are under construction.

Side-lever engine. — The propeller used in the earlier steamships was invariably the paddle-wheel, and the type of engine existing and giving satisfaction on land was naturally adapted at first to rotate

Fio. 1.

these paddle-wheels. Almost all these early enprines were, therefore, of the beam type. In America the beam was generally placed over the crank, while in this country it was placed below the crank. The latter type of engine was known as the side-lever engine. The general arrangement of the side-lever engine is shown in outline in Fig. 1, and it represents the type usually fitted not only in the first steam vessels, but also for some years after.

On the top of the piston-rod is fixed a crosshead with side-rods, s, attached at each end, which, passing down on either side of the cylinder, are connected to the ends, a, of a beam or side-lever, a b, oscillating on a fulcrum or gudgeon at its centre, c. The opposite ends, B, of these side-levers are fitted with journals carrying the crosstail, to the centre of which, one end of the connecting-rod B D is attached, the other end working on the crankpin d. The air-pump e is worked by side-rods from intermediate points in the side -levers, the upper ends of the air-pump side-rods being jointed to the opposite ends of the air-pump crosshead, to the centre of which the air-pump rod is secured. The piston-rod crosshead works in vertical guides to insure parallelism, and the parallel-motion rods used in land beam engines are dispensed with.

Orasshopper engines. — The arrangement of the side-levers was sometimes varied by making them levers of the third order, the gudgeon or fulcrum being at one end and the steam cylinder placed between the gudgeon and connecting-rod. These engines were commonly known as grasshopper engines.

The side-lever type of engine, though very heavy and occup3dng a large space for the power developed, was safe and reliable, securing a sufficient length of connecting-rod, and having its moving parts practically in equilibrium. It consequently continued in general use for a great number of years, but was at length superseded by the direct-acting type, which was lighter and more compact.

Introduction of steam war vessels. — Steam vessels were introduced hito the Royal Navy in the year 1820, when the ' Monkey,' a vessel of 210 tons, was built at Rotherhithe and fitted by Messrs. Boulton & Watt with engines of 80 nominal horse-power. There were two cylinders, about 35^ in. diameter and 3 ft. 6 in. stroke, working at 26 .V revolutions per minute, giving a mean piston speed of 185 ft. per minute. She was followed in 1822 by the ' Active,' of 80 nominal horse-power, by the same firm, and in 1823 by the ' lightning,' of 100 horse-power, by Messrs. Maudslay, and some others whose names appeared for the first time in the Official Navy List for March 1828. These early steam vessels were mainly used for towing and general purposes, and could scarcely be classed as war vessels. Between this date and 1840 seventy other steam vessels were added to the Navy, the majority being fitted with flue boilers and slow-moving side-lever engines worked with steam at a pressure of 4 lbs. per square inch above the atmosphere.

The ' Rhadamanthus,' one of these ships, was fitted with side-lever engines and flue boilers by Messrs. Maudslay, Sons, <k Field in 1832. The nominal horse-power was 220, but the engines were capable of being worked up to 400 I.H.P., or 1*8 times the nominal power. The load on the safety valves was 4 lbs. per square inch, and the number of revolutions per minute when working at full power 17^, giving a mean piston speed of 175 ft. per minute. The total weight of the machinery was 275 tons, or 13*75 cwts. per I.H.P. developed.

Between 1840 and 1850 tubular boilers were introduced. In these

b2 boilers a group of small tubes was substituted for the long winding flue, to convey the heated gases from the furnaces to the chimney. The boilers were thus made lighter and more compact, and the working pressures of steam generally were increased to from 10 to 15 lbs. per square inch above the atmosphere.

Abandonment of side-lever engines. — Attempts were soon made to reduce the space required by the machinery, and the side-levers were abandoned and direct- acting engines fitted for rotating the paddle-wheels. Several arrangements of this kind were fitted, the two best known being the double-cylinder engine by Messrs. Maudslay and the oscillating engine adopted by Messrs. Fenn. Fig. 2 shows the double-cylinder engine^ which consisted of two equal cylinders side by side, the piston-rods from the two cylinders being connected to a single crosshead. In order to get sufficient length of connecting-rod, the crosshead was of peculiar form and pass^ down between the cylinders, having a journal at its lower end, on which one end of the connecting-rod worked, the other end being attached to the crankpin.

Fig. 3 shows the general arrangement of the oscillating engine, which is the simplest and most compact type for driving paddle-wheels. This type of engine, although first fitted for marine purposes by Messrs. Maudslay, Sons, & Field, who in 1828 fitted a pair of oscillating engines into the steamship ' Endeavour,' and subsequently in several other ships, was adopted and perfected by the late eminent engineer, Mr. John Fenn, with whose name it is now generally associated. In these engines the connecting-rod is altogether dispensed with, the upper end of the piston-rod being fitted with brasses to work directly on the crankpin, and the cylinder itself is carried on trunnion bearings, to allow the necessary oscillation to suit the motion of the crank. The trunnions are hollow, and the steam is admitted' to and exhausted from the cylinders through them. In this type of engine space and weight have been economised as far as is possible for paddle-wheel engines, and the majority of engines now made for paddle-wheel vessels are on this plan.

The * Magicienne ' was one of the best specimens of the steam war-vessels of that period. She was fitted with oscillating engines by Messrs. Fenn in 1850. The pressure of steam in the boilers was 14 lbs. per square inch, number of revolutions per minute at full power 20^, giving a mean piston speed of 287 ft. per minute, with a maximum I.H.P. of 1,300. The total weight of the machinery was 275 tons, or 4-23 cwts. per I.H.F.

Defects of paddle-wheels — The paddle-wheel possessed many practical disadvantages which interfered with progress beyond a certain point. Its performance was much affected by the variation of draught of the ship during a voyage, as the coal and stores were consumed, and the paddle-boxes offered resistance to the progress of the vessel. For fighting ships paddle-wheels were particularly unsuitable. The wheels themselves were exposed to danger from shot and shell, and the paddle-boxes interfered seriously with the training and working of the guns, while the shafting and many parts of the engines had to be considerably above the water-line, much of it above the upper deck. The paddle-wheel also is not a form of propeller well adapted for the application of high powers.

Adoption of the screw propeller. — The adoption of the screw propeller in lieu of the paddle-wheel was the most important step taJcen in the progress of marine engineering, for this rendered many subsequent advances possible. Its principal advantages, as compared with the paddle-wheel, are, that it is comparatively little affected by the rolling, or by the variation of the draught of the ship during a voyage, and it is equally capable of application to either great or small powers. It is not exposed to damage by projectiles, and also permits of the engines being kept below the water-line, which is very important in unarmoured warships. Screw engines, whether horizontal or vertical, can be further protected, if necessary, by being kept below the steel armour deck, with armour gratings in the necessary engine-room hatchways and other openings, while in the larger class of war vessels, such as the battleships and large cruisers, their height is so moderate that efficient protection can be given them by armour, even when the engines are vertical and the cylinders above the water-line. With screw engines the decks are also kept clear for the guns.

The substitution of the screw propeller for the paddle-wheel began to grow general about the period 1845-50. The screw propeller had been invented long before, but its practical utility had not been generally recognised, and it was still regarded as being in the experimental stage. The first notable experiments as to the comparative efficiencies of paddle-wheels and screw propellers were made in 1840, when the * Archimedes,' with a screw propeller, beat the paddle-wheel boat * Ariel ' between Dover and Calais by five to six minutes under steam and sail. The ' Archimedes ' afterwards beat the paddle-wheel steamers * Beaver' and * Swallow,' but was beaten slightly by the 'Widgeon.' The Admiralty, in 1843, caused some important experiments to be carried out with the screw ship ' Rattler ' and the paddle-wheel ship * Alecto,' and, in 1849, with the screw ship * Niger ' and paddle-wheel vessel ' Basilisk.' The results in each case were in favour of the screw propeller, and many valuable conclusions were deduced from the trials.

From that time the use of the screw propeller gradually became more general, till at the present day it is almost solely employed for marine propulsion, the paddle-wheel only being applied in special cases. It is not too much to say that ships of the class now traversing the ocean in all directions, both in the royal and mercantile navies, would not have been possible had not the screw superseded the paddle.

Gearing for screw engines and its abandonment. — In order to attain the same speed of ship the screw propeller had to be driven at a much greater speed than the paddle-wheel, and as it was not possible in the then condition of mechanical engineering to drive the pistons at a sufficiently high speed to enable the engine shaft to be connected directly to the propeller shafting, the earlier engines used for working screw propellers were geared, so that the screw shaft was caused to revolve at a much higher rate of speed than the engine shaft. A large spur wheel, keyed on the crank-shaft of the engine, worked into a pinion on the screw propeller shafting, so that the speed of the engine shaft could be multiplied on the screw shaft as might be necessary.

Before long, however, such improvements in workmanship and mechanical details were effected, that the speeds both of piston and mSTORY AND PROGRESS 7

of reyolation could be sufficiently increased to allow direct engines to be fitted In these the gearing is left out, and the crank-shaft connected direct to the screw shafting. In many marine engines at the present day, even of the largest size^ the mean piston speeds are as high as from 800 to 950 ft. per minute at the maximum power, while in the fiist-running engines supplied for torpedo boats and destroyers it rises as high as 1,200 ft. per minute, and in extreme cases to 1,400 ft It is probable that in the future of marine engineering the speeds may be increased even beyond this, in order to attain increased economy.

Horizontal engines. — The paddle-wheel engines were either vertical or inclined ; but when the screw propeller was introduced, and it became possible to place the whole of the propelling apparatus below the water-line, the engine was placed horizontally, and from that time, for about thirty years, the engines of warships were almost always of the horizontal type. One of the great obstacles that had then to be overcome in connecting the crank-shaft of the horizontal engine direct to the screw shafting was the close proximity in which the cylinder was necessarily placed to the centre-line of the ship, owing to the limitation of the beam of the ship, which made it difficult to get a connecting-rod of suitable length to work between the cylinder and the crank.

Trunk engines. — Mr. John Fenn solved this difficulty by his invention of the trunk engine. In this engine a large hollow trunk, cast on or bolted to the piston, and working through a steam-tight staffing-box on the end of the cylinder, was substituted for the piston-rod, and the connecting-rod was attached directly to a journal or gudgeon in the centre of the piston itself, as shown in Fig. 4. Though the use of a large trunk of this description does not at first sight appear desirable, yet the engines of this type have generally worked in a satisfactory manner, and they were amongst the most smoothworking and efficient marine engines employed. With the introduction of high-pressure steam, however, they became obsolete, owing to the difficulty of keeping the trunks in a steam-tight condition.

Eetnm connecting-rod engines. — This kind of engine was adopted by the majority of marine engineering firms to enable the horizontal cylinders to be brought close to the crank -shaft, and, as usually fitted, is shown in Fig. 5. There were two rods to each piston, one passing above, the other below the crank-shaft, to the opposite side of the ship, while the further ends of the piston-rods were fixed to a crosshead, having a journal at its centre, from which the connecting-rod worked back to the crank.

In some later examples, in order to obviate the disadvantage of having more than one stuffing-box for each cylinder, and simplify the design of the piston, a single, piston-rod was fitted, attached to a crosshead between the cylinder and the crank -shaft, from which two rods were carried, one above, the other below, the shaft, to a similar crosshead on the opposite side, as in the ordinary return connecting-rod arrangement.

Burect-acting engines. — The direct-acting engine shown in Fig. 6, having the connecting-rod between the cylinder and the crank, was often employed, especially by Messrs. Humphrys, in the later horizontal examples^ the parts being stowed as compactly as possible in the limited space available, and a short connecting-rod fitted. It is the simplest type, and the most suitable for general work, and, whenever sufficient room can be obtained, it is usually adopted. For vertical engines, with the cylinders at the top working down to the crank- shaft, which are how generally fitted for marine purposes, this type is universally adopted.

Early screw engines. — The majority of steamers, both war and mercantile, built during the years 1850-60, were fitted with horizontal screw propeller engines worked with steam of from 20 to 25 lbs. pressure per square inch. The engines had jet injection condensers, and were not remarkable for economy of fuel, but they were much lighter, and occupied considerably less space, than the paddle-wheel engines that preceded them. The mean piston speed in this type of engine was generally about 400 ft. per minute, and the weight of machinery about 3| cwts. per I.H.P.

Surface condensation. — The adoption of surface condensation, which became general about 1860, formed a most important step in marine engineering. Its value consisted not so much in the economy effected by the avoidance of loss from the brining of boilers, as in the fact that by its eliminating the element of danger resulting from deposit of solid non-conducting matter on the heating surfaces, it rendered possible the use of higher steam pressures in marine boilers, and led eventually to the introduction of cylindrical boilers and compound engines. When surface condensation was first introduced, the old flat-sided boilers, made to fit the section of the ship, were still retained, but were strengthened by fitting additional stays to enable them to carry steam pressures of 30 to 35 lbs. per square inch, and the majority of warships built during the years 1860-70 were fitted with surface-condensing engines worked with steam of this pressure. The piston speeds were also considerably increased, especially in the larger ships in which a long stroke could be obtained. With this type of engine the mean piston speeds varied from 500 to 665 ft. per minute. To promote economy of fuel the cylinders were usually steam-jacketed, and made large enough to allow for considerable expansion at full power, and the boilers were fitted with superheaters. The average weight of the machinery of this type, including the water in boilers and condensers, was about 3 cwts. per I.H.P.

Compound or double expansion engines. — After the introduction of the surface condenser, attention was directed to the use of higher steam pressures and greater expansion of steam, as theoretical considerations showed that considerable gain could thus be effected. The result was that the steam pressure was increased from 30 or 35 lbs. to 60 lbs., while cylindrical boilers were fitted to safely carry the increased pressure, and the engine was changed to the compound type. Compound engines were fitted to nearly flJl warships from 1870 to 1885.

In this type of engine the expansion is conducted in stages ; the steam, after being admitted to a small cylinder and expanding therein, is led to a larger cylinder, where it expands still further prior to exhaust, so that the stresses on the framing and journals are decreased and the loss from liquefaction of steam in the cylinders reduced

HISTOBY AND PROGEESS

CO

P4 to a minimum. The following summary of its advantages is confirmed by experience : —

1. Reduction of the maximum stresses on the framing, shafting, and bearings, and consequent reduction of weight and cost.

2. Increased regularity of turning moment, and consequent increased efficiency of the propeller in the water.

3. Moi^ economical use of the steam in the cylinders and consequent increase of power from a given expenditure of heat.

The working steam pressure in the Royal Navy with this type of engine was originally 60 lbs. per square inch. This has been gradually increased from time to time, till in about the year 1880 it was 90 lbs., while in the last of this type fitted the pressure was increased to 120 lbs.

From the adoption of compound engines and higher steam pressures a considerable economy of fuel at once resulted. The gain in economy by the use even of the 60-lb. compound engines over the ordinary surface-condensing engines worked with steam of 30 lbs. pressure may be taken to be at least 30 per cent.

This gain was well authenticated, and the average amounts claimed by the principal Engineers and Steamship Companies, in reply to questions by an Admiralty Committee in 1872, was 30 to 35 per cent.

Vertical engines. — ^The vertical type of engine, with cylinders at the top and crank-shaft below, was adopted for merchant ships long before it was introduced into the Royal Navy, because it was a necessity in most warships that all the machinery should be kept below the water-line, and horizontal engines alone satisfied this condition. Figs. 7 and 8 show a vertical engine of the type fitted in the mercantile marine. Vertical engines possess many practical advantages over horizontal engines, especially in connection with the working of the cylinders and pistons, and general accessibility of the engine. When, therefore, the twin-screw system was adopted for armour-clad ships, vertical compound engines were fitted, with a middle line water-tight bulkhead separating the two sets. By dividing the power into two parts, each set of engines, even in a ship of great power, would be of moderate dimensions, and although the whole of the machinery might not in all cases be entirely below the water-line, the parts above would be protected, not only by armour plating, but by a body of coal in addition, the coal-bunkers being continued on each side of the engine room. This extension of the use of vertical engines has continued and been applied to all classes of vessel, and special means for protecting the cylinders have often been fitted. At the present time new engines for the Navy are being made vertical for all classes of vessel.

Three-cylinder compound engines. — As the power of compound engines increased the dimensions of the low-pressure cylinders became so great that it was found desirable to fit two low-pressure cylinders instead of one, in consequence of the difficulties experienced in obtaining sound castings of large size, and to keep the size of the reciprocating parts as small as possible. This led to what is known as the three-cylinder compound engine, which is simply a modification of the ordinary two-cylinder compound engine. Figs. 9 and 10 show a vertical compound engine of the three-cylinder type.

Triple expansion engines. — With initial steam pressures above

100 lbs. per square inch, tjie variation of temperature in each cylinder of an ordinary compound engine again becomes great, so that the full economy due to the high pressure cannot be attained in consequence of the loss from liquefaction. It was therefore soon found desirable to extend the compound system, and divide the expansion into three stages, carried out in separate cylinders, so as to i^uce the range of temperature in each.

Engines on this system are usually known as triple expansion or triple compound enghies. They were first introduced by the late Dr., then Mr., A. C. Kirk, of Messrs. R. Napier & Sons, Glasgow, who, in 1874, fitted them on board the s.s; 'Propontis,' to utilise steam of 150 lbs. pressure, supplied by Bowan k Horton's water-tube boilers. These engines gave good economical results, but the boilers unfortunately gave trouble, and were ultimately taken out. Very little further was done in this direction, until, in 1881, Mr. Kirk fitted a set of triple expansion engines on board the s.s. ' Aberdeen,' for the trade to Australia and China. The results in this instance were so satisfactory that other engines of the same type followed, and the system was soon largely adopted in the mercantile marine. Since 1885 the new ships for the Royal Navy have been fitted with triple expansion engines, which type is now the most general for marine purposes. . The steam pressure first used with them in the Navy was 130 lbs., which was gradually increased to 155 lbs. in the year 1887. From this date to 1895 large numbers of triple expansion engines were added to the Navy, all with 155 lbs steam pressure. In the mercantile marine, however, 180 lbs. steam pressure is now largely used, and in many cases 200 lbs.

In the two large cruisers ' Powerful * and * Terrible,' commenced in 1893, and tried in 1896-97, a boiler pressure of 260 lbs. is adopted, reduced to 210 lbs. at the engines, while In cruisers of 1895, and subsequently, these pressures have been increased to 300 and 250 lbs. respectively. Triple expansion engines are fitted, the low-pressure cylinders being divided into two parts.

The gain in economy effected by the triple expansion engine, worked with steam of 130 lbs. to 150 lbs. pressure, over the ordinary compound engine worked at 90 to 100 lbs. pressure, may be taken at from 15 to 20 per cent., while with the higher pressures it will be still greater. Figs. 11 and 12 show the general arrangement of a triple expansion engine.

Quadruple expansion eng^es. — In many cases in the mercantile marine the stage expansion principle is carried still further, and quadruple expansion engines fitted, dividing the expansion into four stages, the boiler pressures being generally 200 lbs. per square inch, and in some cases 250 lbs. per square inch.

These engines are more suitable for the mercantile marine, where the range of powers required from the engines is limited, than for the Navy, where this range is large ; also as regards the Navy generally, evidence does not show that the additional complication thus introduced, and the extra length of engine room required, together with the additional engine friction, is compensated for by a sufficient gain in economy. They are gradually being introduced into the mercantile marine, but in the Navy only one torpedo boat and some smaller craft have been so fitted.

THE MAEUNE STEAM-ENGINE

Improyements in eeonomy. — In consequence of the improvements e£fected, 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 t£is 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 ofder 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, 4IEC., 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 -^s 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 g^erating powers of the boilers have been largely increased. Details of the fittings required for this purpose are given in Chapter Y. The introduction of forced draught has enabled the weight of machinery to be considerably reduced, and the average weight of machineiT for the latest modem warships fitted with circular boilers is about 1} cwt. per I.H.F. developed with moderate forced draught. A lesser weight than this, viz., 1^ cwt., was at one period «Uowed, but this is not now recommended.