r REESE LIBRARY UNIVERSITY OF CALIFORNIA. [ , /90O . Accession No. 80 L]~ 3 LL Cta&sNo. THE MABINE STEAM ENGINE THE MARINE STEAM ENGINE A TEEATISE FOB ENGINEERING STUDENTS YOUNG ENGINEERS, AND OFFICERS OF THE ROYAL NAVY AND MERCANTILE MARINE BY THE LATE EICHAED SENNETT > ENGINEER-IN-CHIEP OP THE NAVY ; FELLOW OP THE ROYAL SCHOOL OP NAVAL ARCHITECTURE AND MARINE ENGINEERING ; MEMBER OP THE INSTITUTIONS OP CIVIL AND MECHANICAL ENGINEERS ; VICE-PRESIDENT OP THE INSTITUTION OP NAVAL ARCHITECTS; LATE INSTRUCTOR AND LECTURER IN MARINE ENGINEERING AT THE ROYAL NAVAL COLLEGE AND HENEY J. OEAM SENIOR ENGINEER INSPECTOR AT THE ADMIRALTY; INSPECTOR OF MACHINERY IN H.M. PLEE MEMBER OP THE INSTITUTION OF CIVIL ENGINEERS MEMBER OF THE INSTITUTION OP NAVAL ARCHITECTS ; LECTURER! ON MARINE ENGINEERING AND LATE INSTRUCTOR IN MARINE ENGINE DESIGN AT THE ROYAL NAVAL COLLEGE ; ETC. WITH NUMEROUS DIAGRAMS 89 PATERNOSTER ROW, LONDON NEW YORK AND BOMBAY A.11 rights reserved 3 if. Bibliographical Note. First Edition, March 1882. Second Edition, with Additions and Corrections, October 1885. Third Edition, Revised and largely Rewritten by H. J. Oram, January 1898. Fourth Edition, with Additions and Modifications, June 1899. INTRODUCTORY NOTE To THE many who knew the late Eichard Bennett, my old friend and pre- decessor in office, and appreciated his work and worth, and regretted the all too early closing of his brilliant career, it will be a source of gratification that the revision of this book has been undertaken by so capable a marine engineer as Inspector of Machinery H. J. Oram, E.N. Mr. Oram has acquired avast amount of special knowledge and experience of marine machinery of the latest types, and it is believed this has been incorporated in the present edition in a manner that will make it of great value to students, young engineers, and officers of the Eoyal and Mercantile Navies in carrying on their studies and duties. A. J. DUESTON, Engineer-in-ChieJ of H.M. Navy. ADMIRALTY, LONDON. NOTE TO NEW EDITION. THE demand for a new edition of this work coming so soon indicates a measure of appreciation of its contents which is highly valued by the writer. In revising for the present edition, the suggestions of reviewers who kindly expressed them have been kept in view as far as possible, and it is hoped in the future to do this more fully. Some modifications have been made, and such additions as the progress of engineering appeared to warrant, such, for example, as the Marine Steam Turbine, the develop- ment of which will be watched with interest. As a figure was given in the preface for the I.H.P. of water-tube boilers con- structed or ordered for the Koyal Navy, it may be mentioned that this figure is now practically 1,500,000, I.H.P. H. J. OKAM. LONDON : April 1899. PBEFACE THIS work, originally written by the late Mr. Eichard Sennett, was never revised, owing at first to the pressure of his official duties, and subsequently by reason of his death, so that it had become obsolete, on account of the very great changes which have taken place in marine engineering, more especially in the naval machinery with which it originally dealt. When considering the preparation either of an independent new work or the thorough revision and modernisation of the old one, the latter course was chosen, as there appeared to be features in the work and its arrangement which offered advantages over others of its kind, although this course has involved practically rewriting it. The style and arrangement have, however, been preserved, and the result appears in the present volume as a practically new book, which it is hoped will be of service to students of engineering, and enjoy a measure of popularity equal to that received and so well deserved by the original work. The amount of printed matter and also the number of illus- trations have been considerably increased, as in the writer's opinion ample illustration is essential to the proper understand- ing of written descriptions of engineering details. The illustra- tions have generally been specially prepared for the work, and alteration of type and size of page have been made to keep the book within convenient size, owing to the increased contents. These illustrations and the subject matter have also been made more general in character, and been drawn from the practice of the most successful makers of mercantile marine, as well as naval, machinery. The fact may be noted that during recent years there has been, as regards the engines of large Vlll THE MARINE STEAM-ENGINE vessels, a much nearer approximation to identity in mercantile and naval practice 'than was usual previously, showing that valuable features have been mutually borrowed. The growing importance of water-tube boilers has caused this subject to be dealt with at considerable length, and this part of the book exemplifies the great changes that have taken place since the work was first published. It may be observed in passing, as indicating the national importance of this section, that in the Koyal Navy there are now about 1,000,000 I.H.P. of water-tube boilers, either built, building, or about to be com- menced. A new feature is the detailed description of the care and management of, and the treatment of defects in, marine engines and boilers, which it is expected will be found of value to students of engineering and young engineers. It is not pretended, how- ever, that this part of the duties of engineers can be adequately learnt from books, for actual experience in the engine rooms will alone completely supply the requisite instruction. What is given will, however, prepare young engineers for such experience, and give information on points which they are at first more or less unacquainted with. In the preparation of a work of this kind, one becomes indebted to many friends for assistance of various kinds, and to these I tender my best thanks, especially to Mr. P. Marrack, E.N., Engineer Inspector, Admiralty, who has kindly read over most of the proof sheets. H. J. OKAM. LONDON, 1898. CONTENTS. INTRODUCTORY. CHAPTER PAGE I. HISTORY AND PROGRESS OP MARINE ENGINEERING . 1 II. WORK AND EFFICIENCY . 19 III. HEAT AND ITS EFFECT ON WATER .... 22 THE BOILER. IV. COMBUSTION AND ECONOMY OF FUEL BOILER EF- FICIENCY 32 V. METHODS OF ACCELERATING THE KATE OF COMBUSTION OF FUEL .... ... 45 VI. PETROLEUM AS FUEL 53 VII. ARRANGEMENT AND EFFICIENCY OF BOILERS WATER TANK BOILERS 57 VIII. WATER-TUBE OR TUBULOUS BOILERS .... 74 IX. BOILER MOUNTINGS AND BOILER-ROOM FITTINGS . . 100 X. CORROSION AND PRESERVATION OF BOILERS . . .127 THE STEAM. XI. EFFICIENCY OF THE STEAM 184 XII. EXPANSION OF STEAM .139 XIII. METHODS OF INCREASING THE EXPANSIVE EFFICIENCY OF STEAM ........ 155 XIV. COMPOUND OR STAGE EXPANSION ENGINES . . . 162 THE MAKINE STEAM-ENGINE THE MECHANISM. PAGE XV. EEGULATING AND EXPANSION VALVES AND GEAR . 166 XVI. SLIDE-VALVES AND FITTINGS 172 XVII. STARTING AND KEVEBSING ARRANGEMENTS . . 197 XVIII. ARRANGEMENT OF THE CYLINDERS OF COMPOUND, TRIPLE, AND QUADRUPLE EXPANSION ENGINES 212 XIX. DETAILS OF CYLINDERS AND ENGINE-ROOM FITTINGS IN CONNECTION 225 XX. CONDENSERS, FEED-WATER FITTINGS, AND UNDER- WATER VALVES ...... 244 XXI. EOTARY MOTION . 268 THE PEOPELLEE. XXII. PROPULSION 287 XXIII. CO-EFFICIENTS AND CURVES OF PERFORMANCE . . 301 XXIV. PADDLE-WHEELS 309 XXV. SCREW-PROPELLERS 315 GENEEAL. XXVI. THE INDICATOR AND INDICATOR DIAGRAMS . . 334 XXVII. PUMPING, WATERTIGHT, AND FIRE ARRANGEMENTS 362 XXVIII. AUXILIARY MACHINERY AND FITTINGS . . . 372 XXIX. EAISING STEAM AND GETTING UNDER WAY . . 416 XXX. MANAGEMENT OF ENGINES UNDER WAY ENGINE AND BOILER DEFECTS 428 XXXI. ENGINES DONE WITH EXAMINATIONS, ADJUST- MENTS, AND GENERAL INFORMATION . . . 444 XXXII. MATERIALS USED IN CONSTRUCTION . . . 450 XXXIII. THEORETICAL INDICATOR DIAGRAMS OF STAGE EX- PANSION ENGINES 459 CONTENTS xi APPENDIX. PAGE (A) Application of the Indicator Diagram to Determine the Stresses on Crank- Shafts. Curves of Twisting Moments 483 (B) Effect of the Inertia of the Eeciprocating Parts of the Engines 485 (C) Extracts from the Board of Trade Eules relating to Machinery and Boilers . . . . . . . 491 (D) Lloyd's Eules for Boilers and Machinery .... 502 INDEX . . .509 THE MAKINE STEAM-ENGINE. CHAPTER I. . HISTORY AND PROGRESS. THE earliest steam engines 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 rota- tive 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 navi- gation. 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 pro- pulsion 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 stern, 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 navigation proving commer- cially successful was in America, where, in 1807, Robert Fulton built a steam vessel called the ' Clermont,' propelled by paddles driven by a Boulton & Watt engine. In 1812 Henry Bell built a vessel called the 'Comet,' 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 2 THE MARINE STEAM-ENGINE 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 exten- sion 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 con- struction. 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 FIG. 1. these paddle-wheels. Almost all these early engines 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 HISTORY AND PROGRESS 3 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. Grasshopper 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 com- monly known as grasshopper engines. The side-lever type of engine, though very heavy and occupying 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 into 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^ 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 Ibs. 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, & 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 Ibs. 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 -7 5 cwts. per I.H.P. developed. Between 1840 arid 1850 tubular boilers were introduced. In these B2 4 THE MAEINE STEAM-ENGINE 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 Ibs. 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. Penn. 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 cross- head. In order to get sufficient length of connecting-rod, the cross- head was of peculiar form and passed 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 oscil- lating engines into the steamship * Endeavour,' and subsequently in several other ships, was adopted and perfected by the late eminent engineer, Mr. John Penn, with whose name it is now generally associated. In these engines the connecting-rod is altogether dis- pensed with, the upper end of the piston-rod being fitted with brasses to work directly on the crankpin, and the cylinder itself is carried 011 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. Penn in 1850. The pressure of steam in the boilers was 14 Ibs. 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.P. Defects of paddle-wheels The paddle-wheel possessed many prac- tical 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. HISTOEY AND PEOGEESS 6 THE MABINE STEAM-ENGINE Adoption of the screw propeller, The adoption of the screw pro- peller in lieu of the paddle-wheel was the most important step taken in the progress of marine engineering, for this rendered many sub- sequent 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 impor- tant in un armoured 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 experi- mental 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 experi- ments 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 HISTOKY AND PEOGKESS 7 of revolution could be sufficiently increased to allow direct engines to be fitted. In these the gearing is left out, and the crank-shaft con- nected 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 fast-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 Penn 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 stuffing-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 smooth- working and efficient marine engines employed. With the introduc- tion of high-pressure steam, however, they became obsolete, owing to the difficulty of keeping the trunks in a steam-tight condition. Return 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 cross- head, 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 cross- head between the cylinder and the crank -shaft, from which two rods were carried, one above, the other below, the shaft, to a similar cross- head on the opposite side, as in the ordinary return connecting-rod arrangement. Direct-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 8 THE MARINE STEAM-ENGINE HISTOEY AND PROGRESS 10 THE MAKINE STEAM-ENGINE 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 now 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 Ibs. 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 com- pound 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 Ibs. 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 con- siderations showed that considerable gain could thus be effected. The result was that the steam pressure was increased from 30 or 35 Ibs. to 60 Ibs., 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 all 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 de- creased and the loss from liquefaction of steam in the cylinders reduced HISTOKY AND PROGRESS 11 12 THE MARINE STEAM-ENGINE 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 in- creased efficiency of the propeller in the water. 3. More economical use of the steam in the cylinders and conse- quent 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 Ibs. per square inch. This has been gradually increased from time to time, till in about the year 1880 it was 90 Ibs., while in the last of this type fitted the pressure was increased to 120 Ibs. 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 Ibs. 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 ques- tions 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 neces- sity 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 obtain- ing sound castings of large size, and to keep the size of the recipro- cating 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 HISTOKY AND PEOGRESS 13 14 THE MARINE STEAM-ENGINE 100 Ibs. per square inch, the 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 reduce the range of temperature in each. Engines on this system are usually known as triple expansion or triple compound engines. 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 Ibs. pres- sure, supplied by Rowan & Horton's water-tube boilers. These engines gave good economical results, but the boilers unfortunately gave trouble, and were ultimately taken out. Yery little further was done in this direction, until, in 1881, Mr. Kirk fitted a set of triple expan- sion 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 Ibs., which was gradually increased to 155 Ibs. in the year 1887. From this date to 1895 large numbers of triple expansion engines were added to the Navy, all with 155 Ibs steam pressure. In the mercantile marine, however, 180 Ibs. steam pressure is now largely used, and in many cases 200 Ibs. In the two large cruisers 'Powerful ' and 'Terrible,' commenced in 1893, and tried in 1896-97, a boiler pressure of 260 Ibs. is adopted, reduced to 210 Ibs. at the engines, while in cruisers of 1895, and subsequently, these pressures have been increased to 300 and 250 Ibs. 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 Ibs. to 150 Ibs. pressure, over the ordinary compound engine worked at 90 to 100 Ibs. 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 engines. 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 Ibs. per square inch, and in some cases 250 Ibs. 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 intro- duced, 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. HISTOEY AND PEOGEESS 15 16 THE MARINE STEAM-ENGINE HISTOKY AND PROGRESS 17 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 exhaust- ing 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, tkc., 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 in- creased. 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 1J cwt. per I.H.P. developed with moderate forced draught. A lesser weight than this, viz., H cwt., was at one period allowed, but this is not now recommended. From this brief sketch a general idea may be formed of the pro- gress 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 Ibs. of coal per horse -power. In modern war- ships, 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 war- ship 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 Ibs. and engines working at 250 Ibs. per square inch. A great 18 THE MARINE STEAM-ENGINE 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. 19 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 avoir- dupois, 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 foot- pound represents the amount of work done in raising a weight of one pound througli 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 x 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 some- times called one inch-ton, and it is equal to 2,240 inch-pounds or **- foot-pounds. The work of lifting one ton one foot is one foot-ton, 12 and so on It will be seen that the different terms used are self- explanatory and are convertible one to another. The foot-pound is, however, the general unit, the others only being employed for con- venience in special cases. 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 c2 20 THE MARINE STEAM-ENGINE involve time as well as distance. Power may be denned 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 Efficiency = _gseful work done Total energy expended 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 is wasted, and these four stages all tend to decrease the total efficiency. 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 trans- mitted to the water in the boiler is, in ordinary cases, not more than fi 7 from f0 to JQ. ^^ B ^ ract ^ on * s C3 ^ e & tne Efficiency of the boiler. 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 repre- sents only a small fraction of the total heat contained in the steam. The fraction varies very considerably, depending on the type of engine, its steam pressure, rate of expansion, &c. say from ^ to ^. In large modern marine engines it may be taken as from to ~. This fraction, o y 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 work ing 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, o o^ Its value is from r~ to ; I, the former being more general, 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 WORK AND EFFICIENCY 21 the Efficiency of the propeller. It may be taken as averaging from 5 6 16 to 16- 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 com- ponents of the resultant efficiency, the efficiency would be : i*it .&-- 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. 22 THE MARINE STEAM-ENGINE CHAPTER III. HEAT AND ITS EFFECT ON WATER. IN order to comprehend the principles on which the construction and performance of the steam-engine depend, and the object of the various improvements that have from time to time been introduced, it is necessary that the true nature and properties of heat should be known. We will, therefore, as concisely as possible, state the principal points relative to this subject, in order that the succeeding chapters may be clearly understood. Temperature, The temperature of a body may be denned as the extent to which it may be capable of communicating sensible heat, or heat that may be felt, to other adjacent bodies. When two bodies of different temperatures are placed in contact with each other, it is a well-known fact that the hotter body becomes cooler and the colder body hotter, till at length the two bodies become of the same temperature, after which no change in the temperature takes place. This is caused by the passage of heat from the hotter to the colder body, and shows clearly that heat is something that can be transferred from one body to another, so as to diminish the amount of heat in the former body and increase it in the latter. Effect of and nature of heat. When heat is added to or abstracted from a body, one of the two following effects is produced : either the temperature of the body is altered, or its state is changed. For example, if heat be added to water under the atmospheric pressure, the temperature is increased until it reaches 212 Fahr. After this the addition of heat does not further increase the temperature, but causes the water to evaporate and become steam that is, it changes the condition from that of a liquid to that of a gas. Again, if heat be abstracted from water, the temperature is reduced till it reaches 32 Fahr., after which the diminution of heat does not further decrease the temperature, but changes the condition of the water from the liquid to the solid state, forming ice. The quantities of heat passing from one body to another can thus be estimated by the effects produced, so that it is clear that heat is something that can be both transferred and measured. The true nature of heat has been determined by experiments on friction. It is a matter of common observation that the work ex- pended in friction is apparently lost that is, it appears no longer in the form of mechanical work ; but at every place where friction occurs, heat is developed, and the greater the friction the greater is the amount of heat produced. Experiments have shown that the amount of heat generated by friction is exactly equivalent to the HEAT AND ITS EFFECT ON WATER 23 amount of work lost, and we therefore infer that heat is of the same nature as mechanical work that is, it is one of the forms of energy. British thermal unit. The unit by which heat is measured is called a thermal unit, and in British measurements represents the quantity of heat necessary to raise one pound of water at its maximum density, which corresponds to a temperature of about 39 Fahr., through one degree Fahr. Joule's equivalent. The honour of determining the exact relation between heat and mechanical work belongs to Mr. Joule, who proved, by an elaborate series of careful experiments on the friction of oil, water, mercury, and other substances, that one thermal unit is equal to 772 foot-pounds of mechanical work l that is, that the quantity of heat necessary to raise the temperature of one pound of water at its maximum density, one degree Fahr., can be made to perform work equal to the raising of 772 Ibs. one foot high. In honour of the discoverer this important constant, 772, expressing the relation be- tween heat and mechanical work, is called Joule's equivalent, and is frequently denoted by the letter J. The convertibility of heat and work, in a definite ratio, is ex- pressed in the following statement, generally known as the mechanical theory of heat, viz. : Heat and mechanical energy are mutually con- vertible, and heat requires for its production, and produces by its ^ disappearance, mechanical energy in the proportion of J72 foot-pounds \ for each unit of heat. This statement forms also \ho~first law of the science of ther mo -dynamics. Communication of heat. Heat may be communicated from one body to another in three different ways, viz., by radiation, conduction, and convection. Kadiation. Radiant heat is given off from hot bodies in straight lines, and the rays of heat are subject to the same laws as the rays of light. As far as the generation of steam is concerned the useful radiation is confined to the furnace, the crowns and sides of which, intercepting the rays of heat from the burning fuel, become themselves heated, and the heat passes through them to the water in the boiler. A consider- able amount of heat is given off by radiation from burning coal, and it is very important, therefore, to intercept this, and to insure that as far as possible the whole of the heat diffused in this way should be trans- mitted, either directly or indirectly, to the water in the boiler, and not wasted on the external air or other bodies. Radiation is an important item to be considered with reference to the economical employment of steam, for it always causes a certain loss of heat, and unless proper precautions are taken this loss may become very considerable. The surfaces of the boilers, steam-pipes, cylinders, &c., when the engines are at work, are very much hotter than the surrounding bodies, and consequently, in order that loss of heat by radiation may be avoided as far as possible, all those surfaces should be clothed with some non-conducting material. Hair-felt has been largely employed for this purpose, and this is usually kept in its place by an outer 1 Subsequent experiment appears to show that the exact value is somewhat (nearly 1 per cent.) higher than this viz., 778. 24 THE MARINE STEAM-ENGINE covering of canvas, wood, or sheet-iron. Preparations of cork and other non-conducting materials have also been used. These substances, however, when applied to very hot surfaces, are in danger of being burnt away, and various incombustible non-conductors, such as asbestos, silicate cotton, fossil meal,