The problem to be solved in designing the machinery of a vessel may be stated in general terms as follows: a given amount of power is to be developed for a given length of time at a minimum of cost. It must be borne in mind, however, that the cost of the developed power does not comprise only the interest on the capital representing the original cost of the machinery, the money value of the fuel consumed, the wear and tear, and other running expenses,—but includes the value of the weight of the machinery and the fuel, and of the space occupied by them, in so far as they reduce the carrying capacity of the vessel. For a steamer designed to trade between certain ports it is easy to determine what ratio the expenses should bear to the carrying capacity, to make the vessel a remunerative investment. The total cost of driving a vessel with a certain power may be calculated in dollars and cents, and compared with the value of her carrying capacity and speed; and the proportions promising the greatest relative economy may thus be determined.
This exact method cannot be applied to naval vessels; the value of the different qualities which a man-of-war must possess, cannot be measured by a commercial standard, nor are these qualities of the same relative importance under all circumstances.
In the following investigation we will not consider vessels designed for special purposes, as torpedo-boats, rams, floating batteries, &c., but have only regard to cruising vessels intended for general service, in which either the armament, or the speed, or the ability to keep the sea a long time becomes, according to circumstances, the most important factor of efficiency. Experience has determined what fraction of the whole weight of the vessel, and how much space is to be allotted to the motive power, to the armament and to the equipment respectively, to render the general efficiency of the cruiser a maximum. The problem to be solved in designing the steam machinery consists in making such use of the space and weight allotted to the machinery and the fuel, as to produce the greatest possible effect; and the efficiency of the machinery is to be measured by a two-fold standard, viz., 1st, by the total distance through which the vessel can steam at various rates of speed, and 2nd, by the highest rate of speed which can be maintained for a certain length of time. The cost of the fuel, an important element in the efficiency of a mercantile vessel, is of relatively small importance in the case of a man-of-war.
There are a number of important conditions not to be lost sight of in the design of the machinery of a cruising man-of-war,—demanded by the varying nature of her service, and imposing many restrictions on the designer. It must be borne in mind that the greatest power which the machinery is capable of developing will be called into use only on exceptional occasions; that, therefore, the vessel must steam most economically with a fraction of the total power;—that to be prepared for all emergencies, the boilers must be able to generate steam rapidly; that the machinery must preserve its efficiency under all the varying conditions of long cruises at any part of the world, with few facilities for repairs, and often for many months without an opportunity for those periodical examinations, cleanings and adjustments, which are so essential for the preservation of the efficiency of all machinery. Besides it is not sufficient to give such structural strength to certain parts of the machinery, especially to the boilers, as is demanded by the stresses produced when working with the greatest power, but the effect of violent concussions by ramming or the explosion of torpedoes, and the chances of penetration by shot, must be taken into consideration. To protect the machinery against this latter contingency in an unarmored vessel, it has to be placed as low as possible, the waterline being generally assumed to be the extreme limit which any vital part of the machinery is allowed to reach. This restriction in regard to height not only necessitates an increase in the space occupied by the machinery in the length and breadth of the vessel, but it often precludes the adoption of arrangements which would insure greater economical efficiency.
These considerations are sufficient to indicate that the economy and efficiency of the steam power of mercantile and naval vessels can not be measured by the same standard, and that there are also essential differences in the choice of the general arrangements and of the details of construction of the boilers of these two classes of vessels.
In modern cruising vessels of the English and French navies, of more than fifteen hundred tons displacement, the total weight of machinery, fully equipped for service, is from twenty to twenty-five percent, of the displacement, and the weight of fuel carried in the bunkers varies between ten and fifteen per cent, of the displacement; and with these proportions, the highest number of indicated horse powers developed by the machinery is from ten to twenty-five per cent, greater than the number of tons representing the displacement of the vessel. In recently constructed vessels of the French navy the proportion of weight of machinery to displacement is, on the whole, somewhat greater than in English vessels, at the expense of their armament and equipment; but in the two fast English despatch vessels, the Iris and the Mercury, the weight of machinery forms nearly twenty-seven per cent., and that of the fuel twenty per cent, of the total displacement, and the highest number of indicated horse-powers developed by the machinery of the Iris was more than twice as great as the number of tons representing the displacement of the vessel.
The weight and space required for a given type of the engines proper may be considered as being very nearly a fixed quantity for a given number of indicated horse-powers. But the weight and space required for boilers capable of furnishing a given power may vary considerably, depending, first, on the form of their shell, the arrangement of their internal parts, and their location in the vessel with regard to each other; secondly, on the rate of combustion and the economic evaporative efficiency of the boilers.
The return tubular boiler, with the tubes arranged directly over the furnaces, combines so many advantages respecting economy in weight and space, and general maintenance, in comparison with its evaporative efficiency, that it is almost universally used on board of sea-going vessels, and in fact it is called the marine boiler par excellence. When this arrangement of tubes is departed from, it invariably entails a loss of space in the vessel, and it is done, therefore, only under exceptional conditions: for instance, in vessels of very light draught it maybe necessary to reduce the height of the boiler by placing the tubes on a level with the furnaces instead of above them, in order to get the boiler below the water-line of the vessel. The vertical tubular boiler of the Martin type, which formed for many years a distinctive feature of our naval steamers, has of late almost disappeared, having given way to the horizontal tubular boiler. We cannot enter farther into the question of the relative merits of these two types of boilers, than to state that, while the vertical water-tube boiler has the advantage of greater economical evaporation, the horizontal tubular boiler is capable of developing more power because it is capable of maintaining a higher rate of combustion, with natural draught.
As far as the form of the shell is concerned, two types of boilers have to be considered, viz., the rectangular boiler and the cylindrical boiler, each possessing peculiar merits. The question, which of these two types is better adapted for naval purposes, is an intricate one, and can be solved only after a thorough, long-continued, practical test. At present we can give only a brief statement of their respective characteristic features, and point out how they bear on the question of efficiency and economy in the performance of naval vessels. Modifications or combinations of these two forms, like the semi-cylindrical, the oval, and the cylindrical lobe boiler, may be used under special conditions; but, in general, they possess the defects of both types without their most characteristic advantages; they need not be considered here any farther. The oval boiler is, however, used very extensively at present in the English Navy.
The rectangular boiler, which depends for its strength almost entirely on the bracing, is nearly universally used in naval vessels as long as the steam pressure does not exceed forty or forty-five pounds per square inch above the atmosphere. The principal advantage connected with the rectangular shell, lies in the fact that it utilizes the whole of the space occupied by it in the vessel, and is easily adapted to any arrangement of the interior parts. Seven furnaces, having an aggregate grate surface of about one hundred and forty square feet, and the required heating surface, viz., from twenty-five to thirty times the area of the grate-surface, may be arranged within a single shell about twenty-five feet long, from nine to ten feet wide, and from nine to ten feet high. This whole space is utilized to the best advantage for the purposes of generating and storing the steam, and little more water is carried than is necessary to ensure proper circulation. It is readily seen, that the greater the area of grate-surface contained in a single shell, the less will be the weight of the boiler relatively to its power; it is, however, impracticable to make marine boilers much longer than twenty-five feet.
When the steam pressure exceeds forty-five pounds per square inch, the use of cylindrical boilers becomes advisable. The cylindrical shell possesses a special fitness for resisting an internal pressure, which produces on every portion of it a uniform tensile stress; in this manner it is exempt from the evils resulting from the varying bending stresses to which, in the rectangular boiler, each portion of the shell lying between adjacent braces is subjected, and which must ultimately cause leaks around the rivets, open the seams and separate the fibres of the metal. The bottom of the cylindrical boilers, inside and outside, can be made much more accessible for examinations and repairs, than is the case with the rectangular boiler. For these two reasons we may assume that, under like conditions of working, the cylindrical shell is more durable than the rectangular shell. It is stated by Lloyd's surveyors as a matter of experience, that in cylindrical boilers the flat ends, which are stayed in the same manner and experience the same strains as rectangular shells, deteriorate much quicker than the cylindrical shell. In the event of perforation by shot, the chances of destruction would probably be equal for both types of boilers; but it may be assumed that the cylindrical boiler will bear violent concussions more safely than the rectangular boiler. The diameter of the cylindrical shell is limited, of course, by the height which the boiler is allowed to attain; and thus, in vessels of relatively light draught, the number of cylindrical boilers has to be increased as their diameter decreases. This subdivision of the boiler power has its advantages, as well as very serious disadvantages. One or more of these boilers may be disabled without crippling fatally the motive power of the vessel. When it is desirable to use only a fraction of the boiler power, it is more economical to disconnect completely several of these boilers, than merely to reduce the grate-surface in the large rectangular boilers. The violence of boiler explosions depends on a variety of circumstances; but it appears that the damage done by them bears some ratio to the quantity of water carried in the boiler, and this quantity is necessarily much less in each of the smaller cylindrical, than in the large rectangular boilers. On the other hand, the great number of separate attachments required by the numerous cylindrical boilers, increase greatly their cost, weight and liability to derangement. But the most serious objection to cylindrical boilers, is the fact that this form does not utilize the space occupied completely. A cylindrical boiler twelve feet in diameter may contain three furnaces, each three feet in diameter. Two of these boilers placed side by side, occupying almost exactly the same space in the length and breadth of the vessel as the above mentioned rectangular boiler, but being about two feet higher, contain fourteen per cent, less grate-surface; and this loss in area of grate, relatively to the area occupied by the boilers on the floor of the vessel, increases rapidly as the diameter of the boilers decreases.
The question whether cylindrical or rectangular boilers should be used in naval cruising vessels, must be determined according to the rule regarding the measure of efficiency, laid down in the beginning of this essay. The saving in fuel, due to the use of steam of a high pressure, must be sufficient to counter-balance the loss in weight and space due to the substitution of the high-pressure cylindrical, for the low pressure rectangular boiler; or to state this point more precisely, with the same aggregate weight and space allowed for boilers and coal, that type of boiler which enables the vessel to steam a greater distance at the same rate of speed, is the preferable one. This question cannot be decided by deductions drawn from theoretical rules, or from the results of special trials of short duration; but a careful comparison of data furnished by repeated trials under all the varying conditions of a cruise can alone give a final answer.
It may be proper to remark here, that the space required for the boilers includes also the fire room. The natural arrangement of the boilers in pairs opposite to each other, with the fire room running between them in a fore and aft direction, will not be departed from, except when, as in some English vessels of recent construction, the hold of the vessel is sub-divided by longitudinal bulkheads into water-tight compartments, and a different arrangement of the boilers becomes a necessity, notwithstanding the loss in space, and the many other inconveniences produced thereby.
With the ordinary types and arrangement of boilers, the space required for them is in the direct ratio of their grate-surface; but this ratio is greater for cylindrical than for rectangular boilers, and increases as the diameter of the cylindrical boilers decreases.
The efficiency of the machinery of a vessel is frequently measured by the number of horse-powers developed per square foot of grate surface. Leaving out of account the efficiency of the engines proper, the power of the boilers is measured by the weight of steam generated in a unit of time, and depends on the proportions of calorimeter and heating surface to grate surface, and on the rate of combustion, or the weight of a given kind of fuel consumed per square foot of grate-surface per hour. The calorimeter and heating surface determine the quantity of heat per unit of weight of fuel available for the purpose of generating steam; by augmenting the heating surface, the weight and bulk of boilers are necessarily increased, and a limit is soon reached where the economical advantage resulting from such an increase vanishes. The rate of combustion determines the total heat available for the evaporation of water in a unit of time; but with given proportions of the boiler, the weight of steam generated per pound of fuel diminishes with an increased rate of combustion. Consequently, when a given weight of steam is to be generated, or in other words, a given amount of power is to be developed for a given length of time, this result may be obtained in two different ways; viz., either large boilers with a low rate of combustion, but high economic evaporative efficiency, or smaller boilers with a higher rate of combustion but lower economic evaporative efficiency, may be used; in the first case, the weight and space required for the boilers will be greater, while the weight and space required for the fuel will be less than in the second case. It is necessary to determine that rate of combustion and those proportions of the boilers, for which the aggregate weight and space required for boilers and fuel, in order to develop a certain power for a given length of time, become a minimum. It is evident that, the longer the time during which the power is to be developed, the greater will be the loss of fuel due to a diminished economic evaporation with an increased rate of combustion: consequently, the rate of combustion, which gives the most economical results in respect of weight and space required, is a decreasing function of the time during which the power is to be developed. This question is discussed in a very instructive manner by Chief Engineer Isherwood, U. S. N., in the second volume of his "Engineering Researches." Basing his conclusions on data derived from the numerous boiler experiments made under his direction, he finds that, measured by economy in the aggregate weight and space required for boilers and fuel, the most advantageous rate of combustion for steaming during 200 consecutive hours, is about twelve pounds of anthracite coal per square foot of grate per hour, when the heating surface of the boiler is twenty-five times the area of the grate surface, and the calorimeter is one-eighth the area of the grate. The tables given in the above mentioned work furnish the ready means to calculate the effects which a change in the rate of combustion, or in the length of the steaming time, has on the economy in space and weight, and thus to determine the most efficient proportions of the boilers, in so far as their efficiency is measured by the length of time during which they are capable of furnishing a given power with a fixed amount of aggregate weight and space required for boilers and fuel.
We have assumed that a second measure of the efficiency of a vessel-of-war is the highest speed which she can maintain for a certain length of time. It may be all-important at times to push the development of power, that is to say the speed of the vessel, up to the highest possible limit, irrespective of economical considerations. To this end the rate of combustion must be increased far beyond the economical limit; and it is an important question, how much the power of a given boiler can be increased by increasing the rate of combustion.
Isherwood states that "when the boilers are placed in the hold of a vessel, and the air has to reach their ash-pits through the restricted hatches of the decks, an air-current from the upper deck to the ash-pits having to be produced at the expense of the draught of the boilers," and "the calorimeter being one-eighth of the grate surface, the heating-surface twenty-five times the grate surface and the chimney sixty feet high above the grates"—"it is found experimentally, that the maximum rate of combustion by natural draught is, for the horizontal fire-tube boilers, sixteen pounds of anthracite per square foot of grate surface per hour." This rate of combustion is 38 per cent, greater, and furnishes 15 per cent, more steam than the rate which was found to be most economical in respect of weight and space required for two hundred hours steaming. Any further increase is impossible with anthracite coal, unless the draught be aided by artificial means.
When, on the other hand, a free burning bituminous or semi-bituminous coal is used as fuel, from sixteen to twenty pounds can be burned economically in marine boilers per square foot of grate per hour with natural draught; and by urging the fires, this quantity may be increased to twenty-seven pounds with certain kinds of coal. For an increased rate of combustion it becomes necessary to increase the proportions of heating-surface and calorimeter to grate-surface, in order to preserve the economic evaporative efficiency of the boiler, and this causes a slight increase in the total weight of the boiler relatively to the area of its grate. The space required to stow the bituminous coal is from five to fifteen per cent greater than that required for an equal weight of anthracite; this greater bulk of the bituminous coal is, however, fully offset by the greater amount of refuse in ashes and clinker produced in the combustion of our anthracite coals. Our Pennsylvania anthracites give on an average from twelve to twenty per cent, of refuse, while the better classes of English semi-bituminous coal produce only from six to twelve per cent, of ashes. The saving in weight and bulk of boilers due to the increased rate of combustion is 80 great, that all the lines of transatlantic steamers use semi-bituminous coal exclusively, although its cost in our market is fully fifty per cent, greater than that of anthracite. The average rates of combustion of a large number of steamers of different lines running between New York and England range between thirteen and nineteen pounds per square foot of grate per hour.
The use of semi-bituminous coals has the additional important advantage for ships of war that steam can be raised with them in a much shorter time than with our hard anthracites; they are, therefore, used on the naval vessels of all foreign, nations, England furnishing the chief supply to most of the European countries. Great care is exercised in selecting these coals according to the service to be performed by the vessels. French naval vessels, during trials and for special service, burn Newcastle and Cardiff coals, mixed in equal proportions; for ordinary service, French coals, mixed in various proportions according to their more or less bituminous character, are used.
No coal is accepted unless it produces less than two per cent, of clinker, and less than thirteen per cent, of soot and ashes. In the English Navy, all cruising vessels burn now a mixture of equal weights of the smokeless, friable Welsh coal and of bituminous North Country coal; for trials and special occasions a carefully picked Welsh coal, containing not more than six per cent, of refuse, is furnished. The English have made repeated trials of burning the hard Welsh anthracites in their naval vessels, but the result was such a large reduction in the rate of combustion, and, consequently, such a falling off in the highest speed of their vessels that the attempt had to be abandoned. In fact, English and French writers generally consider the use of hard anthracite coals as fuel for marine boilers as impracticable.
The selection of anthracite as fuel for our naval vessels was the result of the extensive experiments and investigations made, by authority of Congress, in the years 1842-43 by Professor Walter Johnson of the Smithsonian Institution. Pure anthracite will evaporate more water per pound of fuel in a steam boiler than coals containing various proportions of bituminous substances: its freedom from smoke and relatively small liability to deterioration from exposure are great advantages in favor of its use on board of cruising vessels. But it is evident that means must be provided to increase the ordinarily slow rate of combustion with this coal, if our boilers are to produce the same power per square foot of grate-surface as foreign naval vessels. In the first place the chimney of our boilers should be made as high as practicable; and neither the arrangement of the rigging, nor mere aesthetical considerations should be allowed to limit its height. Special attention should also be paid to giving to the air the freest possible access to the fire room. In the second place special machinery must be provided to increase the draught of the boilers artificially.
The U. S. S. Mackinaw and Eutaw had vertical water-tube boilers of the Martin type; the proportions of heating-surface and calorimeter to grate-surface being respectively as 25.18 to 1, and as 1 to 7.5. With natural draught the highest rate of combustion was 11.67 pounds of Pennsylvania anthracite coal, producing 16.6 per cent, of refuse. Forcing air into the ash pits by means of a fan-blower raised the rate of combustion to twenty seven lbs. of the same kind of coal; and by increasing, in this manner, the rate of combustion 131 per cent, the total horse power of the boilers was increased from 75 to 85 per cent. This method of blowing air directly into closed ash pits is connected with such serious inconveniences that it can be used only under exceptional circumstances. When the fire room is enclosed by tight bulkheads, it is a good plan to produce an increase of pressure in the fire room by forcing air into it. With an open fire room and exhaust fan connected with the chimney would promise good results, but the practical difficulties connected with its application are great. The simplest means of increasing the draught is the steam jet, which may be applied to any boiler, and no naval vessel should be without it. It is true, it is wasteful, but it has the great advantages of occupying no useful room, of being of inexpensive construction, and of not being liable to derangement. Isherwood found that the rate of combustion of anthracite coal in naval boilers might be increased 73.89 per cent, with an expenditure of 9.73 per cent, of the steam by the jet; and that the jet used 8.21 per cent, more steam of the total amount evaporated than the fan, for equal rates of combustion.
The foregoing inquiry will show how important it is, in designing the machinery of naval vessels, to distinguish between the means of attaining the most efficient cruising speed, and the highest possible speed of the vessel; the former must determine the dimensions and proportions to be given to the machinery, so as to secure the greatest efficiency with the allotted weight and space; the latter determines the limit of the capacity and strength of certain parts. Dislère states that for the pursuit of the ordinary merchant fleet in time of war a speed often knots would be sufficient for a cruiser; and Barnaby, Chief Constructor of the British Navy, assumes that cruising vessels of about nineteen hundred tons displacement should attain a speed of not less than thirteen knots on the measured mile. Accepting the opinions of both these authorities as correct, we must give such proportions to the boilers of our ordinary cruising vessels as will enable them to steam for the greatest length of time at a speed of ten knots an hour, with a given space allotted to, and weight of, machinery and fuel; and since for a speed of thirteen knots more than double the power required for ten knots has to be developed, we must provide means which make it possible to increase the rate of combustion correspondingly. For instance, in case the vessel is to be able to steam with the amount of fuel in her bunkers, at the rate often knots an hour for ten or twelve consecutive days, the grate surface would have to be proportioned so as to furnish the required power with a rate of combustion of from seven to eight pounds of anthracite coal; and in order to drive the same vessel at a speed of thirteen knots, the rate of combustion would have to be increased to about twenty four pounds of anthracite coal for the same grate-surface.
Vessels designed for the special purpose of intercepting mail steamers and fighting the fast cruisers of the enemy require not only a higher possible speed, but their machinery should be proportioned so as to attain the greatest efficiency at a relatively higher rate of speed than is done in the former class of vessels.
It may be instructive to illustrate some of the principles laid down in this essay by an example taken from our own naval service. The U. S. S. Quinnebaug, and class, have a load-draught displacement of nineteen hundred and ten tons. Their engines were originally duplicates of those of the four vessels of the Alaska class, but were converted from simple engines into compound engines, using steam of eighty pounds pressure above the atmosphere. The steam is furnished by ten cylindrical boilers having a diameter of eight feet, and a length of eight feet one inch. Each boiler contains a single cylindrical furnace-tube, and the horizontal return-tubes are arranged over and at the sides of the furnace. The aggregate area of grate-surface in the ten boilers is two hundred and forty square feet, and the ratio of heating to grate-surface is 24 to 1. The boilers are placed opposite each other, five on each side of the vessel, with the fire-room running fore and aft, between them. One telescopic smoke pipe, fifty feet high above the level of the grates, is common to all the boilers.
The arrangement of the coal bunkers differs somewhat in the several vessels of this class, but most of them carry one hundred and eighty tons of anthracite coal; one-half of this amount is stowed behind and around the boilers, and the other half is carried on the berth deck, in bunkers forty three feet long, running on each side of the vessel in the wake of the boilers. Neglecting these berth-deck bunkers, the total space occupied by the machinery and the fuel comprises the whole width and seventy-one feet in the length of the vessel below the berth-deck: the length of the engine room is twenty-three feet nine inches; the length of the fire room forty-four feet, and the width of the space occupied by boilers and fire room, twenty five feet nine inches from bulkhead to bulkhead.
The coal carried by the Quinnebaug (one hundred and eighty tons) is 9.4 per cent, of the displacement of the vessel. The total weight of the machinery equipped for service, including water in boilers, stores &c., is three hundred and sixty tons, or 18.8 per cent, of the displacement. The weight of the machinery is distributed as follows: engines, steam pumps, stores &c., including everything abaft the after fire room bulkhead=one hundred and sixty -six tons; boilers and all attachments and appurtenances, including floor plates and coal bunkers=one hundred and sixty tons; water in boilers=thirty-four tons. During the steam trial of the Quinnebaug in Chesapeake Bay, in December, 1878, a speed of 12.9 knots was maintained for one hour; the highest speed logged was 13.21 knots; but the mean speed for six consecutive hours was 11:65 knots, the engines developing 1102.89 I.H.P. with seventy-one pounds of steam in the boilers, and a combustion of 11.6 lbs. of anthracite coal per square foot of grate per hour.
According to these results a speed of ten knots would require 700 I.H.P. to be developed, and could be maintained with a consumption of seven lbs. of anthracite coal per square foot of grate per hour, or eighteen tons a day, and the vessel could steam at this rate for ten days. In order to maintain a speed of thirteen knots, the rate of combustion would have to be raised to about twenty-three lbs. per square foot of grate. The high speed attained during the trial for a limited period of time, shows plainly that the boilers were deficient in power relatively to the capability of the engines. Their low rate of combustion is to be attributed to the contracted space in the back connections, and to the faulty arrangement of the tubes. The influence of this arrangement on the economic and potential evaporation of these boilers, is discussed in an article in the last number (March, 1879) of the Franklin Institute Journal.
The draught through the lower tubes is so sluggish, that they become choked with ashes in a short time, and after a few days' steaming, the rate of combustion falls to nine pounds of anthracite per square foot of grate. Another serious disadvantage of the arrangement of the tubes in this boiler is the fact that it renders the crown sheets of the furnace and the bottom of the boiler practically inaccessible for cleaning. This type of boilers seems to have been adopted in order to place them as low as possible in the vessel, and to reduce the weight of water carried by them to a minimum; these considerations, however, must be regarded as of minor importance, compared with the questions of power and durability. The small height under the berth deck beams of these vessels, barely permits placing boilers one foot larger in diameter into them.
There are building now at the Washington Navy Yard, some cylindrical boilers for the U.S.S, Nipsic. They are nine feet in diameter, and are nearly nine feet long. Each boiler has two cylindrical furnaces thirty-four inches in diameter, and contains thirty-two square feet of grate surface. The tubes are arranged in the usual manner over the furnaces, and the boilers are perfectly accessible above and below the furnaces. The ratio of heating to grate-surface is 25.6 to 1, and of grate to calorimeter, 7.08 to 1; with these proportions and a chimney fifty feet high, the rate of combustion with natural draught, ought to be fourteen pounds of anthracite per square foot of grate. Eight of these boilers would contain two hundred and fifty-six square feet of grate-surface, or 6.Q per cent, more than the Quinnebaug's boilers, and with the above rate of combustion, they should develop eighteen per cent, more power than the Quinnebaug's boilers developed on her trial. At the same time these new boilers would occupy 4.5 feet less room in the length of the vessel; nearly the whole of this space, however, would be required for additional bunker-room, since the greater length of the boilers diminishes the capacity of the side bunkers.
The total weight of these new boilers, fully equipped for service, would be about fifteen tons greater than the weight of the present boilers, entirely on account of the greater quantity of water carried by the larger boilers.
Two rectangular boilers having two hundred and forty square feet of grate-surface in fourteen furnaces, would occupy 18.5 feet less space in the length of the vessel, and would weigh twenty tons less, including water and all attachments, than the present Quinnebaug's boilers. The room saved by the use of rectangular boilers would be sufficient to stow about ninety tons of coal, that is to say double the quantity which the vessel now carries in her hold, or all she carries in her berth deck bunkers. We cannot carry this comparison any farther at present. The foregoing may serve as an illustration of the mode of inquiry to be pursued, in determining the relative merits of different types of boilers. In conclusion we will remark, that, whatever our opinion may be about the merits of the compound and simple engines, we cannot shut our eyes to the fact that there is a growing tendency towards the use of higher pressures of steam for marine engines. Necessity will stimulate invention to overcome the serious difficulties which still limit pressures to eighty pounds per square inch. In case further improvements in the manufacture of steel boiler plates, and the discovery of some means which will effectually prevent the present rapid deterioration of marine boilers, should permit the use of a smaller factor of safety in boiler construction, than is at present found necessary, the reduction in weight would be relatively much greater for the cylindrical than for the rectangular boiler. A reduction in the space occupied by the machinery in the vessel can be attained, as far as it depends on the performance of the boilers, only by an artificial increase in the rate of combustion, by means which do not entail a large expenditure of power, or a great decrease in the economic evaporative efficiency of the boilers.
DISCUSSION.
Lieut. McLean. Mr. Chairman. I move that the Institute tender a vote of thanks to Mr. Roelker for the interesting paper he has read this evening.
The motion was carried unanimously.
Lieut. Mc Lean. I would like to ask Mr. Roelker if he has any data or information in regard to the differences in the economic efficiencies of the high pressure boilers with compound engines and the low pressure boilers with simple expansive engines when compared at low rates of speed and medium pressures. I ask the question because I think he has treated only of the high rates.
P. A. Eng. Roelker. I have no data to offer at present which would be valuable. I have endeavored to show in my paper that the economic efficiency of boilers of naval vessels should not be measured by their economy at the highest rates of speed.
Lieut. Mc Lean. I would like to ask what the life time of these boilers is at 80 pounds pressure? How do they compare in point of economy in five or six years service with rectangular boilers, taking into consideration the high steam pressure required?
P. A. Eng. Roelker. The introduction of compound engines and of these high-pressure boiled into our naval vessels is of comparatively recent date, so that we have not yet sufficient data to say positively what the life time of these boilers will be, in comparison with rectangular low-pressure boilers. Still, there is no reason why the high pressure in itself should cause these boilers to deteriorate quicker. Since the cylindrical form is best adapted to resist internal pressures, and the cylindrical boiler can be made more accessible and all its parts can be adjusted with greater precision to resist the various stresses; I think that the cylindrical boiler, even when used with high pressures, will be found to last longer than the rectangular boiler, provided it be worked under the same conditions as far as corrosion and similar influences are concerned.
Lieut. Tanner. I think that practice has pretty well demonstrated the fact that, in merchant or naval steamers running over regular routes, the cylindrical high pressure boiler has a life equal to if not greater than the rectangular low pressure boiler.
But I wish to ask, Mr. Chairman whether the gentleman does not think that, under the conditions existing in the naval service, where as a rule strange men are put into the tire room at the commencement of a cruise, the life of the cylindrical boiler would be much shorter, relatively, than that of the rectangular low pressure boiler under the management of the same men?
P. A. Eng. Roelker. I do not see why the inexperience of a fireman should shorten the life of our boilers so considerably. One of the principal reasons why the cylindrical boiler has a better chance for life is, that it can be made perfectly accessible in every part. In rectangular boilers there are always a great many narrow water spaces formed by flat surfaces, which have to be stayed every eight or nine inches, and thus access to these parts of the boiler is prevented. Consequently all sorts of deteriorating matters accumulate and rest there. This fact together with the peculiar bending stresses produced upon every part of the shell, is the principal cause of rapid decay, especially at the lower parts, of rectangular boilers. Cylindrical boilers have relatively few such spaces that are not accessible. The interior cannot only be inspected, but can be scraped and cleaned readily. That is a process which does not require especial skill. Therefore I do not see why these boilers should deteriorate more rapidly than the rectangular boilers in the navy, especially when it is proved that they last so much longer or equally as long in the merchant marine.
Of course the cylindrical boilers should be of sufficient diameter to admit of such arrangements as to make all parts accessible. These boilers in some of our vessels are two small, and made so on account of the low decks, and they are thereby deprived of the advantages I have just spoken of.
Lieut. Tanner. I referred more particularly to the cylindrical boilers, as they are constructed for the smaller classes of naval vessels, where the space below the beams, and water line, is so small that the boilers can have but little water over the tubes, the evaporation very rapid, the number of boilers greatly increased, each requiring a high order of intelligence and constant watchfulness on the part of the firemen and water tenders, who strange to the ship, and that type of boiler—would not the danger of burning out tubes, if not more serious accidents, be greater than with the low pressure rectangular boiler under the management of the same men?
In comparing the relative value of the high and low pressure boilers in the merchant marine and naval service, we must take into consideration the fact that in the former, the leading men in the engine and fire room are usually identified with the boilers from the time they are placed in the ship, and soon become thoroughly acquainted with every detail necessary to their safe and economical management.
Under the present system in the navy, these men are enlisted and sent on board when the ship goes into commission; they are strangers to the engineers in charge, to the ship, and perhaps, to that type of machinery.
My question is, would not the chance of accident from the burning out of tubes, etc., be greater than with the low pressure rectangular boiler under the management of the same men?
The Chairman. As I understand the question, it is that the complications of the cylindrical boiler are such, that it requires more care to run it than it does in the case of a rectangular boiler.
Lieut. Tanner. The water space in cylindrical boilers as they are constructed for the smaller classes of naval vessels, is much less than in the rectangular boiler; the evaporation much more rapid, the number of boilers much greater, each requiring the same care and constant watchfulness; the men in the fire room at the beginning of the cruise are strange to the ship and, perhaps, to that type of boiler. All these causes tend greatly to increase the liability to accident, and shortening the life of the boiler.
That is the point I make and ask the gentleman if I am not right?
P.A. Eng. Roelker. That is a point to which I think I made allusion in my paper, saying that the multiplication of boilers and of all other attachments naturally produces complications; and certainly on that account there is greater liability to derangement. Of course wherever the machinery is multiplied, the chances of derangement increase. In these vessels, for instance, which I took as an example, we have ten boilers, and even if we increased the size of the boilers to the greatest admissible diameter, we would require eight separate boilers. If we used rectangular boilers, we would put two boilers into the vessel, and there is no doubt that these eight boilers would require four times as much care as the two boilers. Also, these small boilers carry relatively less water than the others, and the water level will fall much quicker than in the larger boilers. Therefore there is undoubtedly a greater chance of accident from this cause; but the number of accidents which have happened on that account, to my knowledge, have been very few; surprisingly few. The coming down of a crown sheet is not a very unusual thing in any vessel. Merchant vessels are just as liable to it as men-of-war. The thing has happened with rectangular boilers as well as with these small cylindrical boilers. No doubt a greater amount of care is required, and liability to accident is increased, as the number of boilers is multiplied.
Lieut. Schroeder. Mr. Chairman, I think it a great pity that we should renounce an improved boiler, one which we think best, and give as a reason why it should not be adopted, that we have not got sufficiently expert firemen to attend to it. I deem that a question that should not be admitted into the argument.
Lieut. McLean. I wish to ask if at ordinary speeds, say seven or eight knots, the compound engine with its boilers as found in our vessels, is more economical, or at least as economical, in fuel, space, and other essentials, as the simple expansive engine with lighter cylindrical or rectangular boilers? Is not the present system better designed for a high speed at an economical rate, than for a cruising speed to be maintained for a long time?
P.A. Eng. Roelker. As to the relative economy of the high pressure compound, and the low pressure single expansive engines, when working with reduced power, there exists still a great diversity of opinion among engineers; and I have not been able to collect a sufficient number of reliable data to settle this question definitely in my mind. For this reason I have spoken, in the paper which I have read, of the efficiency of boilers without reference to the efficiency of the engines proper. I am quite convinced, however, that the selection of the type of machinery for naval vessels should be determined by the relative economy in developing the power required for, what I called, the most efficient cruising speed in time of war; and while the engines should be proportioned with a special view to the economical development of this power, they must possess sufficient strength and capacity to work off all the steam which the boilers can generate with their highest rate of combustion. The English seem to follow a different course in proportioning the machinery of their naval vessels. They proportion their boilers to steam economically at a low rate of speed, burning about ten pounds of semi-bituminous coal on the square foot of grate per hour; when necessary the rate of combustion of this coal can be increased to twenty-three pounds per hour without the use of artificial draught. But their engines are proportioned so as to work economically with the power due to this high rate of combustion, and in consequence they are wasteful of power and steam at the lower rates of speed, which the vessels have to maintain nearly throughout their whole lifetime. Let us take as an example the vessels of the Garnet class, which are very nearly of the same size as our Quinnebaug class of vessels. Their displacement, at load draught, is eighteen hundred and sixty-four tons, or forty-six tons less than that of the Quinnebaug; they have, however, three feet more beam than the latter vessel. Their grate-surface is almost exactly the same as that of the Quinnebaug, viz.: two hundred and forty-five square feet. The Garnet has sufficient height under her berth deck beams to carry boilers ten feet in diameter, and of these she requires only six, while the Quinnebaug has ten boilers, eight feet in diameter, on account of the small height under her berth deck beams: for this reason, the boilers of the Garnet occupy eleven feet less space in the length of the vessel than the Quinnebaug’s boilers. But the engines of the Garnet are much larger than those of the Quinnebaug, and, apparently, proportioned to work economically, when developing the lightest possible power, viz.: twenty-one hundred indicated horsepowers.
The Garnet has two-cylinder compound engines.
Diameter of high pressure cylinder, 57 inches.
Diameter of low pressure cylinder, 90 inches.
Length of stroke, 33 inches.
Steam pressure in boilers, 60 lbs. per sq. inch.
Highest number of revolutions, 90 per minute.
The main dimensions of the Quinnebaug’s engines are as follows:
Diameter of high pressure cylinder, 42 inches.
Diameter of low pressure cylinder, 64 inches.
Length of stroke, 42 inches.
Steam pressure in boilers, 80 lbs. per sq. inch.
English engineering journals have criticized severely this policy of the English naval authorities, of proportioning the engines of their vessels for a power which can be developed only under the exceptionally favorable conditions of a trial, and which will rarely be approached in the whole afterlife of the vessel.
Lieut. John H. Moore. Mr. Chairman; Mr. Roelker has stated that European navies use semi-bituminous coal in their vessels. I would like to ask if any better results would have been obtained in the recent trials of the Quinnebaug if this semi-bituminous coal had been used.
P. A. Eng. Roelker. More power would undoubtedly have been developed, but the boilers have not the proportions required for the economical use of semi-bituminous coal. This coal experiences great waste and deterioration from transportation and long stowage, and produces much soot which lessens, often greatly, the heating efficiency of the boilers. But for a short trial of speed the use of that coal gives much better results than anthracite. The difference is this, as I have already stated; under the most favorable circumstances, the boilers being constructed with a special view to attaining a high rate of combustion, not more than sixteen pounds of anthracite can be burned on the square foot of grate per hour, with natural draught; while twenty-three or twenty-four pounds of semi-bituminous coal are always burned during the full-power speed trials of English vessels.
Lieut. Moore. During the trial of the Quinnebaug did she have any means of obtaining artificial draught from a jet or fan?
P. A. Eng. Roelker. That I am not quite sure of. Some of the boilers were designed to have a steam jet, but whether they were all supplied with it I cannot tell. There were no fan blowers. In such trials jets are not made use of, because too large an amount of steam would be expended. All the trials which we make, are made with a natural draught in order to test the capabilities of boilers and engines under ordinary conditions; in fact, that is the difference between the trials of our engines and those of the English service. They test their vessels under exceptional conditions; we test ours under the ordinary conditions of service. The English use a fuel that is specially selected for trial purposes. It produces only about six per cent, of refuse, being hand-picked and delivered in the most perfect condition on board the vessel. The firemen have only to throw it into the furnaces and keep the fires properly leveled. A special body of men is selected from the Reserve to do duty as firemen during each trial. Our vessels as soon as they are ready for a cruise, with their stores aboard, steam out with the ordinary coal which is to be used during the cruise. The coal used during this so called full-power speed-trial may be good, bad or indifferent. The firemen, also are generally newly enlisted men, and often possess little experience and skill in firing. On this account we may say that actually we do not even try our vessels under conditions as favorable as obtain ordinarily in the service, because the men profit by their experience in the course of a cruise.
Our naval vessels on foreign stations very often use Welsh coal and I think it would be a good plan for our vessels to try the mixed fuel which is used in the English and French services; that is a mixture, in equal proportions, of this Welsh and of bituminous coals. The bituminous coal acts in binding the small particles of this Welsh coal together and thereby prevents waste of coal through the grate. But whether the substitution of this semi-bituminous coal for anthracite for general cruising purposes would be an improvement, it is hard to tell at present.
Rear Admiral Jenkins, (Vice President.) While in China, we used the Cardiff or Welsh coal in our ships and we found that the British Admiral in charge; of the British Squadron in China, bought a lot of it for the sake of economy. We also used the Japanese coal, called Takasima, and found it cheaper than our own coal.