Read Ebook: Scientific American Supplement No. 299 September 24 1881 by Various

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The author began by referring to a paper read at the Liverpool meeting in 1872, by Mr. F. J. Bramwell, F.R.S., on "The Progress effected in Economy of Fuel in Steam Navigation, considered in Relation to Compound Cylinder Engines and High-pressure Steam;" then proceeded to continue the subject from the date of that meeting, to trace out whether any, and if so what, progress had been made; further, to consider whether or no we have reached the finality so strongly deprecated by Sir Frederick Bramwell in the discussion referred to, and, if not, then in what direction we are to look for further development.

From a table it would seem that the steam pressures are now much higher, the boilers have less heating surface, and the cylinders are much smaller for the indicated horsepower developed than in 1872; and at the same time the average consumption of fuel is reduced from 2.11 lb. to 1.828 lb., or by 13.38 per cent.

MARINE ENGINES.

The author then briefly described the modern marine engine and boiler. The three great types of compound engines may be placed as follows in the order of their general acceptance by the shipowning community: The two-cylinder intermediate-receiver compound engine, having cranks at right angles. The Woolf engine in the tandem form, having generally the high-pressure and low-pressure cylinders in line with each other, but occasionally alongside, and always communicating their power to one crank. Such a pair of engines is used sometimes singly, oftener two pairs together, working side by side to cranks at right angles; recently three pairs together, working to cranks placed 120 deg. apart. The system affords the opportunity of adding yet more engines to the same propeller to an indefinite extent. The three cylinder intermediate-receiver compound engine, with one high and two low-pressure cylinders, the steam passing from the high-pressure cylinder into the receiver, and thence into the two low-pressure cylinders respectively. The cranks are placed at equal angles apart round the crank shaft, so as to balance the forces exerted upon the shaft.

These three types may be said to embrace all the engines now being manufactured in this country for the propulsion of steam vessels by the screw propeller. In their leading principles they also embrace nearly all paddle engines now being built, whether the cylinders be oscillating, fixed vertically, or inclined to the shaft.

The compound engine, in fact, in one of these three forms, may now be said to be universally adopted in this country; and the question of the relative value of simple expansion in one cylinder, and of compound expansion in two or more cylinders, which agitated the minds of some of our leading engineers ten years ago, is now practically solved in favor of the latter.

THE MARINE BOILER.

The marine boiler of to-day is in all its main features the same as it was ten years ago. The single-ended boiler, made with two, three, and sometimes four furnaces, is the simplest form, and for all powers under 500 indicated horse power is the most generally adopted. The double-ended form is largely used. It has been found more economically efficient than the single-ended form, by as much as ten per cent, in the writer's own experience. It is generally adopted for engines of large power, but for small power is inconvenient, owing to its occupying more room lengthwise in the vessel, and also involving two stokeholds and therefore more supervision. At one time great difficulty was found in keeping the bottoms of boilers of this kind tight. Owing to their length, the unequal expansion due to different temperatures at the top and bottom caused severe racking strains on the bottom seams and riveting--so severe in some cases as to rend the plating for a large part of the bottom circumference of the shell. This difficulty has now been to a large extent got over, in consequence of the greater attention given to the form and direction of the water spaces in the boiler itself, so as to induce circulation of water; the introduction of the feed-water at the top instead of near the bottom; the more careful management now usual on the part of engineers; and lastly, the use of larger plates, welded horizontal seams, drilled rivet holes, and more perfect workmanship throughout. A modification of double-ended boiler is that introduced by Mr. Alfred Holt. It has many decided advantages, but is costly to make. The formation of the two ends into separate fire-boxes leaves the bottom of the boiler free to adapt itself to the variations of temperature to which it is exposed. The separation of the furnaces from the combustion chamber, excepting through the opening afforded by a connecting tube, is an advantage in the same direction, and avoids almost entirely the racking strains due to irregular furnace action. The weight of water carried is less, and that of the boiler may also be made less; while the elliptical form of the two ends gives greater steam space.

A type of boiler largely used in her Majesty's Navy, somewhat like a locomotive boiler, is highly efficient in regard to weight and power developed. Many examples have yielded one indicated horse-power in the cylinders for every three square feet of heating surface, under natural draught and with a very moderate height of funnel; and this with a consumption of fuel not exceeding 2 1/2 lb. per indicated horse-power per hour under a working pressure of 60 lb. With the aid of a steam jet in the funnel, the heating surface per indicated horse-power has fallen below 2 1/2 square feet. The large water surface afforded for escape of steam secures almost entire freedom from priming, without the incumbrance of steam domes; and the large combustion chamber allows of the thorough combustion of the gases before their passage through the tubes. The locomotive type of boiler has lately occupied the writer's attention, with a view to its more definite introduction into marine work. The difficulties, however, which lie in the way of applying it to steamers going long voyages are very great. The principal difficulty lies in the necessity of burning a large quantity of fuel in a very limited space and time. This can only be done either by direct pressure or exhaust action applied at the furnace. In other words, we must either exhaust the funnel, which will absorb a large amount of power, but would be comparatively easy of application; or our stokers, as is the case with our miners, must work under a pressure of air.

STEEL BOILERS.

The writer stated that his experience in the manufacture and working of steel boilers was satisfactory. Many steel boilers of sizes varying from six feet diameter to fourteen feet six inches diameter have left the works at St. Peter's since 1877, when the first was made; and in no case has there been a failure of a plate after being put into a boiler, either in the process of manufacture or in working at sea. The mode of working is as follows: For shell plates, from five-eighths inch to seven-eighths inch thick, to warm each to a dark red heat before rolling, having previously drilled a few holes to template for bolting the strakes together; the longitudinal seams are usually lap joints treble riveted, requiring the corners to be thinned, which is done after rolling. The furnace plates are generally welded two plates in length, and flanged to form Adamson rings, and at the back end to meet the tube plate; the back flame-box plates are flanged, also the tube plates and front and back plates; and wherever work is put on to the plate it is annealed before going into the place. The rivet holes are drilled throughout. In the putting together the longitudinal seams of the thicker plates of the shells, great care is always taken to set the upper and under plates for the lap to their proper angle before they are bolted together, a point generally overlooked by the practical boilersmith.

CORROSION OF BOILERS.

The question of corrosion is one which is gradually being answered as time goes on; and so far very satisfactorily for steel. Some steel boilers were examined a few weeks ago which were among the first made; and the superintending engineer reports: "There is no sign of pitting or corrosion in any part of the boiler; the boilers are washed out very carefully every voyage, and very carefully examined, and I cannot trace anything either leaking or eating away. No zinc is used, only care in washing out, drying out, and managing the water." This is the evidence of an engineer with a large number of vessels in his charge. On the other hand, some of the most prominent Liverpool engineers always use zinc, and take care to apply it most strictly. The evidence of one of them is as follows: "We always fix slabs of zinc to most boilers, exposing not less than a surface of one square foot for every twenty indicated horse-power, and distributed throughout the boiler. This zinc we find to be in a state of oxide and crumbling away in about three months. We then renew the whole, and find this will last twelve months or more, when it is renewed again. Meanwhile we have no pitting and no corrosion; but on the contrary, the interior surfaces appear to have taken a coating of oxide of zinc all over, and we have no trouble with them."

HOW THE MARINE ENGINE MAY BE IMPROVED.

Merchant steamers.......................... 480 Royal Navy................................. 300 Engines specially designed for light draught vessels...................................280 Royal Navy, Polyphemus class .................................. 180 Modern locomotive.......................... 140 Torpedo vessels............................. 60

Ordinary marine boilers, including water... 196 Locomotive boilers, including water......... 60

The ordinary marine boiler, encumbered as it is by the regulations of the Board of Trade and of Lloyds' Committee, does not admit of much reduction in the weight of material or of water carried when working. The introduction of steel has reduced the weight by about one-tenth; but it will be the alteration of form to the locomotive, tubulous, or some other type, combined with some method of forced draught, to which we must look for such reductions in weight of material and water as will be of any great commercial value. The engine may be reduced in weight by reducing its size, and this can only be done by increasing the number of revolutions per minute.

It has hitherto been the practice to treat the propeller as dependent upon the size of engines, draught of water, and speed required. This process should be reversed. The propeller's diameter depends on the column of water behind necessary to overcome the resistance in front of it due to the properties of the vessel. This fixed, the speed will then fix the number of revolutions, which will be found much greater than is usual in practice, and from this the size of the engines and boilers will be determined. Great saving in weight can be effected by careful design and judicious selection and adaptation of materials, also by the substitution of trussed framing and a proper mode of securing the engine to the structure of the vessel, as worked out in H.M.S. Nelson, by Mr. A. C. Kirk, of Glasgow, and in the beautifully designed engines by Mr. Thornycroft, in place of the massive cast-iron bedplates and columns of the ordinary engines of commerce. The same may be said of the moving parts. In fine, the hull and engines should be as much as possible one structure; rigidity in one place and elasticity in others are the cause of most of the accidents so costly to the ship-owner; under such conditions mass and solidity cease to be virtues, and the sooner their place is taken by careful design, and the use of the smallest weight of material--of the very best kind for the purpose--consistent with thorough efficiency, the better for all concerned.

CONSUMPTION OF FUEL IN MARINE ENGINES.

Coming to the question of the consumption of fuel, a considerable saving has been effected in nine years, as shown in the following table:

Item. 1872. 1881.

This shows a saving equal to 13.38 per cent, in quantity of fuel consumed. Mr. Marshall then read a letter from Mr. Alfred Holt, of Liverpool, bearing on this subject, in which Mr. Holt spoke favorably of the single-crank engine, and stated his belief that the compound system would ere long be abandoned for the simple engine. He is endeavoring to feel his way to using the steam in one cylinder only, and so far the results have been encouraging, and he is now fitting a 2,200-ton vessel on that system. He is also endeavoring to do without a crank shaft, the forward end of the screw shaft carrying an ordinary crank with overhung pin. This experiment also promises satisfactorily. In his opinion the great improvement of the immediate future is to increase the steam production of our boilers. A ton weight of a locomotive boiler produces as much steam as six tons of an ordinary steamboat boiler.

Mr. Holt speaks of the coal account as one of the minor disbursements of a steamer. He does not give the ratio which coals bear to the total disbursements, but from other reliable sources Mr. Marshall found that, according to the direction of the voyage, it varies from 16 to 20 percent.--or, say, an average of 18 per cent.--of the total disbursements, in a vessel carrying a cargo of 2,500 tons. This will represent to-day about ?3,000 per annum, and in 1872, at equal prices, the cost would have been ?3,750--showing a saving of ?750, equal to a dividend of, say, 3 per cent. on the value of the ship. Again, the cost of coal per mile run for such a vessel, in 1872, would have been at least 16 1/2 d.; to-day it does not exceed 13d.

EVAPORATIVE EFFICIENCY OF MARINE BOILERS.

The marine boiler as now made is very efficient, but if the quantity of steam used be considered in relation to the increased pressure, it will be seen that the boiler of to-day is little if anymore efficient than that of ten years ago. The present boiler has an evaporative efficiency of about 75 per cent., and cannot be much improved so long as air is supplied to the furnace by the natural draught. To increase the efficiency from 75 to 82.5 per cent. would require about double the heating surface, the weight of boiler and water being also doubled, while the gain would be only 10 per cent. Mr. Blechynden's formula, used in Mr. Marshall's works for weights of cylindrical marine boilers of the ordinary type, and for pressures varying from 50 lb. to 150 lb., is as follows:

This formula, if correct--and it is almost strictly so--would give the relative weight of boilers per sq. ft. of heating surface, for 105 lb. and 150 lb. total pressure, assuming we wish to increase the efficiency 10 per cent, as follows:

In other words, the boiler with the higher efficiency would weigh two and a half times that with the lower efficiency. In the case of a vessel of 3,000 tons, with engines and boilers of 1,500 indicated horse power, the introduction of locomotive boilers with forced draught would place at the disposal of the owner 150 tons of cargo space, representing ?1,500 per annum in addition to the present earnings of such a vessel.

MARINE LOCOMOTIVE BOILERS.

Mr. Thornycroft has for some years used the locomotive form of boiler for his steam launches, working them under an air pressure--produced by a fan discharging into a close stokehold--of from 1 in. to 6 in. of water, as may be required. The experiments made gave an evaporation of 7.61 lb. of water from 1 lb. of coal at 212? Fahr., with 2 in. of water pressure, and 6.41 lb. with 6 in. of pressure. These results are low, but it is to be remembered that the heating surface is necessarily small, in order to save weight, and the temperature of the funnel consequently high, ranging from 1,073? at the first pressure, and 1,444? at the 6 in. With the ordinary proportions of locomotive practice the efficiency can be made equal to the best marine boiler when working under the water pressure usual in locomotives, say from 3 in. to 4 in., including funnel draught.

It has fallen to the lot of the writer to fit three vessels recently with boilers worked under pressure in closed stokeholds. The results, even under unfavorable conditions, were very satisfactory. The pressure of air would be represented by 2 in. of water, and the indicated horse power given out by the engines was 2,800, as against 1,875 when working by natural draught, or exactly 50 per cent. gain in power developed.

Mr. Marshall then proceeded to refute the arguments which may be urged against the use of the locomotive boiler at sea, and which we need not reproduce. Coming to the engines, Mr. Marshall said that the total working pressure of to-day may be accepted as 105 lb., or equal to seven atmospheres. If it were boldly accepted that eleven atmospheres, or 165 lb., were to be the standard working pressure, the result would be a gain of 14.55 per cent., provided no counteracting influence came into play. Of course, there are forces which war against the attainment of the full extent of this advantage, viz., the greater condensation in the cylinders and loss in the receiver or passages.

In regard to the former, it may be questioned whether by steamjacketing the high pressure cylinder, correctly proportioning the steam passages, and giving a due amount of compression in both cylinders, this may not be reduced far below the generally received notion; and the latter cause of loss may be considerably reduced in its effect by a more carefully chosen cylinder ratio. The ratio usually adopted, between 3.5 and 4 to 1, whether the pressure be 70 lb. or 90 lb., may well be questioned. With a cylinder ratio of 2.95 to 1, the economic performance is very good, and equal to any with the higher ratio. A lower cylinder ratio has another advantage of considerable value, viz., that the working pressure can be much reduced as the boilers get older, while by giving a greater amount of steam the power may be maintained--at an extra cost of steam, of course, but not so great a cost as with higher ratios. The cut-off in the high-pressure cylinder usually takes place at about 0.6, and the ratio of expansion has decided the ratio of cylinders. The use of separate starting valves in both cylinders obviates that necessity.

The difficulties in the way of taking advantage of the higher economic properties of greater pressures than hitherto used on board ship, are, it is submitted, not insuperable, and it would be to the interest of all that they should be firmly and determinedly met. It may be accepted as an average result that the Woolf engine, as usually arranged, will use 10 per cent. more steam than the receiver engine for the same power.

Of the three-cylinder receiver type the data are insufficient to form a definite opinion upon; but so far the general working of the Arizona is stated to be as good, economically, as any of the two-cylinder receiver class. The surface condenser remains as it was ten years ago, with scarcely a detail altered. In most engines it remains a portion of the framing, and as such adds greatly to the weight of the engine.

It is a question seriously worth consideration whether or no the surface of tubes can be reduced. The practice at present is to make the surface one-half the boiler surface as a minimum, that is, equal to about 2 square feet per indicated horse power. In practice, the writer has found 1.4 square feet per indicated horse power to maintain a steady vacuum of 27 1/2 inches.

Mr. Marshall has just completed six pairs of engines for three twin screw ships, having steel shafts of 10 inches diameter, and has in each case run the engines at 120 revolutions per minute, while indicating 1,380 horse power from each pair for ten to fifteen hours without stopping; and in no case has a single bearing or crank pin warmed or had water applied, the surfaces on examination being perfect. In these engines all working bolts, pins, and rods, except the piston and connecting rods, are of steel, all rods in tension being loaded to 8,000 lb. per square inch. The boilers are of the Navy type, made throughout of Siemens-Martin steel plates, riveted with steel rivets, all holes drilled. Furnaces are welded and flanged; the tubes are of brass. In comparison with an ordinary merchant steamer's iron boilers of the double ended type, they weigh, including water and all appurtenances, as follows:

Double ended Type. Navy Type.

SCREW PROPELLERS.

The screw propeller is still to a great extent an unsolved problem. We have no definite rule by which we can fix the most important factor of the whole, namely, the diameter. Mr. Froude has pointed out that by reducing the diameter, and thus the peripheral friction, we can increase the efficiency; and this is confirmed by cases--of Iris reduced 2 feet 3 inches, and the Arizona reduced 2 feet. This must, of course, be qualified by other considerations. The ship has by her form a definite resistance, and a certain speed is required; if the propeller be made too small in diameter, the ship will not be driven at the required speed, except at serious loss in other directions. This question was too large and complicated to be dealt with here, and should, in the first instance, be made the subject of careful and extended experiment, on which a separate paper should be written.

To sum up the whole. Progress has been made during the past nine years, and in the following particulars:

STEAM FERRY BOATS OF THE PORT OF MARSEILLES.

All those who are acquainted with the Old Port of Marseilles know the inconvenience of communication between one shore and the other, and the high price of ferriage by row boats. To obviate this, Captain Advient has been struck with the happy idea of creating a cheap steam service , thus supplying a genuine want in the modes of locomotion of the city.

The building of these ferry boats, on a system providing for the use of separate hulls, was confided to Messrs. Stapfer, De Duclos & Co., of Marseilles, whose well-known reputation was a sufficient guarantee that the problem would be successfully solved.

There existed difficulties of two natures: The first of these related to the stability of boats such as this, having their engine, boiler, supply of coal, forty passengers who might all occupy one side of the vessel, a central superstructure, with roof; and, finally, all the weight centered on five feet of the deck, with nothing below to counterbalance it except the hollow hulls and two three-foot compartments, each placed toward the central portion of the hulls and designed as fresh-water reservoirs for the steam generator. The second difficulty was to obtain the best utilization possible of a screw placed in the current between the hulls and upon a shaft inclined toward the stern, that is, "stern" by analogy, for there is no distinction of fore and aft in ferry boats.

The conditions of the problem were finally fulfilled to the satisfaction of all concerned, and especially to that of the public.

The hulls, navicular in form and having a flat bottom, are constructed of one-tenth inch iron plate and 40x40 angle iron. Their dimensions are: Length, 33 feet; breadth, 3 1/4 feet; and depth, 5 feet. The internal distance between the two shells is 7 1/4 feet. These hulls, having absolutely water-tight decks, are connected below by tie bars of flat iron, and above by vertical stays 1 foot in length, which serve to support the floor-planks of the deck and boilerplate flooring of the engine-room. The engine-room, which is 19 1/2 feet long by 5 feet wide, is constructed of varnished pitch-pine, with movable side-shutters of teak. The roof, of thin iron plate, is provided with a ventilator to allow of the escape of hot air.

The passengers, to the number of forty or fifty, can move about freely from larboard to starboard, or from stem to stern, or seat themselves on the benches running along the inside of the guard railing on the two sides of the vessel. They are protected from rain by a roof, and from the rays of the sun by a curtain extending along the sides.

Although the usual method of landing is fore and aft, gangways have been provided at the sides for side-landing should it become necessary.

The general appearance of one of these boats may be likened to that of a floating street-car. Finally, a small apartment, provided with benches, is provided for the use of those passengers who might be taken sick, or for office purposes, if need be.

The total weight of one of the boats is divided up as follows:

Total...........................41,800 "

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