Read at the Thirtieth Session of the Institution of Naval Architects, July 26, 1888; the Right Hon. Earl of Ravensworth, President, in the Chair. [Reprinted from the Transactions of the Institution of Naval Architects, 1889.]
The R. Italian ironclad Lepanto underwent recently a series of trials at sea which are interesting, both in themselves and with reference to the size and type of the ship and the power and type of her engines and boilers. It is, in fact, the first time that a power of 16,000 I.H.P. has been developed on board an ironclad, giving her a speed of over 18 knots, and that a large number of locomotive boilers, in connection also with boilers of a different kind, have been worked together with complete success.
I have got permission from His Excellency the Minister of Marine, Mr. B. Brin, the designer of the ship, to put before this institution the results obtained, hoping that their record, enhanced in importance by the considerable amount of attention that this type of ship has attracted from the naval and the engineering world, will be a useful addition to the knowledge already gained on the propulsion of modern war-ships.
A description of the Italia, the sister ship of the Lepanto, was given some time ago in scientific newspapers (Engineering, February 17, 1888, p. 158), and therefore I shall restrict myself here to the principal dimensions and data of the Lepanto that have a bearing on the subject.
The principal dimensions of the Lepanto are as follows:
Length between perpendiculars, 400 ft. 6 in.
Breadth, 72 ft. 9 in.
Depth, moulded, 46 ft.
Mean draught, normal, 28 ft. 4 in.
Area of midship section, 1843 sq. ft.
Displacement, 13,851 tons.
The ship is entirely built of steel, and has no sheathing on her bottom, differing in this respect from the Italia, in which the steel bottom is sheathed with wood and zinc.
The internal divisions are pretty much the same in both ships, with the exception of the boiler-rooms, which are differently arranged on account of the different type of the boilers.
The Italia is fitted with twenty-six boilers of the Admiralty oval marine type, divided into six compartments, three forward the engine-rooms and three aft, each compartment having its own funnel.
On the Lepanto there are also six compartments of boilers, similarly situated, as shown in Fig. 1 (Plate VI), but only the two near the engine-rooms have marine oval boilers, four in each, and the remaining four compartments have locomotive boilers, four in each, making a total number of eight oval marine boilers and sixteen locomotive boilers. Their arrangement is clearly shown on the sketch.
The locomotive boilers, which form, perhaps, the main interesting feature of the machinery, deserve special notice. There are two furnaces in each boiler, separated by a longitudinal water space, which, however, stops short of the tube-plate, leaving a passage between, above the bridge. The furnaces are just as long as the fire-grate, but to prevent the fire damaging the tubes, and to ensure a good combustion of the gases, a high hanging inclined baffle brick bridge is fitted, as usual in railway practice, in each furnace at the end of the fire-grate.
The bottoms of the ash-pits form water-pans to keep the grates cool; the latter are made with longitudinal cast-iron rocking bars.
The oval boilers have three furnaces, each discharging into one common combustion chamber. Their grates have ordinary fire-bars ¾ inch thick, with 5/8 inch interstices. There are four funnels—two for the forward set of boilers, and two for the after one.
In each set the oval and the locomotive boilers have each their own separate funnel.
The boiler-rooms are provided with twenty fans—four in each oval boiler-room, and three in each locomotive-room—capable of maintaining an air pressure over 2 ½ inches of water in the former, and of 4 inches in the latter.
The main engines, four in number, and arranged in four separate compartments at the center of the ship, are of the well-known type of Messrs. Penn, with three equal vertical cylinders, as applied on H.M. ships Ajax and Agamemnon, working compound at moderate power, and direct at full power.
The cylinders are steam-jacketed, and fitted with a double ported flat-slide valve, having an expansion valve working on its back, which allows of any degree of cut-off being fairly obtained.
The main engines work their own air and main feed pumps, the circulating pumps only being, as usual, moved by independent engines.
PARTICULARS OF TRIALS.
The trials were to be made in accordance with the following program, proposed by Messrs. John Penn & Sons, and accepted by the Ministry of Marine:
- A trial with only two oval boilers at work, and the after engines only at work on the compound system, to ascertain the most economical steaming of the ship.
- A trial with the eight oval boilers at work, the four engines working compound.
- Ditto with four engines working direct expansion.
- A forced draught trial with only the after set of engines and boilers at work, the engines working direct.
- A forced draught trial with all eight oval boilers and eight locomotive boilers at work, the four engines working direct.
- A full-power forced draught trial or trials, with all the engines and boilers at work, the engines working direct.
This program was not completely carried out, on account of the ship having been put in commission, which prevented the final 18,000 I.H.P. trial being made. It went, however, far enough to show what can be expected from the engines when working at their full power.
The trials were made along the eastern coast of the Gulf of Genoa, from Spezia to Genoa and back, the two runs being altogether over eighty nautical miles. A portion of the forward run from Spezia to Genoa was taken in each trial, to bring up the engines to the desired speed.
The speed of the ship was ascertained by means of bearings on well-known points on the shore in both runs.
The bottom of the ship was fairly clean, the ship having been docked on March 1, viz., about one month before the trials.
The ship was fully laden in all the trials, with very slight differences of draught.
The indicated horse-power developed was ascertained from the indicator cards of the main engines, without taking any account of the steam used for auxiliary purposes.
The trials were carried out under the direction of Mr. J.W. Fairley and Mr. May, who represented the firm, and of Mr. Holland, the engineer in charge.
All the results obtained and the conditions of trials are figured on the annexed table, and to them the following remarks will serve as an illustration.
One of these remarks is of great importance, as it refers to the behavior of the locomotive boilers, which was perfect.
After the rather discouraging experience with locomotive boilers working in sets on board some ships, as the Flavio Gioja of the Royal Italian Navy, and the Polyphemus of the Royal English Navy, some fear was entertained that similar troubles might be experienced with the Lepanto, in which the difficulty appeared to be still greater, considering the larger number of boilers to be worked together in so many different separate compartments.
But nothing of the kind happened, nay, everything went to prove the contrary.
From the beginning of the preliminary trials, which took place towards the end of last year, the locomotive boilers gave evidence of their good working, which went on increasing trial after trial, so as to be now an established fact.
They never primed nor gave any trouble whatever. The feeding was occasionally uncertain, but the fault was due to air that collected in the main feed pipe. This imperfection was removed, and on the last two trials the feeding was quite satisfactory. After each one of the last three forced draught trials the locomotive boilers had tubes leaking, but in small numbers, and not more, comparatively, than the oval boilers, which, even in this respect, did not behave better. Moreover, there were discrepancies between the different compartments of boilers, locomotive as well as oval, which show that the management of the fires has a good deal to do in this matter.
The ventilation of the locomotive stokehold is excellent. The fans being fitted on top of the boilers, no current of air strikes the floor, and a thorough cool ventilation and forced draught are obtained without any inconvenience whatever from coal dust. The same may be said of the oval boiler stokeholds, where the fans are fitted on the wings behind the boilers, but, although the supply of air is ample, the temperature does not during forced draught fall so low, probably on account of the boilers facing each other. The mean temperature of the oval boiler stokeholds was about 106° against 88° in the locomotive boiler stokeholds, while the atmospheric temperature oscillated about 58°.
The engines worked very satisfactorily all through the trials, without the slightest hitch occurring in any part of the whole machinery. This circumstance helped; no doubt to some extent, the good performance of the boilers, which had never to be checked or hampered when in full swing.
The power of 16,150 I.H.P. on the last trial was developed by the engines with a mean air pressure of 1.9 in. of water in the locomotive boiler stokehold, and of 1.6 in. in the oval boiler stokehold, the coal burnt per square foot of grate per hour being 51 pounds in the former and 38 in the latter. But as, at the preliminary partial trials mentioned above, the oval boilers were worked up to 2 ½ in. of pressure and the locomotive boilers up to 3 ½ in. with perfect success, burning 45 and 68 pounds of coal respectively per square foot of grate per hour, there is evidently room left for more power. The gain in speed which would follow such increase of power is, however, not very great, as is clearly apparent from the results of the last two trials, and still more from the I.H.P. Curve, shown on Fig. 2 (Plate VII), of which something will be said hereafter.
A very good performance was that obtained on the sixth trial (April 28), when, with only two-thirds of the boilers at work, the engines developed over 12,000 I.H.P. (two-thirds of the total power), driving the ship at nearly 17 knots. The cut-off being at 0.175 of the stroke, the steam worked with a ratio of expansion 435, which, from the consumption of water shown by the indicator cards, appears to give the most efficient performance of the engines at great power.
A circumstance deserving notice is that all these trials were carried out with the ship's stokers, who, for the greater part, were not yet trained for forced draught stoking.
Regarding the efficiency of the engines, although the consumption of coal was the lowest on the first trial, when the two after engines were acting compound at very low power with a great ratio of expansion (about 11), this condition of working does not appear to be the most efficient, as the consumption of water shown by the indicator cards was greater on this trial than on the next, when the ratio of expansion was reduced to 3.5 only.
The same thing happens, although in a less marked degree, when the engines act with direct expansion, as there is no material difference in the consumption of steam over four expansions. Below this ratio the consumption increases, as may be seen from the results of the last two trials.
To have more complete data of the ship and engines' performance, besides the trials above mentioned, runs were made on the measured mile to ascertain the speed of the ship corresponding to the lowest possible speed and power of the engines; also the power and speed of the engines for a speed of about 10 knots of the ship.
The results are as follows:
Speed of ship, knots, 2.7 10
Revolutions, 15 55
I.H.P 158.6 2403
Now I beg to call attention to Fig. 2 (Plate VII), in which the results relating to the ship's performance are graphically recorded, in connection with the E. H. P. Curve, as was determined by experiments on the model of the Italia made for the Royal Italian Government by Mr. R. E. Froude at Torquay, by the kind permission of the Admiralty.
The E. H. P. curve aa corresponds to a displacement of 14,784 tons, which is approximately the mean displacement of the Lepanto at the various trials. The Lepanto being a finer ship than the Italia for the same displacement, the ordinates of curve aa should be lowered a little, but considering that the bottom of the Lepanto was not perfectly clean, curve aa may be accepted as sufficiently correct.
bb is the I.H.P. curve.
dd is the " indicated thrust curve," as it results from the I.H.P. curve. The dotted line at the bottom of curve dd is to show the increase of thrust due to the friction of the forward set of engines when they were acting at low powers with the after set.
According to this curve the initial friction of the engine would be about 7.5 per cent of the load at full power.
ff is the "curve of the net resistance of the ship" as it results from the E.H.P. curve.
It will be noticed that undulation characteristic of the E.H.P. curve aa and of the net resistance curve ff at about 16.5 knots is faithfully reproduced on the I.H.P. curve bb and on the indicated thrust curve dd, giving strong evidence of the correctness and importance of the method of investigation devised by the late Mr. Froude.
Curve cc gives the ratio ,E.H.P./I.H.P. = p, viz., the propulsive coefficient or the "net total efficiency of propulsion," which slightly increases at the higher speeds when it approaches to the standard value 0.50.
Curves mm and nn give the "coefficient of performance" for displacement and midship section.
Curve gg gives the ratio between the net resistance of the ship and the indicated thrust.
Curve hh gives a similar ratio when the initial friction of the engines is taken off from the indicated thrust.
All these coefficient curves cc, mm, nn, gg, hh, show more or less an undulation at about the same speed at which there is a marked change on the curve of E.H.P.
Curve rr in Fig. 3 (Plate VII) gives the I.H.P. in function of revolutions.
By the following method of investigation devised by Mr. R.E. Froude, and illustrated in his paper "On the Determination of Dimensions for Screw Propellers," read at the Institution of Naval Architects, 1886, 1 have approximately determined for the maximum speed of 18.38 knots the efficiency of the Lepanto's screw propellers, which would have an abscissa-value 10.75, very close to maximum efficiency. This abscissa-value and the corresponding net total efficiency of propulsion are plotted on Fig. 4 (Plate VI), which is the reproduction of Mr. Froude's standard curve for the efficiency of screw propellers, as illustrated in his paper above mentioned.
With this abscissa-value the true slip of the Lepanto's screw propellers at the speed of 18.38 knots would be about 20 per cent, while the apparent slip is only 2 ½ per cent, leaving 17.28 per cent for the speed of the wake that follows the ship.
Appendix.
To comply with the wish expressed by Mr. F.C. Marshall in his remarks, I have given in Fig. 5 mean specimens of the Lepanto's indicator cards taken during the last three trials.
Regarding the performance of the Flavio Gioja's locomotive boilers, which was not so satisfactory as in the case of the Lepanto, it is necessary to bear in mind that the boilers worked under very different conditions in the two ships, especially on account of the different type of the engines. The Flavio Gioja having trunk engines, the cylinder condensation of steam was much larger than in the Lepanto, with the consequence that the boilers of the Flavio Gioja had to be overworked to give out the required power. This fact goes far enough to account for the difference in the results of the two ships, but some collateral circumstances came also into play, as for instance the less roomy and less comfortable condition of the Flavio Gioja's stokeholds, which prevented the boilers being so regularly and properly worked as in the Lepanto.
On the official trials, the engines of Flavio Gioja developed a mean of 4156 L H. P., at the rate 14.35 I. H. P. per square foot of grate, with a consumption of seven tons of coal per hour, viz., at the rate of 55 pounds of coal per square foot of grate; the cut-off in the cylinders being 0.35, corresponding to a ratio of expansion 2.33. The consumption of steam, as shown by the indicator card, was about 23 pounds per L H. P. per hour, viz. 10 per cent larger than that of the Lepanto for the same output of power per square foot of grate.