Introduction.—Within comparatively recent years, the use of Diesel propulsion for merchant ships has shown a notable and steady increase, and many people have believed that this form 0 Propulsion should also be adopted by warships. It is the purpose of this article first of all to describe briefly the outstanding installations of Diesel plants in wars JPs, then to outline the general considerations in comparing Diesel and steam drive, and finally to analyze the questions of weight and radius of action for the two installations in the case of a light cruiser of 5,000 tons displacement. This discussion will not consider the applicability of Diesel Propulsion to naval auxiliaries, since these ships are more properly considered in relation to existing merchant-ship installations than in relation to combatant ships, so far as their propulsion is concerned.
The primary advantage claimed for Diesel propulsion is decreased fuel consumption, and consequently increased radius of action. The secondary advantages embrace numerous desirable qualities, the number claimed varying with the author; among these may be mentioned decreased vulnerability in action, reduced space and armor belt height, greater duplication of power units, better compartmentation, greater protection from gas attack, and easier smoke prevention. If we could accept all these advantages as advanced by diesel proponents, they would seem to constitute an unanswerable argument in favor of Diesel propulsion. Before, however, considering the factors involved, I wish to describe the most noteworthy of the present Diesel installations in naval ships.
Installations.—There are three main types of Diesel installation which we should consider: (1) the direct drive; (2) the geared drive; and (3) the Diesel-electric drive. All of these drives have been successfully used, and, as in the corresponding steam turbine installations, each type of drive has its own merits and demerits. In addition, we must remember that there are two ways of employing Diesel engines for mam power: either in a ship equipped with Diesels only for main propulsion, or in a ship having Diesel engines of sufficient power for cruising speeds and steam turbines to furnish the power for full speed. The considerations entering into the choice of the type of Diesel drive are discussed later in this article; it is sufficient to mention here that all of the outstanding Diesel installations in warships have employed the geared drive, using Vulcan hydraulic couplings.
Turning now to the existing Diesel plants in warships, we have to consider first of all the German “pocket battleship” Deutschland. This ship, with her sister- ships the Admiral Scheer and the Admiral Graf Spec, was the first large warship with main propulsion consisting exclusively of Diesel engines. Since these ships are today the largest warships propelled by Diesels, their installations are of particular interest.
The Deutschland is driven by two shafts; four engines drive each shaft through a reduction gear, to which the engines are coupled by means of Vulcan hydraulic couplings. The engines are of the 2-cycle, double-acting type with nine cylinders. The hydraulic couplings are necessary on a geared Diesel installation to damp out vibrations; they also provide a convenient means of cutting the engines in or out without shocks. While they involve considerable additional weight, this disadvantage had to be accepted to make possible an installation of such large power. The speed of the engines, in order to keep the weight at a minimum, was made as high as deemed practicable for such large units; they run at 450 r.p.m. while the shafts run at 250 r.p.m. Each group of four engines is controlled from a control station from which, together or separately, the engines can be started, stopped, or reversed. The scavenging air for the main engines is supplied by four engines similar to the main engines, but having only five cylinders; these engines also drive the oil and water pumps. The total power of the main engines themselves is 56,800 horsepower, giving a power per engine of 7,100. Since the ship is all-Diesel, the auxiliaries are of necessity electric-driven; this requires a large generating plant, and hence there are eight Diesel- driven dynamos of 250 kw. each, driven by engines of 375 hp.
The total weight of the engineering installation is 48.5 pounds per shaft horsepower. This weight includes the propellers, propeller shafting, piping, gratings, floor plates, and all auxiliaries. The minimum fuel consumption for the main engines and scavenging engines is .385 pounds per s.hp. per hour; for the dynamo engines it is .394. The cruising radius of the ship is reported as not less than 10,000 miles. From all available information, the operation of the machinery of the Deutschland has been satisfactory in service.
The Deutschland was preceded in the matter of all-Diesel drive by the German gunnery training ship Bremse. This ship, laid down in 1929, is of 1,250 tons displacement and makes 27 knots with a shaft horsepower of 26,000. Thus the Deutschland, of 10,000 tons displacement and making 26 knots on 56,800 s.hp. represents a considerable increase in size. The Bremse installation, as in the later ship, consists of two shafts each driven by four engines connected through Vulcan clutches and reduction gearing. The engines are of roughly half the power of those in the Deutschland, each of them being rated at 3,250 hp. The height of the main engines, from the center line of the crankshaft to the top of the engine, is about 8§ feet. This ship is reported to be extremely noisy under way—a disadvantage which is sometimes difficult to prevent in high powered Diesel installations.
Turning now to the installations in which the Diesel power is only sufficient for cruising at speeds considerably below full power, we find that the German cruiser Karlsruhe was the first ship (1929) to have such an installation. An 800-horsepower Diesel engine geared to each of the two shafts through Vulcan clutches drives the ship below 9 knots; but above this speed the steam plant must be used and the Diesels are useless. To overcome this disadvantage, the next German cruiser to have Diesels for cruising, the Leipzig, adopted a radically different type of installation. This ship has three shafts, of which the two wing shafts are driven by geared turbines with 30,000 s.hp. on each shaft. The center shaft is driven by four Diesel engines, connected through Vulcan clutches to a reduction gear as in the Deutschland. The Diesels furnish 12,000 s.hp. on the center shaft, which is sufficient to drive the ship at 18 knots; above this, the wing shafts must be used. To reduce drag losses, it was found advisable to drive the wing shafts by motors when they were not in use; while this required 500 hp., it was computed that 3,000 hp. in drag losses was saved in so doing. Another problem arose in connection with getting full Diesel power both at cruising speed, when the propellers turn at 100 r.p.m., and at full speed, at 400 r.p.m. The solution to this problem was found in the use of a variable pitch propeller.
Of the more recent German ships, it is to be noted that the light cruiser Nurnberg (1934) has a machinery installation very similar to that of the Leipzig; while the two 26,000-ton battleships Scharnhorst and Gneisenau are also equipped with Diesels for cruising, although details of their installations are not available. However, it is also interesting to note that the two 10,000-ton cruisers now under construction in Germany—the Blucher and the Admiral Hipper—will have geared turbines, as will, according to reports, the 35,000-ton battleships.
The above gives only a very brief description of the most important warships which are now driven in whole or in part by Diesel engines. Many smaller naval vessels, chiefly gunboats or mine layers, have been built for various navies; but since they are of low speed as well as small size, they have no place in this discussion. With an idea, then, of the uses to which Diesels have been put in warship propulsion, we will proceed with an analysis of the advantages and disadvantages of adopting Diesel engines, in whole or in part, for the propulsion of naval vessels in general, and a 5,000-ton cruiser in particular.
General considerations.—Later in this article an attempt will be made to give some idea of the relative merits of Diesel and steam installations as regards total weight of machinery and fuel as compared with radius of action. As mentioned before, these factors are the basis of much of the argument for and against Diesel power. It must not be thought, however, that they are the only factors which should be considered in deciding which type of drive would be most advantageous. While the total power to be installed has an effect on the efficiency of a steam installation, it has an even greater effect on the type of Diesel plant. The power, revolutions per minute, and weight of a Diesel engine are all intimately related; and while a detailed analysis of the relationships of these factors is not within the scope of this article, a few fundamental considerations may be pointed out. The power output of a Diesel engine is dependent upon the pressure in the cylinder, the length of stroke, the size of the cylinder, the number of revolutions, and the number of cylinders. For increased power, one or more of these factors must be increased if we wish to get the increased power in one unit. It is obvious that pressure cannot be indefinitely increased; too great an increase above current practice means a sacrifice in reliability. The length of stroke and the number of revolutions are related to the piston speed; and this also cannot be unduly increased without a sacrifice in reliability. The number of cylinders for a single unit is limited to a certain extent by considerations of torsional vibration; as a general rule, it is not considered advisable to have more than nine cylinders in line unless some damping device is introduced to reduce such vibrations. In order to decrease weight per horsepower, an increase in revolutions is usually resorted to; but since this is related to the length of stroke and the piston speed, we see that for a given speed of piston the revolutions must be lower the larger the engine. These limitations on the size of single units are the determining factors in discarding direct drive for installations of large power; as otherwise the size of the engine to give the desired power per shaft would be prohibitive.
The reduced weight of high speed engines for a given power has led to the advocacy of a multiplicity of small, light, high speed units. It would obviously not be practicable to couple a very large number of such units on two shafts, and hence such an installation would have to employ Diesel-electric drive. The prospect of distributing many such small units about a ship is attractive in that it affords good compartmentation and does away with a large engine-room; but it has several disadvantages. The increased weight of the dynamos and motors would probably offset the saving in weight of the Diesel engines; and a more serious handicap is that such small engines would have to be removed periodically for overhaul due to their high speed. The necessity for such
overhauls is admitted by the advocates of such engines; and it is not hard to imagine the difficulties which would be incurred in a naval ship in attempting to remove, periodically, engines which are preferably located well down in the ship with many obstructions to their removal in the decks above them. Thus the question of engine repair and overhaul is a factor militating against the use of high speed Diesels, and thus against the electric drive which would of necessity use such engines to keep the weight of the machinery within a reasonable figure.
The above considerations, while discussed only briefly, give an idea of the reasons why geared Diesels were adopted for the high-powered installations described in the previous pages. With this in mind, we will proceed to a comparison of the total weights and radii of action of a geared Diesel installation and a steam installation for a 5,000-ton cruiser.
Calculation of weights and radii.—As is well known, the outstanding advantage claimed for Diesel drive is the increased radius of action due to the Diesel’s lower fuel consumption; while the principal disadvantage, for naval vessels especially, is the increased weight of the Diesel installation. In the following pages, an attempt is made to compare a steam installation with a comparable Diesel plant with respect to these two factors. Such a comparison is meaningless unless the total weight of machinery and fuel is considered in relation to the radius of action; for, in naval ships, savings in fuel are valueless if they are more than offset by increased weight of machinery. Also, any such comparison of necessity will vary widely for differing types of ships; hence, for illustrative purposes, only one type of ship will be considered in this analysis. The ship considered is a student’s design of a light cruiser of 5,000 tons standard displacement, with a maximum speed of 32.5 knots and a shaft horsepower of 55,000. The curve of speed versus shaft horsepower (Fig. 1) was computed by standard methods using Taylor’s Standard Series. Calculations were then made for the radius of action possible with each installation, for various values of total weight of machinery and fuel and for various speeds. A sample calculation is given below; it applies to the Diesel installation for a speed of 20 knots and a total weight of machinery and fuel of 1,600 tons.
From Fig. 1, for speed=20 knots, s.hp. = 7,500 =13.6 per cent of full power.
Minimum fuel consumption = .405 lb./s.hp./hour.
From Fig. 2, ratio of fuel consumption at 13.6 per cent of full power to minimum fuel consumption = 1.01. (2)
Actual fuel consumption at 20 knots = 1.01 X.405 =.407 lb./s.hp./hr.
Total fuel consumption =
1.365 tons/hour.
Total weight of machinery and fuel =1,600 tons (assumed).
Weight of machinery =48.5 lb./s.hp. (3)
Total weight of machinery =
=1,190 tons.
Weight of fuel = 1,600-1,190=410 tons.
Radius of action =-
x20=6,000 miles.
Any such calculations are naturally greatly influenced by the assumptions made for the values of minimum fuel consumption, ratio of fuel consumption at various powers to minimum fuel consumption, and the machinery weight. Hence I will attempt to justify the figures used.
- Minimum fuel consumption. For the Diesel installation, the values already given for the main engines and the dynamo engines of the Deutschland were used to find the fuel consumption for all purposes, including main and scavenging engines and dynamos. For the steam plant a value of 0.6 lb./s.hp./ hour is assumed as representative of present good practice. This figure was obtained on the British destroyer Acheron and was exceeded (0.58 lb./s.hp./hour) on the German battleship Gneisenau.
- Ratio of fuel consumption at various speeds to minimum fuel consumption. For the Diesel installation, data from the Deutschland were again used. The curve for the plant as a whole is based on the assumption that at least two engines (one on each shaft) will always be in operation, and that, when possible, the engines in use will be run at their most economical speed. For the steam plant, published data for the cruiser Richmond were used. Since the Richmond is now rapidly becoming obsolescent, this curve doubtless gives higher values of fuel consumption than could be obtained with a modern installation.
- Weight of machinery. The Diesel weight of 48.5 lb./s.hp. has already
mentioned in the description of the Deutschland. This is by far the lightest plant of any that has yet been installed in Diesel-driven ships, and it is doubtful that this figure could be lowered appreciably at the present time. For the steam plant, a weight of 35 lb./s.hp. is assumed; this is based on an article by Rear Admiral H. G. Bowen, U.S.Navy,2 whereinhegives40lb./ s.hp. as an approximate value for a heavy cruiser, and 30 lb./s.hp. for a destroyer.
From a series of calculations similar to the above, two sets of curves were plotted; Fig. 3 gives curves of radius of action plotted against total weight of machinery and fuel, for speeds of 32.5, 26, 20, 15, and 12 knots; and Fig. 4 gives curves of radius of action plotted against speed for total weights of machinery and fuel of 1,500, 1,600, 1,700, and 1,800 tons.
Discussion.-Turning now to Fig. 3, the curves of radius of action versus total weight of machinery and fuel, we find that these curves are straight lines emanating from an origin which represents the weight of the machinery alone. An inspection of the curves shows, as would be expected, an advantage for the steam installation at high speeds and low total weights; conversely, the Diesel plant shows up most favorably for high total weights and low speeds. This would be expected from the fact that at low speeds the fuel consumption per shaft horsepower per hour of the steam plant is rising more sharply than the Diesel curve with decreasing speeds of the ship. The dotted line represents the locus of points where the radius of action of the two installations are the same; to the right of this line, the Diesel has the advantage, while the steam plant is to be preferred for conditions of weight and radius falling to the left of it.
Figure 4 is plotted from the same data as is Fig. 3, but here the co-ordinates are radius of action and speed. As before, the dotted line represents the dividing line between the two installations. These curves show graphically the fact that the advantage of Diesel drive lies in the installations having the larger total weight of machinery and fuel.
From the foregoing discussion and comparisons, it would appear that the use of Diesel engines at cruising speeds, as was done in the Leipzig installation, is a desirable combination of the best features of the two systems of propulsion. It is not within the range of this article to go into all the factors involved in such an installation, but a few of them may be mentioned. It is obviously desirable that the Diesels be useful at both cruising speed and at full power; several choices of installation are possible to permit this. If the Diesels are to furnish power to the shafts at full speed, some arrangement must be used to permit their furnishing their full power at the revolutions both for cruising speed and at those for full speed. The variable pitch propeller, as used in the Leipzig, is one solution of this problem. If a Diesel-electric system is installed, it is possible to use the Diesel power for driving the shafts at cruising speeds; while at full speed, with the turbines furnishing all of the shaft power, the Diesel system can be used to furnish electrical power for turret training or other purposes. In such a system, the weight of the main shaft motors is the only useless weight when traveling at full speed.
It is not the purpose of this article to attempt to reach any definite conclusions regarding whether or not Diesel machinery should be used for a light cruiser such as the one considered, or for any naval vessel. As in so many other choices involved in ship design, the selection of machinery is always the result of a compromise between the desirable characteristics of high speed and large radius of action and the other military features which suffer as the proportionate weight of machinery and fuel is increased. There are also many other factors entering into the question of the advisability of the use of Diesel propulsion in naval ships which have not been mentioned. In ships built to “standard” displacement under treaty limitations, increased machinery weight is less desirable than increased weight of fuel, since the latter is not included in the standard displacement; thus the Diesel plant is at a disadvantage in this respect. Another of the many elements which should be considered is the limited experience of Diesel engine builders in this country in manufacturing engines of large powers, as compared with manufacturers abroad. The present resurgence of merchant shipbuilding in the United States will doubtless result in an impetus being given to the development of marine Diesel engines of large power. What this may mean for the future of warship propulsion only time can tell; but whatever the verdict, we may rest assured that the future will witness great progress in both the main fields of marine propulsion.