Editor’s Note: Submitted in the Prize Essay Contest, 1926. Although not awarded the prize or honorable mention, the article was considered so important by the Board of Control that it was decided that it should be published in the Proceedings.
Everyone following the triumphant progress of the Diesel engine must be struck by the great difference in the rate in which it is being adopted by the mercantile marine and the navy. While in some countries more than half the merchant tonnage laid down is propelled by internal combustion engines, all the naval vessels are still steam-driven, with the exception of submarines and a few auxiliary craft.
The reason for this is clear: the marine Diesel motor, developed on slow cargo steamers, where low revolutions are of paramount importance, has grown to be a heavy, ponderous engine, of undoubted reliability, but of great weight and enormous dimensions. Increase in power is obtained by very slow degrees, and at the cost of still greater weight and size. The introduction of double-acting engines has given but little improvement; every Diesel engine of considerable power yet constructed is totally unsuited for naval use, which is not surprising, since the requirements for a naval vessel are fundamentally different from those for a merchantman. The former wants a sprinter, the latter a stayer.
In these days of rapid progress it is very dangerous to predict that anything will be technically impossible in the future, and it is in no way intended to deny, that it may be possible at some future date to produce an engine, that will fit in below the armor deck of some capital ship or cruiser, that will not exceed the very narrow limit of weight available, and that will develop some 30,000 H.P. or more. However, if this engine is to be reliable, it will have to be evolved from motors that bear no resemblance to it whatever, usually a slow and tedious process, that may well be expected to extend through many years.
Rather than speculate on some more or less distant future, it is proposed to investigate what may be done now. As no capital ships will be built during the next years, we must look for an installation suited for a cruiser, or possibly an aircraft-carrier. This limits the height of the engine to about 15 feet, while a power far in excess of anything yet produced will be necessary on a minimum weight. It is evident that we must turn our attention to fast-running engines of the submarine type, in combination with some form of gear or transmission, which will enable us to use several motors on one shaft.
The biggest submarine engine yet fully tried out in practice is the 3,000 B.H.P. ten-cylinder M.A.N. engine, used on the U-142 and her sister ships. After the war a great number of these engines were available, and many were fitted in merchant vessels running at reduced revolutions and power. They have been doing steady work for several years now, and have given full satisfaction. At present a 6,000 B.H.P. engine seems well practicable. A motor of this power has an overall length of 44 feet, 6 feet breadth, and 11 feet height above center shaft. It develops normally 5,000 B.H.P. at 350 revolutions, and can be forced to 6,000 B.H.P. by supercharging. There are 10 cylinders of about 26 inches diameter and 24 inches stroke (See Jahrbuch der Schiffbautechnischcn Gesellschaft 1920, page 318 ff.). According to press reports Sulzers are constructing a six-cylinder 7,000 B.H.P. engine, of which no further details are published. By using twelve or sixteen of these engines from 70,000-110,000 B.H.P. would be available, the difficulty being to gear these engines onto three or four shafts.
Technically three types of transmission have been tried out, viz. the direct gearing, the Diesel-electric drive and the Diesel-hydraulic drive. The first named has given excellent account of itself on two cargo carriers, the Havelland and the Munsterland, in connection with the 3,000 B.H.P. submarine engines mentioned above. Recently an intermediate liner, the Monte Sarmiento, has been put into commission, and her sister ship, the Monte Olivia, will follow shortly. These vessels have twin screws and are propelled by four motors of 1,750 B.H.P. each, each shaft being driven by two engines. The gears are protected against the uneven turning-moment of the engines by heavy flywheels, calculated to keep the period of vibration well away from the service speed of the engines.
The Diesel-electric drive is well known, and needs no further description.
The Diesel-hydraulic drive implies the use of the Vulcan hydraulic coupling. This very simple device consists of two sets of scoops fixed on the motor shaft, with a loose sleeve between. This sleeve is free to turn on the shaft and is in no way connected with it; it carries two corresponding sets of scoops, and between them a gear pinnion, which meshes with the pinnion mounted on the propeller shaft. The sets of scoops are enclosed in two casings which can be filled with oil, and one of which is provided with fixed guide-blades between the scoops. The oil is set in motion by the scoops mounted on the motor shaft, and carries the secondary scoops along, there being practically no slip. The fixed guide-blades serve to invert the motion of the oil thrown from the primary scoops; in this way reversal of motion is possible independently of the engines, by emptying one casing and filling the other. When both casings are empty there is no connection between propeller-shaft and motor. The coupling absorbs the vibration of the engines and transmits a perfectly even turning- moment, so that the gears work under the same conditions as in turbine-driven vessels.
Full-scale trials on the M.S.S. Vulcan (2,000 tons dw.) and Duisburg (9,500 tons dw.) have shown that it takes only eight to ten seconds to reverse from full speed ahead to full speed astern, and that the efficiency is as great as 97-98 per cent. These figures . have been confirmed in actual practice. Up to now ten vessels have been, or are being fitted with this drive, totalling about 20,000 B.H.P. and 60,000 tons dw.
The direct gearing is not considered suitable for naval work. In order to develop sufficient power it would be necessary to connect three or four engines to one shaft: to start and reverse these engines at the same time would raise insuperable difficulties. Furthermore any small trouble on one of the engines would put a whole shaft out of commission, while a major breakdown (e.g. if a piston seizes), would probably cause the fracture of a shaft or the destruction of the gears. Another disadvantage lies in the fact, that it is not possible to stop one or more engines, when less than full power is wanted.
Between the electric and the hydraulic drive there is in many respects not much to choose, as both have all the advantages of the indirect drive. On the other hand, the Diesel-electric installation probably is heavier and certainly far more complicated. It is less reliable, especially in view of the heavy currents that have to be handled when reversing suddenly. From a military point of view it is more vulnerable.
The advantages of indirect drive, mentioned above, consist of:
- Better protection and watertight subdivision.
- Greater fuel economy.
- Easier maintenance.
- Greater reliability.
- Better maneuvering.
(a) The indirect drive calls for a great number of small units, each installed in a separate compartment. The destruction of one will but slightly impair the efficiency of the rest. The athwartship dimensions of the engines being small, it is possible to adopt longitudinal bulkheads, because the filling-up of one compartment will not cause any serious list. At the same time excellent protection against poison gas is obtained.
(b) Fuel economy is most called for at low, “economical” speeds, when only about 10 per cent or less of full power is used. Under these circumstances the economy of a Diesel engine suffers greatly, mainly because of the lowered mechanical efficiency, and the fuel consumption is raised accordingly. In case of indirect drive one or more engines are disconnected and stopped. For instance, if a ship is run at half its trial speed, about one-tenth of full power will be necessary at one-half of the revolutions for full speed (roughly speaking). If four engines are connected to each shaft, three will be stopped, the remaining one delivering two-fifths of its full output at the reduced revolutions at nearly the same m.e.p. as it was designed for, and at which the best fuel economy is obtained. In case of direct drive, the engines, running at 10 per cent of full power, would have such extravagant fuel consumption, that it would be preferable to stop two or more shafts altogether, dragging the propellers idle through the water. This means a greatly increased resistance, waste of fuel, and consequently reduced radius of action.
(c) Generally speaking three big engines are easier to maintain than twelve smaller ones. This, however, takes no account of the special conditions on warships, where space is cramped, where there is no high skylight, but an armor-grating or a protective deck right over the engines. Under such circumstances it is far easier to handle many light parts than a few heavy ones.
Also, on merchant vessels, the machinery is always running at full power, while in naval vessels it nearly never does; consequently, in case of indirect drive, small repairs, such as cleaning valves, and so forth, may be carried out while at sea on engines that are not in use.
As all engines would be exactly similar, a very complete store of reserve parts can be carried.
(d) If one engine has an x per cent chance of defect during a certain period, four engines will have, ceteris paribus, about 4x per cent chance. However, on the indirect-driven vessel, there will be in most cases but one engine running on each shaft, and this brings down the chance of breakdown to x per cent again. Also, there will be four engines on each shaft, any of which may be used, whereby the risk dwindles to x4 per cent. Of course, this only holds good for cruising, but it is on long cruises that reliability is most needed. On full speed runs the advantage lies the other way. But full speed runs usually last but a relatively short time (at least on cruisers that are patrolling or raiding, and these are the only ones that need be taken into account, see below), during which period x and 4.x are nil to all practical purposes, provided the machinery has been kept in good order. As we have seen, the indirect drive offers every opportunity for fulfilling this stipulation.
Another factor in favor of the indirect drive is the fact that the engines are slightly simpler, because they are not reversible, and are not subject to the brutal changes in temperature caused by expanding hot air in cold cylinders. In fact, in this respect they work under the same circumstances as stationary engines.
(e) In the indirect-driven vessel the number of maneuvers that may successively be carried out is not limited by the capacity of the compressed air reservoirs.
The hydraulic drive gives also the advantage of quick reversing; on the 9,500-ton Duisburg it took but nine to ten seconds from full speed ahead to full speed astern. This may be explained by the fact that only the inertia of the propeller, shaft, and gear has to be overcome, and the full power of the engines is available to do this.
With the electric drive reversing is more complicated and will probably take more time, especially in big installations.
It is rather difficult to form an opinion on the question of space, without a definite project of the vessel into which the machinery has to be fitted. Generally speaking, it may be stated that the athwartship and vertical dimensions of the engines proposed are small, and would present no special difficulties. On the other hand it is not improbable that the total length of machinery-space would somewhat exceed that of a corresponding steam installation. But length is the dimension the designer is least concerned with: an increase in length will lower the horsepower per ton displacement, will increase the metacentric height and stability, and will give more deck space, especially valuable on a vessel that will have to carry airplanes. It is true that it also causes greater ship-bending moments, and higher structural stresses. But a modern vessel is sure to have one or more protective decks against aerial attacks, which, if properly constructed, will give plenty of strength in the places where the highest stresses occur, viz., in the upper parts of the ship.
The all-important question of weight has been kept to the last. As no large installations have been carried out, speculations must be rather vague. The engines themselves need not weigh more than 40 pounds per H.P., this being the weight of a modern submarine engine, such as would be employed. The complete engine Weight, inclusive of shafting, piping, etc., is about 80 to 90 pounds Per H.P. on a submarine. The weight of a proposed high-powered installation is put at 73 pounds per H.P. To this must be added the weight of the gears and hydraulic couplings of which not much is published. On the other hand several factors tend to lower the weight, viz., the elimination of the major part of the compressed air vessels, auxiliary compressors, etc., and the fact that the weight per H.P. of shafting, propellers, piping, etc., is considerably lower in the bigger installations. Still it is not probable that the weight could be reduced below 100 pounds per H.P., and it is quite possible that it will exceed that amount. The weight of a corresponding steam-installation would be about 50 to 60 pounds Per H.P. This seems rather hopeless for the Diesel drive, but the question changes considerably if we take the weight of fuel into account. For the same radius of action the Diesel-driven ship would not be more than one-third* of the fuel of the steamship, which may have as much as 60 pounds per H.P. on board, giving 110 to 120 pounds per H.P. in total. The corresponding figure of the Diesel-driven ship would not be less than 120 pounds per H.P., so that, if all assumptions are made in favor of the latter, the question of weight would not seem to be an insuperable obstacle. It must be kept in mind, however, that this procedure cancels the principal alleged advantage of the Diesel drive; on the assumptions made, the radius of action is the same as that of a steam-driven vessel. Also for the “Washington cruiser” the substitution of engine weight for fuel is impossible, because the former is included in the 10,000 tons displacement and the latter is not. It can only be done on smaller cruisers, such as the proposed British B Class.
(* Note: This fraction is considered justifiable, because at low speeds the steam engine will not run at its best economy, while the Diesel engine (indirect drive) will, because extensive waste-heat recuperation for distilling, etc., is possible, because all stand-by losses are suppressed and because the fuel consumption of a Diesel engine does not increase after a prolonged period of service in the same way that of a steam engine does.)
The matter assumes a different aspect if we allow a slightly inferior speed for the Diesel ship. Suppose a reduction of 2 knots, say from 34 to 32, is allowed. This means that but 75 per cent of the horsepower is needed: such is the price of speed.
The weight of the motor plant, estimated at 100 pounds per H.P. is reduced by 25 per cent, so that an additional fuel supply of 25 pounds per H.P. is available; in other words, the radius of action is more than doubled. It follows that but a very slight reduction in speed is needed to obtain a big increase in radius of action, or to neutralize any excess of motor weight above 100 pounds per H.P.
In a modern steam-driven cruiser a large percentage of the weight of the equipped vessel consists of oil fuel, which is mostly carried in double-bottom tanks. When the oil is in, the center of gravity is low, giving a large metacentric height, which makes the vessel uneasy and jerky in its movements. When the oil is out, the opposite is the case: the vessel is tender and heels easily in a beam wind.
In a motor vessel the major part of the fuel is replaced by additional engine weight, which always contributes in the same way to the stability of the ship; consequently, in light condition, the center of gravity lies lower than in a steamship. Another factor tending to lower the center of gravity is the suppression of funnels and uptakes in the Diesel ship.
A low center of gravity allows a reduction in beam to be made, which is attended by a reduction in hull weight.
On a naval vessel deck space is scarce, especially since the introduction of anti-aircraft batteries and scouting planes. The Diesel drive will bring great improvement in this matter, because there Will be practically no funnels and uptakes.
The steam engine, in its simplest form, consists of two inherent parts: the engine and the boiler. If either is damaged, the whole plant suffers. The matter is more complicated in a large installation, where there may be eight boilers and three or four engines, each in a separate compartment, but still the same principle obtains. It follows, that from a military point of view a diesel-driven ship with twelve engine rooms, each with a self-contained prime mover, is to be preferred to a steamship with eight boiler and four engine rooms, because in the former case the filling-up of one compartment entails the loss of 8 per cent of the propulsive power, while in the latter case 12 1/2 per cent or 25 per cent are lost.
In case of the indirect drive this does not hold quite true, because the destruction of the couplings would disable a shaft. However, the chance of a direct hit is small, and the mere filling-up of the compartment in which the couplings are situated would not cause much damage, as their working would not be impaired. In this respect the electric drive is inferior.
The reliability of the Diesel engine is regarded with much doubt by many practical men, especially by those who have had no actual experience with engines constructed after the war. Of course, the engine in itself is far more complicated than a turbine or triple expansion steam engine, the pressures, stresses, and temperatures much higher; the chances of failure seem multiplied. Still, the Underwriters quote the same insurance premium (on casco and engines) for Diesel- and steam-driven vessels, so that in actual practice the reliability seems to be much the same. Of course, a lightly constructed, high speed, trunk-piston engine is more liable to defect than a heavy, slow-running one of the crosshead type, but the same difference exists between the machinery of a tramp-steamer and that of a modern cruiser or destroyer. The steam plants on these vessels are not so very reliable at all. Only the failures likely to occur are so very familiar, that they are accepted as inevitable and, by a curious process of subconscious .thought, left out of account. Many engineers seem less impressed by a leaky condenser-tube than by a broken valve-spring, although it may cause far more damage and trouble, simply because they are better acquainted with it.
It is easy to enumerate a few notable instances of machinery failure during the war, e.g., the disabling of the Lion by a leak in her feed-water tank and the Queen Elizabeth stripping the blades of one of her turbines at Gibraltar, while urgently needed at the Dardanelles. Also the tale of the German raiders is very instructive. Of these there were six: the Dresden, Leipzig, Nurnberg, Konigsberg, Emden, and Karlsruhe. Constant machinery trouble prevented the Dresden doing any “useful” work as raider, and delivered her, after a game of hide and seek, helplessly into the hands of the enemy. The Leipzig, at the Falkland Islands, was unable to run full speed, because her engines would not stand the strain. The Nurnberg would almost certainly have escaped (vide Nav. Op. I, p. 430) but for the fact that two of her boilers exploded at the beginning of the chase. Urgent repairs prevented the Konigsberg from raiding the Colombo-Aden trade route and forced her to lie up on the Rufuji River. The Emden reports constant boiler trouble. The Karlsruhe, having only just completed her trials, makes the best show. Still she had to lie in mid-ocean with stopped engines for two whole days, in order to carry out repairs, and her first officer reports that the services of a seaplane would have been invaluable, especially when the vessel was coaling or overhauling the engines.
It is difficult to see where the “undoubted reliability” of the steam engine comes in, and how on this account serious objections can be raised against the adoption of the Diesel engine, especially in view of the advantages of the indirect drive, stated above.
As we have seen, a Diesel-driven warship will, ceteris paribus, probably be slightly slower than a steam-driven one. In these days, when ever greater speed is claimed for warships, a new type of prime mover, which brings with it a decrease in speed, will stand a poor chance indeed. But in certain circumstances the steam-driven vessel is slower than her motor-engined rival, and the advantage thus given to the latter may possibly atone for the lower speed she usually has. To speak plainer: the Diesel ship can almost at a moment’s notice work up to full speed, whereas the steamer wants at least an hour to do so. It took the Richmond about one hour to work up from cruising speed to 30 knots (i.e., 4 knots below her trial speed) when she received the message of the forced landing of the Boston, and surely then if ever her boilers would have been strained to the utmost limit. In a Diesel ship the engines could have been started in a very few minutes indeed.
In war time this must be a valuable asset to a ship, especially a cruiser on raiding or patrolling duty. A scout, sent on a well- defined errand, can have her boilers ready by the time any need for full speed is likely to occur; a raider or patrol cannot do so, because of the enormous loss of fuel incurred. On a turbine vessel the commander is always faced with the dilemma: greater preparedness versus greater economy. In most cases a mean course must be adopted, and both suffer. The motor ship can always be run at its best economy, without impairing its preparedness in the least.
It is interesting to show numerically how great the advantage is, thus given to the Diesel ship. Suppose a Diesel-engined raider, with a maximum speed of 32 knots, is sighted at fifteen miles distance by a turbine cruiser capable of doing 34 knots. The raider will run away directly at 32 knots, while the cruiser, not having steam for more than, say, 20 knots, may, by forcing her boilers, succeed in doing twenty-five miles in the first hour of the chase. By that time the Diesel ship will be twenty-two miles away, and out of sight. The steamship now has a 2-knot advantage, but unless she is able by airplane scouting to find out the course her quarry has taken, and is not prevented by the raider’s planes from doing so, she has little chance of finding him. If she can keep on his track, she wants three and a half hours more to reduce the distance to fifteen miles again; after four and a half hours of chase she is just about where she started, and at least another three hours of chase are necessary before an engagement is possible. The raider seems to stand a very good chance indeed of effecting his escape. If the condition of the sea makes it impossible to run at full speed, the advantage of the motor ship is greater still, as the difference in speed will be negligible and she will get a big start at the beginning.
In the opposite case, i.e., if the pursuer is propelled by internal- combustion engines, he can come to grips in less than an hour’s time.
To resume the foregoing pages it may be stated that:
- A Diesel-engined cruiser is technically possible.
- The Diesel drive offers several advantages over the steam drive, e.g., better protection, greater stability, more deck space, greater radius of action.
- A Diesel-engined vessel will probably be slower than a corresponding steam-driven one, this disadvantage being partly offset by the fact that the motor ship can always be ready for full speed without waste of fuel.
This article has been written from a technical point of view. While it has tried to show the advantages and disadvantages to be expected from the adoption of the Diesel drive, it is not intended to go into their relative importance, which is impossible without a clear conception of the service for which the vessel is intended. This is staff work, and beyond the scope of this article.
A treatise on Diesel-engine warships is not complete without some mention being made of the most recent development in this direction, viz., the cruising Diesel engine on a steam-driven vessel. According to press reports a Diesel engine is to replace the cruising turbine on the new British mine-layer Adventure, and one is also provided for in the design of the proposed 17,500-ton French cruisers. In itself the system has much to commend itself: a motor has a low fuel consumption; so for long-distance cruising, one is fitted and the steam machinery is retained for high speed running.
The weight of the engine, etc., would have to be deducted from the fuel supply, and in this way a certain fraction of the gain is lost. We will assume that the vessel has an oil supply of 1,000 tons, and wants 3,000 H.P. to run at cruising speed. The motor will be of a sturdy type as it must be designed to run continually at full power without an overhaul, and will not weigh less than 150 pounds per H.P., or about 225 tons. This means that the radius of action is more than doubled on the assumption that the oil consumption of the motor is one-third of that of the cruising- turbine. If the radius of action is left unaltered, the gain in weight of fuel may be used for additional armor or ordnance, but in that case the radius of action at full speed would be materially curtailed.
In any case it will be reduced by 22 1/8 per cent, but this is probably a matter of minor importance, as in almost every case a large part of the cruise would be run at moderate speeds.
It would be interesting to know how the Diesel engine has been connected to the shafts. In this case the Diesel-electric drive seems to offer many advantages, as it leaves the designer a free hand in the placing of the motor, and it allows one engine to drive all the shafts, so that no losses are incurred by dragging idle propellers through the water.
Of the advantages of the Diesel drive not much remains. The Watertight subdivision is not improved at all, the stability but little, and the ability to work up to full speed quickly is completely lost. If the vessel is running on the Diesel engine with the boilers out and the turbines cold, she will want at least three hours to set her steam plant going, unless the turbines are kept warm by the utilization of the waste heat of the exhaust gases.
The Diesel cruising engine seems only justified on vessels that go on long, but very definite errands, on which the time when full speed will be wanted can be predicted with certainty, that is to say, on exactly the opposite occasions from those when a full Diesel ship can be employed to advantage.