SUBMARINE BOATS
By Commander William Hovgaard
Professor of Naval Design and Construction, Massachusetts Institute of Technology, Boston, Mass. Read to the Royal Canadian Institute, January 23rd, 1915
Part II – A discussion of the principal features of the design of submarine boats.
The "submersible" has already been defined as a submarine boat in which predominant importance is given to the requirements of service in the light condition, while the "submarine" is designed more particularly with regard to the submerged condition. Hence the ship-shaped form of the submersible and the spindle-shaped form of the submarine. The external ship-shaped form of the submersible is however attained without abandoning the advantage of the circular section which is maintained throughout an inner spindle-shaped "strength-hull", because it is the form best adapted to resist the pressures of the water. Between the inner and the outer shell are water-ballast tanks and oil tanks, whence the strength hull may be of small diameter well suited to resist great pressures without going to excessive scantlings. The outer hull, not being exposed to great pressures, may be lightly built and will yet in some measure protect the inner hull against damage by collision. At the same time the ballast and oil tanks may, with a relatively small addition in hull weight, be made very large, whence a great reserve buoyancy and great radius of action can be secured. In a submarine the tanks are chiefly inside the strength hull and cannot, therefore, be very large without unduly increasing the diameter of the hull and hence its tendency to collapse.
Speaking broadly, the submersible has better seagoing qualities and a higher speed on the surface than the submarine, but the form is not so favorable for driving under water. Most early boats were submarines. It was natural that inventors, especially those not acquainted with the requirements of naval service, should direct their efforts especially to navigation under water and that they should underestimate the importance of qualities required for work on the surface. At present the general trend of the development is towards the submersible type. Of recent years, therefore, the submarine, where it is still retained, has approached more and more to the submersible by an enlargement of the superstructure, which at the same time has been given a more ship-shaped form. In English submarines side structures have been added. The reserve buoyancy in the early submarines was only about 5 per cent. of the light displacement, but has been gradually increased to about 18 or 20 per cent. In submersibles, on the other hand, the reserve buoyancy has been reduced from about 72 per cent. in the Narval to about 35 per cent. or less in recent boats.
The submersible has a rather high centre of gravity and in submerged condition a low metacentre (centre of buoyancy), and hence small stiffness on account of the high position of the ballast tanks, while in the surface condition the stiffness is in some cases excessively great owing to the large area of the water-line. The submarine has a lower centre of gravity and, due to the low lying water tanks, it has a high metacentre in the submerged condition and small or moderate stiffness on the surface. The Laurenti type, where the ballast tanks are partly below the strength hull partly above or very high, are intermediate between the Germania and the Holland type in this respect.
In order to obtain sufficient stability in the submerged condition submersibles must generally carry a considerable amount of keel-ballast. This of course is a drawback, but also most submarines carry some ballast. Part of the ballast is generally detachable, often referred to as a "safety keel", to be let go in case of emergency. In passing from the light to the submerged condition and vice versa a point will exist where the stability is a minimum, being reduced by the presence of free water in the tanks. The designer must, therefore, carefully determine the conditions of stability in all intermediate positions in order to satisfy himself that a proper .metacentric height is always maintained. If the stability vanishes at any point the boat may heel over to a considerable angle before equilibrium is restored or may even capsize.
STRENGTH OF HULL.
The hull of a submerged vessel is exposed to an external water pressure which is directly proportional to the depth of immersion. Already at a depth of 200 ft. the pressure is about 100 lbs. per sq. in., and since the depth of water in the ocean is generally more than 10,000 ft., boats cannot be constructed to withstand the pressures at all depths which they may encounter. It is therefore necessary to assign a limit to the pressure head which a boat is required to resist. Generally boats will have no reason to go deeper than required to clear the bottom of vessels on the surface, that is to a depth of about 75 ft., but accidentally they may descend involuntarily to greater depth. Usually the head to which submarine boats are tested is about 150 ft., although in some navies it is as much as 200 ft. A certain margin of safety is of course applied in the construction, but if the boat goes much beyond its test depth it is liable to collapse. In most boats the strength hull is of circular section as stated above.
In most boats the strength hull is of circular section as stated above. The Whitehead boats and other boats of the Holland type have internal frames. As seen from the midship section of the Danish boat Havmanden, the Whitehead boats in some cases have oval sections even amidships, because this form is convenient for boats designed for operating in shallow water. Some boats, such as those of the Germania type, have no frames. Other boats such as those of the Laurenti type appear to use the framework between the inner and outer hull as a means of stiffening the strength hull.
STEERING AND NAVIGATION.
Steering in a horizontal direction takes place as in ordinary vessels, but steering in the vertical plane has caused many difficulties to early inventors. As late as 1901 a German authority, Professor Busley, deprecated the value of submarine boats on that ground.[1] Mr. Holland introduced diving and emerging by inclining the boat at considerable angles, while most other inventors preferred to keep the boat as nearly as possible on an even keel and to effect great changes in depth by pumping water in or out of the boat, by means of horizontal propellers, or by so-called "hydroplanes". The last method is that which at present is mostly used in submersibles. Hydroplanes are similar to rudders sometimes fitted amidships abreast of the centre of gravity of the boat, sometimes placed forward and turned the same way as the aft rudders. In all cases the object is to obtain an upwards or downwards force driving the boat laterally up or down. This method is generally considered safer than the "porpoising" used in the Holland boats. Once the desired depth is attained it is preserved by using the horizontal rudder in the same way as when steering a course on the surface, but with this difference, that even small deviations from the given course line (depth) are not here permissible. For guidance in steering a depth gauge and a clinometer are used. Steering in the vertical plane requires considerable skill and experience.
In order to navigate, the submarine boat must be provided with a reliable compass and, even when submerged, a view of the horizon must be obtainable at any time. An ordinary magnetic compass is not quite reliable even when placed in a conning-tower of bronze, but recently the advent of the gyroscopic compass has provided a means of accurately determining the direction. The faculty of vision when the boat is submerged, as it must be when making an attack, constitutes one of the most important and difficult problems connected with submarine boats. The water is practically opaque and it was therefore necessary in early boats, when going under water, to emerge from time to time so as to obtain a view from the conning-tower, but evidently this mode of navigation was anything but safe, since the presence of the boat was thus revealed to the enemy. Already in the "eighties" and "nineties" optical tubes were introduced of simple construction, invented by Marie Davyin 1854 and gradually perfected. In its simplest form the optical tube had a mirror at each end inclined at 45° to the axis. The tube, being fitted watertight in the top of the boat, projected a foot or two above water when the boat was immersed to a certain depth and thus a view of a part of the horizon might be obtained, but the angle of vision was only one or two degrees, and the image was very imperfect. The mirrors were replaced by prisms, lenses were introduced and during the "nineties"several improvements were made, but not till about ten years ago was any serious progress made. Then in a few years the optical tube, or the "periscope" as it is now usually called, was developed to a high degree of perfection, enabling the submarine boat to perform attacks without showing anything but the top of the periscope occasionally above water, at the same time obtaining a perfect view of the enemy. The periscope was the last link in the chain of inventions which were needed to give to the submarine boat positive military value. The improvements in the periscope comprise a larger field of vision spanning an arc of more than 50°, as large as or greater than that of the human eye, convenience of observation, and the addition of means for measuring distances and indicating directions. The magnification of the object is only about 1.5, which is found to give to the observer the same impression as when using the naked eye. By using the utmost refinements of optical art and science a perfect image of unsurpassed clearness and distinction is obtained. Mechanical power is used for handling the tube, enabling it to be pushed up and down readily and quickly and to be turned round its axis. The length of modern tubes is up to 25 ft. with a diameter of about 6 in. The head of the tube projects from 15 to 22 ft. above the superstructure. Difficulties still exist due to the vibration of the boat and due to spray on the front glass, but they are of secondary importance. Instruments have been constructed by which an all-round view of the horizon can be obtained without turning the tube, but have not proved quite satisfactory.
VARIATIONS IN BUOYANCY.
In order to go from the light to the submerged condition and vice versa it is necessary to admit or to discharge water. Main tanks of great capacity must therefore be found provided with large sea-valves and powerful pumps. Means are provided also by which the water maybe forced out of the tanks by admitting compressed air. Ordinarily the air is not admitted till the boat is on the surface, where only low pressure air is required, but a sufficient store of highly compressed air is carried by means of which the tanks can be emptied very quickly in case of emergency, even when the boat is at a great depth. The time occupied in passing from the surface to the submerged condition should be not more than about from 4 to 5 minutes. This, of course, is a point of great military importance. Since the main tanks are always filled completely when it is desired to dive, it is necessary to fit one or more auxiliary or regulating tanks of moderate capacity to allow for variations in the specific gravity of the water and for changes in the weight of the boat due to consumption of provision, stores, etc. The consumption of fuel-oil is roughly compensated for by admitting water to the bottom of the oil tanks, which are thus always kept completely filled. The buoyancy of the boat in the submerged condition is so adjusted that a small "reserve buoyancy" remains, leaving always a tendency to rise to the surface. This tendency is overcome dynamically when the boat is in motion either by hydroplanes or by giving the boat a slight downward inclination by the head. Small tanks, one at each end of the boat, permit an accurate adjustment of the trim (the longitudinal balance). Special tanks are fitted for compensating for such losses in weight as when a torpedo is fired and another is inserted in the tube. In some boats a so-called "floating tank" is fitted, connected with a continuously acting pump which is worked in conjunction with the horizontal rudders; this tank serves to compensate automatically for incidental variations in buoyancy occurring after the boat has got under way, notably those due to changes in the specific gravity of the water. A considerable amount of buoyancy can be obtained almost instantaneously in case of emergency by the release of the safety-keels referred to above. The superstructure which is above water in the light condition is self-bailing. In some boats it is entirely and permanently open, serving only to provide a raised platform, but in most boats it is a watertight structure provided with large and numerous valves that can be readily closed when the boat is in light condition whereupon the superstructure will add to the reserve buoyancy and the stability.
VENTILATION AND HABITABILITY.
Space is always restricted in a submarine boat. When going on the surface the motor gives off much heat and requires a great amount of air for its combustion. It is unavoidable that some of the products of combustion, carbon monoxide and carbonic oxide, leak out from the engine. Also the men consume oxygen and produce carbonic oxide, and when charging the batteries free hydrogen is liable to be liberated, forming with the air in the boat an explosive mixture. Where the fuel is gasoline or other very volatile oil, it will evaporate at a low temperature and is liable to leak out into the boat; it is poisonous and is capable of forming an explosive mixture with the air. Hence it is necessary to provide very vigorous ventilation when going on the surface. In the submerged condition the problem would appear to be even more difficult. The available air gradually becomes vitiated, but it is found that the crew can live for several hours without any sort of air renewal or means of purification. This is due to the constant leakage which takes place from the compressed air system, a leakage which can never be entirely prevented. If desired, the carbonic acid, which gradually accumulates due to exhalation, may be removed by chemical means, or the foul air may be pumped out. Fresh air can be supplied from the compressed air reservoirs. There is, however, rarely occasion for resorting to such means. A greater difficulty is the escape of gasoline and poisonous fumes from the motor as well as from the battery. There is no convenient test for carbon monoxide suitable for use in submarine boats, whence it has been necessary to use white mice for indicating the presence of this poisonous gas to the effects of which these little animals are very sensitive. White mice breathe much more vigorously than human beings and will absorb carbon monoxide about twenty times as rapidly as man. Hence, long before man feels any discomfort, the mice will show symptoms of distress. When this occurs, and especially when the mice become asphyxiated it is time. to ascend to the surface and to renew the air in the boat.
With proper precautions the crew may remain in the boat with all hatches closed for 12 hours or more, which is generally all that is required, since the boat can go to the surface as soon as darkness sets in.
Life on board a submarine boat is, however, very fatiguing and for this reason the time in which a boat can stay away from its base is very limited. The crew has to be changed at frequent intervals or it must be given time to recuperate, a fact which in many cases may limit the practical endurance of the submarine boat more than the supply of fuel. As matters stand now it may be said that the crew of a submarine boat ought to be relieved after two or three weeks' service.
PROPULSIVE MACHINERY.
For propulsion on the surface the gasoline motor was the first really successful engine. It was light, occupied small space as compared with the steam machinery and the combustion of fuel-oil was not more than about one-half pound per H.P. hour as against at least 1 1/2 lb. per H.P. hour for steam machinery with oil-fed boilers. For small boats, of a displacement of from 100 to 300 ts. where weight and space were very restricted, the gasoline engine offered the best solution, but the dangers by this volatile oil soon made it necessary to introduce heavy oil motors, although they were in several respects inferior to the gasoline motors. While the latter are easy to start, special means are required for starting the former, and the consumption of fuel in heavy-oil motors, as for instance of the Koerting type, was about twice as great as in the gasoline motors. The last step in the development was the introduction of the Diesel engine, which although rather heavy and likewise burning heavy oil, has a consumption of fuel somewhat less than that of the gasoline engine. The nominal radius of action of recent boats of the largest size, driven by Diesel motors, is given as from 3000 to 5000 miles. The speed on the surface has attained 16 kts. In several boats and the designed speed in some boats now under construction is 18 or 20 kts.
The Diesel engine is the motor which to-day finds most favour in submarineboats, but with the increasing size of boats and the claims to higher speed it becomes increasingly difficult to produce motors of sufficient power. Units of from 800 to 1200 H.P. are under construction and there are usually two and in some boats three propellers. Many difficulties are met with and failures have occurred, whence steam power has been preferred in some boats as for instance in the French submersibles Gustave Zédé and Nereide of 1000 ts. displacement, which are to make 20 kts. Steam machinery has the advantages of reliability and durability, but it occupies much space and the problem of getting rid of the heat is difficult to solve. The radius of action obtainable with steam power on a given weight of fuel is much smaller than with Diesel motors.
The weight of Diesel engines as fitted in submarine boats is about.65 lbs. per H.P. as compared with about 50 lbs. per H.P. for gasoline engines and from 50 to 60 lbs. per H.P. for steam machinery inclusive of propellers and shafts.
For under-water propulsion electric power derived from a storage battery of lead accumulators still offers the best solution. Since the first appearance of these cells they have been improved upon in many technical details, and are now reliable and durable. They will stand complete charging and discharging more than 400 times, and under ordinary peace service conditions if carefully handled they may beexpected to last about 5 or 6 years. The weight per H.P.-hour including outfit is by discharge in 3 1/2 hours about 80 lbs., practically the same as in early accumulators. Lead cells permit great variations in power and are at their best at low rates of discharge, a most valuable quality for submerged work. They can be stowed low in the boat and add thus considerably to the stability. They occupy about 0.4 cb. ft. per H.P.-hour, i.e., less than any other source of energy at present available for this purpose.
Attempts have been made to introduce accumulators of different type but so far without success. The Edison alcalic iron-nickel cells are about 10 per cent. lighter, but occupy at least 25 per cent. more space; they cannot be so rapidly discharged as the lead accumulators. They cost more than three times as much, but are more durable. It appears that recently improvements have been made and better results are claimed by the makers. Practical service tests are required to determine the relative merits of these cells.
The total energy accumulated by storage batteries is necessarily small and rarely allows more than a speed of about 10 kts. for 3 or 4 hours. Recently boats have been designed for 11 or 12 kts. The radius of action at maximum speed of large boats is about 30 or 40 miles, at reduced speed about 100 miles. The excessive weight of the plant for underwater propulsion is the more unfortunate, since the weight available for propulsion is already very small as compared with that in ordinary torpedo-boats. The reason for this is that the hull weight is extremely great, occupying about 20 per cent. more of the total displacement than in an ordinary torpedo-boat. Only about 30 per cent. of the displacement of a submarine boat can be devoted to machinery and fuel as against about 50 per cent. in a torpedo-boat. Moreover the plant for under water propulsion comes as an extra addition. It is evident, therefore, that submarine boats can never compete with torpedo-boats as to speed and radius of action. Great efforts are being made to devise a type of machinery that can be used both on the surface and submerged and especially one by which the propulsion under water does not entail any extra weight, but no satisfactory solution has yet been obtained. Any process based on combustion involves the storage of atmospheric air or oxygen, but a storage of these gases in sufficient quantities for underwater propulsion requires excessive weight and space. The discharge of the products of combustion is liable to reveal the presence of the boat.
M. d'Equevilley has proposed a solution which is being tried in the French submersible Charles Brun and probably also in a German boat.[2] He uses an ordinary boiler with oil fuel and a steam engine on the surface, but when the boat dives under water the exhaust steam is led to a concentrated lye of sodic hydrate (NaHO) which absorbs the steam under strong evolution of heat and thus serves as fuel in a secondary"soda boiler". This process goes on till the lye is saturated. When the boat comes to the surface and steam is available from the primary boiler, the soda lye may again be concentrated by evaporation of the water which it has absorbed, and the boat is ready for another submerged run. This plan offers the advantages that there is no change of motor, the same engine being used under water as on the surface, and there are no products of combustion. The machinery can be forced without difficulty and relatively high power attained both in light and submerged condition. No electric motor is needed. On the other hand, the system requires the addition of special soda-boilers and a hot water reservoir.; the plant occupies so much space that the available weight cannot be fully utilised; the centre of gravity of the machinery is high and requires extra ballast to be carried; the radius of action on the surface is necessarily smaller than with an explosion motor; there is likely to be a strong corrosion due to the soda, and isolation for heat will probably cause difficulties. The soda-boiler plant appears, however, more promising than other power plants so far proposed for this purpose.
ARMAMENT.
The principal armament of submarine boats is the Whitehead torpedo. English boats of the F-class are said to carry six 21 in. tubesand French boats of the latest type eight tubes. Recently large submarine boats have been equipped with an armament of light guns in disappearing mountings. Later English boats carry two 3-in. guns so mounted that they can be used against air-craft as well as against other vessels. When not in use, the guns and mounts are housed in the superstructure.
Attempts have been made to design mine-laying submarine boats, a problem which is evidently of considerable interest. As far as known, Russia is the only power that prior to the war had built a boat for this purpose, viz., the Krab, designed for dropping mines when in surface condition. A boat so designed that mines could be dropped when in the submerged condition would be of greater value, but there are technical difficulties in releasing mines under water, in compensating for their weight and in determining their exact location.
SIGNALLING.
The faculty of communicating with other vessels whether on the surface or submerged is one of great military importance for the submarine boat. For service on the surface wireless telegraphy has been successfully used for several years, but for submerged service it is only quite recently that means of signalling has been devised which seem to promise good results. It was at first, when the submarine bell was invented, attempted to use it for signalling, but it was found that itwas not well suited for sending messages by the Morse system. No practicable solution was discovered till an Austro-Hungarian physicist, Mr. H. C. Berger, showed the way by his experiments undertaken in the Danube at Budapest on the transmission of longitudinal vibrations through water. A wire of 2 in. in diameter was set into vibrations by the friction of a hand-driven silk-wheel moistened with alcohol whereby a clear and sustained note was produced, capable of being sent in dots and dashes of the Morse code. The wire was fastened to a plate in contact with the water, and was anchored at the other end to some fixture, but the tension of the wire was immaterial. The identical apparatus used by Berger was fitted in one of the United States submarine boats in 1911 and readable signals were transmitted over a range of two miles. Still better results were obtained with steel ribbons and power-driven exciters, by means of which distinct signals were transmitted over a distance of 10 miles. Recently electrically-worked oscillators, invented by Professor R. A. Fessenden, have been used instead of the wire ribbons and have given very promising results. This mode of signalling is sometimes referred to as the "submarine wireless system" , but it must be distinctly understood that the transmission through the water takes place entirely by sound waves emanating from a diaphragm plate which may be part of the ship's side. The receiver is a similar plate in another ship similarly connected. This appliance is now being developed by the Submarine Signalling Company of Boston and, apparently, with considerable success.
SAFETY, SALVAGE, AND TRANSPORTATION.
As a consequence of the numerous and serious accidents which have befallen submarine boats of recent years, much has been done to increase the safety of this craft. The hull is subdivided more minutely than formerly by bulkheads of sufficient strength to withstand the maximum water pressure. A buoy provided with telephone connection is fitted in the superstructure and can be sent to the surface in case of emergency, enabling communication to be established with the outside world. In some boats the men are provided with diving suits and helmets, enabling them to escape or to remain for a longer time in the boat when it is flooded. Great precautions are taken to prevent the fumes from the storage battery from entering the working rooms of the boat. The battery is in many boats placed in an entirely separate, airtight, well ventilated compartment. Vessels of special type, "salvage docks", are built for the purpose of raising the boats when they have sunk to the bottom in damaged condition. Shackles are fitted on the top of the boats for this purpose.
Special vessels are constructed also for the transportation of submarine boats.
SIZE.
From the moment that submarine boats were taken into practical service, claims to increased seagoing capability, speed, radius of action, and better living conditions on board were advanced by the naval officers. These claims could be best met by an increase in size and we can understand, therefore, that size has steadily increased ever since the beginning of the century. Boats were then less than 100 ts. Fully submerged and are now being built of about 1200 ts. displacement. The reason why the displacement has not increased much faster is chiefly the difficulty of providing motors for propulsion of sufficient power. By an increase in size, moreover, the boats become more difficult to handle under water, especially where the depth is small, but probably this difficulty would be of secondary importance for ocean-going boats, should such boats become a reality. The high cost of large boats will put an early limit to their number, the price per ton being almost three times as high as for battleships.
MILITARY VALUE.
The great military value of submarine boats has been demonstratedin the present war. At the present stage of development submarineboats afford not only the best means of defence of our own harbors and coasts, but may be used also for offensive purposes in the open sea and on the coasts of an enemy up to a distance of some five hundred miles from their base. The large boats of about 1200 ts. Displacement now under construction probably have a still greater radius of action.
A peculiarity of the submarine boat is its faculty to carry out an attack with relatively small risk once it has gotten into position, in which respect it differs radically from the ordinary torpedo boat which must always be prepared for great and almost unavoidable sacrifices in order to carry out a successful attack. The greatest difficulty with a submarine boat is to bring it into position for attack. The future development of the submarine boat is likely to be steady but slow. In the meantime it is probable that also the means of attack and defence possessed by the battleships against submarine attack will progress. Evidently, the first point for the battleship is to detect the submarine boat. Once detected before it has reached within striking range the submarine boat can generally be avoided, because its speed under water is so slow relative to that of the battleship. The detection of a submerged boat is, however, a difficult matter, the only visible point being the head of the periscope, which needs to be shown above the surface only from time to time. In still water the periscope is fairly visible by the wake which it makes on the surface when emerging, but in rough and misty weather it is extremely difficult to detect. Detection from air-craft is under certain circumstances fairly easy and this mode seems to promise a great deal. When the periscope is discovered, it will be at once subject to a hail of projectiles from light guns, and if it is hit the boat will be blind and helpless. If, after that, the boat shows its conning tower above the surface, it will generally be exposed to destruction by artillery fire.
Seaplanes or other types of air-craft may possibly become a dangerous enemy of the submarine boat not only in helping to detect it but also by direct attack with bombs. When the boat is submerged it is quite helpless against bomb dropping. Even very light bombs are likely to prove destructive, and since the air-craft is in no danger of counterattackf rom the submarine boat, it can go very low and hitting should not be a difficult matter. The submarine boat cannot even observe a seaplane when immediately over it. A further development of the seaplane is, therefore, likely to prove extremely dangerous to the submarine boat. Numerous small patrol-boats properly equipped for attacking the submarines with gun, ram and other weapons are likely to prove effective in the vicinity of the coasts.
While the active or offensive means of defence are in this case as elsewhere the most effective, the battleship possesses means of defence of a passive nature such as watertight subdivision, elastic bulkheads and underwater armour, which may be still further developed, but experiments and war experience are required to throw light on the problems involved. The superior speed of battleships is of course in itself a means of protection. On account of the present limited range of submarine boats and perhaps especially on account of the limit to the endurance of the personnel, they do not render the powerfully armed and well protected artillery ships superfluous. Large seagoing battleships and battle cruisers are yet required in order to control the ocean, but as matters stand now, the smaller enclosed seas such as the Baltic, the North Sea, the Mediterranean, the Yellow Sea and other similar waters may be practically controlled by submarines and by light, fast vessels. In the presence of an active enemy well provided with submarine boats large vessels cannot operate in such seas except under the greatest precautions, going at high speed and using all possible means of defence.
[1] [Back] Transactions of the Institution of Naval Architects. London, 1901, p. 188-189.
[2] [Back] Jahrbuch der Schltfbautechnischen Gesellschaft, 1913, pp. 131-138.
This page was first posted 23 May 2003.