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Motion of the Ship at Sea

The behaviour of a vessel under the stresses and strains to which she is subjected by the seas is a matter which continually exercises the skill and ingenuity of naval architects



AN ONCOMING SEA will lift the bows of a ship if she has sufficient buoyancy


























AN ONCOMING SEA will lift the bows of a ship if she has sufficient buoyancy. If she has not, and takes a volume of water on board forward, when the sea lifts the stern the propellers are often brought out of the water. This causes the engine to race, because the resistance of the water to the screws is suddenly removed. Considerable damage may be caused to the engines in this way. Above is the Patrician, a Harrison Line vessel of 5,742 tons gross, built in 1917 as the Saint Jerome. She has a length of 423 feet, a beam of 56 feet and a depth of 23 ft. 8 in.




MODERN scientists and naval architects are constantly striving to solve the complex problems which arise from the motion of a ship at sea. Since it is impossible to foresee all the conditions of wind and weather that can influence a ship on service, the solutions are not easily found.


The average passenger is inclined to divide the motion of the ship into two categories. Pitching is the movement of the ends of the ship, and rolling is the motion athwartships. The naval architect has to go farther; while rolling is always rolling, the fore-and-aft motion caused by waves moving at right angles to the length of the ship is further divided into heaving and pitching.


In scientific language, pitching is the oscillatory rotational motion about a transverse axis through the centre of gravity. Heaving is the vertical oscillating movement of the centre of gravity as the ship passes through the waves. Although pitching is not excessive when the ship’s course is oblique to the waves, it is accompanied by a certain amount of rolling, and the two combine in a corkscrew-like motion which can be exceedingly unpleasant. The pitching motion is not nearly as distressing to a passenger as the rolling, but it means much more to the designer, for its strain on the fabric of the ship is much greater.


The main strains caused on the structure of a ship by pitching are of two kinds. “Hogging” occurs, when the centre of the ship is supported by a wave and the two ends are in the air and tend to droop. “Sagging” takes place when the two ends are supported and the centre of the ship is without support. The bending stresses are always greatest amidships, where the vessel is less able to take the increased stress than forward and aft. This alternate hogging and sagging is called “working”. In the old days of wooden ships it was plainly audible in any heavy weather, and would often open up the seams and cause the vessel to founder.


Modern steel ships are more strongly built, but there is still the element of risk. When the whole fabric of a ship is “working”, rivet heads are liable to. be shorn off by the moving plates, and fifty rivets displaced in a ship of average size may mean the admission of 300 tons of water an hour. An opening one inch square, 25 feet below the waterline, will admit about 30 tons an hour. The various underwater openings of a modern ship, necessary for cooling the condensers and for other purposes, may easily be seriously damaged as the ship “works”.


There is also the possibility of damage by blows received from the sea. A heavy sea striking a ship, generally when she is pitching, will send a shock through the whole structure, followed by the rapid vibration of the elastic portions of the hull. Such a shock may be produced by forcing a ship through the waves at such a speed that she has not time to rise and fall with successive seas, but cleaves her way through the wave crests on an even keel. Every wave so thwarted hits the ship’s bow and the process, known as “pile-driving”, may be bad for the ship.


A slower ship being driven through a head sea will often “slam”. By coming down on a rising wave, or for some other reason, her bow will receive from the sea a heavy blow which is felt all over the ship. The engineer’s main concern is that the pitching of the ship is liable to damage his engines. If the bow goes down the stern will rise, especially if a heavy weight of water comes on board forward and presses the bow still farther down. If the ship lifts her propellers out of the water, or into surface water which has poor powers of resistance, they will race at a tremendous speed. Unless prompt measures are taken the engines may be damaged. Governors and other devices reduce the risk, but it is still present.


The designers of men-of-war have also to consider the utility of the ship as a fighting machine. The bow guns are generally among the most important of the ship’s armament, but if they are constantly smothered in heavy spray or green water they cannot be fought efficiently, and excessive pitching is therefore a, serious disadvantage.


For many years the favourite ship form was known as the “cod’s head and mackerel tail”, with the greatest beam well forward, so that the bow had plenty of buoyancy to resist pitching. For many years the flare above the water-line has been favoured. The sides of a ship are curved outwards in a manner which not only increases the resistance to the downward plunge, but also supplies increasing buoyancy to the ship as the water rises up towards the deck.


A number of specially fast ships, men-of-war and merchantmen, have recently adopted the bulb bow, which first came into general prominence when it was fitted to the North German Lloyd record-breakers Bremen and Europa. This bulb, always submerged, not only gives increased buoyancy forward, keeping the screws down and the bow up, but also is claimed to reduce resistance by keeping the displacement of water well below the water-line.


Directly opposed to this in principle is the Maierform hull (as used on the “Scharnhorst” and the “Kirnwood”), in which the forefoot is completely cut away. This hull form is designed to let the particles of displaced water pass from the bow to the stern of the ship by the shortest possible path, giving a ship considerably less wetted surface than one of the same displacement but of normal design.


All these factors make it desirable for the architect to evolve the steadiest possible ship, but it was many years before the subject received any proper attention. The first man to treat it seriously was William Froude, who began to investigate the motion of ships in 1856, before the Great Eastern was launched. It was intended to make her the most popular passenger carrier in the world. Nothing could do that, but in the attempt Froude made researches, and came to decisions which have not yet been displaced.


One reason why the subject had received so little attention before Froude’s time was that almost every ship, even the most powerful steamers, had a certain amount of sail. Sail always has a steadying effect, acting in the air much as bilge keels do in the water. This steadying influence was one of the principal reasons why sail was retained in steam warships and certain steamers for so long. after the improvements in the machinery had made the sail unnecessary for propulsive purposes.


THE BULBOUS BOW has the effect of diminishing pitching by giving increased buoyancy forwardIn the early days of ironclads many ships in foreign navies as well as in the Royal Navy were gravely deficient in stability. Some of these vessels, although safe enough for harbour work, could not be described as ready to go anywhere. H.M.S. Captain, which capsized with terrible loss of life in 1869, was the worst instance of many, and it was proved that the question of stability was closely linked with the rolling motion which Froude was investigating.





THE BULBOUS BOW has the effect of diminishing pitching by giving increased buoyancy forward. This form of bow first came into general prominence on the appearance of the Norddeutscher Lloyd liners Bremen and Europa. The bulb can be clearly seen in this photograph of the Europa at her launch.





As the man-of-war developed slowly towards its present stage the designer’s problems became more and more difficult. He was forced to depart from the well-proved, full-lined hull which had suited sailing ships for centuries. By putting a huge weight of armour plating well above water he upset all the accepted principles of ballasting. The Navy demanded more and more speed, and to supply it the builder had to fine down the lines below water more and more.


The centre of gravity — the point at which the weight of the ship can be assumed to act downwards — is located much higher in a warship, especially in an armoured vessel, than in a merchant ship.


One of the first things that a designer has to do is to work out the position of the centre of gravity and also the metacentre, the point about which the centre of buoyancy swings when the ship rolls or heels. The distance between the centre of gravity and the metacentre is the metacentric height of the ship, and this is all-important in connexion with her stability or rolling. The metacentre must be above the centre of gravity if the ship is to be stable.


It is surprising, perhaps, that a “stiff” ship with great metacentric height, offering most resistance to any inclination, generally rolls most in a seaway. A “tender” ship, however, with a small metacentric height, easily heeled, is usually steadier.


From 1871 to 1876 Froude carried out a long series of rolling experiments in still water, using every naval vessel available. He produced the rolling effect by moving weights. Ingenious self-recording apparatus was devised to derive the utmost value from these experiments, and the Navy became more and more interested.


Suddenly there came a greater demand for information. The Navy was building the biggest and most heavily armoured warship in the world, H.M.S. Inflexible, carrying the biggest muzzle-loading guns in heavy turrets on deck, and having her sides protected by 24 in. of iron. She was a wonderful ship, arousing as much attention in her day as the Dreadnought did thirty years later. The Navy, however, was anxious. In spite of her heavy brig rig and enormous spread of canvas with its steadying influence, thoughtful officers were not at all sure how the new vessel would behave at sea.


Stability Experiments


Her design was revolutionary. To get the maximum fire of four big guns ahead, astern, or on either broadside, the two turrets were not placed in the centre line but in echelon, and well out towards the sides of the ship. Their big guns were muzzle-loading, and as it was impossible to build a turret big enough to permit them to be loaded within its walls, two ports had to be cut in the deck close to each turret.


When it was desired to load the guns, the turret was revolved until they came over these ports. They were then depressed so that the loading operations could be carried out from the deck below, where powerful rams were fitted. If a ship so designed was to roll heavily, her decks would be awash and it would be impossible to use the only sizeable guns in the ship for fear of flooding out the magazines through the loading ports. Steadiness was of the greatest importance in the Inflexible, the most expensive ship of her day. With her distribution of weights the experts were doubtful whether she could secure the desired steadiness.


For several years Mr. Froude and his young assistant, Mr. Philip Watts, who was eventually to become Sir Philip Watts and to design all the Navy’s men-of-war, carried out a number of experiments. It was necessary to ascertain how much the Inflexible would roll, and to check the tendency to do so. This was accomplished by fitting huge water chambers into the hull of the ship. By moving the water from side to side against the motion of the ship they checked the tendency to roll. Once the water in the tanks, the depth of which was carefully calculated, was set in motion by the ship, it moved automatically, exerting a powerful damping motion.


THE ROLLING MOVEMENT of a ship at sea is probably more distressing to passengers than the pitching movement
























THE ROLLING MOVEMENT of a ship at sea is probably more distressing to passengers than the pitching movement, but is not such a source of anxiety to naval architects, because it does not exert such a strain on the vessel. Stability, however, is specially important for warships and passenger liners, and various types of anti-rolling devices have been invented. This photograph shows the Dramatist, 5,443 tons gross, built in 1920. She has a length of 410 feet, with a beam of 52 ft. 4 in. and a depth of 30 ft. 6 in.




These early experiments in the Inflexible led to the elaborate anti-rolling tanks of later years.

The trouble with the Inflexible was that her heavy weight of armour pressed her down. When Sir William White designed the Royal Sovereign type of battleship, in the 1880s, he gave it a good freeboard and beautiful lines, so that it had plenty of stability. Ships of this type, however, were too buoyant, H.M.S. Resolution in particular rolling so much that she earned the nickname of “Rolling Rezzy”. Great improvements were effected by the fitting of bilge keels. These were longitudinal strips of steel projecting from the ship where the bottom turned into the side, and they check rolling by offering great resistance to the water. In spite of these improvements, however, the motion of a big capital ship is so ponderous that it is almost impossible for her to accommodate herself perfectly to every wave motion. Some of the most modern fighting ships roll horribly.


The placing of weights is of the greatest importance, whether the ship is a man-of-war or a merchantman. Free weight, capable of moving from side to side as the ship rolls, is dangerous unless it is kept under strict control, as it is in anti-rolling tanks.


The shape of the hull may do much to check rolling. The development of the steamer had been in progress for many years before the present almost box-like shape, with a flat bottom, became general. During the process nearly all steamers carried canvas for steadying purposes, but to-day it is not considered necessary.


Many schemes to prevent rolling were tried, especially on the route across the English Channel, which was becoming more and more popular among people who did not mind paying for their comfort. One of the most popular principles was to adapt the basic design of a native catamaran with its two hulls. Early in the seventeenth century a vessel of this type was tried on the Irish Sea route, and the experiments continued, with varying success, until the Gemini was built in 1850.


She had two separate hulls, 157 feet long, with a single paddle between. Either hull was nearly 9 feet wide and the space between them was rather greater. The deck, which covered the hulls and joined them firmly together, was nearly 27 feet long. The Gemini was sufficiently successful to encourage the production of others, and the twin-hulled Channel steamers Castalia (1874) and Calais-Douvres (1875) were built (see also the chapter “Novelties in Ship Design”). Sir Henry Bessemer’s own uncomfortable Channel crossing in 1868 prompted him to bring his inventive genius on to the problem and the famous Bessemer was built to his design.


From Vice-Admiral Popoff’s circular ironclads, designed to give Russian conscript sailors a perfectly steady gun platform, developed the famous Imperial yacht Livadia — “The Summer House on a Turbot” — built on the Clyde. Some of these freaks checked rolling, or even eliminated it, but their disadvantages in other directions were so overwhelming that they were useless as ships.


Development of the Bilge Keel


Far more practical was the development of the bilge keel, which produces remarkable results when judiciously used. If it is too big, or wrongly placed in relation to the stream of water along the ship’s side, it will seriously reduce the speed and affect the handiness of the ship. If it is carefully designed it can be fitted even into such ships as destroyers, which are intended primarily for speed. To do this the bilge keel has to be carefully calculated and bilge keels of surprisingly small size can make a big difference.


Nobody knows to whom the credit for the invention of the bilge keel should be given, but in some form the principle has been used for checking rolling for many years.


The Euphrates class of Indian troopship was built in the 1860s, with a specially light draught for passing through the Suez Canal. These ships were heavy rollers, but bilge keels added on Froude’s advice made a big difference. In 1872 Froude carried out a series of interesting trials outside Plymouth breakwater with H.M. steam sloops Greyhound and Perseus, one having bilge keels 3 ft. 6 in. wide by 100 feet long, the other having none. The biggest roll that could be noted in the stabilized ship was 11½ degrees, but the other rolled to about 23 degrees.


Another name closely associated with the development of the bilge keel is that of Mr. Dyson Weston, who in the 1870s was made the Marine Superintendent of the African Steamship Company. That concern owned a steamer, the Elmira, which had been built to cross the bars of West African ports.


Her draught was shallow and the bar keel, which was still customary, was replaced by a plate. She rolled so terribly that she was regarded as being positively dangerous, and nobody would travel in her if it could possibly be avoided. The directors decided to sell her, but their loss was considerable, and it started Weston thinking. Froude’s experiments were then in their early stages and had been principally confined to men-of-war. Weston adapted the principles to merchant ships, and vastly improved the comfort of the traders to the west coast of Africa.


THE PITCHING MOTION OF A SHIP is caused by the lifting of the stern and the bows alternatel


















THE PITCHING MOTION OF A SHIP is caused by the lifting of the stern and the bows alternately. The Poona, above, is running through a heavy sea which will aggravate the pitching by pressing the bows farther down. The Poona, built in 1905, was a P. and O. vessel of 7,626 tons gross. She had a length of 479 ft. 8 in., a beam of 57 ft. 3 in. and a depth of 32 ft. 7 in.




In 1901 the early destroyer H.M.S. Star was used to test the effect of bilge keels on ships of her type. The Admiralty feared that these might seriously reduce her speed. They began with tests in the basin of one of the dockyards. Fifty brawny bluejackets were added to her normal crew, and they spent their days running from side to side as fast as possible, while the scientists made their observations of the ship’s maximum heeling and rolling capacity. Further tests were then carried out at sea, after bilge keels had been fitted. It was found that if they were judiciously designed bilge keels could be used to great advantage.


One of the greatest rivals of the bilge keel, for the purpose of steadying ships, is the tank. In H.M.S. Inflexible some experiments were carried out in face of great difficulties. Two water chambers were fitted, one forward and one aft, stretching across the ship between the armoured and the main decks. The after chamber was wanted for stores and the space had to be surrendered. The experiments on the forward chamber were suspended for the bombardment of Alexandria. The tank proved capable, later, of reducing the ship’s rolling by nearly forty per cent, an excellent performance, and Sir Philip Watts continued the experiments and produced striking results. H.M.S. Edinburgh was used for experimental purposes in the 1880s, her water chamber being 14 feet across and containing 210 tons of water.


All these experiments, however, were rather primitive, and the scientific development of the tank is usually associated with a German inventor, Dr. H. Frahm, who developed anti-rolling tanks at the famous Blohm and Voss yard at Hamburg. He used a tank resembling a capital U, built athwartships and extending from side to side of the ship. In this the water column could oscillate with the same number of swings per minute as were peculiar to the ship herself. It had a great effect and was fitted in a number of German and British passenger liners.


The Cunard Company tried Frahm’s invention with the Laconia of 1911. Tanks containing 320 tons of water were built into the cross bunker. Results were sufficiently encouraging for the Aquitania to be given a similar tank amidships.


In the early part of the twentieth century the gyroscope became a matter of scientific interest. Dr. Ernest Schlick determined to use it for reducing rolling. His apparatus consisted of a heavy fly-wheel fixed to a vertical axis and installed in a case which was mounted on a shaft athwartships. When the wheel was made to rotate rapidly it had the usual effect of the gyroscope in checking any movement from its set level. The Schlick system was fitted to the MacBrayne steamer Lochiel in 1908, and to several other small ships. In this experimental form it produced interesting and generally satisfactory results.


In theory the perfect gyroscopic disk should weigh about one-fifth of the total weight of the ship. In the worst weather it should then be able to reduce a thirty per cent roll to nearly nothing, but that is impracticable. The stabilizer need only apply sufficient influence to overcome the rolling movement arising as each single wave produces its effect. The beginning of the movement is small, and if the gyro deals effectively with that, its result is remarkable.


Gyro stabilizers have been fitted in many ships of many types, but perhaps the one that has attracted most attention is the Sperry installation in the Italian record-breaker Conte di Savoia. This consists of three gyroscopes, each having a wheel 13 feet in diameter, weighing 175 tons. Their damping effect is considerable.


HEAVY SEAS STRIKING A SHIP will send severe shocks through her whole structure
























HEAVY SEAS STRIKING A SHIP will send severe shocks through her whole structure, If the ship is driving through a head sea with such speed that she has no time to rise and fall with successive seas, every wave will strike the bows with force. On the other hand, if the ship is slow, her bows will often fall on a rising sea and suffer a sudden shock in this way. Illustrated above is the Engineer, a vessel of 5,883 tons gross, built in 1908. She was 399 feet long, with a beam of 51 feet and a depth of 26 ft. 8 in.


Click here to see the photogravure supplement to this chapter.


You can read more on “How the Modern Ship is Tested”, “Novelties in Ship Design” and

“Ship Types and Deck Arrangement” on this website.