The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

330 arrows in Fig. 2. But, since these forces no longer act in the same vertical ‘straight line, it is clear that a turning force, technically called a moment, is in- troduced, the intensity of this force de- pending upon the weight of the ship and the distance GZ between the center of gravity and the center of buoyancy. The turning moment acting on the vessel in Fig. 2 obviously tends to re- store her to the original position and she is therefore said to be in_ stable equilibrium. Now suppose that the center of gravity is raised from the _ position shown in Fig 1, say by pumping out a ballast tank and placing a large number of passengers on an upper deck. The new situation thus created is indicated in Fig. 3. The center of gravity having been raised to the position G*, as before the weight of the vessel acts downward through this point and the upward forces of the water acts upward through the center of buoyancy. How- ever, it will be noted that the conditions of equilibrium are the reverse of those existing in Fig. 2, and the turning force or moment tends to roll the vessel over further instead of to return her to an upright position. In Fig. 3 the ship is in unstable equilibrium. Metacenter Furthermore if we draw a vertical line through B*, the center of buoy- ancy, -it will cut the center line of the vessel at some such pointas M@. This point M is called the metacenter from the Greek word meta meaning limit. It will also be noted that. in Fig. 2;.-in: a condition of stable equilibrium, G, the center of gravity, is below M, the metacenter, whereas in Fig. 3, under a condition of unstable equilibrium, G’, the center of gravity, is above M, the metacenter. It will also be observed that the distance between G and M is a measure of the stability or instability of the vessel; in other words the farther G is below M, the more stable the ship must be and the further it is above M the more unstable the vessel is. Under all circumstances, if the vessel is stable, and if she is to return to an even keel after listing, M, the metacenter, must be above the center of gravity. The dis- tance GM or GM is called the metacentric height. When the center of gravity rises to a position above the metacenter, producing a condition of instability, the vessel is said to have a negative metacentric height. Te is obvious that a passenger vessel should be so designed that it cannot have a negative metacentric height under~ any possible conditions of ballasting or load- ing. George Nicol in his book entitled “Ship Construction and Calculations,” makes the following interesting state- THE MARINE REVIEW ment: “The great importance of the points G and M are manifest and a ship master ought to know for every condi- tion of lading of his vessel, in which he may have to put to sea, what GM or metacentric height he has available. A knowledge of a_ vessel’s metacentric height is useful for many purposes. It is an excellent guide, for instance, in determining whether or not a_ vessel may be safely shifted in harbor, whether’ ballast tanks should be filled, or in a case of a vessel carrying oil in bulk how the loading of cargo proceeded with. Besides the foregoing, if the vessel be of a known type, the metacentric height will furnish a good basis from which to predict the probable. nature of her stability at large angles of inclination.” Continuing the author states, “The question of a minimum value of GM has been the subject of much debate and difference of opinion. Those who have favored a large value have been confronted with the fact that. great stiffness conduces to bad behavior at sea. On the other hand, a very. small value indicates a crank vessel and maybe, although not necessarily so, an altogether unsafe one. The only secure manner of dealing with vessels in this respect is to compare them with others whose performances at sea are known and to adopt values of metacentric height thus suggested. There seems to be a concensus of opinion in favor of limiting the minimum value of GM (metacentric height) in steamers of about medium size to one foot when filled with a homogeneous cargo which just brings them to the load waterline. Cases are on record of vessels having given a good account of themselves with a smaller metacentric height, when loaded as above. In one oft quoted instance, the metacentric height was as low as 0.6 feet, yet the vessel proved herself in every way a good sea boat. The natural feeling however is to have a margin on the side of safety and this is considered to be provided in ocean going steamers when the metacentric height has the minimum value given above.” Calculations Not Difficult For angles of inclination up to 10 degrees the metacentric height may be computed by comparatively simple meth- ods with which every ship master should be familiar. At large angles of in- clination, however, the position of the metacenter changes rapidly. Neverthe- less, the physical laws at work are un- changed and the calculations necessary to determine the stability of a vessel at a large angle of inclination ‘are not sufficiently difficult to discourage a t should be 5 September, 1915 man having the intelligence of the aver- age master or mate of a seagoing ship. In the design of a vessel, the two most important factors affecting the metacentric height are the beam and freeboard. In similar shaped vessels the metacentric height varies as the square of the beam; in other words, if we have two vessels which are similar except that one has 20 feet and the other 40 feet beam, the latter will be four times as stable as the former. The free- board, or the distance between the waterline and the lowest opening in the hull through which free water might be admitted if the vessel listed, also has a powerful influence on the stability. It is readily seen that if a vessel has a high freeboard and is stable, the further she lists the greater will be the force tending to restore her to an upright position. If the freeboard is low, how- ever, free water may be admitted to the hull after a slight initial list, rapidly lowering the metacenter and eventually resulting in an accident. The position of the center of gravity of course has a tremendous influence on the stability of a vessel. Further- more, to a great extent the position of this point is. dependent upon the nature of the stowage. A ship master may therefore often make the stability of his vessel what he pleases. If he finds that she is deficient in stability, he cannot correct the defect by increasing the “beam or the freeboard, but he can, it may be, stow heavy weights low down in the hold and lighter ones higher up and by thus lowering the center of gravity attain a satisfactory result. a To Salve Emden | The Australian commonwealth gov- ernment has called for tenders for the salvage of the German cruiser EmpEN, as she lies beached and_bat- tered in the Cocos islands. The mini- ster of defence recently announced that, “tenders are invited for the sal- vage of the EmpEN as she now lies stranded on North Keeling Island. Tenderers must undertake to | for- ward to the navy office at Melbourne, and hand over, free of charge, all guns and gun-mountings, torpedoes. and torpedo tubes, fire-control instru- | ments and apparatus, money in what- ever form, and all confidential books and documents that may be salved. Should the ship herself be salved and brought into port, the commonwealth government is to have the option of purchasing the ship at a price to be determined by arbitration in the event of any dispute arising. All informa- tion available at the navy office as to present condition will be supplied on application to the naval secretary. Tenders must state terms and time.”