April lo, 1884] 



NA TURE 



56- 



lieight is 6 inches for the curve A, and I foot for the curve B. 

 It is obvious that, in judging of tlie safety of small metacentric 

 height for such a vessel, the range of stability is an important 

 factor to be considered. The range necessary for seaworthiness 

 largely determines and often fixes the limit below which the 

 metacentric height should not be reduced in such a type of 

 \essel and in many others. 



But there are very large numbers of steamers, such as passenger 

 liners and cargo steamers, of the spar and awning-deck classes, 

 which generally have very large ranges of stability, and large 

 lighting moments at great angles of inclination, whatever the 

 metacentric height may be ; and in many cases, even with no 

 metacentric height at all. In such cases the minimum meta- 

 centric height which is essential to safety and efficiency has to be 

 determined by entirely different considerations from those which 

 apply to war ships, and those classes of mercantile steamers 

 whose stability is of the character shown by the curves in Fig. 2. 

 When we have to deal with vessels which even with no meta- 

 centric height will return to the upright, provided water does not 

 get into the ship, and no large weights shift, whatever angle of 

 inclination may be reached, the conditions of the problem are 

 entirely changed. The principal oljject which then has to be 



ANGLES OF INCLINATION 



considered is to prevent too easy an inclination from the upright 

 by the action of the wind and other forces which may operate 

 upon her ; and the question mainly turns upon what may fairly 

 l)e considered sufficient for this purpose. 



Many persons have been surprised on first learning how little 

 metacentric height many high-sided mercantile steamers are in 

 the habit of working with in safety. There are many steamers 

 of the spar and awning-deck classes employed in carrying homo- 

 geneous cargoes, which have been performing their work for 

 years, not only with perfect safety but without showing any 

 signs of what nautical men call tenderness, the metacentric 

 heights of which, during certain periods of their voyages, are 

 frequently not more than 8 inches or even 6 inches. The latter 

 figure may probably be regarded as about the minimum which 

 such vessels approach without indicating to those on board that 

 they are becoming unduly tender : but it is quite certain that 

 many never show any such signs, and appear to be perfectly safe 

 with 8 inches of metacentric height. 



Vessels of this class have curves of stability of which those 

 shown in Figs. 3 and 4 are types. The curves in Fig. 3 are for 



a spar-decked steamer 31S feet by 40 feet by 22 feet. The load 

 draught is 23 feet 6 inches, and dis])lacement 5760 tons ; the 

 freeboard being 8 feet 6 inches. Those in Fig. 4 are also for a 

 spar-decked steamer 220 feet by 30 feet by 23 feet. The load 

 draught is 16 feet, and displacement 2000 tons ; the freeboard 

 being 8 feet 6 inches. The curves marked A in each of these 

 figures are constructed for 6 inches of metacentric height, and 

 those marked B for i foot, in order that they may be compared 

 with the corresponding curves in Fig. 2. The metacentric 

 height of 6 inches is about what each of these vessels would 

 have if laden to the draughts named with homogeneous cargoes, 

 such as they frequently carry ; and the metacentric heights of 

 I foot are obtained by leaving a portion of such cargo out of the 

 'tween decks, and replacing it by an equal weight of ballast in 

 the bottom. 



It will be seen that the increase of righting moment in Figs. 3 

 and 4 continues up to a very large angle of inclination. This 

 increase of righting moment tends to prevent dangerous inclina- 

 tions being reached, while the smallness of the metacentric 

 height causes such vessels to be very easy and comfortable in a 

 seaway. Some steamers whose stability is of this character are 

 vessels which carry cargoes liable to shift, such as grain or coals, 

 and it may be thought that with cargoes of this class a small 

 nietacentric height is particularly unsafe, and that considerable 

 initial stiffness is necessary to prevent any danger arising through 

 shifting of cargo. Any opinions that may be formed upon this 

 point are necessarily more or less speculative, as we have but little 

 exact information to go by ; but it should be borne in mind, in 

 considering the question of initial stiffness in connection with 

 shifting cargoes, that, although such stiffness increases the resist- 

 ance to inclination, it increases at the same time the tendency to 

 roll, and to displace or shift the cargo. 



The question of the minimum metacentric height which may 

 be regarded as consistent with safety in those types of ships 

 w-here it is not governed in any degree by the necessity of pro- 

 viding range of stability, as shown by Figs. 3 and 4, is a sub- 

 ject which has never been much discussed, and which, on account 

 of its important and immediate bearing upon the safety of many 

 vessels at sea, is, in my opinion, deserving of the consideration 

 of this Institution. If any of the remarks contained in this paper 

 should serve to elicit opinions, information, or facts bearing upon 

 the subject, my purpose in making them will be answered. 



I may add, in conclusion, that the following are the main 

 points which I have desired to lay before the Institution in this 

 paper : — (i) The form in which the results of stability calcula- 

 tions can be put before owners and masters of mercantile 

 steamers, so as to be of the greatest practical use in loading 

 such steamers, and regulating their stability in accordance with 

 the requirements that may arise ; (2) the fundamental differ- 

 ence which exists between the relation of righting moments at 

 large angles of inclination and range of stability to metacentric 

 height in the various types of steamers, as shown by Figs. 2, 3, 

 and 4, such relation making it necessary to fix the minimum 

 metacentric height that should be allowed with due regard to 

 the righting moments at large angles of inclination in some cases 

 and unnecessary to do so in others ; and {3) the minimum meta- 

 centric height that may be regarded as consistent with safety in 

 cases where range of stability and the righting moments at large 

 angles of inclination are so ample as not to call for consideration. 

 The two latter points are so intimately connected with the first 

 that they naturally require to be considered along with it. 



T/fE INSTITUTION OF NAJ'AL ARCHITECTS 

 'T'HE Institution of Naval Architects held its twenty-fifth 

 Session at the Rooms of the Society of Arts on April 2, 3, 

 and 4, Lord Ravensworth in the chair. Whilst the papers read 

 were of course mainly on technical questions of naval construc- 

 tion, equipment, &c., some of them possessed points of general 

 scientific interest, of which a brief account may be given. The 

 President's address dealt mainly with what may be called the 

 economic side of the shipping industry, dwelling on such points 

 as the Merchant Shipping Bill, the length of time occupied in 

 building ships of war, the depression of the carrying trade, &c. 

 Passing on to the papers contributed, the first read was by Mr. 

 J. D. .Samuda on the Riachiielo, a steel armour-clad twin screw 

 turret-ship of 6000 tons displacement, and 6000 horse-power, 

 lately built by his firm, for the Brazilian Government. 



The second paper, by Mr. A. F. Yarrow, was on an Electrical 

 Launch tried last year both on the Thames and on the Danube, 



