56o 



NATURE 



\_April lo, 1884 



July last, that these opinions largely prevailed and were 

 erroneous. 



Prof. Elgar ivas called upon to make some investigati ns re- 

 specting the stability possessed by the Daphne at the time of the 

 disaster which happened to her, and lo give evidence respecting 

 the same. He afterwards pointed out, in a letter to the Tunes 

 of September i last, some of Ihe considerations which obviou-ly 

 apply to light draught stability. The first, which it appears had 

 never before been stated, is that any homogeneous floating body 

 which is symmetrical about the three principal axes at the centre 

 of gravity — such as a rectangular prism or an ellipsoid — will 

 have the same moment of stability at equal angles of inclination, 

 whether floating at a light draught with a snail volume below 

 water, or at a deep draught with a similar volume above water. 

 For instance, if a horn geneous pri-im of symmetrical cross- 

 section 5 feet high floit at a draught of I fvot, it will then have 

 preci-iely the same moment of stability at equal angles of inclina- 

 tion, and consequently the same curve of stability through lut, as 

 if it were loide 1 — without altering the position of the centre o' 

 gravity — till it had 4 feet draught of water, and I fo it of free 

 bjard. Fr )m this it follows that, in such elementary forms of 

 floating bodies, lightness of draught has the same effect upon 

 stability as IdwucnS of freeboard ; and if a 1 iw freebjard is un- 

 favourable to stability, so also, and precisely to the same extent, 

 is a correspondingly light draught of water. This proposi:ion 

 can be made still more general, a< it applies to homogeneous 

 bodies of any form of cross-section which revolve about an 

 horizontal axis fixed only in direction. From this may be 

 deduced the results given by Atw'ood in his papers read before 

 the Royal Society in 1796 and 179S respecting the positions of 

 equilibrium and other peculiarities cjunected with the stability 

 of floating b jdies. 



In considering the stabiliiy of a ship at various draughts of water, 

 and comparing it with th it of the cla^s of figures above described, 

 modifications require to be made for the departure from sym- 

 metry of form, anl for the extent to which the vertical p sition 

 of the centre of gravity differs from what it would be if the 

 external surface inclojcd a homogeneous volume. Prof. Elgar 

 has done this with great fullness of derail in his paper, and 

 ^hows, by means of curves, how the stability varies with draught 

 of water at con tant angles of inclination in various geometrical 

 forms of floating bodies, and in a large passenger steamei' of 

 oi-dinary type. The curves thus dealt with are curves of righting 

 moments, a'ld njt merely cirves of lengths of righting arm. 

 The ordinary curve of stability is u ually made for lengths of 

 righting arm, because the displacement is const" nt, and the same 

 curve therefore gives upon different scales, either lengths of 

 righting arm or righting moments. In the vertical or cross 

 curves of stability, however, such as are now being dealt with, 

 draught, and therefjre displacement, is one of the variable quin- 

 lities, and curves of righting moments are of a very different 

 character from curves of righting arm. Complete cross curves 

 for a ship, from which ordinary curves of stability can imme- 

 diately be obtained for any draught of water and position of 

 centre of gravity, can be constructed in a few days with the aid 

 of Amsler's mechanical integrator. 



Prof. Elgar shows conclusively the nece sity in many cases of 

 regarding the stability of a ship from thepiintof view of varia- 

 tion of righting moment with draught of water, the angle of 

 inclination being constant, instead of from that of variati n of 

 righting moment with angle of inclination, the draught being 

 cjnstint, as is usually done ; or rather of considering the sab- 

 ject from both points of view instea I of almost exclusively fron 

 the latter. He also shows that it is necessary to investigate, 

 more fully than has formerly been done, the moments and range 

 of stability of ships and other structures that may be intended 

 to float at very light draughts of w a'er. 



Prof. Elgar's second paper was read last week at the meeting 

 of the Institute of Naval Architects ; its title was "The Use of 

 Stability Calculations in Regulating the Loading of Steamers." 



The stability of ships. Prof. Elgar went on to say, is a subject 

 that has received a considerable amount of theoretical investiga- 

 tion during recent years. The general character of the stability 

 of certain classes of ships, and the circumstances which affect it, 

 have been largely ascertained and made known ; while the 

 methods of performing the requisite calculations — especially when 

 large angles of inclination are being dealt with — have been 

 greatly improved. Curves of stability have been constructed 



and made public for nurherous ships of various classes, both for 

 war and mercantile purposes. 



The results of the investigations that have thus Ijeen made 

 are of great value to naval architects and men of science, and 

 enable them to know much more respecting the actual stability 

 often possessed by ships than was possible with the imperfect 

 data available in former years. In the case of ships that are 

 built for purely war and some other special purposes, the 

 ordinary stability calculations enable instructions to be readily 

 framed respecting the stability they possess under ordinary 

 working conditions, or in such critical circumstances as may 

 possibly occur during their career. Any risks of instability that 

 may exist, or arise in certain contingencies, may be ascertained, 

 and the precautionary measures necessary for counteracting them 

 devised and pointed out. 



The problem that has to be dealt with in advising those in 

 charge of war ships how to effectually guard against instability, 

 is well within the grasp of the naval architect. In such vessels 

 the loading is mainly of a permanent character, while that part 

 of it which is subject to variation, such as coals, stores, ammu- 

 nition, &c., varies in a manner which can be readily taken into 

 account in the calculations. Curves of stability that are con- 

 structed for war ships for three leading conditions, viz. (l) the 

 fuUy-Iaden condition ; (2) the same, but with all the coals con- 

 sumed ; and (3) the light condition with all coals, ammunition, 

 and consumable stores expended, are usually sufficient to enable 

 full instructions to be framed for the prevention of instability. 

 In some war ships there are other critical conditions which may 

 require consideration, such as the possible injury and laying 

 open to the sea, of compartments not protected by armour ; but 

 in all these cases the conditions are comparatively fixed, and may 

 be allowed for in the calculations. When curves of stability 

 have once been constructed for a war ship to represent the 

 various critical conditions to which she may be subjected, they 

 are always ajiplicable, and may be relied upon to furnish, at 

 any time, a safe guide to her stability. 



In the case of mercantile steamers, however, except such as 

 carry no appreciable weight of cargo, the problem of how to 

 apply the results of stability calculations to the guidance of 

 those who have to work and stow them is of an entirely dif- 

 ferent character. The naval architect cannot control, or even 

 estimate, the amounts and positions of centre of gravity of the 

 various items of weight that make up the loading to anything 

 like the same degree of certainty as in war ships. There are 

 many steamers afloat in which the cargo is nearly or quite twice 

 the total weight of the vessel, together with her machinery and 

 equipment. In such cases the naval architect can only control 

 in the design about one-third of the total weight of the vessel 

 and her cargo, leaving the remaining two-thirds in the hands of 

 the owner, master, or stevedore. It is obvious, therefore, that 

 whatever may be the qualities of the empty vessel in respect of 

 stability, these may be greatly modified or entirely altered by 

 the manner in which she is loaded. It is the loading to which 

 we must look in the large proportion of cargo-carrying steamers 

 for the due preservation of such stability as is necessary for 

 safety at sea. 



It is in this direction also that we have to look for the cause 

 of a great many of those losses which have occurred at sea 

 during recent years, and to which attention was first prominently 

 called by Mr. B. Martell, the Chief Surveyor of Lloyd's Register 

 Society, in a paper read before this Institution in 1880 upon the 

 causes of unseaworthiness in merchant steamers. Mr. Martell 

 attributed, and quite rightly so, a great many of the losses of 

 steamers to instability ; and there can be no doubt that this 

 cause of loss still continues to operate very largely. The 

 evidence given at Board of Trade inquiries in cases of missing 

 steamers is constantly pointing to instability as the cause of loss, 

 although the full meaning and weight of the evidence may not 

 always be fully and accurately appreciated at these inquiries. It 

 often diverts attention from the main cause of loss to say that 

 it occurred because the ship was unstable. The fact is, that the 

 ship has frequently so little to do with the matter, and the stowage 

 so much, that it is the latter which should be blamed for the 

 instability, and not the ship herself. When a ship is built for 

 a particular trade and for the purpose of carrying certain specific 

 cargoes she may then, of course, be so designed as to be quite 

 stable, in all conditions, while thus employed ; but when vessels 

 are built, as they often are, to dimensions fixed by owners, for 

 general trading purposes, it is seldom possible for the designer 

 to provide against instability arising in some possible or con- 



