562 



NATURE 



\_Apr2l lO, 1884 



average densities of the general goods for various ports is arrived 

 at after a little experience, and the same system adopted. The 

 main point is, to state what space, if any, must be left unfilled 

 in the 'tween deck cargo spaces, with the different descriptions of 

 cargo, and what ballast, if any, is necessary if the vessel is to 

 be loaded to her maximum draught ; (5) if the consumption of 

 the coal diminishes the stability materially, as is often tlie case 

 in some classes of steamers, to call prominent attention to this 

 fact, in order that the captain may not be misled by finding his 

 ship appear to be rather stiff on commencing a voyage. The 

 possible consumption of coal is, of course, taken into account in 

 fixing upon the limits that should be imposed upon the stowage 

 in all the conditions named ; and (6) if there appear to lie any 

 circumstances in which a tendency towards instability may arise 

 they are described, and suitable precautions suggested. I Vielieve 

 that Lloyd's Register Society, in fixing a load-line for vessels that 

 may in some conditions be laden so as to have insufficient 

 stability, describe the stowage that is requisite for safety in 

 somewhat similar terms io the above. 



General particulars, such as these, respecting the character of 

 a ship's stability in different conditions, may be made to convey 

 all the information that is necessary for the effective prevention 

 of instability, and I find that they are appreciated by owners 

 and masters, and actually used as a guide in the loading of 

 ships. They may be made to fully define all the essential points 

 upon which stability depends, and are expressed in a form and 

 language that is understood by those who have to use them. 

 This is shown by the fact that telegrams are sometimes received 

 from foreign ports respecting ships which are to be laden with 

 cargoes somewhat different from those to which the specific 

 instructions apply, describing the cargoes that are to be carried, 

 and asking whether any different arrangement of ballast or 

 proportion of weight in the 'tween decks from what has been 

 prescribed for some other cargo is necessary. Such inquiries 

 show that intelligent use is being made of the information sup- 

 plied, and that it is being utilised for practical guidance in 

 loading. 



One of the main reasons why it is better to give information 

 in this simple foi-m is that it obviously fits in with a shipmaster's 

 own practical modes of thought and ideas respecting stability. 

 It is a mistake to suppose, because owners and masters can- 

 not express their views respecting the stability of ships in scientific 

 language, that they therefore have no views that are worth any- 

 thing. The fact is, that the masters of ships very often have 

 quite correct ideas respecting the stability of their vessels and 

 how to load them. If they see a vessel quite empty in dock, 

 and observe the effect of moving weights in and out when light, 

 they often acquire as much knowledge of her stability in the 

 light condition as is requisite for all purposes of safety and efficient 

 working. They also, by means of experience obtained in load- 

 ing, frequently get to know as much about the stability of certain 

 classes of vessels in the laden condition as is necessary for prac- 

 tical purposes, and certainly for all purposes of safety. Whether 

 sufficient knowledge can be gained in this way or not for all 

 possible requirements depends largely upon the type and pecu- 

 liarities of a vessel. As a rule, it is all that is applied to the 

 purpose, and there can be no doubt that in many cases it may be 

 sufficient. It is in vessels which contain elements of danger 

 that cannot be discovered in this practical manner that a different 

 and more scientific mode of treatment becomes requisite. 



The proper use of stability calculations is not to supersede or 

 interfere with that knowledge of a vessel's qualities which may 

 be gained by experience but to supplement and complete it in 

 certain cases where it may be necessary. As an illustration I 

 may refer to the small range of stability sometimes found to be 

 possessed by deep vessels of low freeboard. The discovery of 

 the dangers to which such ships are liable may perhaps be suc- 

 cessfully made in some instances by simply observing their 

 behaviour at sea ; but probably it is more often made only wlien 

 the ship capsizes. Then, again, many ships become unstalile at 

 sea through the consumption of their bunker coal, particularly 

 when a large portion of such coal is carried, as it sometimes is, 

 in a reserve bunker under the lower deck. There are cases in 

 which the metacentric heights of cargo-carrying steamers are 

 reduced by ij feet by the mere consumption of the bunker coal. 

 In such cases instability may very readily arise at sea in a manner 

 of which the captain is unable to form any accurate conception 

 when merely judging by the results of his own experience. Tliis 

 is particularly likely to be the case when alterations are made in 

 bunkers, or when portions of the hold are added as reserve 



bunkers for enabling voyages of longer duration to be made than 

 have previously been contemplated. I certainly believe, as the 

 result _of an examination of the stability of many mercantile 

 steamers, that a great number of vessels are lost at sea from each 

 of these causes, viz. through capsizing on account of low free- 

 board and consequent small range of stability, and also through 

 loss of stability by reason of the consumption of coal. In both 

 of these classes of cases the danger is aggravated if the ships are 

 flush-decked, without any or with but small water-tight erections 

 above the upper deck. 



■ It is very difficult to make a complete analysis of the various 

 causes of loss at sea, and to show conclusively what is the 

 relative mortality of vessels of various types and different descrip- 

 tions of cargo. The difficulty is due to the fact that the Board 

 of Trade returns are not compiled in a manner which enables all 

 the necessary information to be extracted from them. So far as 

 it is possible to judge, however, by the particulars available, it 

 appears that the types of steamers that are least subject to 

 mysterious losses at sea are those which have long ranges of 

 water-tight erections on deck, and are therefore least liable to 

 become unstable. I believe that the comparative immunity against 

 loss which appears to be possessed by many efficiently built and 

 protected "well-deck" steamers, is largely due not only to their 

 comparatively low centre of gravity of cargo, but to the righting 

 power furnished at large angles of inclination by their extensive 

 deck erections. This is undoubtedly the case, notwithstanding 

 the fact that seas may break into the well, and often fill it witl> 

 water. It may be somewhat startling to persons familiar with 

 the loading of flush-decked steamers, to find many well-decked 

 vessels making voyages across the Atlantic with portions of their 

 decks so near to the water as they sometimes carry them ; but 

 a little examination suffices to show that the fact of the water 

 entering a properly constructed and fitted and moderately sized 

 well cannot do much to endanger the safety of the ship. Any 

 effect it may have upon the stability is only at small angles of 

 inclination. 



In order to show how small is the effect of water in the well 

 of an ordinary first-class steamer of this type upon her stability, 

 I have given two curves of stability in Fig. I for such a vessel. 

 That marked P is for the condition of no water being in the 

 well till the vessel is inclined sufficiently for the edge of the 

 deck to become immersed, and that marked Q for the con- 

 dition of the well being filled with water before the inclinatioii 

 commences. Mr. Martell was good enough to have these 

 curves calculated for me, in order that I might have them in 

 time for the reading of this paper. They are for a raised 

 quarter-deck vessel 257 feet by 35 feet 6 inches by 18 feet 6 inches, 

 with a well 60 feet in length, and bulwarks over 5 feet high ; the 

 freeboard amidships to the main deck being 2 feet 2 inches. Prior 

 to the water entering the well the vessel is assumed to be at her 

 usual trim of about a foot by the stern, and a correction is made 

 for the change of trim caused by the filling of the well. No 

 allowance is made for the quantity of water that would be thrown 

 out of the well by the movements of the ship, but it is assumed 

 to be possible to completely fill it with water to the height of the 

 rail at the fore end of the bridge, and for no other way of escape 

 to exist for the water but that of pouring over the rail as the 

 vessel inclines. The freeing ports and scuppers are not assumed 

 to have any effect in clearing the deck of water. The weight of 

 water which the well will hold when the vessel is upright is 186 

 tons, but when she is inclined to 10° it will only hold 98 tons, 

 and when inclined to 20° it becomes reduced to 28 tons. These 

 figures and the curves in Fig. i show that water in the well of 

 such a vessel cannot materially affect her stability after a small 

 angle of inclination has been reached, and that so far as stability 

 is concerned the well cannot be regarded as a serious element 

 of danger. 



A practical point of great importance in determining the 

 amount of stability a ship sliould possess at sea is the minimum 

 metacentric height that may be regarded as sufficient for safety. 

 Different types of vessels have quite different characteristics 

 in respect of stability. War ships, and some classes of 

 merchant steamers, require large metacentric heights in order to 

 insure sufficient righting moments at moderate angles of inclina- 

 tion, and a safe range of stability. The curves of stability 

 given in Fig. 2 apply to such a case. Those curves belong to a 

 typical three-decked steamer, without any water-tight deck 

 erections, 280 feet by 34 feet 6 inches, by 24 feet 6 inclies. 

 The mean load draught is 22 feet 6 inches, and displacemeui 

 4400 tons ; the freeboard being 5 feet 4 inches. The metacentri. 



