Stability with Draught of Water in Ships. 217 



opinions previously existing upon the point, which were usually 

 based upon mere surmise or vague impression, and often influenced 

 by prejudice, can now be corrected by means of exact and conclusive 

 data. The effect of low freeboard upon stability has latterly been 

 largely made known and discussed. 



It has been seen that the mere act of setting off the righting 

 moments obtained by Atwood's formula, at various angles of in- 

 clination, as ordinates of a curve, and thus obtaining a complete 

 graphic representation of the variation of righting moment with 

 angle of inclination, threw a flood of light upon the general problem 

 of a ship's stability, and enabled it to be far more comprehensively and 

 accurately treated than before. It also enabled definite and instruc- 

 tive generalisations to be made which were previously impossible. 

 It will be observed, however, that the curves of stability referred to 

 only deal with the matter so far as certain fixed draughts of water 

 and positions of centre of gravity are concerned. A curve of stability 

 for a given draught of water and position of centre of gravity ceases 

 to be applicable if changes are made in the weight and consequent 

 draught of water of a ship, or the position of the centre of gravity, or 

 both. 



In the case of war-ships, curves of stability are usually constructed 

 for three conditions. 1st, for the draught of water and position of 

 centre of gravity of the vessel when fully laden ; 2nd, the same, but 

 with all coal consumed ; and, 3rd, when quite light, and without any 

 coal, ammunition, or consumable stores. In certain cases there may 

 be special conditions for which additional curves are required, 

 but usually the above are all that are considered necessary. The 

 stability at intermediate points is not often supposed, or found in 

 practice, to vary sufficiently to call for further attention. In mercantile 

 steamers, however, which are often launched in a very light condition 

 and are constructed for carrying heavy cargoes, and large supplies 

 of coal, the question of stability requires a more exhaustive, and 

 somewhat different, mode of treatment. It becomes necessary, in fact, 

 to make another step in the same direction as was taken when the 

 righting moments for various angles of inclination, described by 

 Atwood, were represented as a whole for a fixed draught of water and 

 loading by means of a curve of stability. What is still further 

 required to complete the representation of a ship's stability is to show 

 how the curves of stability themselves vary with draught of water 

 and position of centre of gravity, and to record this variation in a 

 form that will render it easy to construct curves of stability for any 

 specific draughts of water and positions of centre of gravity. 



The curves of stability of merchant steamers are required for so 

 many different draughts of water and positions of centre of gravity, 

 that I have found it convenient, in practice, after obtaining curves 



