STABILITY^ PROPULSIONj AND SEA-GOING QUALITIES OF SHIPS. 31 



proportion as if the inertia of the rolling mass -were increased in the ratio of 

 unity to 



and periodic roUing in smooth water becoming impossible when g c^ is equal 

 to or greater than 4 jx l''\ 



Of Easy and Uneasy SJiips. 



There is much vagueness in the use of these terms. They are generally 

 applied promiscuously to the practical hindrance caused by motion to the 

 persons engaged in working or manoeuvring the ship, to the inconvenience 

 felt b}'^ passengers, to the straining of a ship's structure, or the tendency to 

 shift her cargo, or to break away half-fastened weights, like boats or guns. 



These all appear to depend in varying proportions on the following exact 

 data: — 



The extent or amplitude of angular motion. 



The rapidity of angular motion. 



The acceleration of linear motion. 



Eut the rapidity of linear motion and the angular acceleration (except so 

 far as this affects bending stress, or as it involves linear acceleration at a 

 distance from the instantaneous axis) do not ax^x^car to have much practical 

 influence. 



In still water the only motion ■v\'hich is sufficiently great to cause incon- 

 venience is that of rolling. EoUing sometimes produces as secondary pheno- 

 mena both pitching and dipping ; but neither of these are sufficient in extent, 

 in stm water, to produce inconvenience. The roIHng, however, may be con- 

 siderable, especially in the case of a ship going vmsteadily before the wind. 

 But if the water itself be oscillating, even moderately, or if there be a gusty 

 wind, then a synchronism between any two of the five movements — the wind, 

 the waves, the rolhng, the pitching, or the dipping* or even (to a lesser ex- 

 tent) their concord at regular intervals — may cause them to enhance the effects 

 one of another to such an extent as to become inconvenient, and in certain 

 cases dangerous. In the case of a thoroughly uneasy ship in the most un- 

 favourable circumstances, the axis of angular motion may assume any and 

 every position, and the linear acceleration may take all conceivable directions; 

 but although any particular point may describe the most irregular curves, 

 both in form and speed, relatively to the vessel's course, yet the chief source 

 of practical danger in open water depends upon the accumulation of motion 

 arising from synchronism. 



It appears to have been generally observed that vessels which have a short 

 period of rolling, also roll through large angles. In this way the uneasiness 

 of the rolling undergoes a double increase as the period diminishes. Further 

 and more exact experiment is required before we can say how far it is con- 

 nected by synchronism with wave-motion, or whether it is an independent 

 phenomenon. Our present theories do not show it to be a necessary conse- 

 quence of rolling in smooth water. 



Waves. 



We do not consider it necessary to go into a formal discussion of this sub- 

 ject. As regards the behaviour of ships, it is quite sufficient to assume that 



* Dipping is the name given to the vertical oscillation of the ship as a whole relatiTely 

 to the surface of the water. 



