STABILITY, PROPULSION, AND SEA-GOING QUALITIES OF SHIPS. 35 



to her strength and durability, and the safety of her lading, and, in ships of 

 war, to the power of working giins in rough weather. 



A ship whose course is either oblique or transverse to the wave-crests is 

 made by the waves to perform a series of longitudinal oscillations, which 

 may be c&RcA passive 2)itchi7ig and scending. 



In all the oscillatory movements which a ship performs among waves, two 

 series of oscillations are combined — those in which the ship keeps time with 

 the waves, being her i}assive or forced oscillations, and those which she per- 

 forms in periods depending on her own mass and figure, as in smooth water, 

 being what may be called her free oscillations. The tendency and ultimate 

 effect of the resistance of the water is to destroy the free oscillations after a 

 certain time, so that the forced oscillations alone are permanent. 



Passive heaving, or the motion of a ship when each of her particles per- 

 forms an orbital motion, similar and equal to that of a certain particle of the 

 water in which she floats, takes place when the ship floats amongst waves 

 ■without having progressive motion. 



The progression of the ship, when under way, alters the action of the 

 waves upon her in various ways, which depend mainly upon the apparent 

 period of the waves relatively to the ship (that is, the interval of time between 

 the arrival of two successive crests at the ship) , and upon the apparent slope 

 of the effective wave-surface in a direction athwart the ship, the latter 

 circumstance being connected mainly with forced rolling oscillations. 



When the apparent periodic time of the waves is modified by the pro- 

 gressive motion of the ship, the time during which the forces act which 

 produce the heaving motion of the ship is altered in the ratio of the appa- 

 rent period to the true period ; and the extent of the heaving motion is also 

 altered in a proportion which, for moderate deviations of the apparent from 

 the true period, varies nearly as the square of that ratio. This law, how- 

 ever, does not continue to hold for a very great increase of the apparent 

 period, the extent of heaving being loss than the ratio first mentioned. 



Hence the heaving motion of a ship is more extensive than that of the 

 effective wave- surface, when the angle made by her course with the direc- 

 tion of advance of the waves is acute, and less extensive when that angle is 

 obtuse. 



Yawing, or swerving of the vessel from side to side by oscillation about an 

 upright axis, is, when produced by the waves, the effect of the lateral sway- 

 ing, which forms the horizontal component of the heaving motion, taking 

 place with different velocities, or in opposite directions at the bow and stern 

 of the vessel. The forces producing it are greatest when her course lies 

 diagonally with respect to the direction of advance of the waves. 



Tor reasons already stated, a very light and stiff ship tends to float like a 

 raft roHiug luith the waves, and assuming at every instant the same slope 

 with the effective wave-surface. 



Let a board, having very little inertia, and no stability, be placed so as 

 to float upright in smooth water ; then, when the water is agitated by 

 waves, that board wiU accompany the motions of the originally iipright 

 columns of water — that is to say, it will roll against the ivaves, inchning at 

 every instant in a direction contrary to the slope of the effective wave- 

 surface. 



It has been shown by ilr. Scott Eussell * tliat the condition of the broad 

 and rounded ])arts of a ship, and of her huU between wind and water, is 



* Trans. I. N. A. for 18G3, 



d2 



