STABILITY, PROPULSION, AND SEA-GOING QUALITIES OF SHIPS. 21 



have been put forth ; but they do not agree in their form or in their result, 

 and the credit of each consequently rests, as a practical matter, on the repu- 

 tation of its author. 



Resistance considered in Detail. 



It cannot be said that our knowledge of the detailed phenomena which 

 accompany the motion of a floating body through the water extends far 

 below the surface of the liquid. Meanwhile the following things appear to 

 be known. 



Por any vessel driven through the water by any power which does not re- 

 act on the fluid there must be a certain movement of the surrounding Liquid, 

 chiefly in the direction of the vessel's motion, which shall be suflicicnt to 

 absorb the work done by the propelling force ; for this is really nothing else 

 than the work done by the power in overcoming the resistance. Much of 

 this is masked by oscillatory movement. Now the setting up of an oscillation 

 involves an expenditure of work; but the maintenance of the oscillation, 

 once established, is independent of the force which caused it, to just the 

 same extent that it takes work to set a pendulum swinging, but once set 

 going, the continuance does not dejiend on the starting force. It follows 

 that making a wave takes up propelling work ; but that a wave once started 

 maintains itself, or dies out, as the case may be, independently of the pro- 

 peller, which it can only affect by getting in its wa3\ 



In a vessel of good form thrust through a fluid, we first meet with a head- 

 pressure wliich relieves itself by the formation of a head sAvell, which dis- 

 perses itself all round if time be allowed for it, either by the sharpness of 

 the vessel's entrance, or by a slow rate of advance. This fixes a limit of 

 speed, which cannot be advantageously exceeded, dependent on the length of 

 entrance as well as on its form, — on the length alone if the form fulfil cer- 

 tain conditions. If the vessel be pushed beyond the speed of dispersion of 

 this wave, it has to be pushed up hiU at a loss of useful work. 



The frictional resistance of the surface of the ship also canies a stream of 

 water in the direction of the ship's motion. In fact, nearly the whole work 

 of friction is expended on producing this stream, which forms a part of the 

 ship's wake. 



The necesssity of filling up the vacuum which would otherwise be left in 

 rear of the ship also produces a following stream, accompanied with waves. 



In vessels driven at a speed beyond what is suited to their form and dimen- 

 sions, there are also supernumerary waves, an accoirnt of which will be found 

 in Professor Hankine's writings already referred to. 



In vessels of unfair form there will further be violent eddies or whii'lpools, 

 as well as extra waves. Seeing that it takes an expenditure of work to make 

 these, it is clear that least resistance means least disturbance. In reality 

 very little is known about these eddies. Their surface-action has been 

 observed, and may easily be seen in dirty water, with froth esj^ecially ; but 

 their extent in depth, and their amplitude as the depth increases, are utterly 

 unknown ; and the other phenomena arc not sufficiently wcU understood to 

 admit of the eff'ect of these being got at by exhaustion, that is to say, by 

 being equated to the unexjjlained residue from the effects of the other known 

 causes. Very little, again, is known about the direction in which the replace- 

 ment aft takes place. The water may of covu'se pour in eit'ner laterally or 

 from behind, or it may well up from underneath as a wave. More or less, 

 it probably does all three, and the projiortion in which it does each is among- 

 the things which neither experiment nor theory has as yet revealed. 



