SECT. 5J WIND WAVES 683 



the harbour mouth and yet it must not be allowed to accumulate there and 

 block the channel. 



For a full discussion of coastal engineering problems the reader is referred to 

 the Proceedings of the Conferences on Coastal Engineering, 1950 to 1954. A 

 recent review and bibliography of the topic is given by Silvester (1959). 



8. Ships and Waves 



The study of ship motion in waves provides at the present time one of the 

 major incentives to wave research. In this section, sufficient account of the 

 subject will be given to explain why this is so and to show what type of in- 

 formation about waves is required. For more detailed information, the reader 

 is referred to the Proceedings of the Symposium on the Behaviour of Ships in a 

 Seaway, held at Wageningen in 1957 (see note at beginning of references), to a 

 monograph by Korvin-Kroukovsky (1958), or to a brief survey by Cartwright 

 (1958). 



A ship encountering a storm at sea first loses speed due to increased re- 

 sistance, then, as the storm gets worse, the motion gets dangerous and engine 

 power has to be reduced. A small ship may have to "heave-to", making no 

 progress at all. An idea of the importance of the loss of time involved can be 

 gained from the fact that it has been found economic to alter the routes of 

 ships to avoid storms, even though this may considerably increase the number 

 of miles covered. The U.S. Navy Hydrographic Department have been routing 

 ships in the Atlantic and Pacific in this way, and estimate a mean saving of 

 14 h in time and about S2,000 per passage. Apart from the loss of speed, storms 

 cause considerable damage, and, of about 300 ships of one sort or another lost 

 at sea each year, quite a high proportion must be lost by wave damage. It 

 seems likely that with more knowledge of the factors governing ship motion, 

 ships could be designed which are safer, more comfortable and capable of 

 maintaining higher speeds in storms. 



Rolling can nowadays be greatly reduced by the use of stabilizers, but no 

 successful stabilizer for pitch or heave has been designed, and these remain 

 the most important motions to consider. Useful results can be obtained by 

 considering the ship response to be linear, though non-linear effects are im- 

 portant when the amplitude of motion becomes large, and have to be taken 

 into account when more accuracy is required. In the linear case, the total 

 motion in any mode (e.g. pitch) is assumed to be the linear superposition of the 

 responses to the individual components in the wave spectrum, taking account 

 of frequency and direction. (It is convenient to think of the wave spectrum as 

 being composed of a large number of discrete components, though in fact the 

 spectrum is continuous : see section 2 of this Chapter.) 



The response -etf a ship to a component wave depends on two factors. The 

 ship will have a resonance at a particular frequency, typically about 1 cycle in 

 5 sec for the pitching of a medium-sized ship, though often with very high 



