690 



BARBER AND TUCKER 



[chap. 19 



would thus not measure short waves approaching from the opposite side of 

 the ship, and the height of waves approaching from its own side would be 

 increased by reflexion. Two measuring heads are therefore mounted on opposite 

 sides of the ship and their output is averaged. 



The response of the instrument drops for waves of shorter wavelength owing 

 to the pressure units having to be mounted considerably below the water line. 

 The shape and motion of the ship makes calculation of this effect difficult, but 

 Korvin-Kroukovsky (1955) has found theoretically that, for short waves, the 

 pressure fluctuations P at a point on the hull of a ship (in equivalent head of 

 water) should be related to the surface amplitude A by approximately 



P = ^ exp ( - 2kz), 



(18) 



where A; = wave number = 27r/ wavelength, and z== depth of the point below the 

 water line. 



Fig. 13. The principle of operation of the shipborne wave recorder. (After Tucker, 1956a, 

 Fig. 1.) 



Cartwright (in press) has compared the spectrum measured by a shipborne 

 wave recorder in a ship steaming at 9 knots with that measured using a buoy, 

 and finds that the above expression expresses the response fairly well, but that 

 better agreement is obtained if the factor 2 is increased to about 2.5. 



In spite of this uncertainty in the high-frequency response of the instrument, 

 shipborne wave recorders have been widely used on account of their ease of 

 operation, and the fact that they measure the waves at the position of the ship, 

 which is convenient for ship-motion research. 



C. Measurement and Analysis of Wave Records 



The principles and statistics of the measurement and analysis of wave 

 records have been discussed in Chapter 15. They will be discussed here briefly 

 from a physical point of view and with emphasis on practical methods. 



The simplest form of analysis is to measure a mean wave height and mean 

 wave period. The "classical" method is to count the number N of crests in a 

 record, and then measure the mean height Hy^ of the highest NjS waves, the 



