The shipboard equipment consists of a transformer; rectifier and 

 filter; and a single- channel, magnetic pen recorder. 



A calibration curve is necessary to convert apparent wave heights 

 to true wave heights. This is necessary because the wave staff has its 

 own vertical motion relative to the wave motion. (See Reference VII- 32.) 



Although the wave staff is difficult to operate in rough weather, it 

 operates satisfactorily in moderate weather. Generally, however, it is 

 difficult to keep a ship from imparting some of its motion to the float- 

 ing instrument through the cables linking the wave staff to the on- deck 

 equipment. 



b. Floating accelerometer on a raft 



This recorder was developed by Dorrestein at the Dutch 

 Meteorological Institute for measuring waves in the open sea (Refer- 

 ence VII-6). It utilizes an accelerometer placed on a small raft which 

 is connected with a ship by a conducting cable. The accelerometer 

 supplies a signal which corresponds essentially to the vertical acceler- 

 ation of the sea surface. This signal is integrated twice, and the inte- 

 grated recording is made aboard ship. The frequency reponse is 

 constant to periods as small as 1.5 seconds. The recorder has some 

 of the disadvantages of the wave staff in that it is a floating instrument 

 and tends to float away with the current. The advantages are that it 

 is relatively small and can be handled by two men, its weight being 

 about 45 pounds. This instrument provided the basic idea for the 

 instrument developed by the David Taylor Model Basin. 



c. Telemetering wave buoy (splashnik) 



The wave buoy, developed at the David Taylor Model Basin, 

 is similar to the device developed at the Dutch Meteorological Insti- 

 tute. However, one significant difference is that this unit transmits 

 its vertical acceleration signals back to a ship by radio, eliminating 

 any ship motion from being imparted to the buoy. The system consists 

 of a buoy assembly containing a transducer and transmitter, a wide- 

 band FM receiver, an electronic low-pass filter, and a recorder. 

 The transmitter sends a signal that varies in frequency proportional 

 to the acceleration experienced by the buoy assembly. This signal is 

 received by an antenna mounted on a ship and is fed into the FM 

 receiver. In the receiver the frequency changes are converted to a 

 varying d.c. voltage which is proportional to the acceleration. The 

 d.c. voltage is placed on the input of the adjustable low-pass filter 

 which cuts off the signals produced by the surface chop of the sea, 



VII-10 



