Unstable Motion of Free Spar Buoys in Waves 



Picture of pitching 



Figure 8 shows the behavior in rolling of the type 14 

 buoy . Once more one remarks that rolling occurs only if the wave 

 frequency lies between two values one close to fz , the other close 

 to 2 f _ . For a frequency of 0. 240 Hz rolling occurs only if the wave 



height is larger than some critical value (f = 0. 20 m), but in 

 contrary to the type 1 buoy , the half-amplitude of rolling increases 

 monotically with the wave height up to f = 1 m . The experiment 

 could not be conducted beyond this value , for the heaving motion 

 became too large and the model was periodically submerged by the 

 waves . 



V - EXPERIMENTS IN REGULAR WAVES : STUDY OF THE 

 VERTICAL MOTIONS AND OF PITCHING - 



5,1- Experimental conditions - 



The just mentioned phenomenon of rolling prevented any 

 valid measurement of the pitch angle by photographic method . The 

 experimental set up was then modified to eliminate the rolling 

 motion . The rod on top of the model was set between two guides 

 made of two horizontal steel wires parallel to the direction of wave 

 propagation and located one under the other . It has been verified on 

 a short number of points that this set-up did not perturb the value of 

 the half -amplitude of pitching and heaving . 



1017 



