Verhagen 



II. GENERAL OBSERVATIONS OF TEST RESULTS 



A study of the test results on the behavior of moored floating 

 bodies In Irregular seas revealed the following general observations: 



1. The horlzonted modes of motion -- surge, sway and yaw -- 

 show two separate frequency regions, A low frequency 

 region corresponding to the low natural frequencies of the 

 moored system, and a frequency region corresponding to 

 those of the energy containing waves. 



2. The long periodic motion Is excited by waves or by a wave 

 group with amplitudes high compared to the mean wave 

 height. In the considered cases, where a linear stiffness 

 of the mooring system Is employed, It appeared that the 

 amplitude of the long periodic motion for a given vessel 

 and mooring system Is proportional to the square of the 

 significant wave height divided by the mean wave period 

 ^wi/s/^ for various long crested seaways coming from a 

 given direction, 



3. For a given body and mooring system tested In various 

 seaways no clear relation could be discovered between the 

 time averaged excursion from the equilibrium position In 

 still water and the amplitude of the long periodic motion. 



These observations are obtained from extensive model tests conducted 

 In the Seakeeplng Laboratory of the N. S.M.B. at Wagenlngen. 



The behavior Is not unique to moored vessels. Also the towing 

 force of a vessel towed In Irregular seas show the same tendency as 

 well as for Instance, the slow oscillations In torque and thrust of the 

 propeller of a self-propelled model as observed In the seaJceeplng 

 model tests. 



m. DISCUSSION OF THE RESULTS 



The third point of the above mentioned observations deserves 

 particular attention. The drifting force on a floating body In regular 

 waves -- the time averaged position of the body Is fixed In space -- 

 Is dependent on the joint action of waves and body motions. The 

 force Is proportional to the square of the wave height and dependent 

 on the phase between wave and vessel motion. 



If we consider an Irregular wave as build up of a regular wave 

 whose amplitude Is a slowly varying function of time (slow as com- 

 pared to the wave period) and a stochastic variable phase, the cor- 

 responding energy spectrum will be narrow. The drifting force on the 

 floating body In that case will show the same dependency of the time 

 as the square of the wave amplitude. The amplitude of modulation 

 will be the same order of magnitude as the time averaged drift force. 



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