Verhagen 



The slow drifting oscillations of a moored vessel are caused 

 by forces due to the reflection of the waves against the fixed obstacle. 



The remarkable observation that the amplitude of the long- 

 periodic motion for a given body and mooring systern^in various 

 seaways of given direction is proportional to Cwu3 /"^ ^^^ ^°^ ^® 

 explained as follows: The force due to reflection of Irregular waves 

 against a fixed obstacle is proportional to Cwi/3 ^^ the body 

 dimensions (L) in the wave direction is not too small compared to 

 the mean wave length X. (L/\ > 2 it)* A "high" wave can be defined 

 more formally as a wave with amplitude a fixed number times the 

 significant wave height. The change of occurrence ger unit time of 

 our so called "high" wave is then proportional to l/T provided all 

 of the considered random waves are Gaussian distributed, are at 

 least distributed in the sanne way. If the combination of floating 

 body and miooring system is considered as a mass-spring system 

 with linear stiffness and damping coefficients the resulting long- 

 periodic m.otions will be proportional to ?,wi/3 /T. In consequence 

 of the many assumptions made in the above reasoning, one shoxild 

 be careful to adapt the explajiation without reservation, A firm 

 foundation is needed. 



IV. MODEL TEST RESULTS 



An extensive test program has been carried out in the Sea- 

 keeping Laboratory of the N, S.M.B. on a number of models in order 

 to obtain systematic information on the station-keeping abilities of 

 moored vessels in a seaway. Some results of motion tests on one 

 of the vessels will be given in this paper. 



The vessel is moored between four horizontal linear springs 

 and tested in various seaways approaching from ahead and from 

 abeami. A sketch of the mooring system is given below. 



Beam seas 



958 



