predicted dip in transmission just above a B/L (beam/wavelength) of 

 0.2. There is also some difference at higher B/L ratios. 



The theory predicts that the part of the transmitted wave which 

 would result where the body is rigidly fixed is almost 1 for a B/L less 

 than 0.1 and drops rapidly at higher B/L ratios to the point where it is 

 of little consequence beyond 0.2. Waves generated by the breakwater mo- 

 tions play an increasing role for B/L ratios above 0.15. Heave motion 

 is the major contributor to the transmitted wave in the very narrow band 

 of B/L between 0.15 and 0.18 with a predicted heave resonance at a B/L 

 of about 0.18. The dip occurs because the waves generated by heave and 

 sway motions are almost 180° out of phase and cancel each other out. At 

 B/L ratios above 0.25, sway motion assumes an increasingly dominant role. 

 Roll motions are small throughout and generate only very small waves. 



(2) Rectangular Breakwater . A breakwater of rectangular 

 cross section with the same beam and draft as the proposed Oak Harbor 

 breakwater was tested at the University of Washington by Nece and Richey 

 (1972). Results for the water depth of 29.5 feet are shown in Figure 



6. 



Again the agreement is reasonable. Further experiments with this 

 model have confirmed the existence of the trough at a B/L of 0.2. How- 

 ever, this phenomenon can be observed only for very small wave heights. 

 For practical purposes, the dip may be smoothed over considerably. The 

 major discrepancy is at the high B/L ratios where the theory shows con- 

 siderably greater transmission than is actually measured in the model 

 tests. Since the transmitted wave is almost totally a result of sway 

 motion, the problem must lie in the wave predicted by this motion. 



Over the entire range of wavelengths of interest, the predicted 

 results follow the pattern previously discussed for the proposed Oak 

 Harbor breakwater. The transmitted wave is almost completely a result 

 of fixed-body transmission followed by regions of heave resonance, heave 

 and sway cancellation, and finally, sway wave generation as the B/L 

 increases. 



It is interesting to note that there is very little difference be- 

 tween the open-well breakwater and the closed rectangle of the same 

 overall dimensions. 



(3) Rectangular Breakwater Tested by Sutko and Haden . In 

 some recent experiments Sutko and Haden (1974) have examined the effect 

 that restricting breakwater motions has on the transmission coefficient. 

 They used a rectangular breakwater model with a beam-to-draft ratio of 

 1.5. Plexiglas end assemblies were used to restrict the breakwater 

 motions. 



Figure 7 shows the transmission coefficient when the breakwater is 

 restricted to sway motions only. Here, the transmitted wave contains a 

 component resulting from the fixed-body transmission and a component 



25 



