resulting from the wave generated by the sway motion. At very low B/L 

 ratios the fixed-body transmission is the more important component. At 

 a B/L of about 0.1 both the fixed-body transmission and the sway-generat- 

 ed wave are of equal importance. At B/L ratios higher than 0.215, the 

 wave generated by sway motion dominates. The agreement between theory 

 and experiment is quite reasonable for this case. 



A comparison when the breakwater is restricted to heave motion only 

 is shown in Figure 8. There is clearly a discrepancy in this case be- 

 tween measured and theoretically predicted transmission coefficient near 

 the B/L ratio of 0.13. As a matter of fact, the theory predicts a heave 

 resonance in this region which does not seem to be supported by the mea- 

 sured data. 



In examining the mechanism used to restrain the breakwater motion, 

 it seemed possible that this apparatus was introducing damping into the 

 system. To test this supposition, transmission coefficients were com- 

 puted with the calculated hydrodynamic damping increased by an arbitrary 

 amount. The major effect of increasing the damping was to decrease the 

 transmission near the heave resonance region. With damping at three 

 times the hydrodynamic value, the results were quite close to the experi- 

 mental measurements. Increasing the damping beyond this had very lit- 

 tle additional effect on the predicted transmission coefficient. The 

 scatter which appears in the experimental data in this region is a 

 further indication that some nonrepeatable effect may be influencing 

 the experimental results. So long as the additional damping is included 

 in the theoretical calculations, the results compare well with experi- 

 mental measurements. 



Figure 9 shows a comparison between model measurements when the 

 model is unrestrained except by a horizontal mooring cable and the 

 theoretically predicted results without mooring restraints. The theo- 

 retical results are characteristic of the rectangular breakwater with 

 the dip in transmission near a B/L equal to 0.2. The pattern of inter- 

 actions between motion-generated waves and fixed-body transmission is 

 similar to the previous description. The agreement between these re- 

 sults indicates that the theoretical model may also yield the correct 

 results when the model is free to heave only. At least further experi- 

 mental investigation is warranted. 



(4) Alaska-Type Breakwater . The State of Alaska has embarked 

 on an ambit iousprogram for constructing moorages using floating break- 

 waters. As part of a Sea Grant project the University of Washington has 

 been studying the performance of this type breakwater. A theoretically 

 predicted transmission coefficient and the transmission coefficient mea- 

 sured in model tests are shown in Figure 10. The model tests were con- 

 ducted using very small incident waves (wave heights on the order of 0.2 

 to 0.3 feet at prototype scale). Results for larger wave slopes were 

 not included in the figure but do show the same trends with lower values 

 of transmission coefficient. Theoretical predictions without added 

 damping and with double the hydrodynamic damping are shown in Figure 10. 



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