optimum width of the offset sections. Figure 191(d) shows that, as the sec- 

 tion width was decreased from a value of one-half wavelength, the coefficient 

 of transmission, C t , increased from a distinct optimum value to the point of 

 no actual offset width. 



Figure 192 shows the offset breakwater performance in wind-generated waves 

 in terms of the ratio of wavelength-to-offset distance between the reflecting 

 surfaces. Theory indicates that the resultant forces and moments acting on 

 the vertical reflecting surfaces of the offset breakwater are much less than 

 for a vertical, fixed wall. Theoretically, the reduction in force will be a 

 maximum if the offset walls are a distance apart equal to one-half the inci- 

 dent wavelength. Figure 192 shows that a maximum average attenuation occurred 

 at a wavelength 1.7 to 1.8 times the offset distance. Sethness and Moore 

 (1973) considered this in relatively good agreement with the simple theory 

 used in the development. Considering the complexities of prototype situa- 

 tions, the agreement indicated that model data subjected to even simple 

 analysis can be considered quite useful. The test results generate a well- 

 defined family of curves which would allow for reasonable accuracy in the 

 design of an offset floating breakwater. When the dominant design wave has 

 been determined, Sethness and Moore's (1973) theory can be applied to deter- 

 mine the geometry of the acceptable offset breakwater. 



1-50 2 .00 2.50 



Ratio of Wavelength-to-Offset Distance, 



3.00 



L/D 



Figure 192. 



L/D Q , and 



depth of penetration, h, on coefficient of transmission, C t , 



for offset floating breakwater configuration (after Sethness and 

 Moore, 1973). 



252 



