Theories of water wave transmission past fixed barriers indicate that the 

 transmitted wave heights depend on the depth of barrier below the water 

 surface. In Ofuya's (1968) tests to determine the influence of the depths of 

 vertical curtain on wave damping of the A-frame, the masses of two breakwaters 

 and their mass radii of gyration were held constant. This was achieved by 

 maintaining equal lengths of vertical curtain but having different depths of 

 submergence below Stillwater level. It was assumed that the slight difference 

 in the location of the center of mass of the two breakwaters had only minimal 

 effects on their motions. Figure 110 shows the effect of this relative 

 penetration depth, h /d, on transmission coefficient, C t « The transmission 

 coefficient for an h /d value of 0.47 exceeds that for h /d of 0.23 by 

 only about 10 percent, in relatively deep water. However, in an intermediate 

 depth, the difference becomes more than 15 percent. In shallow water, the 

 influence of depth of vertical curtain on CL is expected to be significant 

 since kinetic energy concentration is approximately distributed uniformly 

 throughout the water depth. In deep water, the A-frame breakwater with 

 an h /d value of 0.25 can achieve transmission coefficients of 0.4 or less, 

 depending on the cylinder spacing. 



Brebner and Ofuya (1968) investigated two variations of the four-cylinder 

 A-frame floating breakwater, using either a chain connection between the 

 cylinders or a welded connection. Figure 111 shows the wave attenuation 

 characteristics of the two variations. Wave damping by the chain-connected 

 cylinders decreased with decreasing values of L 9 /L , which may be partly 

 attributed to the fact that a decrease in values of L „/L causes both 

 cylinders to become more located in regions of large kinetic energy con- 

 centration. The experiments indicated that, since both cylinders move inde- 

 pendently, the waves generated by the lower cylinders augment those produced 

 by the basic A-frame unit (although this effect is not intuitive. At cerain 

 values of h /h , the structure moved in the seaward direction and, hence, 

 the mooring cable remained slack. The seaward movement, which made the 

 mooring forces a minor measurement, can be partly attributed to the inertial 

 effects of the lower cylinder. A greater effectiveness of the four-cylinder 

 A-frame can be achieved by welding the cylinders. Unlike the chain-connected 

 system, this system experienced no seaward movement. 



The horizontal force components of the mooring force parameter, Fh /yH?L, 

 are presented in Figure 112 as a function of the relative width of the break- 

 water, L/L , for the two-cylinder A-frame floating breakwater evaluated by 

 Ofuya (1968). The data for the four-cylinder A-frame are shown in Figure 113 

 for cylinder spacings of L = 0.54 foot and L = 1.41 feet, respectively. In 

 Figure 112, both peak and average forces for the two-cylinder A-frame are 

 shown with the average forces defined here as the average over 70 cycles of 

 loading. It was evident from the frequency distribution of the resulting 



162 



