The A-frame floating breakwater was found to be an effective means of 

 damping wave action, provided the breakwater width, 2L is not less than 

 about 75 percent of the length of the maximum design wave. The efficiency of 

 the A-frame increases rapidly as the ratio of breakwater width-to-wavelength 

 increases until the ratio reaches approximately 0.75. The efficiency then 

 decreases gradually until the ratio is approximately 1.75, and again gradually 

 increases with further increases in the ratio W/L. The efficiency is very 

 low when the breakwater width is less than 25 percent of the wavelength, 

 increases rapidly as the breakwater width-to-wavelength increases from 25 to 

 60 percent, and reaches a maximum when the width is about 75 percent of the 

 wavelength. 



This breakwater has three distinct types of motion as the breakwater 

 width-to-wavelength ratio changes. With small ratios, the motion is mainly a 

 rocking motion with the breakwater pontoons moving with the wave. With inter- 

 mediate ratios, the windward pontoon moves up and down with the wave while the 

 leeward pontoon rotates back and forth as if it were hinged on its longitu- 

 dinal axis. This is the most efficient action and occurs when the wavelength 

 is approximately equal to the breakwater width. With larger ratios, both 

 pontoons move up and down, as well as rotate about the center wall. 



The diffraction pattern of the waves (shown in Fig. 107 as total wave 

 height amplification) as they pass the breakwater is similar for all wave- 

 lengths and breakwater widths. The greatest reduction in height is immedi- 

 ately downstream from the central part of the breakwater; the reduction 

 becomes less toward the ends of the breakwater, and as the waves move farther 

 downstream. Generally, there is a slight buildup of the waves off the ends of 

 the breakwater and downstream, as well as in the reflected wave area upstream. 

 A buildup also occurs some distance downstream from the breakwater center. 

 Waves at this location (several breakwater widths downstream) are, in some 

 cases, equal to the approaching incident wave; reflected waves upstream are 

 about 150 percent of the incident wave. 



b. Queen's University Hydraulic Laboratories Investigations . The A-frame 

 floating breakwater, evaluated in two-dimensional wave tests by Ofuya (1968) 

 and Brebner and Ofuya (1968), consists essentially of a central thin, vertical 

 rigid wooden curtain with two symmetrically located circular aluminum cylin- 

 ders connected to the vertical board at intervals by thin rods. The depth of 

 the vertical board below the water surface can vary; the height of the curtain 

 above the water surface must be sufficient to prevent wave breaking above the 

 curtain. Variations of the cylinder spacing indicate changes in the radius of 

 gyration of the structure about a lateral axis through its center of gravity. 

 The use of four-cylinder pontoons is a variation of the basic unit. Two 

 different locations of the mooring line attachment were investigated. The 

 line attached to the bottom of the veritcal curtain is considered the most 

 effective. The line attachment to a cylinder was abandoned because the system 

 produced irregular and jerky breakwater motions. 



The effect of wave steepness, H-/L, on transmission coefficient, C(-, is 

 shown in Figure 108. These data show the general trend of steep waves experi- 

 encing greater attenuation than waves of lower steepness. However, there 

 appears to be deviations from this general trend for certain values of L/L , 

 where L is the distance from the vertical curtain to the pontoon. Experi- 

 ments on wave transmission past fixed, thin vertical barriers extending into 



159 



