b. Field Studies . In 1971, a full-size Seabreaker unit 131 feet long was 

 constructed and then towed off an open beach into the English Channel by a 65- 

 foot fishing vessel with a 160-horsepower engine. Towing 110 miles was not 

 difficult at an average speed of more than 4 knots. The unit was moored in 

 Stokes Bay, with a fetch of 3.5 miles in tidal streams up to about 2 knots. 

 The prototype section of Seabreaker remained on station (Fig. 180) for 2 years 

 before being towed to another location for further tests. Although neither 

 attenuation nor mooring data were available, Hasler (1974) contends that the 

 Seabreaker attained up to 60-percent wave height reduction at the design wave 

 in both wind-driven waves and steep swell. 





Ss. 



yflp' 



Figure 180. Prototype Seabreaker floating breakwater positioned 

 in Stokes Bay, England (after Hasler, 1974). 



2. Harris Floating Breakwater. 



In the late 1960's, the Harris floating breakwater configuration was 

 developed to accentuate hydrodynamic instabilities in the waveform that result 

 in turbulent breaking and energy dissipation (Harris and Webber, 1968; Harris 

 and Thomas, 1974). The structure, which also presents a thin, horizontal 

 barrier to the wave motion, consists of a floating slab with a width compar- 

 able to the length of the wave to be attenuated. Various arrangements of 

 slots and mass damping have improved performance. The Harris breakwater acts 

 primarily by inhibiting the vertical component of orbital motion, with a 

 secondary action of energy dissipation by wave breaking and eddy formation. 

 There is also a slight degree of reflection (Shaw, 1973). 



Harris and Thomas (1974) tested a 1:10 scale model of the Harris floating 

 breakwater designed for the Port of Le Havre, England, in Stokes Bay. The 

 prototype was required to be effective in seas up to 10 meters (33 feet) in 

 height; hence, the test site required seas up to 1 meter (3 feet) in height, 

 with otherwise relatively calm conditions. Prevailing winds in the Stokes Bay 

 region are from the southwest with a fetch distance of about 3.5 miles. In 

 winds up to 25 knots, wave heights of 1 meter can be expected. Disadvantages 

 at the test site included relatively strong tidal currents with only brief 



241 



