0.0 ■ 



0.2 ■ 



S 0.4 ■ 



0.6 



.5 0.8 



S> 1.0 



Overall Performancefthis test] 

 K°k Energy Reduction. 



"55 ffl oV 



Frequency [CPS] 



IB 35 15 T 



Wavelength [M] 



Figure 184. Performance characteristics of Harris floating breakwater, 

 developed from incident and transmitted wave spectra at 

 Stokes Bay, England (after Harris and Thomas, 1974). 



The mooring forces in the lines attached to the breakwater were measured, 

 using a lever and spring balance, to determine the horizontal and vertical 

 angle of each mooring line, relative to the breakwater. This permitted the 

 resultant mooring force on the breakwater to be calculated vectorially. Wind 

 strengths were estimated, using the Beaufort scale, by an experienced observer 

 and verified from local weather stations. The tidal current velocity was 

 measured to within 0.05 knot by current meters. The mooring forces determined 

 for wind effects and tidal current effects are presented in Figure 185 (a and 

 b). Part of the scatter may be attributed to the effect of the breakwater 

 orientation, and a part may be due to a lack of correlation between wind and 

 waves. A wind of a given strength could be accompanied by a wide range of sea 

 conditions for adjacent sources. The data on the effect of tidal currents on 

 the mooring forces varied by about 30 percent on either side of the mean 

 value, and was mainly attributed to breakwater orientation in the direction of 

 the tidal stream. 



The essential purpose of the Stokes Bay experiment was to expose the 

 floating structure to the free interplay of the many variables on the open 

 ocean (Harris and Thomas, 1974). Performance was found to be most sensitive 

 to the area of solid slab per foot of breakwater length, but less sensitive to 

 overall width; i.e. , for a given area, performance was improved if that area 

 can be formed over a larger width. The area per foot of length, chosen as the 

 principle parameter, is also closely related to cost of structure. The per- 

 formance of a long breakwater was substantially better than that of a short 

 one, due to longitudinal stiffness; the performance approached that of a 

 rigidly fixed breakwater. Mooring forces were relatively low, about 2 percent 

 of the deadweight of the structure. 



244 



