Table 2. Properties of single- and double-pontoon 

 floating breakwaters (after Ofuya, 1968). 



d 2 /d 1 



Ut. 

 (lb) 



Radius of 



gyration 



(ft) 



Metacentric 



height 



(ft) 



Natural 



period 



V T r 



Heaving 

 (s) 



Rolling 

 (s) 







Single pontoon 



0.45 



45 



0.475 



1.023 



0.52 



0.52 



1.00 



0.58 



58 



0.470 



0.814 



0.59 



0.58 



1.02 



0.92 



92 



0.468 



0.556 



0.74 



0.70 



1.07 



Double pontoon 



0.32 



20 



0.539 



1.600 



0.44 



0.47 



0.94 



0.44 



27 



0.530 



1.182 



0.52 



0.54 



0.95 



0.48 



29 



0.528 



1.073 



0.54 



0.57 



0.96 



and 2.0 feet in height. It was desired that wave heights within the proposed 

 basin not exceed 0.5 foot. Soil studies of the east and south sides of the 

 proposed basin showed an unsuitable foundation for the construction of rubble- 

 mound or composite-type breakwaters. Therefore, because of the poor foun- 

 dation conditions and relatively large range of tides, a 2,509-f oot-long 

 floating catamaran-type breakwater was proposed. 



The potential structure consisted of rectangular wooden modules, each 

 approximately 42.5 feet long, 10 feet wide, and 7.2 feet deep, fastened 

 together to obtain the required breakwater length. Each module had a wooden 

 framework covered with wood decking on the top and sides, concrete beams for 

 ballast, and polystyrene for flotation. The proposed mooring system consisted 

 of either chain mooring lines fastened to concrete anchors both seaward and 

 shoreward of the breakwater, or piles placed between the breakwater modules. 

 Davidson (1971) conducted two-dimensional model tests to obtain wave 

 attenuation characteristics and mooring forces for the proposed structure 

 (Fig. 32). Tests were conducted on a 1:10 scale model specifically to 

 determine (a) the effectiveness of the proposed structure in reducing the 

 existing wave heights, (b) the mooring forces for both the chain- and the 

 pile-type mooring systems, (c) whether or not the proposed breakwater and 

 mooring system would oscillate in resonance with the existing wave conditions, 

 and (d) the natural period of oscillation of the proposed breakwater while 

 unrestrained in still water. 



One module of the proposed Oak Harbor, Washington, floating breakwater was 

 reproduced, using a linear scale of 1:10. In the model, the chain mooring 

 system consisted of two anchor chains on each side (both the sea and harbor) 

 of the breakwater module. Each chain was fastened to strain-gage measuring 

 devices on the bottom of the test flume to measure the forces in the mooring 

 lines. The pile mooring system consisted of a pile on each end of each 

 module. Each of these piles was fitted with a strain gage at the bottom of 

 the flume that was calibrated to measure the seaside and harborside forces in 

 the direction parallel to that of wave travel. 



a. Transmission Tests with Chain Mooring System. Tests were conducted at 

 the 10- and 29.5-foot water depths for the selected wave conditions. The flo- 

 tation depth of the modules at the 29.5-foot water depth was 5.0 feet; each of 

 the four anchors had an initial tension force of about 2,200 pounds (about 100 



64 



