2. Double Pontoons . 



The double-pontoon floating breakwater (Fig. 29) consists of two massive 

 units of rectangular cross sections which are rigidly connected at intervals 

 only. The essentially open interior allows turbulent energy dissipation 

 between the separate single-pontoon sections. The design concept is based on 

 an attempt to dampen waves by primarily wave reflection from a massive struc- 

 ture with a large radius of gyration which experiences only small displace- 

 ments, the turbulence mechanism playing a secondary role. 



18 in 



Figure 29. Double-pontoon floating breakwater (after Ofuya, 1968). 



Ofuya (1968) conducted experimental studies to determine the effectiveness 

 of the double-pontoon concept of floating breakwater on wave attenuation; 

 these data are shown in Figure 30. The coefficient of transmission, C t , is 

 also shown versus the ratio of structure period of oscillation, T , to wave 

 period, T (Fig. 31). These data show that the effectiveness of the break- 

 water in wave damping increases with increase of mass (which involves slight 

 reductions in the values of radii of gyration). The similarity between the 

 results of the single pontoon and the double pontoon is evident by comparing 

 Figures 21 and 30. The comparison indicates the single pontoon is signifi- 

 cantly better, from the wave transmission standpoint, for values of L/d 

 greater than approximately 1.7; however, the significance becomes less appar- 

 ent at lower values of L/d. The geometric properties of both the single and 

 the double pontoons are presented in Table 2 for comparison. 



3. Catamaran-Type (Twin Pontoons). 



When the development of a small-boat basin at Oak Harbor, Washington, was 

 proposed, attention was directed toward the establishment of effective break- 

 water designs in this region where maximum tide elevation is +14.5 feet mean 

 lower low water (MLLW). The harbor, which is primarily a pleasure boat and 

 small fishing boat haven, is located on the mainland side of Whidbey Island in 

 Puget Sound, and is therefore not directly exposed to large ocean waves. The 

 maximum waves from the exposed directious range up to 3.5 seconds in period 



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