^z: 



y ~v v % 



z^s^-^s. stzti: " ., z s. x x. 



Figure 179. Seabreaker floating breakwater (.after Hasler, 1974). 



Before 1963, the surfaces of most floating breakwater designs were essen- 

 tially vertical for reflection purposes. As a result of Easier' s (1974) 

 preliminary tests, a floating breakwater operating on the principal of a 

 single horizontal plate to be held ideally at the level of the wave troughs 

 and as motionless as possible, was developed. The general arrangement of the 

 Seabreaker contains a main float, A, which provides a flat plate effect 

 (Fig. 179). This unit has a length-to-width ratio of about 17:1, with only 

 about 20- to 25-percent reserve buoyancy. When prevented from heaving bodily, 

 its horizontal surfaces interfere with the vertical component of the orbital 

 motion of the water particles within the wave. The back wall, B, reduces 

 overtopping by wave crests and, in heavy seas, traps a considerable amount of 

 water on the deck. This causes the float to trim lower in the water, and 

 pours continuously off the seaward edge, creating a local surface current 

 which opposes the motion of the waves. Both these effects appeared to Hasler 

 (1974) to improve attenuation. The girder, C, of tubular steel, limits the 

 flexing of float, A, both in longitudinal bending and in torsion. It was 

 intended that the Seabreaker ride as high in the water as possible; hence, it 

 was designed to be as light as economically feasible. The outrigger float, 

 D, carried on three cantilever arms, E, was added supplementally to reduce 

 rolling and prevent risk of capsizing. It also contributes to the wave atten- 

 uation by blocking short-period waves created when the crests of large waves 

 overtop the back wall and plunge between the main float and the outrigger. 



a. Model Investigations. A small model 20 feet long was tested in a 

 three-dimensional wave basin, using uniform wave trains of varying periods and 

 heights, and moored at angles between 0° and 40° with the wave crests; reflec- 

 tion, transmission, and mooring forces were determined. A second larger model 

 36 feet long was constructed and moored in a harbor with a fetch of about 1 

 mile, and tested in natural wind-driven waves. The incident and transmitted 

 waves were recorded by motion-picture camera, and simultaneous strain-gage 

 readings were obtained of the loads on the main girder. Mooring loads were 

 not measured, but were apparently modest; the model remained on station 

 unattended for months without either dragging anchor or breaking a mooring 

 line. The entire mooring arrangement consisted of two polyester ropes (1/4- 

 inch diameter) with a breaking load limit of 1,200 pounds when new. 



240 



