system required to reduce ship oscillations sufficiently to meet the 

 surge criteria for different classes of ships and loading conditions. 

 Unfortunately, little reliable data are available from which to obtain 

 the answers to this problem. More theoretical and experimental investi- 

 gations are needed to provide the design engineer with the necessary tools 

 to solve the harbor-basin and ship mooring design problems. 



Theoretically, the problems of designing small-craft harbors are 

 closely related to those problems for the relatively large, commercial 

 vessel harbor, but the short -period part of the wave energy spectrum is 

 the important consideration when the harbor is used to berth small craft. 

 This is especially true for marinas where the boats are for recreational 

 use and are moored by the average boater and left unattended except for 

 the weekend or longer periods of time. However, more prototype wave data 

 are generally available, mostly from hindcast studies, and with the proper 

 selection of breakwater position, length, crest height, and degree of im- 

 perviousness, and if adequate space is allowed for the use of wave absorb- 

 ers in the critical parts of the harbor perimeter, the wave climate in the 

 berthing areas can usually be reduced satisfactorily. The problems of 

 devising satisfactory mooring methods for small craft are not as intrac- 

 table as are those for the larger ships, although the problems are some- 

 what similar in their theoretical aspects. Raichlen (1968) provides some 

 valuable information for use as a guide in mooring small craft, but addi- 

 tional studies are needed to place the solution of small-craft mooring 

 problems on a sounder scientific basis. 



2. Similitude Relations . 



a. Geometrically Similar (Undistorted-Scale) Models. Ideally, all 

 harbor wave action model studies that are performed to determine optimum 

 plans for providing adequate wave protection for moored vessels should be 

 conducted using models constructed geometrically similar to their proto- 

 type harbors. Fortunately, the size and depth of most harbors and the 

 order of magnitude of the horizontal dimensions of short-period storm 

 waves (periods usually range from about 2 to 5 seconds for lakes of small 

 to moderate size, about 5 to 10 seconds for large lakes and near-local 

 ocean storm waves, and about 10 to 20 seconds for severe storm waves 

 generated in ocean areas located a considerable distance from the harbor) 

 are such that undistorted-scale models can be used. Few harbors are of 

 a size and depth to be feasible for use in undistorted-scale models for 

 intermediate- and long-period waves. In nature, surface wind waves are 

 propagated by the restoring force of gravity, and surface tension and 

 friction forces, although present, are not usually of sufficient magni- 

 tude to affect wave action significantly within the areas reproduced by 

 scale models. Thus, harbor wave action models where short-period wind 

 waves cause a problem are designed in accordance with the Froude model 

 law and are constructed geometrically similar to their prototypes. After 

 the linear scale, L-^, has been selected, the model-to-prototype relation- 

 ships necessary for model design, construction, and operation, the inter- 

 pretation of model results and the transference of model test data to 

 corresponding prototype' units can be derived in terms of the linear scale 



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