206 LAUNCHING OF SHIPS IN RESTRICTED WATERS. 



Although previous experience with these brakes was sufficient to determine the 

 number to be required in launching the California, the exact pull required from the 

 brakes, collectively and individually, and the actual force required to stop the ship 

 were not definitely known. There were also other questions on which more com- 

 plete information than was available was considered necessary, such as the actual 

 pivoting point at various tides, the depth of water required to accommodate the 

 deepest dip of the stern, the behavior and clearance of the forefoot on leaving the 

 end of the ways, etc. It was therefore decided to construct and try out a launch- 

 ing model based on the law of comparison similar to model tank experiments. This 

 model could therefore be launched at will and as often as required, varying the con- 

 ditions to suit those expected at the time of the launching. 



The model was constructed with a length ratio of i : 96 (scale ^-inch equals 

 I foot), as giving a craft which was easy to handle, yet sufficiently large to make 

 possible a fair degree of accuracy in the results. Briefly, the dimensions of tRe 

 model are as follows : 



Length 6 feet 3 inches. 



Beam 12 inches. 



Weight (approximate) 39 pounds. 



Material — Wood, hollow, finished in spar varnish. 



Displacement, longitudinal position of center of gravity and longitudinal 

 moment of inertia may be varied at will. 



The general construction of the model is shown on Plate 83. 



The tank and the framework supporting it, shown in the plates, require no 

 special comment. Fresh water is used, and the contour of the river bed is repre- 

 sented by a layer of gravel and sand on the bottom of the tank. The water area 

 represents the width of the channel, 1,230 feet, by a certain portion of its length, 

 320 feet. A modified form of hook gauge records the tide level in feet and tenths. 

 The ground ways (with camber), the ship, the cofferdam bulkheads, crane piers, 

 etc., are, of course, all reproduced exactly to scale. 



To obtain correct results by the method of comparison, it is necessary for the 

 model to run off the ways and through the water at its "corresponding speed." By 

 running on a system of rails and steel wheels with hardened pivots it was possible 

 for the model to accelerate itself at the required rate without the application of an}- 

 external force. A central rail under the keel of the model runs on two large flanged 

 wheels, one under the vessel and one at the end of the ways ; two wheels at the fore 

 poppets run on two rails which represent the ribbands of the ground ways. This 

 system of mounting the model on three points was suggested by Mr. Percy A. Hill- 

 house and Mr. Wm. H. Riddlesworth in a paper presented by them at the Fifty- 

 eighth Session of the Institution of Naval Architects, March 29, 191 5, as being de- 

 cidedly preferable to mounting on six wheels, especially when the ways were cam- 

 bered. The forward keel wheel of the California model supports the weight until 



