20 METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 



axe. After all blocking has been removed, the launching master, standing amidships 

 on the inboard side, gives a signal, and all the lines are cut. The trigger is kicked 

 out by the dagger which falls clear, and the hull slides down the ways. 



The system just described was the one used at Mobile. Under the remainder 

 of the ship there were thirty-five launching ways 8 feet and 6 inches on centers, 

 each coming under a frame. The packing was built up to extend across three 

 ways, and there were four sets to a transverse section of the ship, exclusive of the 

 keel packing, which is individual for each way. These sets do not line up trans- 

 versely, but each successive one starts one way further astern, so that one trans- 

 verse set of packing rests on six different ways. For example, numbering the ways 

 back from the bow, the first complete outboard packing rests on ways 7, 8 and 9. 

 The set just inside it is on ways 8, 9 and 10; the one inside that on 9, 10 and 11 ; 

 and the inboard set on 10, 11 and 12. This staggered arrangement was used so 

 that there would be a greater certainty of all sets of packing starting to move at the 

 same time. 



At San Francisco the details differed materially from those used at Mobile. In- 

 stead of the conventional dagger and trigger release, they used a cradle both fore 

 and aft. After the load had been taken on the packing, lashings from these cradles 

 to deadmen restrained the tendency of the hvill to slide. It is also to be noted that 

 the packing extends across two ways instead of three, as was the case at Mobile. 



This packing was built up in five transverse sets, one under the center keelson, 

 two under the bilges, and two under the longitudinal bulkheads. The sets were not 

 staggered, but two pieces of 2 by 8 ran transversely, connecting the five units of 

 each set. 



At the date of writing no 7,500-ton tankers have been launched at any yards 

 other than Mobile and Oakland. At Jacksonville and San Diego, the method used 

 at Mobile, with possible slight variations, will be followed. 



The chief difference in the two methods described is in the releasing device. 

 Mobile, with the dagger-trigger method, followed very closely the conventional side- 

 launching procedure used almost exclusively on the Great Lakes and to some extent 

 along the Atlantic seaboard. This method is just as positive and probably a little 

 safer than the cradle scheme used at Oakland. In the latter it is necessary to use 

 heavy timbers, in fairly long lengths, rigidly fastened together. Experiments con- 

 ducted by the Concrete Ship Section on the sideways launching of steel ships show 

 that a velocity of 23 feet a second is attained. It is obvious that if any of the 

 cradle timbers should work loose and jam that the force of a 5,000-ton hull mov- 

 ing at this speed would be more than ample to punch the timber through the shell. 

 In the trigger arrangement no such long sticks are present. This actually happened 

 on the side-launching of a concrete barge when a cradle timber broke, jammed in 

 the mud just forward of the launching ways, and ruptured the shell of the vessel. 



Some of the dimensions and figures of the Mobile launching system follow : — 



