26 METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 



compared with the $225 to $300 per ton cost for steel tankers built during the same 



period. 



In Fig. 44, Plate 12, is given a time progress curve for the different items in 

 the construction of the 7,500-ton tanker, hull 1,663, constructed by the San Fran- 

 cisco Shipbuilding Company at Oakland, Cal. The total time required for this ship 

 was the shortest of any of the larger hulls so far built, but the relative proportion 

 of the total time consumed on each item, as well as the time of starting and com- 

 pleting each item and the arrangement with each other, is probably typical. A time 

 progress curve showing comparative rate of progress on each of the ships being 

 built in the Government Agency concrete shipyards is shown in Fig. 45, Plate 13. 

 It was originally intended that construction should proceed much more rapidly than 

 is here indicated, and when work started on the earlier hulls the progress was 

 greater. This extra speed necessitated a great deal of overtime work, and costs 

 were sacrificed to speed. With the signing of the armistice the policy of the Emer- 

 gency Fleet Corporation was changed, and any work being done at high cost in 

 order to give more rapid progress was stopped, overtime work was discontinued, 

 and lower costs made the paramount issue in construction. This naturally slowed 

 up the rate of construction, and the curves shown are to be interpreted in this light. 

 Another item which prevented fast construction was the inexperience of everyone 

 concerned in the work. At Mobile and San Francisco a third ship is now being 

 built by the same organization which have already completed two others. These 

 are hull numbers 1,664 and 1,717. Both are now about half complete, and if the 

 average rate of construction is maintained, they will be built in about 60 per cent of 

 the time of the first ships at these yards. 



STATUS OF CONCRETE SHIPS. 



The concrete ship is a practical structure and, if properly designed and built, 

 will resist the normal stresses to which ships are exposed at sea. 



It has, however, one inherent weakness. The 4-inch shell of a concrete ship 

 will not resist local impact of moderate intensity to the same extent as the shell of a 

 steel ship of the same capacity. Tests were made on both concrete and steel panels 

 designed to duplicate a section of the shell of a concrete and steel ship of similar 

 size. These tests showed that impact loads applied between frames would only dent 

 the shell of a steel ship and possibly loosen some rivets, while loads of similar in- 

 tensity would shatter the concrete shell between frames. In cases of severe impact 

 where the resistance of the frames is an important factor it is believed that the con- 

 crete ship will show equal, if not greater resistance than a steel ship on account of 

 its greater mass. Up to the present writing, no concrete ships have been exposed 

 to collision or severe impact, therefore the behavior of a concrete ship under such 

 conditions is unknown. As a compensating feature, it has been found that concrete 

 ship hulls are more easily and cheaply repaired than steel ship hulls. 



On account of this more friable character of the shell of a concrete ship, it is 



