70 FI^OATING DRY DOCKS IN THE UNITED STATES. 



if the structure is properly cared for, be of practically indefinite duration. It is 

 equally true that old fashioned ' ' quick^consumption ' ' is not more rapid in its 

 ravages than the deterioration of a steel dock if it fails to receive proper " medical ' ' 

 attention. This attention is expensive and the demand for it is constant. The 

 United States Government has had experience with only three docks of this type, 

 the oldest having been in service only eight years. Data based upon even this 

 meager experience would be of great value, but are impossible to obtain owing 

 to the unfortunate system of accoimting which, while accurate as to fvmds expended 

 and well adapted to the business needs of the Department, is of little value to the 

 professional man, in that it offers no way to differentiate accurately between 

 repairs and structural maintenance charges, and charges due to operation or oper- 

 ating maintenance. 



Lyonel Clark, the noted EngHsh designer, has given about the only pubHshed 

 data on maintenance cost* with which the writer is familiar. From these data, 

 derived from experience with eight docks designed by him, the maintenance cost 

 should run from . 7 to i . 7 of i per cent, of the first cost annually. Mr. Clark assumes 

 a life of fifty years for his structure, on the prediction that any type of dock will 

 become obsolete during such a period. On this assumption he proposes the estab- 

 lishment of a I per cent, sinking fund to charge off the cost of the structure at the 

 termination of its usefulness. Had we an appreciable merchant marine, any- 

 thing approaching the above figures would certainly not make a bad showing as 

 a commercial proposition. 



While experience with our own naval docks may not as yet confirm or dis- 

 affirm Mr. Clark's estimates of annual maintenance cost, we are in position to 

 state very definitely just what parts of a steel floating dock are most liable to 

 deterioration and the most difficult to protect. It may be said that the side walls 

 or towers above the light draught fine are practically immune from any but weather- 

 ing effects and general wear and tear. The outside skin plating shows compara- 

 tively little deterioration and it is even a matter of doubt if the covering of marine 

 growths collected in most waters does not afford better protection than frequent 

 self-docking, scraping and painting. The pontoon deck on a large dock causes 

 considerable trouble owing to the fact that the plates are never true planes, and 

 the numerous little hollows hold pools of water. This starts rust under any paint 

 so far tried. It will be seai, therefore, that the outside of a steel dock does not 

 suffer extraordinary deterioration and is comparatively easily cared for, but, on 

 the other hand, the inside — and in particular the inside of pontoons — presents 

 an entirely dift'erent set of conditions. Continually wet, inaccessible, dark, filled 

 with stagnant air, it is almost impossible to paint them properly, corrosion takes 

 place at a rapid rate, and everlasting trouble and difficulty, to say nothing of 

 expense, is the logical result. It ma}' not be an exaggeration to assert that nine- 

 tenths of the entire deterioration, and the same proportion of upkeep charges, 



*Proc. Inst. C. E., Vol. CLXI, session 1904-5, part III. 



