FLOATING DRY DOCKS IN THE UNITED STATES. 67 



steel pontoons must be self-docked at reasonable intervals for cleaning and paint- 

 ing. With wooden pontoons these intervals can be much longer, as there is no 

 rusting to contend with, and by keeping the wooden pontoons wet the rotting of 

 the wood can be prevented. The steel side walls can always be preserved in 

 place without disabling the dock. 



The combination also permits of a dock of much greater capacity than in 

 wood alone. The pontoons become merely beams for transmitting the weight to 

 the side walls, and, when of steel, these can be designed and constructed to take any 

 stress safely. There would probably be found a point where steel pontoons would 

 be more economical than wood in first cost, but it would be considerably beyond 

 the capacity of a purely wooden dock. 



If a dock with wooden pontoons were located in waters where the toredo is 

 plentiful and active, some attention would have to be given to the protection of 

 the wood from the inside of the pontoon, especially if there were periods when the 

 dock was inactive. The new graving dock at the Norfolk Navy Yard was flooded 

 for several months before the caisson could be placed and the dock pumped out 

 the oak keel blocks had been placed in the dock by the original contractor, and 

 when the dock was pumped out it was found that the toredo had cut into the blocks 

 about one inch. The greatest danger for the wooden pontoon would be at the 

 bottom where there is always water, and this would also be the hardest place to 

 inspect. 



The link and wedge-pin connection between pontoons and side walls is 

 extremely simple, rapid and effective. If made from steel they are presumably 

 forged from the solid and have no weak point. By making them of soft steel and 

 finally quenching in oil from a dark cherry-red, a metal with a high elastic limit and 

 great toughness at the same time would be produced. 



The preservation of the outside and inside of an all-steel floating dock, or of 

 one with steel side walls only, is a matter of importance, and, with a commercial 

 dock, a question of econom)^ as well. The Government, in this country at least, 

 has had the most experience in this matter. With the New Orleans dock in fresh 

 water, and the Pensacola dock in salt water, a coal tar paint has been found to 

 be the most enduring, as well as the least expensive preservative that could be 

 applied to the outside. Red lead has not proven satisfactory on the inside of 

 these docks and the coal tar paint is now being tried. The fumes from this paint 

 are severe on the painter's ej'^es in a closed space, but this difficulty can be over- 

 come by equipping the painter v/ith a diver's or a fireman's smoke helmet. 



Were it not for the impossibility of preserving and repairing the bottom, there 

 are many who would prefer the solid dock to any self-docking type. In an effort 

 to meet this case, the writer has proposed the use of a caisson, the general prin- 

 ciples of which are shown in the accompanying sketch, Plate 27. The general propo- 

 sition is not new, and to this class of appliances Mr. Lyonel Clark, the English float- 

 ing dry-dock expert, has given the appropriate name of "limpet." The new 

 feature in this caisson'Js the supplemental chamber shown in the sketch as having 



