68 FLOATING DRY DOCKS IN THE UNITED STATES. 



a circular cross-section. The supplemental chamber gives increased control over 

 the "limpet;" in sinking, the buoyancy of the combination can be first destroyed, 

 thereby giving easy handling under water. The most important function of the 

 supplemental chamber, however, is the insuring of a perfect seal against the bottom 

 of the dock before the main chamber is pumped out. 



When the revival in American shipping comes, the floating dry dock will 

 immediately take a place of prominence and importance. Commercially it is an 

 investment, the cost of which can be very closely estimated, and the time of com- 

 pletion of which can be accurately foretold, neither of which can be said of a grav- 

 ing dock. 



Mr. Donnelly: — I should like to express my appreciation of Mr. Cunning- 

 ham's contribution to the discussion, and in this connection would like to call 

 attention to the fact that the difficulty with toredo in the basin dry dock to which 

 he refers was probably due to the fact that the imder- water structure was directly 

 exposed to simlight. In the examinations which I have made of the interiors of 

 floating dry docks and pontoons I have never found any evidence of animal life. 

 I consider this due to the fact that they were closed structures into which no light 

 penetrated. 



When the sectional dock referred to in the paper was repaired there was 

 removed from the interior of the pontoons from one inch to eighteen inches of 

 mud and silt and it was thought by some, in consequence of the presence of this 

 material, that the bottom or lower timbers would be injured by worms or other 

 marine life. Nothing of the kind, however, was discovered and, when cleaned 

 out, the bottom and lower timbers were foimd to be in perfect condition. When 

 it is understood that these pontoons had not been cleaned out in twenty years, 

 the permanency of such a structure is quite apparent. 



The Chairman: — Is there any further discussion on this paper? 



Mr. Thayer: — Mr. Donnelly has asked for information upon composite 

 docks. I do not know any detailed information except on one dock that I designed 

 for the Government of Venezuela about five years ago; that cost fifty-five dollars 

 per ton of Hfting power, but it would not have cost as much as that if it had been 

 erected in the United States. 



Mr. H. PenTon, Member (Commmiicated) : — With regard to the question 

 of maintenance of all iron or steel floating docks, I might say that when the float- 

 ing dock of the Great Lakes Engineering Works, Detroit, referred to on page 62, 

 was under consideration, the writer being at that time identified with that yard, 

 the question of type and material was long and earnestly debated, the more so 

 because we had less concern as to corrosion and maintenance than obtains in coast 

 practise. In reaching a conclusion the longevity of the U. S. S. Wolverine, referred 

 to in the Transactions for 1909, had a prominent part. We also had before us the 



