FLOATING DOCKS. 293 



caissons, may be classed as floating docks proper; structures 

 having sides varying in height, with open ends, or with one 

 side only, and of less displacement than the base, as pontoon 

 docks; and closed or open rectangular structures, as pontoons 

 simply. 



Durability. The life of a floating dock of any class will 

 depend greatly on the facilities for, and the care and attention 

 bestowed upon, periodical repairs, cleaning and painting of the 

 submerged parts. These operations may be performed by 

 careening or canting ; beaching, when there is sufficient rise and 

 fall of tide; raising the submerged parts out of the water; 

 or by floating the dock into a shallow basin and closing the 

 entrance. 1 



Strains. The buoyancy of the empty chambers produces 

 a strain over the basement while supporting a ship on its keel 

 similar to that of a beam resting on a central fulcrum with 

 a uniform pressure on the lower surface acting upwards, which 

 is equal to the weight of the ship supported. 2 



Applicability. In considering the applicability of a floating 

 dock, a crucial point would be the attainment of a site 

 sufficiently protected as to ensure a ship being placed and 

 lifted under all conditions of weather. If such a site is not 

 available, a floating dock of any class can hardly be considered 

 suitable. 



Conditions affecting Design. The following remarks on the 

 conditions to be observed in designing an open-ended floating 

 or pontoon dock are from Mr. Taylor's paper on "Floating 

 Docks," in the Transactions of the East Coast Institute of Engi- 

 neers and Shipbuilders. 



" The under-deck displacement should be greater than the 

 combined weight of ship and dock. 



" The dock should be sufficiently strong not only to support 

 the intended maximum ship, but so strong that it will not 

 be injured if the ship has been built or damaged in such a way 

 that a great part of the ship's weight rests either amidships or 

 at the ends. 



" The sides should be made as low and as narrow as efficiency 

 will permit, so that a wide working platform may be secured, 

 and light and air have access to the ship, and that the top weight 

 and displacement be a minimum. 



1 M.P.LC.E., vol. xxxi. p. 301. Ibid., p. 300. 



