480
DOCK ENGINEERING.
to increase its stability ; gates and caissons are quite unnecessary and are
rarely used in connection with the rectangular section.
Herr Howaldt, of Kiel, advocates a system of composite docks which
he has devised, the frames being of iron or steel and the deck and bottom
sides of wood. He states, as the result of his experience, that while with
metal plating, the girders must not be more than 2 feet apart, with planks
of pitchpine or beech, 4 inches thick, the frames can be placed 4 feet
apart, without the least deflection in the panels. The advantages claimed
for the system are economy in construction and maintenance (wood requiring
less attention than iron), and a certain amount of natural flotation, which
reduces the pumping power required. This last contention is of doubtful
value: the bulk of a wooden ship largely discounts its natural flotation.
Fig. 464. —Off-shore Dock.
The restriction in beam-accommodation imposed upon a double-sided
dock led Messrs. Clark and Standfield, about the year 1877, to design the
depositing dock, in which one of the vertical sides is suppressed. This has
given rise to two varieties, according to the means adopted for maintaining
equilibrium. The term Depositing Dock (fig. 463) is applied to a dock
freely floating and balanced by an outrigger. A similar dock connected
with the shore by means of hinged arms attached to strong columns, is
known as the Off-shore Dock (fig. 464). The off-shore dock is niuch more