: PORT ECONOMICS

work, whereas a floating dock of similar capacity could
probably be constructed within nine or ten months.
Another point of convenience is that a floating dock can be
constructed in any suitable locality, where there is a ship-
building yard, and transported to its ultimate destination
with simply the additional expense for towage and insur-
ance. Many floating docks for the colonies have been
constructed in home shipyards.

The advantage reverts to the graving dock in regard to
maintenance and upkeep. A stone, or concrete, graving
dock is practically indestructible, whereas the life of a
floating dock, like that of other iron or steel structures,
constantly afloat, is necessarily limited. The length of its
life will vary with situation and circumstances, some
waters being more destructive than others. The average
life may fairly be reckoned at thirty to forty years, though
there is no reason why, with regular re-painting and reason-
able care and attention, this period should not be consider-
ably exceeded. The longevity of a graving dock has the
attendant drawback that the dock becomes obsolete in
time, or rather, by reason of developments in ship con-
struction, incapable of receiving the largest class of vessel in
being, but there still remains a field of usefulness for it in
connection with the less important class of vessels to which
it is appropriate. The enlargement of a graving dock to
keep pace with the growth in size of ships is rarely
practicable. In most cases, the expense is prohibitive.

As regards working cost, the principal factor is pumping,
which, in the case of a floating dock, is very much less
than that in the case of a graving dock—it may be as much
as three or four times less. For a floating dock, only so
much work is required to be done as is necessary to raise
the vessel and the deck of the pontoon above water level.
The graving dock chamber, on the other hand, has always

to be completely emptied, no matter what may be the size
of the vessel occupying it, and the smaller the vessel,
the greater the quantity of water to be pumped. This

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