b.. PORT ECONOMICS
to the usual radial movement. This luffing movement
may be carried out separately, or simultaneously with the
radial movement, and, as under ordinary conditions, the
inward travel of the jib would entail the useless raising
of the load at the end of the hook, with a consequent
waste of power, a special type of level luffing crane has
been devised and is in general use, whereby compensating
gearing lowers the lifting rope to an equal extent with the
raising of the jib head, the result being that the load after
it has been raised above the hatchway coaming travels
along a horizontal path to the point at which it is to be
lowered for deposit. There are various systems of level
luffing gear on the market, some of them of a very ingenious
nature, into which, however, we have not space to enter
here.
Quay cranes have usually a load capacity of from 1 to
3 tons. Some special cranes of higher capacity are also
to be found at ports, generally fixed instead of movable.
As already mentioned, there are other appliances for
cargo handling besides the quay crane (and the ship's
booms), but we shall not notice them in detail, as they are
more generally furnished by private enterprise, and, in this
chapter, we are considering chiefly the obligations of a port
authority.
As a means of dealing with some exceptionally heavy
lifts of an occasional character, such as a ship’s tail shaft,
locomotives, ordnance and the like, cranes of high lifting
power are provided by port authorities, generally afloat,
mounted on pontoons, so as to be mobile and to be able to
function on the off-side of a vessel without interfering with
quayside operations. The powers of such cranes run up
to very high figures. Floating cranes of 50 or 60 tons
lifting capacity are common. One hundred ton cranes are
not unusual. The maximum power is about 250 tons.
At the other end of the scale, it is found a convenient
arrangement at some French ports—notably Rouen—to
employ a number of floating cranes of 2 to 3 tons capacity
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