Transfer of vessels longitudinally or sideways
from railway dry dock cradles has been accomplished since early
in the century. Transfer of heavier ships has become popular in
the last 30 to 40 years because of the needs created by prefabricated
ship construction and assembly-line procedures and also the attractiveness
of multiple docking and winter storage possibilities.
The relatively low cost of transfer systems using one railway
dry dock as the basic lifting and launching facility makes transfer
very advantageous for large-scale developments. The cost varies
greatly depending on arrangement, number of berths, and selectivity
required, but in general one transfer berth including a transfer
cradle will cost from 10% to 30% of the cost of the railway itself.
Transfer of a ship on a horizontal plane applies the same principles
of rolling and hauling used in railway dry docks, but with the
difference that no component of gravity pull needs to be overcome
- only friction forces. Thus the effort needed is only about 20%
of that required to pull a ship up a 5% incline.
The transfer system for a ship consists of a series of ways
bearing on the ground or on pile foundations, which support one
or more cars or cradles carrying the ship. Suitable blocking is
provided between the ship and cradle to give headroom and clearance
for working on the hull and to distribute the vessel weight as
much as possible. The transfer cars are either fitted with wheels
or travel on a free-roller system to keep friction low. For vessels
up to about 400 tons, flanged wheels traveling on heavy crane
rails can be used, but beyond this capacity the free roller system
is preferable because of the heavy loads. Up to 1500 tons capacity,
the choice between side and longitudinal transfer is largely a
matter of convenience with respect to site conditions. For smaller
vessels, the longitudinal system has proved very economical, especially
if only a single spur track is used.
Large railways using heavy multiple-chain hauling machines are
not well adapted to end transfer because the transfer cradles
would have to pass over the machinery itself.
Also, at sites with little or no tide variation, track extensions
are necessary, which usually preclude the use of longitudinal
When full selectivity of storage spaces provided by two-directional
transfer is required, then the cost of end transfer is about equal
to the cost of side transfer.
With two-directional transfer, the second direction of movement
is accomplished at a lower level than the cradle deck level of
the initial transfer. This permits the car for the one direction
to be superposed on the car for other direction.
Side transfer can be provided for an existing cradle where the
ship's keel is on a declivity. This addition to an older railway
dry dock is generally limited to only one or two berths. It has
permitted shipyards with old World War II cradles to greatly increase
their capacity at very modest cost. It has also enabled yards
that formerly launched ships on greased ways to abandon the risky
and costly launch in favor of the controlled lowering, with the
added advantage of near horizontal construction.
Next: Hauling Machines