The railway dry dock differs from both the floating dry dock and the basin
or graving dry dock in that it is a mechanical means of hoisting
a ship out of water to an elevation above the highest tides, where
it rests on a structure fixed to the ground. The vertical lift
is similar except that its platform is not supported directly
by the ground.
The railway has a cradle that is lowered into the water along
an inclined track until the vessel to be drydocked can be floated
over it. The cradle is then hauled up the track with the vessel,
which grounds out on suitable blocks, until the cradle deck is
clear of the water. The cradle is designed to support the docked
vessel and to distribute its weight to the track.
The track must be constructed with a smooth gradient and supported
by an adequate foundation. The track and foundation provide the
basic support of the cradle at all times. If the ship exerts heavy
load concentrations, these are transmitted directly to the track
and foundation by the cradle. Therefore the foundation, track,
roller or wheel system, and cradle structure must be designed
to take the highest expected concentration of load. In a railway
dry dock, the line of the keel blocks can be different from the
line of the track so as to conform to the more common trim of
vessels. Thus, the slope of the track may be gentle or steep to
suit local conditions, yet the vessels are lifted on practically
an even keel.
The declivity of the track may vary from one in two to one in
thirty, depending on site conditions, available space, river or
tidal currents, variations in water level, and desired lifting
A major improvement was the development of track constructed
on a vertical circular curve so as to cause the cradle to rotate
as it travels from outshore to inshore. This rotation makes it
possible for the keel blocks and cradle deck to be on a declivity
when submerged and horizontal in the up position, for ease of
side or end transfer.
In a few instances, all of the characteristics and restrictions
dovetail naturally. The railways at Boulogne-sur-Mer, France,
have curved tracks which are steep at the outshore end because
they have limited longitudinal space and limited water depth at
high tide; an important fraction of the vessels of interest there
draw much more water aft than forward.
It is very important when lifting a ship out of water that the
top of the keel blocks be close to parallel to the ship's keel
when contact is made; otherwise tipping forces develop which,
in severe cases, could make the ship unstable.
The sequence of events in drydocking is that first the vessel
is firmly grounded on keel blocks; then while it is still largely
waterborne, the bilge supports are placed against the hull, after
which the cradle is pulled inshore, making all of the vessel accessible.
Hauling up the inclined track is accomplished by means of powerful
machinery and chains, which pull the cradle with its superimposed
vessel on a system of free rollers or wheels. The speed and economy
of this operation represent an advantage of railway dry docks
over other systems.
The hauling machine and chains are sensitive only to the total
live and dead load and to friction associated with the rollers
or wheels. When more than one chain is used, the chain loading
must be equalized to assure that no single chain is overstressed
because of variation in chain pitch.
Railway dry dock cradles are of three types of construction:
all wood, composite, and steel. In the composite type the cradle
superstructure is steel, with the transverse beams, deck, docking
platforms, and blocks of wood; in the steel cradle, only the deck
and blocks are wood. Most new cradles are steel. Of whichever
type, the cradle is a structure of unique design in that it must
have strength and stability and yet at the same time by flexible
in longitudinal bending and in torsion to accommodate the irregularities
of the track and the ship.
Tracks may be of the two-, three-, or four-way type. Usually
the portion above water is concrete; the submerged portion used
to be timber, effectively protected against marine borers as necessary,
but nowadays, for cost reasons, steel tracks are more and more
If vessels are to be repaired with dispatch and efficiency,
there must be free access for light and air, so that workmen may
properly execute their tasks. A railway dry dock places a vessel
above the level of the yard, and since it is open on all sides,
there is free circulation of air and good illumination. The deck
is substantially at yard level independent of tide, and materials
and staging can be handled conveniently from the adjacent yard.
Equipment and material can be moved easily on the cradle when
there is a flush deck, made possible by providing recessed metal
troughs for the bilge block chains to run in (not desirable in
regions of heavy snow and freezing if the dock must be operated
Workmen can step directly from the dock to the yard, and all
work can be readily supervised. Under such conditions the most
efficient work is possible.
Control of fixed charges and operating costs, and efficiency
in repairing vessels are vital elements in successful shipyard
operation. In the most favorable size range, the cost of a railway
dry dock is ordinarily about one-half or less than that of a floating
dry dock, and an even smaller fraction of that of a basin dock.
The economic life of a railway dry dock is about the same as that
of a floating dock constructed of similar materials.
In the absence of marked inflation, an annual reserve of 4 percent
is usually adequate to meet ordinary maintenance, periodic repair,
and ample sinking fund requirements.
Next: Side haul railway dry