Page 306 - Ship Construction.DJ Eyres 6Ed

Ch25-H8070.fm Page 295 Wednesday, October 18, 2006 7:00 AM
Cargo Access, Handling and Restraint 295
portable deck and common in ro-ro ferries. Hoistable decks are lowered
from and stowed at the deckhead by hoist wires led through a hydraulic
jigger winch. Folding decks stow at the sides and ends of ship spaces and
are generally hydraulically lowered into the horizontal position. Lloyd’s
Register include requirements for movable decks in their Rules and if the
ship is fitted with portable decks complying with these rules at the owners
or builders request the class notation ‘movable decks’ may be assigned.
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Scissors Lift
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Cargo can be lowered or raised between decks or to the hold by means of a
scissors lift which is often fitted in ro-ro ships as an alternative to internal
ramps, it taking up less room. The hydraulic cylinder powered scissors lift is
also often designed to transfer heavy unit loads.

Cargo Restraint

In ro-ro and container ships the lashing of cargo is an important safety
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consideration and usually calls for fittings which will permit rapid and easy
but effective securing of the cargo because of short ship turn around times.
The shipbuilder is responsible for the deck and perhaps hatch fittings for
the securing devices and will look to the ship operator for guidance on their
type and positions. On the decks of ro-ro ships where the direction of lash-
ing is unpredictable and vehicles must transverse the fitting a cloverleaf
deck socket in conjunction with an elephants foot type of end lashing is
popular (see Figure 25.5).
Containers have very little strength in any direction other than vertically
through the corner posts thus it is necessary to provide substantial support
to the containers when they are on the ship. Stowage of containers is with
their longer dimension fore and aft since the ship motion transmitted to
cargo is greater in rolling than pitching and it is therefore prudent to limit
any possible cargo movement within the container to the shorter transverse
dimension. Also of course when off loading the fore and aft container is
more easily received by road or rail transport. Below decks containers are
restrained in vertical cell guides which are typically 150 × 150 × 12 angles
and they are structurally supported so that any dynamic forces other than
purely vertical are transmitted as much as possible through the ships struc-
ture and not into the containers. The cell guides are not to form an integral
part of the ships structure, they are to be so designed that they do not carry
the main hull stresses. Where four container corners are adjacent the cell
guides may be built into a composite pillar (see Figure 25.5). The clearance
between container and cell guide is critical. If it is too small the container