Question
Describe the
arrangement of tank top and double bottom in the machinery space making
particular reference to the structure and scantlings below the main engine.
Show the method adopted in the arrangement of D.B. tanks to avoid contamination
of fresh water, fuel oil and lube oil stored in D.B. tanks.
Answer
Double bottom in the
machinery space
In the machinery space other
factors must be taken into account. Forces of pulsating nature are transmitted
through the structure due to the general out of balance forces of the machinery
parts.
The machinery seats must be
extremely well supported to prevent any movement of the machinery.
Additional girders are
fitted in the double bottom and the thickness of the tank top increased under
the engine in an attempt to reduce the possibility of movement which could
cause severe vibration in the ship. For similar reasons the shaft and propeller
must be well supported.
Great care must be taken in
the machinery space to ensure that the main and auxiliary machinery are
efficiently supported. Weak supports may cause damage to the machinery, while
large unsupported panels of plating may lead to vibration of the structure.
The main engine bedplate is
bolted through a tank top plate which is about 40 mm thick and is continuous to
the thrust block seating.
A girder is fitted on each
side of the bedplate in such a way that the holding down bolts pass through the
top angle of the girder.
In welded ships a horizontal
flat is sometimes fitted to the top of the girder in way of the holding-down
bolts.
In motor ships where a drain
tank is required under the machinery, a cofferdam is fitted giving access to
the holding down bolts and isolating the drain from the remainder of the double
bottom tanks.
Additional longitudinal
girders are fitted in way of heavy auxiliary machinery such as generators.
Access to these D\B tanks
and cofferdam is provided by watertight manhole door.
Docking plugs are fitted in
all D\B yanks for complete draining in drydock for
purpose of inspection
Air pipes and sounding pipes
are also fitted to D\B tank and these tanks are tested by maximum service pressure head