Page 172 - Ship Construction.DJ Eyres 6Ed

Ch16-H8070.fm Page 161 Wednesday, October 18, 2006 6:57 AM

16
Bottom Structure

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Originally ships were constructed with single bottoms, liquid fuels and fresh
water being contained within separately constructed tanks. The double
bottom structure which provides increased safety in the event of bottom
shell damage, and also provides liquid tank space low down in the ship,
only evolved during the early part of the twentieth century. Today only small
vessels such as tugs, ferries, and cargo ships of less than 500 gross tonnage
have a single bottom construction. Larger ocean – going vessels are fitted
with some form of double bottom.

ึ
Keels

At the centre line of the bottom structure is located the keel, which is often
said to form the backbone of the ship. This contributes substantially to the
longitudinal strength and effectively distributes local loading caused when
docking the ship. The commonest form of keel is that known as the ‘flat
plate’ keel, and this is fitted in the majority of ocean-going and other vessels
(see Figure 16.1(a)). A form of keel found on smaller vessels is the bar keel
(Figure 16.1(b)). The bar keel may be fitted in trawlers, tugs, etc., and is
also found in smaller ferries.
Where grounding is possible this type of keel is suitable with its massive
scantlings, but there is always a problem of the increased draft with no
additional cargo capacity. If a double bottom is fitted the keel is almost
inevitably of the flat plate type, and bar keels are more often associated
with open floors, but a flat plate keel can be fitted in way of open floors.
Duct keels (Figure 16.1(c)) are provided in the double bottoms of some
vessels. These run from the forward engine room bulkhead to the collision
bulkhead and are utilized to carry the double bottom piping. The piping is
then accessible when cargo is loaded, an entrance to the duct being
provided at the forward end of the engine room. No duct is required aft of
the engine room as the piping may be carried in the shaft tunnel. A width of
not more than 2.0 m is allowed for the duct, and strengthening is provided
at the tank top and keel plate to maintain continuity of strength of the
transverse floors.