Page 174 - Ship Construction.DJ Eyres 6Ed

Ch16-H8070.fm Page 163 Wednesday, October 18, 2006 6:57 AM
Bottom Structure 163
Single Bottom Structure

In smaller ships having single bottoms the vertical plate open floors are
fitted at every frame space and are stiffened at their upper edge. A centre
line girder is fitted and one side girder is fitted each side of the centre line
where the beam is less than 10 m. Where the beam is between 10 and 17 m
two side girders are fitted and if any bottom shell panel has a width to
Double Bottom Structureศึกษาเท่านั้น---
length ratio greater than four additional continuous or intercostal stiffeners
are fitted. The continuous centre and intercostal side girders are stiffened
at their upper edge and extend as far forward and aft as possible.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
The single bottom structure is shown in Figure 16.2 and for clarity a
3-dimensional representation of the structure is also provided to illustrate
those members which are continuous or intercostal. Both single and double
bottoms have continuous and intercostal material and there is often some
confusion in the student’s mind as to what is implied by these terms.
A wood ceiling may be fitted across the top of the floors if cargoes are
to be carried but this does not constitute an inner bottom offering any
protection if the outer bottom shell is damaged.

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An inner bottom (or tank top) may be provided at a minimum height above
the bottom shell, and maintained watertight to the bilges. This provides a
considerable margin of safety, since in the event of bottom shell damage
only the double bottom space may be flooded. The space is not wasted
but utilized to carry oil fuel and fresh water required for the ship, as well as
providing ballast capacity.
The minimum depth of the double bottom in a ship will depend on the
classification society’s requirement for the depth of centre girder. It may
be deeper to give the required capacities of oil fuel, fresh water, and
water ballast to be carried in the bottom. Water ballast bottom tanks are
commonly provided right forward and aft for trimming purposes and if
necessary the depth of the double bottom may be increased in these
regions. In way of the machinery spaces the double bottom depth is also
increased to provide appreciable capacities of lubricating oil and fuel oil.
The increase in height of the inner bottom is always by a gradual taper in
the longitudinal direction, no sudden discontinuities in the structure
being tolerated.
Double bottoms may be framed longitudinally or transversely (see Figure
16.3), but where the ship’s length exceeds 120 m it is considered desirable to
adopt longitudinal framing. The explanation of this is that on longer ships
tests and experience have shown that there is a tendency for the inner bottom
and bottom shell to buckle if welded transverse framing is adopted. This