440

NICHOLAS'S SEAMANSHIP

NAUTICAL KNOWLEDGE

as compared with a single bottomed ship and relieving the owner of
certain charges which are levierl on the registered cubic capacity of
the ship.

Fig. 21.—Cellular Double Bottom.
The main feature of &C.D.B are indicated in the figure.
A is a continuous centre line longitudinal girder, a^ and a2 being
continuous double angles connecting its upper and lower edges to the
inner bottom plating and to the flat plate keel respectively.
B is an intercostal side girder, its ends being connected to the
floors by vertical angles &3, and its top and bottom edges to the inner
and outer bottom plating by the intercostal angles \ and b2 respectively.
The floor plate F is continuous from the centre girder to the margin
plate, having a stiffening angle /x on its top edge, which also provides
the rivet connection to the inner bottom plating D; and the stiffening
angle /2, which is shown joggled in our drawing, on its lower edge, to
provide a rivet connection to the outer bottom plating. A short
vertical angle/3 connects the inner edge of the floor to the centre girder,
and f± connects its outer edge to the margin plate C.
G is the margin plate forming the side boundary of the inner bottom;
its top edge is flanged and riveted to the inner bottom plating. This
bottom part of the vessel is constructed first and forms a perfectly
watertight unit. The frame legs are placed in position as the work
progresses. The frames, in the figure, appear to be of bulb angle
section which forms a rib just as efficient as the frame and reversed
frame section; it saves the labour and expense of having to bend the
reversed frame to exactly the same curvature as the frame and then
riveting the two together; it is easier to clean and paint, thus keeping
down corrosion, and the bulb stiffens the edge of the frame just as
effectively as a reversed frame.
The tank side bracket H is riveted to the frame as indicated by the
dotted line; the top edge of the bracket plate is stiffened by a flange
and connected to the side or the margin plate by means of the vertical
angle h. C is a continuous angle along its lower edge to connect ifc to