4:28 NICHOLLS'S BJSAMAtfSHIP AMD NAUTICAL KNOWLEDGE

with a neutral area D E between; and if the hull of the ship were not
made strong enough to resist these stresses she would bend downwards
at the middle of her length This is called "sagging/*

When the ship is supported amidships on the crest of a single wave
as at A (Figure 6), with the ends B and C unsupported, the stresses are
changed to tension on the upper edge and compression on the lower
edge with a neutral area D E between, and if the hull were not made
strong enough to resist these stresses she would bend upwards and suffer
excessive stresses at the middle of her length. This is called "hogging "
These longitudinal stresses occur alternately when a ship is among
waves and the ship girder has to be specially strengthened and stiffened
along the length of her strength deck by means of "sheer" strake,
"stringers,*5 etc., and along the lower edge by means of centre keel, side
"keelsons" and "longitudinals" in order to make the hull strong
enough to withstand all normal hogging and sagging stresses.

Fig. 6.—Hogging.
Transverse Stresses.—The ship is also subjected to transverse racking
stresses when rolling in a seaway, the tendency of which is to cause
distortion at the corners of the box-shaped girder. A simple transverse
section consists of a "frame," or rib, extending the whole girth of the
ship, the top ends being held firmly in position by means of a transverse
beam. The beam is connected to the frame by means of a bracket called
a beam "knee," which is made triangular in shape and of sufficient
depth and breadth to get a good rivet connection to the beam and
to the frame.
The strain of racking is likely to be more evident at the top corners
than at the bottom part of a transverse section as the upper works are
relatively weaker owing to deck; openings and other sources of weakness
in the higher parts of the structure, whereas the "floor" and "bilge"
brackets make the bottom part of the section more solid and rigid.
Figure 7 shows excessive distortion. The dotted lines represent
the original shape of the vessel. The frames, pillars and bottom