Page 142 - Introduction to Naval Architecture

128 STRENGTH

THE STANDARD STATIC LONGITUDINAL STRENGTH
APPROACH

The ship is assumed to be poised, in a state of equilibrium, on a
trochoidal wave of length equal to that of the ship. Clearly this is a
situation that can never occur in practice but the results can be used to
indicate the maximum bending moments the ship is likely to
experience in waves. The choice of wave height is important. To a first
order it can be assumed that bending moments will be proportional to
wave height. Two heights have been commonly used L/20 and
05
0.607(£) where L is in metres. In recent years the latter has been
more generally used because it was felt to represent more closely the
wave proportions likely to be met in deep oceans. Steeper waves have
been used for smaller vessels operating in areas such as the North Sea.
It is a matter for the naval architect to decide in the light of the
intended service areas of the ship.

Figure 7,2 Ship on wave

Two conditions are considered, one with a wave crest amidships and
the other with wave crests at the ends of the ship. In the former the ship
will hog and in the latter it will sag. In some cases the hogging and
sagging was exaggerated by modifying the mass distribution. There was
little point as the calculation was a comparative one only and the
resulting condition was an artificial one. It has not been done for some
years. By moving the ship to various positions in relation to the wave
crest the cycle of bending moment experienced by the ship can be
computed.
The bending moments obtained include the still water moments. It
is useful to separate the two as, whilst the still water bending moment
depends upon the mass distribution besides the buoyancy distribution,
the bending moment due to the waves themselves depends only on the
geometry of the ship and wave.