Page 141 - Introduction to Naval Architecture

STRENGTH I2/

Figure 7.1 Loads on ship section

produce a local deformation of structure. The hydrostatic loads tend to
dish plating between the supporting frames and longitudinals. The
deck grillages must support the loads of equipment and cargo.
Thus there are three contributions made by items of structure,
namely to the longitudinal, transverse and local strength. Longitudinal
strength in a seaway is considered first.

FORGES ON A SHIP IN A SEAWAY
The mass distribution is the same in waves as in still water assuming the
same loading condition. The differences in the forces acting are the
buoyancy forces and the inertia forces on the masses arising from the
motion accelerations, mainly those due to pitch and heave. For the
present the latter are ignored and the problem is treated as a quasi-
static one by considering the ship balanced on a wave. The buoyancy
forces vary from those in still water by virtue of the different draughts
at each point along the length due to the wave profile and the pressure
changes with depth due to the orbital motion of the wave particles. This
latter, the Smith effect referred to in Chapter 5, is usually ignored these
days in the standard calculation to be described next. Ignoring the
dynamic forces and the Smith effect does not matter as the results are
used for comparison with figures from previous, successful, ships
calculated in the same way.
The standard calculation is one that was used for many years.
Nowadays the naval architect can extend the programs for predicting
ship motion to give the forces acting on the ship. Such calculations
have been compared with data from model experiments and full scale
trials and found to correlate quite well.