Page 159 - Introduction to Naval Architecture
P. 159
STRENGTH 145
The second moment about the new NA is:
2
Stress in deck as aluminium = 0.322 X 71.15 = 22.91 MN/m
The superstructure efficiency relates to the effect of the super-
structure on the stress in the upper deck of the main hull. The
new stress in that deck, with the superstructure in place, is given
2
as 55 MN/m . If the superstructure had been fully effective it
would have been:
With no superstructure
the stress was
Hence the superstructure
efficiency
Stresses associated with the standard calculation
The arbitrary nature of the standard strength calculation has already
been discussed. Any stresses derived from it can have no meaning in
absolute terms. That is they are not the stresses one would expect to
measure on a ship at sea. Over the years, by comparison with previously
successful designs, certain values of the derived stresses have been
established as acceptable. Because the comparison is made with other
ships, the stress levels are often expressed in terms of the ship's
principal dimensions.Two formulae which although superficially quite
different yield similar stresses are:
