Steels_ Metallurgy and Applications, Third Edition

172 Steels: Metallurgy and Applications

together, give the structure the required stiffness or section modulus. In general,

the plate thicknesses are related to the length of a ship, assuming that the depth

and breadth conform to a reasonably fixed ratio of its length. For example, the

ratio of length to depth is not expected to exceed 16:1 and the length to breadth

ratio is generally greater than 5:1.

Naval architects have to legislate for the worst situation that a ship is likely to

encounter due to wave action, namely a wave with a length equal to that of the

ship. As illustrated in Figure 2.17, this can give rise to two extreme conditions

of stress:

1. Suspension at the middle position putting the deck in tension (hogging).


2. A wave at either end putting the bottom in tension (sagging).

Thus the area of the ship that is given greatest attention in design is the middle
section or midships, since this is the area which is subjected to greatest stress and
deflection. For this reason, the plate thicknesses in this region, designated 0.4L
amidships, are heavier than those required towards the ends of the ship. Between
the two extremes, classification societies quote a taper in terms of percentage
decrease in thickness per metre so as to avoid abrupt changes in section. The
longitudinal strength of an I-beam is located in the flanges and the thickness of the
deck and bottom plating are greater than those in the sides of a ship. However, the
longitudinal strength is not derived solely from the deck and bottom plating and

(a)

(b)

0.4L

= Amidships

(c)

L -

Figure 2.17 Schematic illustration - hull deflection