Page 342 - Practical Ship Design

Structural Design 299

10.5.3 Spacing of bulkheads

In bulk carriers and tankers the spacing of main watertight bulkheads as well as
meeting any subdivision requirement should, if possible, be so arranged that the
lengths of holds or tanks are such that they are multiples of the web frame spacing,
thereby minimising the number of web frames..

10.5.4 Hatch arrangement
On ships with large cargo hatches, only the deck plating and associated longi-
tudinals outside the hatch coamings contribute to the upper flange of the ship
girder. Purely from the point of view of efficient structure and economy in steel-
weight, hatches should not therefore be any wider than is necessary for efficient
cargo handling - although whether a cargo handling enthusiast would be willing
to settle for anything less than a 100% spotting ability is open to question.
Recent bulk carrier casualties suggest that more importance should be attached
to the structural strength of hatch covers and their ability to maintain watertight
integrity in extreme conditions, than has been the case in the past. This applies
particularly to the foremost hatches.

10.5.5 Alignment of structure
Other deck openings such as stairwells and access hatches should, if possible, be
kept inside the line of the main hatch or engine casing openings and their longest
dimension should, if possible, be in the fore and aft direction. If there are a series of
such openings as on a passenger ship or a warship, these should be lined up so that
a minimum number of longitudinals have to be cut.

10.5.6 Use of higher tensile steel
The use of higher tensile steel is - certainly on larger ships -one of the best ways
of reducing weight and although the material cost per tonne is higher, the reduced
tonnage usually means the total material cost is reduced and in some cases there
can also be a reduction in labour cost. Table 10.1 shows the respective strengths of
mild steel and the various higher tensile steels.
When higher tensile steel is used, Lloyds hull girder section modulus can be
multiplied by a factor k,, where

k, = 24510 or 0.72, whichever is greater.
For local scantling requirements of plating, stiffeners etc the corresponding
multiplier is:

kI- = 23510