Page 379 - Practical Ship Design

336 Chapter 12

Righting
and
heeling
levels

0 40
Angle of heel

Volumetric heeling moment
Fig. 12.1. Grain stability. L, = ; Ld0 =0.8~4,
Displacement x stowage factor

12.2.3 Basis of grain shijit calculations

Calculations are based on the following assumptions.
(i) That in filled compartments there will be a void under all boundary surfaces
having an inclination of less than 30" to the horizontal. A formula for the
depth of void is given, but in design practice the lesson is that the bottom of
top-side wing tanks should be inclined at an angle of at least 30".
(ii) That within filled and trimmed hatchways the void is to be taken as 150
mm below the lowest point of the hatch or the top of the hatch side
coaming whichever is lower. The volume of any open void within the
hatch cover to be added. There are other rules which apply if the hatchway
is not trimmed.
(iii) The grain shift moment is to be calculated based on the grain surface being
at 15" to the horizontal in the void spaces of filled compartments and at 25"
to the horizontal in partly filled spaces. The calculated transverse heeling
moment is to be multiplied by 1.12 to allow for the effect of the accompany-
ing vertical shift.
(iv) If necessary the partially filled compartment can be overstowed with bagged
grain to eliminate the free surface. If this is done the bagged grain must be
tightly stowed and extend to a height of 1/16 of the breadth of the grain free
surface or 1.2 m, whichever is greater.

It musl be emphasised that this is only an outline of the rules. The detailed rules
have several other requirements.