Page 377 - Practical Ship Design

334 Chapter 12

in The Merchant Shipping (Load Line) Rules 1968. The standards apply to all
conditions of loading. The standards are:
(a) The area under the curve of righting levers (GZcurve) shall not be less than:
(i) 0.055 metre-radians (mrad) up to an angle of 30";
(ii) 0.09 mrad up to an angle of 40" or the angle at which the lower edges
of any openings in the hull, superstructure or deckhouses, being
openings which cannot be closed weathertight, become immersed if
that angle is less.
(iii) 0.03 mrad between an angle of heel of 30" and one of 40" or such
lesser angle as referred to in (ii)
(b) The righting lever (GZ) shall be at least 0.20 m at an angle of heel equal to or
greater than 30" (in many cases a reduction in this angle can be accepted
subject to the areas under the curve being increased)
(c) The maximum righting lever (GZ) shall occur at an angle of heel not less
than 30"
(d) The initial transverse metacentric height shall not be less than 0.15 m. In the
case of a ship carrying a timber deck cargo which complies with sub-
paragraph (a) by taking into account the volume of the timber deck cargo
the initial transverse metacentric height shall be not less than 0.05 m.

12.1.3 Znfluence of these standards on design
These standards are easy to meet in almost all new designs with difficulty occurring
only when operational reasons impose unusual dimensional constraints.
The large angle requirements of (a), (b) and (c) can generally be assured by
keeping the ratios B/D and T/D within the ranges suggested in Figs. 3.9 and 3.10.
The initial stability requirement of (d) should also follow unless there is an unusual
amount of top weight. Notwithstanding this, the stability should be checked in
detail at as early as possible a stage in the design and should be rechecked when the
design nears completion and while any changes that such a check shows to be
necessary can still be made without undue expense.
Because the standards are not difficult to meet, it is wise to exceed them quite
comfortably, at least in the initial design phase and the author would much prefer
to see all ships having a GM in the worst service condition of 0.40 m.

12.1.4 Thixotropic cargoes
In 512.1.1 (ii) it was noted that higher intact stability standards are required for
certain special cargoes. Two of these types of cargo which require special stability
investigations are grain and dredge spoil. The reason is the same in both cases and is
the fact that these cargoes are thixotropic which means that they are liable to move
when the ship rolls or heels and act to a greater or lesser degree as a free surface.