2.3 Severe wind and rolling criterion (weather criterion)
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2.3 Severe wind and rolling criterion (weather criterion)

  2.3.1 The ability of a ship to withstand the combined effects of beam wind and rolling shall be demonstrated, with reference to figure 2.3.1 as follows:

  • .1 the ship is subjected to a steady wind pressure acting perpendicular to the ship's centreline which results in a steady wind heeling lever (l w1 );

  • .2 from the resultant angle of equilibrium (), the ship is assumed to roll owing to wave action to an angle of roll () to windward. The angle of heel under action of steady wind () should not exceed 16° or 80% of the angle of deck edge immersion, whichever is less;

  • .3 the ship is then subjected to a gust wind pressure which results in a gust wind heeling lever (lw2 ); and

  • .4 under these circumstances, area b shall be equal to or greater than area a, as indicated in figure 2.3.1 below:

 where the angles in figure 2.3.1 are defined as follows

= angle of heel under action of steady wind
= angle of roll to windward due to wave action (see 2.3.1.2, 2.3.4 and footnote)
= angle of down-flooding (ϕf) or 50° or ϕc, whichever is less,
where:
= angle of heel at which openings in the hull, superstructures or deckhouses which cannot be closed weathertight immerse. In applying this criterion, small openings through which progressive flooding cannot take place need not be considered as open
= angle of second intercept between wind heeling lever lw2 and GZ curves.

  2.3.2 The wind heeling levers lw1 and lw2 referred to in 2.3.1.1 and 2.3.1.3 are constant values at all angles of inclination and shall be calculated as follows:

 where:

P = wind pressure of 504 Pa. The value of P used for ships in restricted service may be reduced subject to the approval of the Administration
A = projected lateral area of the portion of the ship and deck cargo above the waterline (m2)
Z = vertical distance from the centre of A to the centre of the underwater lateral area or approximately to a point at one half the mean draught (m)
Δ = displacement (t)
g = gravitational acceleration of 9.81 m/s2.

  2.3.3 Alternative means for determining the wind heeling lever (lw1 ) may be accepted, to the satisfaction of the Administration, as an equivalent to calculation in 2.3.2. When such alternative tests are carried out, reference shall be made based on the Guidelines developed by the Organizationfootnote. The wind velocity used in the tests shall be 26 m/s in full scale with uniform velocity profile. The value of wind velocity used for ships in restricted services may be reduced to the satisfaction of the Administration.

  2.3.4 The angle of roll ()footnotereferred to in 2.3.1.2 shall be calculated as follows:

 where:

  • X1 = factor as shown in table 2.3.4-1

  • X2 = factor as shown in table 2.3.4-2

  • k = factor as follows:

    • k = 1.0 for round-bilged ship having no bilge or bar keels

    • k = 0.7 for a ship having sharp bilges

    • k = as shown in table 2.3.4-3 for a ship having bilge keels, a bar keel or both

  • r = 0.73 + 0.6 OG/d

    • with:

  • d = mean moulded draught of the ship (m)

  • s = factor as shown in table 2.3.4-4, where T is the ship roll natural period. In absence of sufficient information, the following approximate formula can be used:

where:

  • The symbols in tables 2.3.4-1, 2.3.4-2, 2.3.4-3 and 2.3.4-4 and the formula for the rolling period are defined as follows:

  • Lwl = length of the ship at waterline (m)

  • B = moulded breadth of the ship (m)

  • d = mean moulded draught of the ship (m)

  • CB = block coefficient (-)

  • Ak = total overall area of bilge keels, or area of the lateral projection of the bar keel, or sum of these areas (mē)

  • GM = metacentric height corrected for free surface effect (m).

Table 2.3.4-1 Values of factor X1

B/d X 1
≤ 2.4 1.0
2.5 0.98
2.6 0.96
2.7 0.95
2.8 0.93
2.9 0.91
3.0 0.90
3.1 0.88
3.2 0.86
3.4 0.82
≥ 3.5 0.80

Table 2.3.4-2 Values of factor X2

CB X 2
≤ 0.45 0.75
0.50 0.82
0.55 0.89
0.60 0.95
0.65 0.97
≥ 0.70 1.00

Table 2.3.4-3 Values of factor k

 k
0 1.0
1.0 0.98
1.5 0.95
2.0 0.88
2.5 0.79
3.0 0.74
3.5 0.72
≥ 4.0 0.70

Table 2.3.4-4 values of factor s

T s
≤ 6 0.100
7 0.098
8 0.093
12 0.065
14 0.053
16 0.044
18 0.038
≥ 20 0.035

 (Intermediate values in these tables shall be obtained by linear interpolation)

  2.3.5 The tables and formulae described in 2.3.4 are based on data from ships having:

  • .1 B/d smaller than 3.5;

  • .2 (KG/d-1) between - 0.3 and 0.5; and

  • .3 T smaller than 20 s.

 For ships with parameters outside of the above limits the angle of roll (ϕ1) may be determined with model experiments of a subject ship with the procedure described in MSC.1/Circ.1200 as the alternative. In addition, the Administration may accept such alternative determinations for any ship, if deemed appropriate.

Parent topic: Chapter 2 – General Criteria
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