14.6 Intact Stability Monohulls

 (1) Curves of statical stability (GZ curves) for at least the Loaded Departure with 100% consumables and the Loaded Arrival with 10% consumables should be produced.

(2) The GZ curves required by (1) should have a positive range of not less than 90º, where the ‘Sail Area Displacement Ratio’ is greater than 10 calculated as follows:

• ▽ = Vessel displacement in meters cubed (m³)

• A_sails = is the area of the full upwind sail plan, including sail overlaps in square meters (m)²

(3) For vessels where the ‘Sail Area Displacement Ratio’ is less than 10 calculated as per (2), Where a range

of less than 90º exists, the wind speed required to capsize should be calculated to be more than 38 knots as follows:

The heel angle resulting from a steady wind heeling moment corresponds to the intersection of the righting and heeling arm curves, so the heeling arm at the point

of capsize is defined where the heeling arm curve is tangential to the GZ curve.

The heeling arm curve is defined by the formula:

HA_Ɵ = HA₀(cosƟ)^1.3

Where

• HA_Ɵ = Heeling arm at any given angle Ɵ

• HA₀ = Heeling arm at 0° where heeling arm curve is tangential to the GZ curve

V is calculated by the formula:

Where

• V = Apparent wind speed in knots

• v = Apparent wind speed in meters per second (m/s)

• ρ = Density of Air (assumed to be 1.22) 

• Δ = Vessel displacement in kilograms (kg)

• A_sails = is the area of the full upwind sail plan, including sail overlaps in square meters (m²)

• h_sails = is the height of the centroid of the sail plan above half the draft in meters (m)

• C_sails = is the maximum sail heeling force coefficient, assumed to be 1.75 (unless proven otherwise)

• A_hull = is the profile area of the hull and superstructure in square meters (m²)

• h_hull = is the height of the centroid of the hull and superstructure area above half the draft in meters (m)

• C_hull = is the hull heeling force coefficient, assumed to be 1.0 (unless proven otherwise)

(4) In addition to the requirements of (2) or (3), the angle of steady heel should be greater than 15 degrees (see figure). The angle of steady heel is obtained from the intersection of a "derived wind heeling lever" curve with the GZ curve required by (1).

• In the figure:-

• 'dwhl' = the "derived wind heeling lever" at any angle θ°

  • = 0.5 x WLO x Cos¹³θ

• where

Noting That:

• WLO= is the magnitude of the actual wind heeling lever at 0º which would cause the vessel to heel to the 'down flooding angle' θ_f or 60º whichever is least.

• GZ_f = is the lever of the vessel's GZ at the down flooding angle (θ_f) or 60º whichever is least.

• θ_d = is the angle at which the 'derived wind heeling' curve intersects the GZ curve. (If θ_d is less than 15º the vessel will be considered as having insufficient stability for the purpose of the Code).

• θ_f = the 'down-flooding angle' is the angle of heel causing immersion of the lower edge of openings having an aggregate area, in square meters, greater than:-

• = where Δ = vessels displacement in tonnes

All regularly used openings for access and for ventilation should be considered when determining the downflooding angle. No opening regardless of size which may lead to progressive flooding should be immersed at an angle of heel of less than 40°. Air pipes to tanks can, however, be disregarded.

If, as a result of immersion of openings in a superstructure, a vessel cannot meet the required standard, those superstructure openings may be ignored and the openings in the weather deck used instead to determine θf. In such cases the GZ curve should be derived without the benefit of the buoyancy of the superstructure.

It might be noted that provided the vessel complies with the requirements of (1) to (4) and is sailed with an angle of heel which is no greater than the’ derived angle of heel', it should be capable of withstanding a wind gust equal to 1.4 times the actual wind velocity (i.e. twice the actual wind pressure) without immersing the’ down-flooding openings', or heeling to an angle greater than 60°.