2.6 Buoyancy and stability in the displacement mode following damage

  2.6.1 The requirements of this section apply to all permitted conditions of loading.

  2.6.2 For the purpose of making damage stability calculations, the volume and surface permeabilities shall be, in general, as follows:

  2.6.3 Notwithstanding 2.6.2, permeability determined by direct calculation shall be used where a more onerous condition results, and may be used where a less onerous condition results from that provided according to 2.6.2.

  2.6.4 The Administration may permit the use of low-density foam or other media to provide buoyancy in void spaces, provided that satisfactory evidence is provided that any such proposed medium is the most suitable alternative and is:

• .1 of closed-cell form if foam, or otherwise impervious to water absorption;^footnote

• .2 structurally stable under service conditions;

• .3 chemically inert in relation to structural materials with which it is in contact or other substances with which the medium is likely to be in contact (reference is made to 7.4.3.8); and

• .4 properly secured in place and easily removable for inspection of the void spaces.

  2.6.5 The Administration may permit void bottom spaces to be fitted within the watertight envelope of the hull without the provision of a bilge system or air pipes provided that:

• .1 the structure is capable of withstanding the pressure head after any of the damages required by this section;

• .2 when carrying out a damage stability calculation in accordance with the requirements of this section, any void space adjacent to the damaged zone shall be included in the calculation and the criteria in 2.6, 2.13 and 2.15 complied with;

• .3 the means by which water which has leaked into the void space is to be removed shall be included in the craft operating manual required by chapter 18; and

• .4 adequate ventilation is provided for inspection of the space under consideration as required by 2.2.1.2.

• .5 void spaces filled with foam or modular buoyancy elements or any space without a venting system are considered to be void spaces for the purposes of this paragraph, provided such foam or elements fully comply with 2.6.4.

  2.6.6 Any damage of a lesser extent than that postulated in 2.6.7 to 2.6.11, as applicable, which would result in a more severe condition shall also be investigated.

  2.6.7 Extent of side damage

The following side damage shall be assumed anywhere on the periphery of the craft:

• .1 the longitudinal extent of damage shall be 0.75 ∇^1/3 , or (3 m + 0.225 ∇^1/3 ), or 11 m, whichever is the least;

• .2 the transverse extent of penetration into the craft shall be 0.2 ∇^1/3 . However, where the craft is fitted with inflated skirts or with non-buoyant side structures, the transverse extent of penetration shall be at least 0.12 ∇^1/3 into the main buoyancy hull or tank structure; and

• .3 the vertical extent of damage shall be taken for the full vertical extent of the craft,

The damages described in this paragraph shall be assumed to have the shape of a parallelepiped.^footnote Applying this to figure 2.6.7 a, the inboard face at its mid-length shall be tangential to, or otherwise touching in a least 2 places, the surface corresponding to the specified transverse extent of penetration, as illustrated in figure 2.6.7 a.

Side damage shall not transversely penetrate a greater distance than the extent of 0.2∇^1/3 at the design waterline, except where a lesser extent is provided for in 2.6.7.2. Refer to figures 2.6.7b and 2.6.7c.

If considering a multihull, the periphery of the craft is considered to only be the surface of the shell encompassed by the outboard surface of the outermost hull at any given section.

Figure 2.6.7.a

Figure 2.6.7.b

Figure 2.6.7.c

  2.6.8 Extent of bow and stern damage

  2.6.8.1 The following extents of damage are to be applied to bow and stern, as illustrated in figure 2.6.8:

• .1 at the fore end, damage to the area defined as A_bow in 4.4.1, the aft limit of which being a transverse vertical plane, provided that this area need not extend further aft from the forward extremity of the craft's watertight envelope than the distance defined in 2.6.7.1; and

• .2 at the aft end, damage to the area aft of a transverse vertical plane at a distance 0.2∇^1/3 forward of the aft extremity of the watertight envelope of the hull.

  2.6.8.2 The provisions of 2.6.6 in relation to damage of lesser extent remain applicable to such damage.

  2.6.9 Extent of bottom damage in areas vulnerable to raking damage

  2.6.9.1 Application

• .1 Any part of the surface of the hull(s) is considered to be vulnerable to raking damage if:

  • .1.1 it is in contact with the water at 90% of maximum speed in smooth water, and

  • .1.2 it also lies below two planes which are perpendicular to the craft centreline plane and at heights as shown in figure 2.6.9.1.

  For multihulls, individual hulls shall be considered separately.

• .2 Raking damage shall be assumed to occur along any fore-and-aft line on the surface of the hull(s) between the keel and the upper limit defined in the figure below:

• .3 Damage shall not be applied at the same time as that defined in 2.6.7 or 2.6.10.

Figure 2.6.9.1

 where: T = maximum draught of the hull (each hull considered individually in the case of multihulls) to the design waterline, excluding any nonbuoyant structure, provided that structures such as single plate skegs or solid metal appendages shall be considered to be non-buoyant and thus excluded.

  2.6.9.2 Extent

  2.6.9.2.1 Two different longitudinal extents shall be considered separately:

• .1 55% of the length L, measured from the most forward point of the underwater buoyant volume of each hull; and

• .2 a percentage of the length L, applied anywhere in the length of the craft, equal to 35% for craft where L = 50m and over and equal to ( L/2 + 10)% for craft where L is less than 50m.

  2.6.9.2.2 Except as provided below, the penetration normal to the shell shall be 0.04 ∇^1/3 or 0.5 m, whichever is the lesser, in association with a girth along the shell equal to 0.1∇^1/3, where ∇ is the volume of displacement corresponding to the design waterline (m³). However, this penetration or girth shall under no circumstances extend above the vertical extent of the vulnerable area as specified in 2.6.9.1.1.

  2.6.9.2.3 The shape of damage shall be assumed to be rectangular in the transverse plane as illustrated in figure 2.6.9.2 below. Damage is to be assumed at a series of sections within the defined longitudinal extent in accordance with figure 2.6.9.2, the mid-point of the damaged girth being maintained at a constant distance from the centreline throughout that longitudinal extent.

Figure 2.6.9.2

  2.6.10 Extent of bottom damage in areas not vulnerable to raking damage

  2.6.10.1 Application

This applies to all parts of the hull(s) below the design waterline which are not defined as vulnerable to raking damage in 2.6.9.1. Damage shall not be applied at the same time as that defined in 2.6.7 or 2.6.9.

  2.6.10.2 Extent

The following extent of damage shall be assumed:

• .1 the length of damage in the fore-and-aft direction shall be 0.75 ∇^1/3 , or (3 m + 0.225 ∇^1/3), or 11 m whichever is the least;

• .2 the athwartships girth of damage shall be 0.2 ∇^1/3;

• .3 the depth of penetration normal to the shell shall be 0.02 ∇^1/3,

• .4 the shape of damage shall be assumed to be rectangular in the plane of the shell of the craft, and rectangular in the transverse plane as illustrated in figure 2.6.9.2.

  2.6.11 In applying 2.6.9 and 2.6.10 to multihull craft, an obstruction at or below the design waterline of up to 7 m width shall be considered in determining the number of hulls damaged at any one time. The requirement of 2.6.6 shall also be applied.

  2.6.12 Following any of the postulated damages detailed in 2.6.6 to 2.6.11, the craft in still water shall have sufficient buoyancy and positive stability to simultaneously ensure that:

• .1 for all craft other than amphibious air-cushion vehicles, after flooding has ceased and a state of equilibrium has been reached, the final waterline is below the level of any opening through which further flooding could take place by at least 50% of the significant wave height corresponding to the worst intended conditions;

• .2 for amphibious air-cushion vehicles, after flooding has ceased and a state of equilibrium has been reached, the final waterline is below the level of any opening through which further flooding could take place by at least 25% of the significant wave height corresponding to the worst intended conditions;

• .3 there is a positive freeboard from the damage waterline to survival craft embarkation positions;

• .4 essential emergency equipment, emergency radios, power supplies and public address systems needed for organizing the evacuation remain accessible and operational; and

• .5 the residual stability of craft meets the appropriate criteria as laid out in annexes 7 and 8 according to table 2.3.4. Within the range of positive stability governed by the criteria of annexes 7 and 8, no unprotected opening shall be submerged.

  2.6.13 Downflooding openings referred to in 2.6.12.1 and 2.6.12.2 shall include doors and hatches which are used for damage control or evacuation procedures, but may exclude those which are closed by means of weathertight doors and hatch covers and not used for damage control or evacuation procedures.