4.11 Double Bottoms:

(1) Subject to paragraphs (a) to (c), a double bottom shall be fitted extending from the collision bulkhead to the afterpeak bulkhead, as far as this is practicable and compatible with the design and proper working of the ship-

• (a) in ships of 50 metres and upwards but less than 61 m in length a double bottom shall be fitted at least from the machinery space to the forepeak bulkhead, or as near thereto as practicable;

• (b) in ships of 61 metres and upwards but less than 76 m in length a double bottom shall be fitted at least outside the machinery space, and shall extend to the fore and after peak bulkheads, or as near thereto as practicable;

• (c) subject to paragraph (c), in ships of 76 metres in length and upwards, a double bottom shall be fitted amidships, and shall extend to the fore and after peak bulkheads, or as near thereto as practicable; and

• (d) for vessels assessed in accordance with the probabilistic means in accordance with Parts B-1 through B-4 of SOLAS, where it is deemed that the installation of a double bottom is impracticable in accordance with paragraph (c), the vessel must be able to demonstrate compliance with the enhanced survivability criteria defined in section 4.30 of the Code, following the occurrence of bottom damage in the area concerned.

(2) Where a double bottom is required to be fitted the inner bottom shall be continued out to the ship’s sides in such a manner as to protect the bottom to the turn of the bilge; such protection will be deemed satisfactory if the inner bottom is not lower at any part than a plane parallel with the keel line and which is located not less than a vertical distance h measured from the keel line, as calculated by the formula-

• h = B/20,

provided that in no case is the value of h to be less than 760 millimetres and need not be taken as more than 2,000 millimetres.

(3) Small wells constructed in the double bottom in connection with drainage arrangements of storage spaces, etc., shall not extend downward more than necessary, provided that-

• (a) a well extending to the outer bottom is, however, permitted at the after end of the shaft tunnel;

• (b) other wells (e.g., for lubricating oil under main engines) may be permitted by the Administration if satisfied that the arrangements give protection equivalent to that afforded by a double bottom complying with this regulation; and

• (c) in no case shall the vertical distance from the bottom of such a well to a plane coinciding with the keel line be less than 500 millimetres.

(4) A double bottom need not be fitted in way of watertight tanks, including dry tanks of moderate size, provided the safety of the ship is not impaired in the event of bottom or side damage.

(5) Any part of a passenger ship that is not fitted with a double bottom in accordance with subsections (1) or (4) shall be capable of withstanding bottom damages, as specified in section 4.11(7), in that part of the ship.

(6) In the case of unusual bottom arrangements in a passenger ship it shall be demonstrated that the ship is capable of withstanding bottom damages as specified in section 4.11(7).

(7) Compliance with subsections (5) or (6) is to be achieved by demonstrating that si, when calculated in accordance with section 4.8 is not less than 1 for all service conditions when subject to a bottom damage assumed at any position along the ship’s bottom in accordance with the following conditions and with an extent specified in subsection (2) for the affected part of the ship-

• (a) Flooding of such spaces shall not render emergency power and lighting, internal communication, signals or other emergency devices inoperable in other parts of the ship.

• (b) Assumed extent of damage shall be as follows-

• Extent	For 0.3 L from the forward perpendicular of the ship	Any other part of the ship
  Longitudinal extent	1/3 L2/3 or 14.5 metres, whichever is less	1/3 L2/3 or 14.5 metres, whichever is less
  Transverse extent	B/6 or 10 metres, whichever is less	B/6 or 5 metres, whichever is less
  Vertical extent, measured from the keel line	B/20 or 2 metres, whichever is less	B/20 or 2 metres, whichever is less

• (c) If any damage of a lesser extent than the maximum damage specified in section 4.11(7)(b) would result in a more severe condition, such damage should be considered.

(8) In case of large lower compartments in passenger ships-

• (a) the Administration may require an increased double bottom height of not more than B/10 or 3 metres, whichever is less, measured from the keel line;

• (b) alternatively, bottom damages may be calculated for these areas, in accordance with subsection (7), but assuming an increased vertical extent.