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Regulation 6 Special Rules Concerning Subdivision {a) Where in a portion or portions of a ship the watertight bulkheads aie carried to a higher deck than in the remainder of the ship, and it is desired to take advantage of this higher extension of the bulkheads in calculating the floodable length, separate margin lines may be used for each such portion of the ship provided that — (i) The sides of the ship are extended throughout the. ship's length to the deck corresponding to the upper margin line and all openings in the shell plating below this deck throughout the length of the ship are treated as being below a margin line, for the purposes of Regulation 13 ; and (ii) The two compartments adjacent to the " step " in the bulkhead deck are each within the permissible length corresponding to their respective margin lines and, in addition, their combined length does not exceed twice the permissible length based on the lower margin line. (b) (i) A compartment may exceed the permissible length determined by the rules of Regulation 5 provided the combined length of each pair of adjacent compartments to which the compartment in question is common does not exceed either the floodable length or twice the permissible length, whichever is the less. (ii) If one of the two adjacent compartments is situated inside the machinery space, and the second is situated outside the machinery space, and the average permeability of the portion of the ship in which the second is situated differs from that of the machinery space, the combined length of the two compartments shall be adjusted to the mean average permeability of the two portions of the ship in which the compartments are situated. (iii) Where the two adjacent compartments have different factors of subdivision, the combined length of the two compartments shall be determined proportionately. (c) In ships 430 feet (or 131 metres) in length and upwards, one of the main transverse bulkheads abaft the forepeak shall be fitted at a distance from the forward perpendicular which is not greater than the permissible length. (d) A main transverse bulkhead may be recessed provided that all parts of the recess lie inboard of vertical surfaces on both sides of the ship, situated at a distance from the shell plating equal to one-fifth the breadth of the ship, as defined in Regulation 2, and measured at right angles to the centre line at the level of the deepest subdivision loadline. Any part of a recess which lies outside these limits shall be dealt with as a step in accordance with the following paragraph. (e) A main transverse bulkhead may be stepped provided that it meets one of the following conditions : (i) The combined length of the two compartments, separated by the bulkhead in question, does not exceed either 90 per cent, of the floodable length or twice the permissible length, except that in ships having a factor of subdivision greater than -9, the combined length of the two compartments in question shall not exceed the permissible length.

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