Hull Construction Regulations — 12
12 (a) In determining the floodable length, a uniform average permeability shall be used throughout the whole length of each of the following portions of the ship below the margin line : (i) the machinery space, (ii) the portion forward of the machinery space, and (iii) the portion abaft the machinery space; (b) the assumptions of permeability appropriate to each Class of ship, which shall be taken into account for each of the above portions of the ship, are laid down in sections 3 and 8 of this Schedule; (c) for a given ship, therefore, three flooding curves must in general be drawn more or less completely, corresponding to the three different permeabilities, the significant part of each curve depending on the position of the bulkheads bounding the machinery space; (d) the size and shape of a flooding curve depend principally on the freeboard ratio and on the assumed permeability; they depend, also, to a smaller extent, on the character of the lines of the vessel and on the sheer of the margin lines forward and aft; using the same vertical and horizontal scales for percentage of length, the ends of a flooding curve terminate on straight lines drawn through points in the base line representing the position of the fore and after perpendiculars, at an angle θ, where tan θ = 2; these lines are called the forward and after terminals respectively; (e) in order to determine curves of floodable length for any ship the Standard Diagrams should be used; these give floodable lengths (for the two permeabilities 60 per cent and 100 per cent), for a definite standard family of ship forms of differing block coefficients, freeboard ratios and sheer ratios; the floodable lengths obtained are in every case to be set off at right angles to the base line of the floodable length curve; for the two permeabilities mentioned, the curves of floodable length for any vessel of the standard form can be obtained directly from the cross curves given in the Plates, by the method indicated on Plate II; while for any other permeability the appropriate curve may be obtained (including terminal points) as follows: if 1 1 be the floodable length at the point considered for permeability 100 per cent and 1 2 be the floodable length at the point considered for permeability 60 per cent then the floodable length 1 3 at that point for a permeability µ will be given by 1 3 = 1 1 + 3 2 (1 2 - 1 1 ) ( 100 µ - 1) (f) a convenient method of arranging the work to obtain the required curves is indicated in Specimen 2; (g) if the ship under consideration conforms to standard type, that is to say, if the coefficients (see Specimen 1) agree with those given on Plates XXVI and XXVII for the standard form, the floodable length curve determined as above will hold good for the ship; if, however, there are differences in these respects, the curve obtained as above should be modified as follows: let A, Fig. 1, mark the fore-and-aft position of the centre of flotation of mean waterplane of the standard form, and A 1 , that of the corresponding point for the ship under consideration, the horizontal distance between them being mL; take any point P on the standard form curve, distant χ from A, the ratio ( χ L ) being represented by p, so that the value of p varies according to the position of P; the longitudinal position of the point P 1 , on the new curve, corresponding to the point P on the old, is given by χ 1 = χ × n 1 n × a a 1 ÷ (1 ± m ap n ) where n and n 1 are the moment of inertia coefficients of the mean water plane of the standard form and the new form respectively; the length of the ordinate M 1 p 1 is given by M 1 P 1 = MP × a 1 a x ß ß 1 x (1 ± m ap n ) where a and a 1 are the area coefficients of the mean water planes of the standard form and the new form respectively and ß and ß 1 are the sectional area coefficients of the standard form and the new form respectively at MP and M 1 P 1 ; the sign to be used in the last factor of the above expressions will be + when the cen