Floodable Length
Centroid of Centroid of Centroid of volume Vo to volume Vi to lost buoyancy waterline WoLo waterline W, Li
Figure 4.36
waterline is tangent to a line drawn 76 mm (3 inches) below the bulkhead deck at side. This line is called the margin line. The jloodable length at any point along the length of the ship is the length, with that point as centre, which can be flooded without immersing any part of the margin line when the ship has no list.
Take the ship shown in Figure 4.36 using subscripts 0 and 1 to denote the intact ship data for the intact and damaged waterlines. Loss of buoyancy = Vi  V0 and this must be at such a position that Bj moves back to B0 so that B is again below G. Hence:
This then gives the centroid of the lost buoyancy and, knowing (Vj V0) it is possible to convert this into a length of ship that can be flooded. The calculation would be one of reiteration until reasonable figures are obtained.
The calculations can be repeated for a series of waterlines tangent to the margin line at different positions along the length. This will lead to a curve of floodable length as in Figure 4.37. The ordinate
Figure 4.37 Floodable length at any point represents the length which can be flooded with the centre at the point concerned. Thus if I is the floodable length at some point the positions of bulkheads giving the required compartment length are given by setting off distances 1/2 either side of the point. The lines at the ends of the curves, called the forward and after terminals will be at an angle tan12 to the base if the base and ordinate scales are the same.
The permeabilities of compartments will affect the floodable length and it is usual to work out average permeability figures for the machinery spaces and for each of the two regions forward and aft.
Figure 4.37 Floodable length
For'd end permeability = /x3
Aft end Machinery space per meability » fj., permeability »/x2
Figure 438 Floodable length with permeability
For'd end permeability = /x3
Aft end Machinery space per meability » fj., permeability »/x2
Figure 438 Floodable length with permeability
This leads to three curves for the complete ship as shown in Figure 4.38. The condition that a ship should be able to float with any one compartment open to the sea is a minimum requirement for ocean going passenger ships. The Merchant Shipping Regulations set out formulae for calculating permeabilities and a factor of subdivision which must be applied to the floodable length curves giving permissible length. The permissible length is the product of the floodable length and the factor of subdivision. The factor of subdivision depends upon the length of the ship and a criterion of service numeral or more simply criterion numeral. This numeral represents the criterion of service of the ship and takes account of the number of passengers, the volumes of the machinery and accommodation spaces and the total ship volume. It decreases in a regular and continuous manner with the ship length and factors related to whether the ship carries predominantly cargo or passengers. Broadly, the factor of subdivision ensures that one, two or three compartments can be flooded before the margin line is immersed leading to what are called one, two or threecompartment ships. That is, compartment standard is the inverse of the factor of subdivision. In general terms the factor of subdivision decreases with length of ship and is lower for passenger ships than cargo ships.
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