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The main purpose of the calculation is to find the freeboard draught (maximum allowable draught). The rule reference is the International Convention of Load Lines, 1966.

The first step in freeboard calculation is deciding the type of freeboard. There are two kinds, Type 'A' and Type 'B'.

Usually Type 'A' ships are the ones designed to carry liquid cargoes in bulk, where the cargo, tanks have small assess openings. Because of small access to the cargo and lack of floodable volume after damage in case of a loaded ship, such ships can have a low freeboard compared to Type 'B' ships. Such a ship necessarily has the following features:

  • High integrity of the exposed deck, and
  • High degree of safety against flooding, resulting from the low permeability of loaded cargo spaces and the degree of subdivision usually provided. 

Type 'B' ships are all the other ships other than Type 'A', and these have larger freeboard (or lower maximum draught).

Oil tankers are usually Type A ships. But sometimes a tanker can be taken as a Type B ship, with larger freeboard when the cargo capacity is the limiting factor. In such tankers with a normal density cargo, the maximum dead weight would be with a lower draught than the Type A ship, and near the Type B ship. In choosing Type B notation, the stringent damage stability calculation by ICLL need not be satisfied.

As a considerable variety of ships will come within the Type ‘B’ category, a reduction or increase from the basic table Type ‘B’ freeboard is made in the following cases:

  • Vessels having hatchways fitted with portable beams and covers on exposed freeboard or raised quarter decks, and within 25 per cent of the ship’s length from the FP on exposed superstructure decks, are to have the basic freeboard increased.
  • Vessels having steel weathertight covers fitted with gaskets and clamping devices, improved measures for the protection of the crew, better-freeing arrangements, and satisfactory sub-division characteristics may obtain a reduction in the basic freeboard given for a Type ‘B’ ship. This reduction may be increased up to the total difference between the values for Type ‘A’ and Type ‘B’ basic freeboards. The Type ‘B’ ship, which is effectively adopting Type ‘A’ basic freeboard, is referred to as a Type ‘B-100’ and its final calculated freeboard will be almost the same as that for a Type ‘A’ ship. 
  • Other Type ‘B’ vessels which comply with not such severe sub-division requirements can be assigned a basic freeboard reduced by up to 60 per cent of the difference between ‘B’ and ‘A’ basic values.
Obtaining the maximum possible draft can be important in many Type‘B’ vessels, and careful consideration at the initial design stage with regard to sub-division requirements can result in the ship being able to load to deeper drafts. This is particularly the case with bulk carriers since these vessels can often be designed to obtain the ‘B-60’ freeboards; and where this is impossible some reduction in freeboard may still be possible. The Convention allows freeboards between that assigned to a Type ‘B’ and a Type ‘B-60’ where it can be established that a one compartment standard of sub-division can be obtained at the draft of a Type ‘B’ vessel, but not at the draft of a Type ‘B-60’.

Ore carriers of normal layout arranged with two longitudinal bulkheads and having the side compartments as water ballast tanks are particularly suited to the assignment of Type ‘B-100’ freeboards, where the bulkhead positions are carefully arranged. In the case of Type ‘A’, Type ‘B-100’, and Type ‘B-60’ vessels over 225 m the machinery space is also to be treated as a floodable compartment. The full sub-division requirements are given below.

Damage is assumed as being for the full depth of the ship, with a penetration of 1/5 the beam clear of main transverse bulkheads. After flooding the final water-line is to be below the lower edge of any opening through which progressive flooding may take place. The maximum angle of heel is to be15°, and the metacentric height in the flooded condition should be positive. 

Type             Length                        Sub-division requirements
A               Less than 150m               None

A               Greater than 150m          To withstand the flooding of                        Less than 225m               any compartment within the                                                                cargo tank length which is                                                                  designed to be empty when the                                                            ship is loaded to the summer
                                                          water-line at an assumed                                                                      permeability of 0.95
A              Greater than 225m           As above, but the machinery                                                               space also to be treated as a                                                                 floodable compartment with an
                                                         assumed permeability of 0.85

B+                    —                            None

B                      —                            None

B–60            100 to 225m                To withstand the flooding of                                                               any single damaged
                                                         compartment within the cargo                                                             hold length at an assumed                                                                   permeability of 0.95

B–60          Greater than 225m        As above, but the machinery                                                              space also to be treated as a                                                                floodable compartment at an
                                                        assumed permeability of 0.85

B–100            100 to 225m            To withstand the flooding of any                                                         two adjacent fore and aft                                                                     compartments within the cargo
                                                       hold length at an assumed                                                                   permeability of 0.95

B–100         Greater than 225m     As above, but the machinery                                                              space, taken alone, also to be                                                              treated as a floodable                                                                          compartment at an assumed                                                                permeability of 0.85.

Having decided the type of ship, the computation of freeboard is
comparatively simple, a number of corrections being applied to the rule basic freeboard given for Type ‘A’ and Type ‘B’ ships against length of ship. The length (L) is defined as 96 per cent of the total length on the water-line at 85 per cent of the least moulded depth, or as the length measured from the fore side of the stem to the axis of the rudder stock on the water-line, if that is greater.

The corrections to the basic freeboard are as follows:

Flush Deck Correction: The basic freeboard for a Type ‘B’ ship of not more than 100 m in length having superstructures with an effective length of up to 35 per cent of the freeboard length (L) is increased by:
                              7.5(100-L)(0.35 - E/L) mm

where E= effective length of superstructure in metres.

Block Coefficient Correction: Where the block coefficient Cb exceeds 0.68, the basic freeboard is multiplied by the ratio
                                 Cb + 0.68
Cb is defined in the rules as ∇/(L.B.d), where ∇ is the moulded displacement at a draft d, which is 85 per cent of the least moulded depth.

Depth Correction: The depth (D) for freeboard is given in the rules. 
  • Where D exceeds L/15 the freeboard is increased by (D–(L/15)) Rmm, 
  • where R is L/0.48 at lengths less than 120 m and 250 at lengths of 120 m and above. 
  • Where D is less than L/15 no reduction is made, except in the case of a ship with an enclosed superstructure covering at least 0.6L amidships. 
This deduction, where allowed, is at the rate described above.

Superstructure Correction: Where the effective length of the superstructure is 1.0L, the freeboard may be reduced by 350 mm at 24 m length of ship, 860 mm at 85 m lengths, and 1070 mm at 122 m length and above. Deductions at intermediate lengths are obtained by linear interpolation. Where the total effective length of superstructures and trunks is less than 1.0L the deduction is a percentage of the above. These percentages are given in tabular form in the rules, and the associated notes give corrections for size of forecastles with Type ‘B’ ships.

Sheer Correction: The area under the actual sheer curve is compared with the area under a standard parabolic sheer curve, the aft ordinate of which (SA) is given by 25(L/3+10) mm and the forward ordinate (SF) by 2SA mm. Where a poop or fo’c’sle is of greater than standard height, an addition to the sheer of the freeboard deck may be made.

The correction for deficiency or excess of sheer is the difference between the actual sheer and the standard sheer multiplied by (0.75 – S/2L) where S is the total mean enclosed length of superstructure. Where the sheer is less than standard the correction is added to the freeboard. If the sheer is in excess of standard a deduction may be made from the freeboard if the superstructure covers 0.1L abaft and 0.1L forward of amidships. No deduction for excess sheer may be made if no superstructure covers amidships. Where superstructure covers less than 0.1L abaft and forward of amidships the deduction is obtained by linear interpolation. The maximum deduction for excess sheer is limited to 125 mm per 100 m of length.

If the above corrections are made to the basic freeboard, the final calculated freeboard will correspond to the maximum geometric summer draft for the vessel. The final freeboard may, however, be increased if the bow height is insufficient, or the owners request the assignment of freeboards corresponding to a draft which is less than the maximum possible.


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