Regulation 27 - Types of ships
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Statutory Documents - IMO Publications and Documents - International Conventions - Load Lines, 1966/1988 - International Convention on Load Lines, 1966, as Amended by the Protocol of 1988 - Annex I - Regulations for Determining Load Lines - Chapter III - Freeboards - Regulation 27 - Types of ships

Regulation 27 - Types of ships

  (1) For the purposes of freeboard computation, ships shall be divided into type 'A' and type B.

Type 'A' ships

  (2) A type A ship is one which:

  • (a) is designed to carry only liquid cargoes in bulk;

  • (b) has a high integrity of the exposed deck with only small access openings to cargo compartments, closed by watertight gasketed covers of steel or equivalent material; and

  • (c) has low permeability of loaded cargo compartments.

  (3) A type 'A' ship, if over 150 m in length, to which a freeboard less than type B has been assigned, when loaded in accordance with the requirements of paragraph (11), shall be able to withstand the flooding of any compartment or compartments, with an assumed permeability of 0.95, consequent upon the damage assumptions specified in paragraph (12), and shall remain afloat in a satisfactory condition of equilibrium, as specified in paragraph (13). In such a ship, the machinery space shall be treated as a floodable compartment, but with a permeability of 0.85.

  (4) A type 'A' ship shall be assigned a freeboard not less than that based on table 28.1.

Type 'B' ships

  (5) All ships which do not come within the provisions regarding type 'A' ships in paragraphs (2) and (3) shall be considered as type 'B' ships.

  (6) Type 'B' ships, which in position 1 have hatch covers which are permitted by the Administration to comply with the requirements of regulation 15 (other than paragraph (6)) or which are fitted with securing arrangements accepted under the provisions of regulation 16(6), shall be assigned freeboards based upon the values given in table 28.2, increased by the values given in table 27.1:

Table 27.1 Freeboard increase over tabular freeboard for type B ships, for ships with hatch covers complying with the provisions of regulation 15 (other than paragraph (6))

Length of ship (m) Freeboard increase (mm) Length of ship (m) Freeboard increase (mm) Length of ship (m) Freeboard increase (mm)
108 and below 50 139 175 170 290
109 52 140 181 171 292
110 55 141 186 172 294
111 57 142 191 173 297
112 59 143 196 174 299
113 62 144 201 175 301
114 64 145 206 176 304
115 68 146 210 177 306
116 70 147 215 178 308
117 73 148 219 179 311
118 76 149 224 180 313
119 80 150 228 181 315
120 84 151 232 182 318
121 87 152 236 183 320
122 91 153 240 184 322
123 95 154 244 185 325
124 99 155 247 186 327
125 103 156 251 187 329
126 108 157 254 188 332
127 112 158 258 189 334
128 116 159 261 190 336
129 121 160 264 191 339
130 126 161 267 192 341
131 131 162 270 193 343
132 136 163 273 194 346
133 142 164 275 195 348
134 147 165 278 196 350
135 153 166 280 197 353
136 159 167 283 198 355
137 164 168 285 199 357
138 170 169 287 200 358
Freeboards at intermediate lengths of ship shall be obtained by linear interpolation.
Ships above 200 m in length shall be dealt with by the Administration.

  (7) Type 'B' ships, which in position 1 have hatchways fitted with hatch covers complying with the requirements of regulation 16(2) through (5), shall, except as provided in paragraphs (8) to (13) inclusive, be assigned freeboards based on table 28.2.

  (8) Any type 'B' ship of over 100 m in length may be assigned freeboards less than those required under paragraph (7), provided that, in relation to the amount of reduction granted, the Administration is satisfied that:

  • (a) the measures provided for the protection of the crew are adequate;

  • (b) the freeing arrangements are adequate;

  • (c) the covers in position 1 and 2 comply with the provisions of regulation 16(1) through (5) and (7); and

  • (d) the ship, when loaded in accordance with the requirements of paragraph (11), shall be able to withstand the flooding of any compartment or compartments, with an assumed permeability of 0.95, consequent upon the damage assumptions specified in paragraph (12), and shall remain afloat in a satisfactory condition of equilibrium, as specified in paragraph (13). In such a ship, if over 150 m in length, the machinery space shall be treated as a floodable compartment, but with a permeability of 0.85.

  (9) In calculating the freeboards for type B ships which comply with the requirements of paragraphs (8), (11), (12) and (13), the values from table 28.2 shall not be reduced by more than 60% of the difference between the tabular values in tables 28.1 and 28.2 for the appropriate ship lengths.

  (10)

  • (a) The reduction in tabular freeboard allowed under paragraph (9) may be increased up to the total difference between the values in table 28.1 and those in table 28.2 on condition that the ship complies with the requirements of:

    • (i) regulation 26, other than paragraph (5), as if it were a type 'A' ship;

    • (ii) paragraphs (8), (11) and (13); and

    • (iii) paragraph (12), provided that throughout the length of the ship any one transverse bulkhead will be assumed to be damaged, such that two adjacent fore and aft compartments shall be flooded simultaneously, except that such damage will not apply to the boundary bulkheads of a machinery space.

  • (b) In such a ship, if over 150 m in length, the machinery space shall be treated as a floodable compartment, but with a permeability of 0.85.

Initial condition of loading

  (11) The initial condition of loading before flooding shall be determined as follows:

  • (a) The ship is loaded to its summer load waterline on an imaginary even keel.

  • (b) When calculating the vertical centre of gravity, the following principles apply:

    • (i) homogeneous cargo is carried;

    • (ii) all cargo compartments, except those referred to under subparagraph (iii), but including compartments intended to be partially filled, shall be considered fully loaded except that in the case of fluid cargoes each compartment shall be treated as 98% full;

    • (iii) if the ship is intended to operate at its summer load waterline with empty compartments, such compartments shall be considered empty, provided the height of the centre of gravity so calculated is not less than as calculated under subparagraph (ii);

    • (iv) 50 per cent of the ship's total capacity of tanks and spaces fitted to contain each type of consumables and stores is allowed for. It shall be assumed that for each type of liquid at least one transverse pair or a single centreline tank has maximum free surface, and the tank or combination of tanks to be taken into account shall be those where the effect of free surfaces is the greatest; in each tank the centre of gravity of the contents shall be taken at the centre of volume of the tank. The remaining tanks shall be assumed either completely empty or completely filled, and the distribution of consumable liquids between these tanks shall be effected so as to obtain the greatest possible height above the keel for the centre of gravity;

    • (v) Ballast water tanks shall normally be considered to be empty and no free surface correction shall be made for them.

    • (vi) Alternative treatment for free surface may be considered when developing the final condition for application of damage specified in regulation 27(12):

      • (aa) Method 1 (appropriate to virtual corrections). The virtual centre of gravity for the initial condition is determined as follows:

        • I the loading condition shall be developed in accordance with paragraphs (i) to (iv);

        • ii the correction for the free surfaces is added to the vertical centre of gravity;

        • iii one virtual initial condition with all compartments empty is generated on summer load line draught with level trim, using the vertical centre of gravity from the above loading condition; and

        • iv the damage cases will be checked for compliance with the damage stability criteria using the above initial condition.

      • (bb) Method 2 (appropriate to the use of actual free surface moments according to the assumed tank fillings for damage case). The virtual centre of gravity for the initial condition is determined as follows:

        • i the loading condition shall be developed in accordance with paragraphs (i) to (iv);

        • ii one virtual initial condition for each damage case with liquid-filled compartments may be generated on summer load line draught with level trim, using the initial virtual condition with filled compartments generated on summer load line draught with level trim. Using the vertical centre of gravity and free surface correction from the above loading condition separate calculations for each damage case are performed, only the liquid-filled compartments to be damaged are left empty before damage; and

        • iii the damage cases will be checked for compliance with the damage stability criteria using above initial conditions (one initial condition for each damage case)."

    • (vii) weights shall be calculated on the basis of the following values for specific gravities:

        salt water 1.025
        fresh water 1.000
        oil fuel 0.950
        diesel oil 0.900
        lubricating oil 0.900.

Damage assumptions

  (12) The following principles regarding the character of the assumed damage apply:

  • (a) The vertical extent of damage in all cases is assumed to be from the base line upwards without limit.

  • (b) The transverse extent of damage is equal to B/5 or 11.5 m, whichever is the lesser, measured inboard from the side of the ship perpendicularly to the centreline at the level of the summer load waterline.

  • (c) If damage of a lesser extent than specified in subparagraphs (a) and (b) results in a more severe condition, such lesser extent shall be assumed.

  • (d) Except where otherwise required by paragraph (10)(a), the flooding shall be confined to a single compartment between adjacent transverse bulkheads, provided that the inner longitudinal boundary of the compartment is not in a position within the transverse extent of assumed damage. Transverse boundary bulkheads of wing tanks, which do not extend over the full breadth of the ship shall be assumed not to be damaged, provided that they extend beyond the transverse extent of assumed damage prescribed in subparagraph (b).

  • If in a transverse bulkhead there are steps or recesses of not more than 3 m in length, located within the transverse extent of assumed damage as defined in subparagraph (b), such transverse bulkhead may be considered intact and the adjacent compartment may be floodable singly. If, however, within the transverse extent of assumed damage there is a step or recess of more than 3 m in length in a transverse bulkhead, the two compartments adjacent to this bulkhead shall be considered as flooded. The step formed by the afterpeak bulkhead and the afterpeak tank top shall not be regarded as a step for the purpose of this regulation.

  • (e) Where a main transverse bulkhead is located within the transverse extent of assumed damage and is stepped in way of a double bottom or side tank by more than 3 m, the double bottom or side tanks adjacent to the stepped portion of the main transverse bulkhead shall be considered as flooded simultaneously. If this side tank has openings into one or several holds, such as grain feeding holes, such hold or holds shall be considered as flooded simultaneously. Similarly, in a ship designed for the carriage of fluid cargoes, if a side tank has openings into adjacent compartments, such adjacent compartments shall be considered as empty and as being flooded simultaneously. This provision is applicable even where such openings are fitted with closing appliances, except in the case of sluice valves fitted in bulkheads between tanks and where the valves are controlled from the deck. Manhole covers with closely spaced bolts are considered equivalent to the unpierced bulkhead, except in the case of openings in topside tanks making the topside tanks common to the holds.

  • (f) Where the flooding of any two adjacent fore and aft compartments is envisaged, main transverse watertight bulkheads shall be spaced at least 1/3 L2/3 or 14.5 m, whichever is the lesser, in order to be considered effective. Where transverse bulkheads are spaced at a lesser distance, one or more of these bulkheads shall be assumed as non-existent in order to achieve the minimum spacing between bulkheads.

Condition of equilibrium

  (13) The condition of equilibrium after flooding shall be regarded as satisfactory provided:

  • (a) The final waterline after flooding, taking into account sinkage, heel and trim, is below the lower edge of any opening through which progressive downflooding may take place. Such openings shall include air pipes, ventilators (even if they comply with regulation 19(4)) and openings which are closed by means of weathertight doors (even if they comply with regulation 12) or hatch covers (even if they comply with regulation 16(1) through (5)), and may exclude those openings closed by means of manhole covers and flush scuttles (which comply with regulation 18), cargo hatch covers of the type described in regulation 27(2), remotely operated sliding watertight doors, and sidescuttles of the non-opening type (which comply with regulation 23). However, in the case of doors separating a main machinery space from a steering gear compartment, watertight doors may be of a hinged, quick-acting type kept closed at sea whilst not in use, provided also that the lower sill of such doors is above the summer load waterline.

  • (b) If pipes, ducts or tunnels are situated within the assumed extent of damage penetration as defined in paragraph (12)(b), arrangements shall be made so that progressive flooding cannot thereby extend to compartments other than those assumed to be floodable in the calculation for each case of damage.

  • (c) The angle of heel due to unsymmetrical flooding does not exceed 15. If no part of the deck is immersed, an angle of heel of up to 17 may be accepted.

  • (d) The metacentric height in the flooded condition is positive.

  • (e) When any part of the deck outside the compartment assumed flooded in a particular case of damage is immersed, or in any case where the margin of stability in the flooded condition may be considered doubtful, the residual stability is to be investigated. It may be regarded as sufficient if the righting lever curve has a minimum range of 20 beyond the position of equilibrium with a maximum righting lever of at least 0.1 m within this range. The area under the righting lever curve within this range shall be not less than 0.0175 m.rad. The Administration shall give consideration to the potential hazard presented by protected or unprotected openings which may become temporarily immersed within the range of residual stability.

  • (f) The Administration is satisfied that the stability is sufficient during intermediate stages of flooding.

  • (g) Compliance with the residual stability criteria specified in paragraphs (a), (c), (d) and (e) above is not required to be demonstrated in service loading conditions using a stability instrument, stability software or other approved method.

Ships without means of propulsion

  (14) A lighter, barge or other ship without independent means of propulsion shall be assigned a freeboard in accordance with the provisions of these regulations. Barges which meet the requirements of paragraphs (2) and (3) may be assigned type 'A' freeboards:

  • (a) The Administration should especially consider the stability of barges with cargo on the weather deck. Deck cargo can only be carried on barges to which the ordinary type 'B' freeboard is assigned.

  • (b) However, in the case of barges which are unmanned, the requirements of regulations 25, 26(3), 26(4) and 39 shall not apply.

  • (c) Such unmanned barges which have on the freeboard deck only small access openings closed by watertight gasketed covers of steel or equivalent material may be assigned a freeboard 25% less than those calculated in accordance with these regulations.


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