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Stability is one of the most important safety features of ships, and in particular of small ships which tend to suffer from insufficient stability which could lead to capsizing the vessel and loss of the crew. It is, therefore, essential to design a ship with adequate stability and to maintain it in all conditions of loading during its operation.

The International Conference on Safety of Life at Sea, 1960, recognizing the importance of the stability of ships, recommended that IMCO should undertake studies on intact stability of passenger ships, cargo ships and fishing vessels, with a view to formulating such international standards as may appear necessary.

In pursuance of the above recommendation, the Inter-Governmental Maritime Consultative Organization established in 1960 the Sub-Committee on Sub-division and Stability which was charged, among others, with the task of studying the intact stability of passenger ships and cargo ships.

As a result of the Sub-Committee’s comprehensive studies on existing national requirements, on results of analyses of intact stability casualty records and on stability calculations of ships which have operated successfully, a Recommendation on Intact Stability for Passenger and Cargo Ships under 100 metres in length was drawn up. This Recommendation was approved by the Maritime Safety Committee in March 1968 and adopted by the Assembly of the Organization at its fourth extraordinary session in November 1968.

By Resolution A. 167(ES.IV) the Assembly invited all governments concerned to take steps to give effect to the Recommendation as soon as possible unless they are fully satisfied that their national stability requirements supported by long operating experience already ensure adequate stability for particular types and sizes of ships. The Assembly at the same time requested the Maritime Safety Committee to continue the study on this subject and to develop improved stability criteria.

As regards fishing vessels, due to their special constructional features and specific conditions under which they operate, a separate recommendation on intact stability of fishing vessels has been formulated (Assembly Resolution A.168(ES.IV). This recommendation is issued as a separate IMCO publication.

The Organization is continuing its studies on the stability criteria, paying particular attention to the effect of external forces on stability. Studies on stability of special types of ships, apart from fishing vessels, such as ships carrying timber deck cargo, container ships, hydrofoil boats and air cushion vehicles, as well as drilling rigs and production platforms, are also in progress.

 
RECOMMENDATION ON INTACT STABILITY FOR
PASSENGER AND CARGO SHIPS UNDER 100 METRES IN LENGTH

1. Scope ^

1.1. The provisions given hereunder are recommended for new decked sea-going passenger and cargo ships (other than fishing vessels) under 100 metres in length.

1.2. Administrations are invited to adopt, for all conditions of loading, the stability criteria given in 5, unless they are satisfied that operating experience justifies departures therefrom.

2. General Precautions against Capsizing ^

2.1. Compliance with the stability criteria does not ensure immunity against capsizing regardless of the circumstances or absolve the master from his responsibilities. Masters should therefore exercise prudence and good seamanship having regard to the season of the year, weather forecasts and the navigational zone and should take the appropriate action as to speed and course warranted by the prevailing circumstances.

2.2. Care should be taken that the cargo allocated to the ship is capable of being stowed so that compliance with the criteria can be achieved. If necessary the amount should be limited to the extent that ballast weight may be required.

2.3. Before a voyage commences care should be taken to ensure that the cargo and sizeable pieces of equipment have been properly stowed or lashed so as to minimize the possibility of both longitudinal and lateral shifting while at sea, under the effect of acceleration caused by rolling and pitching.

3. Calculation of Stability Curves ^

The methods and procedures employed for calculating stability righting arms should be in accordance with Appendix I, and the degree of accuracy obtained should be acceptable to the Administration.

4. Assessment of Compliance with Criteria ^

4.1. For the purpose of assessing in general whether the criteria are met, stability curves should be drawn for the main loading conditions intended by the owner in respect of the ship’s operations.

4.2. If the owner does not supply sufficiently detailed information regarding such loading conditions, calculations should be made for the standard conditions given in Appendix II.

4.3. In all cases calculations should be based on the assumptions shown in Appendix II.

5. Recommended Criteria ^

5.1. The following criteria are recommended for passenger and cargo ships:

1. The area under the righting lever curve (GZ curve) should not be less than 0.055 metre-radians up to q = 30° angle of heel and not less than 0.09 metre-radians up to q = 40° or the angle of flooding q _f if this angle is less than 40°.
   
   Additionally, the area under the righting lever curve (GZ curve) between the angles of heel of 30° and 40° or between 30° and q _f, if this angle is less than 40°, should not be less than 0.003 metre-radians.
   
   
2. The righting lever GZ should be at least 0.20m. at an angle of heel equal to or greater than 30°
   
   
3. The maximum righting arm should occur at an angle of heel preferably exceeding 30° but not less than 25°
   
   
4. The initial metacentric height GM₀ should not be less than 0. 15m.

5.2. For ships loaded with timber deck cargoes and provided that the cargo extends longitudinally between superstructures transversely for the full beam of ship after due allowance for a rounded gunwale not exceeding 4 per cent of the breadth of the ship and/or securing the supporting uprights and which remains securely fixed at large angle of heel, an Administration may apply the following criteria in substitution for criteria given in 5.1 above:

1. the area under the righting lever (GZ curve) should not be less than
   0.08 metre-radians up to q = 40°or the angle of flooding if this angle is less than 40.
   
   *q _f is an angle of heel at which openings in the hull, superstructures or deckhouses which cannot be closed weathertight immerse. In applying this criterion, small openings through which progressive flooding cannot take place need not be considered as open.
2. The maximum value of the righting lever (GZ) should be at least 0.25 m.
   
   
3. at all times during a voyage the metacentric height GM₀ should be positive after correction for the free surface effects of liquid in tanks, and, where appropriate, the absorption of water by the deck cargo and/or ice accretion on the exposed surfaces. Additionally, in the departure condition the metacentric height should be not less than 0.10 m.

5.3. The following additional criteria are recommended for passenger ships:

1. The angle of heel on account of crowding of passengers to one side as defined in Appendix II 2. (9) should not exceed 10°.
   
   
2. The angle of heel on account of turning should not exceed 10° when calculated using the following formula:
   
   
   where:
   M_R = heeling moment in metre-tons,
   V_o = service speed in m./sec.,
   L = length of ship at waterline in m.,
   D = displacement in metric tons,
   d = mean draught in m. ,
   KG = height of centre of gravity above keel in m.

5.4. The criteria mentioned in 5.1 and 5.2 fix minimum values, but no maximum values are recommended. It is advisable to avoid excessive values, since these might lead to acceleration forces which could be prejudicial to the ship, its complement, its equipment and to the safe carriage of the cargo.

5.5. Where anti-rolling devices are installed in a ship the Administration should be satisfied that the above criteria can be maintained when the devices are in operation.

5.6. A number of influences such as beam wind on ships with large windage area, icing of topsides, water trapped on deck, rolling characteristics, following seas, etc., adversely affect stability and the Administration is advised to take these into account so far as is deemed necessary.

5.7. Regard should be paid to the possible adverse effects on stability where certain bulk cargoes are carried. In this connection attention should be paid to the Code of Safe Practice for Bulk Cargoes. Ships carrying grain in bulk should comply with the criteria mentioned in 5.1 in addition to the stability requirements in Chapter VI of the International Convention for the Safety of Life at Sea, 1960.

6. Inclining Test ^

6.1. When construction is finished, each ship should undergo an inclining test, actual displacement and co-ordinates of the centre of gravity being determined for the light ship condition.

6.2. The Administration may allow the inclining test of an individual ship to be dispensed with, provided basic stability data are available from the inclining test of a sister ship.

7. Stability Information ^

7.1. The master of any ship to which the present Recommendation applies should receive information which will enable him to assess with ease and certainty the stability of his ship in different service conditions. A duplicate of this information should be communicated to the Administration.

7.2. Stability information should comprise:

1. Stability characteristics of typical loading conditions;
   
2. Information in the form of tables or diagrams which will enable the master to assess the stability of his ship and verify whether it is sufficient in all loading conditions differing from the standard ones. This information should include, in particular, a curve or table giving, as a function of the draughts, the required initial metacentric height GM₀ (or any other stability parameter) which ensures that the stability is in compliance with the criteria given in 5.1 above;
   
3. Information on the proper use of anti-miling devices if these are installed in the ship;
   
4. Additionally, information enabling the ship’s master to determine the initial metacentric height GM₀ by means of rolling test as described in the Annex to the Memorandum to Administrations reproduced at Appendix III would be desirable;
   
5. Notes on the corrections to be made to the initial metacentric height GM₀ to take account of free surface liquids.
   
6. For ships carrying timber deck cargoes the Administration may deem it necessary that the master be given information setting out the changes in deck cargo from that shown in the loading conditions, when the permeability of the deck cargo is significantly different from 25 per cent.
   
7. For ships carrying timber deck cargoes conditions should be shown indicating the maximum permissible amount of deck cargo having regard to the lightest stowage rate likely to be met in service.

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