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Conditions of stability, hull strength, draft and trim of the vessel at sea and at port

The Master shall ensure that the conditions of stability, hull strength, draft and trim of the vessel at sea and on arrival / departure at / from port and during loading / unloading cargo, bunkering and water ballast exchange, have been worked out, to secure safety of the vessel. He shall confirm the safety of the vessel by proper GM, stress and other factors as being within appropriate Limits.

stability failure
Stability Calculation failure may lead to disaster

In case of any deviation from the limits, in spite of best efforts from vessel (including transfer of ballast), the Master should contact the vessel operator immediately to rectify the situation, and also advise the Company.

Stress and Stability Calculation

The Master shall use the approved (or designated by Company) loading computer for calculating stability, hull strength, draft, trim and other necessary parameters. However it is strongly recommended to compare the calculation results of both loading computer (the class approved one and the company designated one) when stability and/or hull strength remains nearly the limit of specified safety range before departing the last port for ocean passage.

Where there is no Class approved loading computer (as on some older ships), the loading program in use shall be verified on a regular basis, by inputting standard loading condition from the Trim & Stability Booklet and comparing results.

The Master shall also be familiar with the manual calculations of hull strength and stability, draft and trim using the Trim & Stability Booklet (Loading Manual). If computer loading program is not functional or reliable, manual calculations are to be done and record kept as similar to computer printouts.

The loading computer (approved or otherwise) must be tested at least once every 3 months by simulated loading condition excerpted from the Loading Manual and results compared.

Stress and Stability Maintenance

At no time, the vessel is to exceed the various restrictions prescribed in the Loading manual. These would relate to the drafts, trim, stability, bending moments, shearing forces, torsional moments (if applicable), and other instructions mentioned in the Loading and related manuals.

It shall be confirmed beforehand that bending moments and shearing forces during various stages of cargo loading / unloading, ballasting / de-ballasting and bunkering fall and are always within the specified safety range.

Since allowable bending moment and shearing forces in port are set to a higher value than those at sea, values set for in port shall not be used for calculation of respective values at sea
GM & GZ Curves

On containerships, the minimum GM to be maintained as specified in the Loading Manual. When the GM is calculated to be less than 1.0 m, it is recommended to verify it from the rolling period.

On tankers, the maximum GM as specified in the Loading Manual is not to be exceeded when part loading cargo in cargo oil tanks.

Excessive GM shall be avoided as far as possible, since it can cause intense rolling with possible hull and cargo damage. Ample stability shall be maintained to withstand any influences caused by weather and sea conditions, tug boat operations, heeling due to helm orders, cargo handling and other factors. To decide if a calculated GM is proper or not, assurance of ample GZ shall be confirmed by means of GZ Curves in the loading manual.

Container ship loaded condition

Draft and Trim

At no time shall the mid-ship draft exceed the draft decided under the International Convention of Load Lines ( always taking into account changes in applicable zones and seawater salinity during the voyage)

Forward draft shall be not less than the minimum draft specified in the loading manual to avoid slamming in heavy weather.

It is desirable to have the aft draft sufficient to submerge the propeller.

Drafts during the voyage, shall be such that, both the voyage and port restricted drafts are not exceeded and the minimum Under-keel Clearance is maintained. IMO Visibility shall be always maintained.

Free Surface Effect

To minimize free surface effect, tanks should as far as practicable be full or empty unless they are in use.

Aside from reducing the meta-centric height, during rolling, partly filled tanks can cause sloshing and damage to bulkheads or similar structures in tanks.

On certain tankers, care must be paid to the instructions in the loading manual which prohibit cargo tanks carrying cargo with a density greater than 1.0 to be partly loaded, unless the GM is within a certain limit.

Angle of heel, heel – A steady angle of heel created by an external force, such as wind or waves.

Angle of list, list – A steady angle of heel created by forces within the ship. For example, when the ship is inclined due to her asymmetric construction, or by shifting a weight transversely within the ship. The list reduces of ship’s stability. Therefore it is essential to keep the ship upright at all times by a symmetrical distribution of masses.

Angle of loll – The angle at which a ship with a negative initial metacentric height will lie at rest in still water. In a seaway, such a ship will oscillate between the angle of loll on SB and the one on PS. Depending upon external forces such as wind and waves a ship may suddenly flop over from PS to SB and then back again to PS. Such abrupt oscillation, different from a continuous roll, is characteristic for negative metacentric heights.

An angle of loll can be corrected only by lowering the centre of gravity, not by moving loads transversely. This can be done by moving weight downwards, adding water ballast in double bottom tanks or removing weight above the ship vertical centre of gravity. Where empty ballast tanks are available these will afford the simplest means of lowering the ship’s centre of gravity. The correct procedure is to add ballast on the low side of the ship. The first effect will be to increase the angle of heel and to cause a loss of stability due to the free surface of the water, but this effect is soon cancelled and the angle of heel will rapidly decrease.

Stacking Weights & Lashing Strengths ( Container ships)

The stacking weights and lashing strengths shall be maintained within limits stipulated in the Loading manual.

Relevant Forms

Checklist for calculating stability and hull strength for cargo ship

Reference and related regulations


Trim & Stability Booklet

Vessels Loading Manual ( if in addition to above)

Ballast Water Management Plan

Related Regulations:

SOLAS Convention

(Chapter 1, Part B-1, Regulation 25-8, Stability Information)

Ship Stability Regulations (JAPAN)

NK Rules and Regulations for the Construction and Classification of Ships, such as Part U, Chapter 2, Stability Requirements

IMO Resolution A. 167

IMO Resolution A. 562

Terminal Regulations

International Convention on Load Lines.

Tankers ETAS manuals

Damage Control Stability Booklet ( if applicable)

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