Your understanding of ship stability is mostly assessed by written exam, but there is a chance you may also be asked about stability during your oral exam. You will not be asked to carry out mathematical calculations; you will be examined on your understanding of principles.
Relevant documents and M-notices: SOLAS Chapter II-1 B-1, International Code on Intact Stability, MGN 513, MSC.267(85)
From MIN 653 -Deck Oral Exam Syllabus
Knowledge of the effect of cargo, including heavy lifts, on the seaworthiness and stability of the ship.
Understanding the factors that can affect the ships stability (input of incorrect weights (mis declared weights), ice on deck (accretion), Interpretation of data from loading instrument ( GZ curve), understanding IMO minimum intact stability criteria).
Working knowledge and application of stability, trim and stress tables, diagrams and stress-calculating equipment.
You must be ready to explain and demonstrate an understanding of stability, including but not limited to the following:
Effect on G during loading, discharging and moving weights
Causes of list
Difference between list and loll and the methods of correction.
Changes in stability during the voyage
Free surface and the dangers and effect at small angles of heel
Effect of tank subdivision and density on free surface
Allowance for the effect of free surface
The terms relating to statical stability
GZ curves
Own vessel's state of stability
What is stability?
Stability is the ability of the vessel to remain in upright equilibrium, and to return to the upright when heeled by an external force.
Some definitions
Metacentre - This is shown on diagrams as ‘M’. It is the point through which a line through the heeled centre of buoyancy crosses through a line through the upright centre of buoyancy.
Centre of gravity - This is shown on diagrams as ‘G’. It is the point through which all gravity is calculated to push down.
Metacentric height - This is known as ‘GM’ and is the fundamental measure of stability. It is the difference between KM (the distance from the keel plate to the metacentre) and KG (the distance from the keel plate to the centre of gravity), in meters. A greater GM indicates greater resistance to heeling via a larger righting lever (GZ).
Centre of buoyancy - This is shown on diagrams as ‘B’. It is at the centre of the underwater mass of the vessel. If the vessel is heeled by external forces and the shape of the underwater mass of the vessel changes, B moves.
Righting lever, or righting arm - Shown on diagrams as ‘GZ’. This is the distance between the lines of force of the upward-acting B and the downward-acting G.

Z - This is a point through which the new B is said to be acting upwards.
KM - the distance from the keel plate to the metacentre. This is given to us by the architect.
KG - the distance from the keel plate to the centre of gravity. This is calculated by performing an inclining test on the vessel.
KG(f) - the KG taking into account the virtual rise in G created by the Free Surface Effect.
Free surface effect (FSE) - This is the loss of stability due to liquids (and small solids that act like a liquid e.g. grain) that are free to move. For our purposes this is expressed as a virtual rise in G. The new G is often annotated as Gfluid.
TPC - Tonnes Per Centimetre. This is the mass that must be loaded, at any draft, to change the vessels draft by 1cm in salt water (with a relative density of 1.025t/m3).
MCTC - This is the moment to change the vessels total trim by 1cm.
Heeled - This is when a vessel has been moved from a stable equilibrium by an external force.
Heeling forces - Wind, waves, and turning motions can all heel a vessel.
List - This is when a vessel is at an angle other than upright as a result of internal factors; the uneven distribution of weight (G is ‘off centre’) creates the capsizing lever and no external force is required. Once G and B are in the same vertical plane at an angle of list the vessel is stable.
Loll - This is pronounced ‘lull’. An angle of loll is when a vessel reaches equilibrium at an angle other than upright. This happens when a vessel in unstable equilibrium is heeled by an initial external force; a capsizing lever develops, which continues to heel the vessel until G and B come into vertical alignment. The vessel will remain at this angle until acted on by another external force. A vessel which has an angle of loll on each side is said to ‘flop’ between these.
Downflooding angle - This is the angle at which water will begin to enter the vessel, via a hatchway or similar. At this point the FSE will increase.
Stability information supplied to the master
The Merchant Shipping (Load Lines) Act 1998 Regulation 32:
The owner of every ship to which these Regulations apply shall provide, for the guidance of the master, information relating to the stability of the ship in accordance with this regulation. The information shall be in the form of a book which shall be kept on the ship at all times in the custody of the master.
The act goes on to state that the information to be contained in the book shall be as per MSN 1752 (schedule 6). For now, know that there is a booklet you can consult that contains every bit of intact stability information you might need. The stability booklets for certain ships need to be approved by the administration:
Oil tankers over 100m in length
Single deck bulk carriers over 100m in length
Single deck dry cargo ships over 100m in length
Container ships over 125m in length
Bulk carriers over 150m in length
From the MCA advice to surveyors regarding this book:
It is recommended that the stability information provided for the master should comprise only the minimum necessary to enable him to assess the stability of the vessel in any condition of loading. In this regard, it is considered that the most direct method of assessing both intact and damaged stability is through the use of critical KG(f) data, which shall comprise curves of allowable KG(f) vs draught over the range of operational trims.
Stiff and tender
The bigger the GM, the ‘stiffer’ the vessel is said to be.
The smaller the GM, the more ‘tender’ a vessel is said to be.
A tender vessel is slower to right herself and will have a long, lazy rolling period. It will be possible to heel this vessel to a much greater angle than a stiffer vessel and she will take longer to come back to her upright position.
A stiff vessel is better able to oppose heeling forces and will quickly come back to an upright position. A stiff vessel can have a short, ‘snappy’ or violent rolling motion. In some ways this can be worse than the longer rolling period of a tender vessel; the snappier motion could damage cargo, injure passengers and crew and cause damage to the vessel (both the hull via racking forces and the mast and communications array).
Equilibriums explained
Stable - G below M. Can also be expressed as KG being less than KM. This vessel will right itself if heeled and is said to be in a stable equilibrium.
Neutral - G and M in the same place. KG = KM. Eventually M moves and a small righting lever develops. This vessel is in a neutral equilibrium.
Unstable - M below G. Can also be expressed as KG being greater than KM. This is very dangerous; a capsizing lever will develop until the vessel reaches an angle of loll. This vessel is in an unstable equilibrium.
GZ
A positive GZ will only form when a vessel in a stable equilibrium is heeled. The GZ lever opposes the external heeling force and ultimately brings the vessel back upright. GZ curves will be covered by the next article in this series.
The affect of cargo, including heavy lifts, on vessel stability
Cargo will affect a vessels stability depending on
Where it is stowed
Whether it is solid or liquid
Cargo stowed higher than the centre of gravity will raise the centre of gravity, thus reducing GM and making the ship more tender. Cargo stowed below the centre of gravity will lower the centre of gravity, making the ship stiffer.
It goes without saying that cargo loaded off-centre will cause the vessel to list.
Remember; the centre of gravity moves towards any loaded weight and away from any discharged weight.
Liquid cargoes will reduce GM by increasing the Free Surface Effect. This applies to any and all liquids in containers (unless those tanks are 'pressed up’) and to some cargoes that can act as a liquid, like grain.
During loading
When loading cargo using a shoreside crane, the cargo only affects the stability of the vessel when it is landed onboard. When loading cargo with the vessels own derricks or crane however, the shift in G will occur when the derrick or crane takes the weight of the cargo, and the weight will be applied at the height of the crane. This can result in a significant increase in G and thus reduction in GM.
All other factors being equal, the heavier a lift, the greater will be the affect on G.
Changes in stability during the voyage
As the vessel consumes fuel, water and stores the centre of gravity will move away from the reduced weights.
Liquefaction
From MGN 513:
In a dry, granular, cargo the individual particles are in contact with each other such that frictional forces prevent them sliding over one another. However, if there is enough moisture present then there is the potential for the cargo to behave like a liquid. This is because settling of the cargo occurs under the influences of vibration, over stowage and the motion of the ship. As such, the spaces between the particles reduce in size with an accompanying increase in water pressure between the particles. This results in a reduction in friction between the particles and can allow the cargo to shift suddenly.
Cargoes liable to liquify are classed as Group A cargoes under the IMSBC.
Transportable moisture limit
Cargoes liable to liquify are assigned a Transportable Moisture Limit (TML). The moisture content must be below this value for a cargo to be loaded and carried. Various tests exist to find the moisture content of a cargo.
How to reduce free-surface effect
At the design/construction stage, tanks can be fitted with longitudinal subdivisions or with baffles (bulkheads with holes in them). These either stop liquid from moving or slow its movement.
Practically speaking, there are two options for the seafarer when it comes to reducing free surface effect; either empty tanks or press them up.
Water on deck also induces a free surface effect; ensure that all freeing ports, draining holes and scuppers are clear and unobstructed.
Ice accretion
Ice accretion occurs when a vessel encounters below-zero degrees air temperatures and cold seawater and is made worse by high winds, rain, fog and spray. As ice builds up on the superstructure of the vessel (generally at the forward end) G rises and GM reduces.
The first step in dealing with ice accretion is to reduce ice accretion; reduce speed if running into the weather in order to reduce spray, turn on heaters up forward to increase the temperature of the vessel and consider sending crew out with steam lances, hammers, salt etc. to physically remove the ice.
If facing dangerous ice accretion it would be possible to ballast low in the vessel, possibly even taking her below her marks, though this should be a last resort.
Fixing an angle of loll
To bring a lolled vessel upright, load on the low side and/or discharge on the high side. This is counterintuitive, and the loll will initially worse, but eventually a righting lever will develop. Once this has happened, load low on the high side and/or discharge high on the low side.
Inclining test
Inclining tests are required for all passenger vessels and all cargo vessels of 24m of more in length, under SOLAS Chapter II-1 B-1 Regulation 5. Known weighs are added to an upright vessel following launch, and shifted transversely across the deck. The deflection of the vessel form upright is recorded. If any significant alterations are made to the vessel then she will need to be inclined again. It may be possible to dispense with the need for an inclining test, for example if a sister ship has already been inclined.
MCAQs
How do we find G?
How do we find M?
Draw a diagram of an inclined vessel, showing the righting lever GZ.
Explain how the size of GM affects stability.
Explain the difference between a stiff ship and a tender ship?
Explain the three possible equilibriums of a vessel?
What happens to G when a weight is loaded onto the vessel?
What happens to G when a weight is discharged from the vessel?
Does a full tank have free surface effect?
What information would you find in the stability book (check MSN 1752)