Why ships float, why they return upright, and why some never do
Use the links below to jump to any section:
- Introduction – Stability Is About Behaviour, Not Numbers
- Why Ships Float at All
- Weight, Buoyancy, and Equilibrium
- What “Stable” Actually Means
- The Role of the Centre of Gravity (G)
- The Role of the Centre of Buoyancy (B)
- Righting Moment – How a Ship Comes Back Upright
- Initial Stability vs Overall Stability
- Why Some Ships Feel “Stiff” and Others “Tender”
- Stability Is Dynamic, Not Static
- Why Stability Failures Are Often Sudden
- Stability as a Professional Responsibility
- Closing Perspective
1. Introduction – Stability Is About Behaviour, Not Numbers
Stability is often taught as a collection of formulas, curves, and criteria. That approach creates officers who can pass exams but do not truly understand why ships behave the way they do.
In reality, stability is about how a ship reacts when something disturbs it.
Wind, waves, turning forces, cargo movement, flooding — all of these apply moments to the hull. Stability is the ship’s ability to resist those moments and recover.
Numbers describe stability.
They do not create it.
2. Why Ships Float at All
A ship floats because it displaces water.
This is not a maritime convention — it is basic physics. A floating body displaces a volume of water whose weight equals the total weight of the body itself.
If the ship weighs 50,000 tonnes, it must displace 50,000 tonnes of water to float. If it weighs more, it sinks deeper until enough water is displaced to balance it — or until it reaches the seabed.
This balance is not optional. It is enforced continuously by gravity.
3. Weight, Buoyancy, and Equilibrium
Two vertical forces act on every ship at all times.
Weight acts downward through the ship’s centre of gravity (G).
Buoyancy acts upward through the ship’s centre of buoyancy (B).
When these forces are aligned vertically, the ship is in equilibrium.
When they are not aligned — for example, when wind or waves heel the ship — a rotational moment is created. Stability is the ship’s response to that moment.
4. What “Stable” Actually Means
A stable ship is not one that does not move.
A stable ship is one that, when disturbed, develops a force that tends to return it toward upright.
If a disturbance causes the ship to heel and the resulting forces push it further over instead of back, the ship is unstable — regardless of how good the numbers once looked.
This distinction is critical.
Stability is not the absence of motion.
It is the presence of recovery.
5. The Role of the Centre of Gravity (G)
The centre of gravity is the point through which the total weight of the ship acts.
Every weight onboard affects it:
- cargo
- ballast
- fuel
- stores
- crew
- water in tanks
When weights are added high up, G rises.
When weights are added low down, G lowers.
When weights move sideways, G shifts sideways.
The ship does not “know” what the weights are — it only reacts to where G ends up.
This is why careless loading and poor ballast management silently destroy stability long before any alarm sounds.
6. The Role of the Centre of Buoyancy (B)
The centre of buoyancy is the centroid of the underwater volume of the hull.
Unlike G, B moves when the ship heels.
As the ship heels, the underwater shape changes. More volume appears on one side, less on the other. The centre of buoyancy shifts toward the side with more immersed volume.
This movement is fundamental to stability. It is what allows buoyancy to create a righting lever.
No movement of B means no restoring force.
7. Righting Moment – How a Ship Comes Back Upright
When a ship heels, G remains fixed relative to the ship, while B moves with the underwater shape.
This separation creates a horizontal distance between the lines of action of weight and buoyancy. That distance is called the righting lever (GZ).
The force of buoyancy acting through that lever produces a righting moment, which attempts to rotate the ship back upright.
If the righting moment is strong, the ship recovers quickly.
If it is weak, recovery is slow or incomplete.
If it reverses, capsize becomes inevitable.
Stability lives or dies in this lever.
8. Initial Stability vs Overall Stability
Initial stability refers to how the ship behaves at small angles of heel, usually the first few degrees.
This is often described using GM (metacentric height), which measures how quickly righting moment builds at small angles.
However, initial stability is not the whole story.
A ship can have:
- high GM (stiff at small angles)
- poor righting energy at larger angles
Such a ship feels “safe” initially but may capsize once a certain angle is exceeded.
Overall stability depends on the entire righting curve, not just the first part of it.
9. Why Some Ships Feel “Stiff” and Others “Tender”
A stiff ship has strong initial stability. It resists heeling and snaps back upright quickly. This often produces fast, uncomfortable rolling.
A tender ship has weaker initial stability. It heels more easily and rolls slowly, often feeling more comfortable — until limits are reached.
Neither condition is inherently safe or unsafe.
Safety depends on:
- available righting energy
- range of stability
- loading condition
- operational environment
Comfort is not a reliable indicator of survivability.
10. Stability Is Dynamic, Not Static
Stability changes continuously.
As fuel burns, G moves.
As ballast transfers, G moves.
As cargo is loaded or discharged, G moves.
As tanks slacken, free surface raises G.
The ship you sail is not the ship you loaded.
This is why stability must be understood as a process, not a departure condition.
11. Why Stability Failures Are Often Sudden
Stability failures rarely provide gradual warning.
A ship may operate apparently normally until a critical threshold is crossed. Beyond that point, righting moment may collapse rapidly.
This is why stability accidents often feel “instantaneous” in reports.
In reality, the failure was being prepared silently — through loading decisions, ballast changes, or assumptions — long before the final trigger occurred.
12. Stability as a Professional Responsibility
Stability is not delegated to software, terminals, or surveyors.
Ultimately:
- cadets learn it
- officers calculate it
- masters sign for it
- shore staff investigate it
The laws of physics do not recognise job titles.
When stability is lost, accountability always finds its way back to the decisions that shaped G and B.
13. Closing Perspective
Stability is not a set of numbers to be complied with.
It is a physical relationship between weight and water that determines whether a ship comes back when the sea tries to knock it down.
Every calculation you will learn later exists to describe one simple question:
“If the ship is pushed, will it recover — and how much energy does it have left?”
Understanding that question first is what separates trained professionals from people who only follow screens.
14. Knowledge Check – Stability Fundamentals
Before moving on, use the questions below to test whether the physical meaning of stability is clear.
These are not trick questions. They are designed to reveal gaps in understanding that will cause problems later when calculations are introduced.Take time to think through each answer. If you cannot explain it in your own words, revisit the relevant section.
Conceptual Understanding
- What does stability describe about a ship’s behaviour rather than its condition?
- Why does a ship float, and what physical law enforces this continuously?
- What must be true about forces acting through G and B for a ship to be in equilibrium?
- Why is a ship that “does not move much” not necessarily a stable ship?
- What physically changes when a ship heels that allows a righting moment to develop?
Centres and Forces
- Why does the centre of gravity (G) not move when a ship heels, but the centre of buoyancy (B) does?
- What happens to G when weight is added high up compared to low down?
- Why does careless loading often damage stability long before problems are visible?
- What creates the righting lever (GZ), and why is it critical?
- What does it mean if the righting lever reduces instead of increases with heel?
Stability Behaviour
- Why can a ship with strong initial stability still be unsafe overall?
- What is the difference between a “stiff” ship and a “tender” ship in terms of motion and risk?
- Why is comfort a poor indicator of survivability?
- How do fuel consumption and ballast changes affect stability during a voyage?
- Why do many stability failures appear sudden even though the causes developed slowly?
Professional Responsibility
- Why is stability considered a process rather than a departure condition?
- Who ultimately carries responsibility for a ship’s stability, regardless of software or advisors?
- Why are stability accidents rarely caused by a single mistake?
- What is the real question every stability calculation is trying to answer?
- Why is understanding stability physics more important than memorising formulas?
15. Knowledge Check – Model Answers
Use these answers to verify understanding, not to memorise wording.
If your explanation differs in wording but matches the meaning, that is acceptable.
- Stability describes how a ship responds to disturbance and whether it tends to return upright.
- A ship floats because it displaces water equal in weight to the ship; this is enforced by buoyancy and gravity.
- The lines of action of weight and buoyancy must be vertically aligned.
- A ship can appear steady but still lack sufficient righting ability once disturbed.
- The underwater shape changes, causing the centre of buoyancy to shift sideways.
- G is fixed by weight distribution, while B depends on the underwater volume which changes with heel.
- G rises when weight is added high and lowers when weight is added low.
- Because G shifts silently and stability margins reduce without obvious warning signs.
- GZ is created by the horizontal separation between weight and buoyancy forces and generates the righting moment.
- It indicates loss of restoring force and increasing risk of capsize.
- Initial stability only describes small angles; it says nothing about behaviour at larger angles.
- A stiff ship resists heel strongly and rolls quickly; a tender ship heels easily and rolls slowly.
- Comfort relates to motion, not available righting energy or survivability.
- They change the position of G over time, altering stability continuously.
- Because stability margin collapses rapidly once a critical threshold is crossed.
- Because stability changes with loading, fuel use, ballast transfer, and environmental forces.
- The Master carries ultimate responsibility.
- Because they develop through cumulative decisions rather than a single error.
- Whether the ship can recover from disturbance and how much energy it has to do so.
- Because formulas describe behaviour — they do not create it.
Tags
ship stability · buoyancy · centre of gravity · righting moment · cadet training · maritime fundamentals · cargo operations