| Term | Definition |
|---|---|
| G — Centre of Gravity | The point through which the total weight of the vessel acts downwards. Fixed by the distribution of weights on board. Moving weight upward raises G; moving weight down lowers G |
| B — Centre of Buoyancy | The geometric centre of the underwater volume of the hull. Moves as the vessel heels — always moves toward the low side |
| M — Metacentre | The point about which a vessel heels for small angles of inclination. A fixed reference point for initial stability calculations |
| GM — Metacentric Height | The vertical distance between G and M. The primary measure of initial stability. GM must be positive (G below M) for stability |
| GZ — Righting Lever | The horizontal distance between the lines of action of weight (through G) and buoyancy (through B) when the vessel is heeled. GZ creates a righting moment that returns the vessel to upright |
| Action | Effect on G | Effect on stability |
|---|---|---|
| Adding weight high up | G rises | GM decreases — stability reduced |
| Adding weight low down | G falls | GM increases — stability improved |
| Removing weight high up | G falls | GM increases — stability improved |
| Removing weight low down | G rises | GM decreases — stability reduced |
| Consuming fuel from double bottom | G rises | GM decreases — monitor throughout voyage |
| Filling ballast tanks | G falls (if low tanks) | GM increases |
The free surface effect reduces the effective GM by an amount equal to the free surface moment divided by the vessel's displacement. Key points:
- The size of the free surface effect depends on the breadth of the tank (cubed) — wide, shallow tanks are very dangerous
- The depth of liquid in the tank does not matter — a nearly empty tank has almost the same free surface effect as a half-full tank
- Multiple slack tanks — the effects are cumulative and can be catastrophic
- Keep tanks either full or empty — avoid slack tanks, particularly at sea
- Fuel burn during a passage progressively slacks down fuel tanks — monitor GM throughout
Freeboard is the distance from the waterline to the lowest point where water could enter the vessel. Low freeboard:
- Reduces the range of positive stability — vessel may become unstable at smaller angles of heel
- Increases the risk of water ingress on deck in a seaway
- Deck flooding adds free surface effect and increases topweight — accelerates stability loss
- Freeing ports and scuppers must be clear — blocked scuppers trap deck water and dramatically reduce stability
| Opening | Requirement |
|---|---|
| Watertight doors | Closed at sea unless specifically required to be open for operational reasons — and only then with supervision |
| Hatches and companionways | Closed and secured in heavy weather or when water is on deck |
| Portholes and windows | Secured in heavy weather — a single open porthole at the waterline can flood a compartment rapidly |
| Scuppers and freeing ports | Clear and unobstructed at all times — blocked scuppers trap water on deck |
| Sea cocks and through-hulls | Inspected regularly — closed when not in use if below the waterline |
| Area | Actions |
|---|---|
| Deck | Secure all loose equipment, lines, fenders, deck furniture. Lash tenders and boats. Close all hatches, scuttles, and openings. Rig jacklines and safety nets if required |
| Interior | Secure all loose items — galley, saloon, cabins. Close and dog all watertight doors. Inspect bilges — pumps operational. Stow all breakables |
| Bridge | Plot nearest port of refuge on chart. Obtain latest weather forecast. Reduce speed as appropriate. Switch to manual steering. Post lookout. Engines to standby. Call Master |
| Machinery | Check bilge pumps operational. Reduce fuel consumption if endurance is a factor. Advise engine room of conditions |
| Crew and guests | Advise of conditions — restrict movement on deck. Issue seasickness medication if required. Muster and account for all persons |
| Stability | Check tank status — avoid or minimise slack tanks. Consider ballasting if stability is marginal. Check current GM against stability booklet |
The oral expects you to run this arithmetic out loud. Displacement 500 t, KG 6.20 m, KM 7.00 m (assume KM unchanged for the small change). You load a 10 t tender on the sundeck at Kg 12.0 m:
| Step | Working | Result |
|---|---|---|
| Moments about the keel | (500 × 6.20) + (10 × 12.0) = 3100 + 120 | 3220 t·m |
| New displacement | 500 + 10 | 510 t |
| New KG | 3220 ÷ 510 | 6.31 m |
| New GM | 7.00 − 6.31 | 0.69 m (was 0.80 m) |
Ten tonnes stowed high cost 11 cm of GM. Add a free surface correction for any slack tank — FSC = free surface moment ÷ displacement — and the fluid GM is the number that describes how she will actually behave. The drill page generates infinite variations of this calculation; the method is what the examiner marks.
| Feature | What it tells you |
|---|---|
| Initial slope | Proportional to GM — a tangent at the origin passes through the GM value at 57.3°. Steep = stiff, shallow = tender |
| Maximum GZ (and its angle) | The largest righting lever she can generate, and how far she must heel to find it |
| Angle of vanishing stability | Where the curve recrosses zero — beyond it the lever capsizes rather than rights. The edge of the envelope |
| Area under the curve | Dynamic stability — the energy she can absorb from wind and waves before capsize. What actually matters when a squall delivers energy, not a steady moment |
Diagnosis first: a vessel with a steady heel that, when you shift weight across, flops through upright and settles at the same angle the other side has an angle of LOLL — negative GM — not a list. A list (G off the centreline, positive GM) corrects with a transverse weight shift and stays corrected. Treating loll as list is the classic fatal error: every transfer makes the flop through upright more violent.
The cure is getting G down, not moving weight sideways:
| Order | Action | Why this order |
|---|---|---|
| 1 | Eliminate free surfaces — press up or empty slack tanks | Free surface is a virtual rise of G; killing it may restore positive GM by itself |
| 2 | Fill LOW tanks — double bottoms — starting with the tank on the LOW side | Filling the high side first lifts that side's buoyancy demand and can whip her violently across to the other loll angle |
| 3 | Slowly, one tank at a time, watching her respond | Each tank is a controlled experiment; two at once is a guess |
Rule of thumb: T ≈ 0.8 B / √GM (T in seconds, beam B in metres). A 9 m beam yacht with GM 0.8 m rolls in about 8 seconds; let GM erode to 0.35 m and the period stretches past 12 — long, lazy, with a hesitation at the end of each roll as the righting lever runs out of conviction. A crisp quick roll is a stiff ship: safe but violent — brutal accelerations that break lashings and injure crew. The watchkeeping skill is noticing the CHARACTER of the roll change mid-passage and asking why: fuel burned from low tanks (G rising), free surfaces multiplying, weight migrated upwards — then verifying with the calculation, not the feeling.
Wave encounter period matches the natural roll period — each wave arrives in step and adds energy, so rolls build in groups, each worse than the last, out of proportion to the sea. Classic in beam and quartering seas.
In head or following seas: the waterplane (and so stability) changes as bow and stern pitch over crests and troughs, pumping the roll at roughly half the roll period. Rarer, violent, and famous for surprising well-found vessels.
Damage stability is bought with bulkheads: flood one compartment of a properly subdivided vessel and she settles, trims and survives — provided the boundaries hold, which is why watertight doors are closed at sea and every opening in a bulkhead is glanded. The downflooding angle — the heel at which the first non-weathertight opening immerses — caps the usable GZ curve: vents, doors and hatches left open in a seaway quietly delete the top of your stability range.
The maximum KG / minimum GM curve in the stability book is the whole subject distilled into a go/no-go line: for each displacement it gives the worst KG at which all required criteria are still met. Any proposed condition — heavy lift, deck cargo, refit addition — is checked with one comparison: calculated KG (with free surface corrections) against the limiting KG for that displacement. Below the line: compliant. Above it: the answer is no, in writing, before the crane takes the weight.
Your vessel takes a heavy lift with the crane. When is stability worst — and why?
Why does firefighting threaten stability, and what do you do about it?
What is GM and why does it matter?
What is free surface effect and how do you manage it?
You are at sea and heavy weather is forecast. What preparations do you make?