Stability & Heavy Weather

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

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:

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.

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:

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.

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.