Is she stable enough? Criteria, roll period and warning signs
1The lesson in plain words
Use the approved booklet to check stability. At sea, a slow, hanging roll or an unexplained heel means investigate and act promptly.
What the criteria protect
The approved conditions meet a minimum GM, a minimum GZ (righting lever) at 30° or more, a maximum lever at 25° or more, and minimum areas under the GZ curve, which measure the energy she can absorb. A weather criterion checks a gust while she rolls. You don't recompute these at sea: you use the applicable booklet limits within their approved conditions.
Roll period as a check
With high GM she is stiff and tends to roll quickly. With low positive GM she is tender and tends to roll slowly. The IS Code estimates a full roll as T = 2CB ÷ √GM, in seconds. Use B (breadth), d (mean draught), L (waterline length) and GM in metres. Use free-surface-corrected GM, called fluid GM. The coefficient is C = 0.373 + 0.023(B/d) − 0.043(L/100). Rearranging gives GM ≈ (2CB/T)².
Use it with care
Time several full natural rolls, from one side to the other and back, in calm water, and average them. This is a rough check: waves, slack tanks and moorings disturb it; loading changes can alter mass distribution and C. Under comparable calm-water conditions, a longer natural roll can indicate lower GM. Investigate changed loading and conditions; the estimate cannot establish compliance.
Signs of low GM
She may heel strongly to a small wind or rudder angle, roll slowly or hang at the roll ends. A list is a steady heel from off-centre weight with positive GM. Loll occurs with negative upright GM: she may rest at an angle or flop between sides.
What to do
Stop lifting or hauling, reduce speed and avoid sharp turns. Report and seek help. Follow the booklet's recovery plan; lower or remove high weights, ice or deck water only where safe. If ballasting is permitted, follow its low-tank sequence. Heel may initially increase; check the downflooding margin, the remaining heel before water enters openings, and monitor heel and free surface. Fill the partner tank only as directed. Never transfer ballast to the high side of a lolling ship.
2Step by step
Worked example: timing the roll
- 1
Data: B 9.0 m, d 3.2 m, L 48 m. C = 0.373 + 0.023 × 2.81 − 0.043 × 0.48 = 0.417.
Use waterline length and mean draught.
- 2
In calm harbour water, ten full natural rolls take 70 s, so T = 7.0 s.
Average many rolls; one is not enough.
- 3
GM ≈ (2 × 0.417 × 9.0 ÷ 7.0)² = 1.07² = 1.15 m.
A rough estimate; compare with the applicable approved limit.
- 4
For comparison, assuming C and B remain unchanged in comparable calm-water conditions, T = 10.0 s gives GM ≈ (2 × 0.417 × 9.0 ÷ 10.0)² = 0.56 m.
The estimated GM is about half; investigate loading and conditions, then check the booklet.
3Key data
- IS Code roll period
- T = 2 × C × B ÷ √GM (s)
- Coefficient C
- 0.373 + 0.023 (B ÷ d) − 0.043 (L ÷ 100)
- GM from a timed roll
- GM ≈ (2 × C × B ÷ T)²
- Trend
- Longer comparable natural rolls may indicate lower GM
4Key points
- 1
Use the applicable booklet limits within their approved draught, trim and operating conditions.
- 2
Roll period: T = 2CB ÷ √GM; compare natural rolls under comparable conditions, allowing for changes in C.
- 3
Slow, hanging rolls or flopping warn of low or negative GM; distinguish list from loll.
- 4
Follow the approved loll-recovery plan; monitor heel and downflooding margin. Never transfer ballast to the high side.
5Frequent mistakes
Wrong: A slow, gentle roll means a comfortable, safe ship.
Right: A longer natural roll may indicate lower GM. Compare calm-water conditions and loading, investigate changes and use the approved booklet.
Wrong: Correct a list with no off-centre weight by pumping ballast to the high side.
Right: It may be loll from negative GM. Follow the approved recovery plan; transferring ballast to the high side can make her flop further the other way.
Wrong: A timed roll proves the criteria are met.
Right: It only estimates GM roughly. The criteria come from the booklet calculation.
6Practice case & exam keys
Late in a winter trip your small cargo ship has burned most of her double-bottom fuel and carries ice on deck. In calm water she lies 6° to starboard, then flops to 6° to port after a turn. No cargo has shifted.
Show the answer
Suspect loll. Stop heeling operations, reduce speed, avoid sharp turns and seek help. Follow the approved recovery plan: remove high weight only where safe. If ballasting is permitted, fill a narrow, divided low-side double-bottom tank first. Heel may initially increase; check the downflooding margin. Fill its partner only as the plan directs. Do not transfer ballast to the high side. Monitor heel and reassess stability throughout.
Exam keys
- IS Code roll period: T = 2CB ÷ √GM, with C = 0.373 + 0.023(B/d) − 0.043(L/100).
- Signs of low GM: slow natural roll or hanging at roll ends. List: off-centre weight with positive GM; loll: negative upright GM.
- Loll: follow the approved recovery plan; low-side ballasting may increase heel initially. Check downflooding margin and free surface; never transfer ballast to the high side.
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