Synchronous and parametric rolling
1The lesson in plain words
Heavy rolling builds when the waves meet you at your roll period, or at half of it. Change speed or heading to break the rhythm.
Know your roll period
The natural roll period TR is the time for one full roll, from one side to the other and back. Time several rolls in calm conditions, or use approved ship-specific loading information. GM, metacentric height, indicates initial resistance to heeling. For the same ship with similar mass distribution, smaller GM generally means a longer TR. Size and mass distribution also matter, so a long TR alone does not prove marginal stability. Re-check it for each loading condition.
Synchronous rolling
Synchronous rolling builds when the encounter period matches the roll period: TE ≈ TR. Each wave pushes the ship just as she rolls that way, like pushing a swing. In following and quartering seas, TE is longer and may match a long TR. Example: Tw = 10 s at 12 knots in a following sea gives TE ≈ 16.7 s; with TR = 17 s, change speed or course.
Parametric rolling
Parametric rolling comes from stability changing as waves pass along the hull: less with a crest amidships, more with a trough amidships. It builds when TE ≈ TR or, in the classic case, TE ≈ 0.5 TR, when stability dips twice in every roll. Ships with fine, flared bows and wide, flat sterns, such as large container ships and car carriers, are most at risk, in head and bow seas as well as following seas. Large rolls can build within a few cycles.
Worked example
A large container ship has TR = 26 s, so 0.5 TR = 13 s. In head seas with Tw = 14 s, at 14 knots TE = 588 ÷ (42 + 14) = 10.5 s. Slowing to 3 knots gives TE = 588 ÷ 45 ≈ 13.1 s, close to half TR. Slowing has increased parametric-rolling risk. Choose a safe speed or heading change to move TE away from both 13 s and 26 s, while checking the ship's response.
Countermeasures and design
Watch for rolling that builds in step with the pitching, or rolls much larger to one side. Change speed, heading or both safely, then re-check TE against TR and 0.5 TR. Check other hazards and monitor the response; use motion monitoring or anti-roll systems where fitted. IMO's MSC.1/Circ.1627 (2020) introduces second-generation intact stability criteria for five failure modes: dead ship, excessive acceleration, pure loss of stability, parametric rolling, and surf-riding and broaching. They may support ship-specific operational guidance. Use that guidance where available.
2Key data
- Synchronous rolling
- TE ≈ TR
- Parametric rolling
- TE ≈ TR or TE ≈ 0.5 TR
- Worked example
- TR 26 s; head sea Tw 14 s at 3 kn → TE ≈ 13.1 s ≈ 0.5 TR
- Long roll period
- Lower GM generally lengthens TR on the same ship in like conditions
- Design criteria
- MSC.1/Circ.1627 (2020): five failure modes
3Key points
- 1
Check TR for your loading; a long TR alone does not prove marginal stability. Use approved stability information.
- 2
Synchronous rolling at TE ≈ TR; parametric rolling at TE ≈ TR or 0.5 TR.
- 3
Slowing in head seas can push a big ship into parametric resonance.
- 4
Change speed, heading or both; move TE away from TR and 0.5 TR, check other hazards and monitor the response.
4Frequent mistakes
Wrong: Parametric rolling only happens in beam seas.
Right: It typically occurs in head, bow, following and quartering seas, when TE is near TR or half of TR.
Wrong: Slowing right down head to sea always stops the rolling.
Right: Slowing lengthens TE in head seas and can bring it to 0.5 TR. Check TE before and after.
5Practice case & exam keys
Your car carrier, TR = 22 s, heads into a swell of Tw = 12 s at 6 knots. She starts rolling 15°, more each cycle, in step with the pitching. What is happening and what do you do?
Show the answer
TE = 432 ÷ (36 + 6) ≈ 10.3 s, near 0.5 TR = 11 s: suspect parametric rolling. Change speed, heading or both to move TE away from 11 s and 22 s. At the unchanged head-sea heading, 14 knots gives TE ≈ 8.6 s. Assess slamming, steering and other limits before choosing a change. Follow ship-specific guidance and monitor whether rolling decreases; reassess if it persists. Inform the engine room and log the action.
Exam keys
- Synchronous: TE ≈ TR. Parametric: TE ≈ TR or 0.5 TR.
- Container ships and car carriers: parametric roll in head seas at low speed.
- MSC.1/Circ.1627: second-generation criteria, five failure modes; may support ship-specific operational guidance.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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