Scrubber Retrofit on a Ro-Pax Ferry: Stability Margin Erosion
A 135 m ro-pax ferry on a Hong Kong–Macau route displaces 6,400 t in its worst operating condition and carries 850 passengers and 1,200 lane-metres of vehicles. Its approved stability booklet gives KG 8.20 m, KM 9.48 m and a limiting intact GM of 1.20 m; the damage-stability limiting GM is also 1.20 m. The owner has contracted to retrofit two hybrid scrubber towers, each 55 t with VCG 16.0 m above baseline, and a 30 t battery room at VCG 3.0 m. Two wash-water tanks, each 8 m long and 6 m broad, will normally be part full; the yard's calculation quotes a combined free-surface moment of 300 t·m. The yard's naval architect states that “GM remains positive, so the modification is acceptable” and asks the owner's surveyor to sign the stability booklet amendment before the inclining experiment. The vessel is due to enter service in three weeks and the berth has been sold out.
1. Calculate the new effective GM and determine whether the modification meets the stated limiting GM. State your assumptions and show the arithmetic.
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Take the original displacement as 6,400 t and original KG as 8.20 m, so original solid GM = KM − KG = 9.48 − 8.20 = 1.28 m.
Added weight moment:
- Scrubbers: 2 × 55 × 16.0 = 1,760 t·m
- Battery room: 30 × 3.0 = 90 t·m
- Total added moment = 1,850 t·m
New displacement = 6,400 + 110 + 30 = 6,540 t. New KG = (6,400 × 8.20 + 1,850) / 6,540 = (52,480 + 1,850) / 6,540 = 54,330 / 6,540 = 8.307 m.
Solid GM = 9.48 − 8.307 = 1.173 m. Free-surface correction = 300 / 6,540 = 0.046 m. Effective GM = 1.173 − 0.046 = 1.127 m.
The limiting GM is 1.20 m, so the modification fails by about 0.073 m even before refining KM at the deeper draught. “Positive GM” is not the acceptance criterion. Assumptions: KM is approximately unchanged at this small displacement increase; 300 t·m is the maximum free-surface moment; no ballast or cargo change is included. A full hydrostatic check at 6,540 t could refine KM slightly but cannot recover 73 mm of GM.
2. Explain why the intact GM calculation alone is insufficient and what damage-stability and verification work the surveyor should require before signing the amended booklet.
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Intact GM is only one acceptance gate. This is a passenger ship, so the applicable damage-stability standard in SOLAS II-1 Part B/B-1 for its build date must still be satisfied after the retrofit. That assessment uses attained index A versus required index R and survival factors for each damage case, including residual freeboard, heel, range and openings. The retrofit raises KG and adds free-surface moments, so damage cases that previously passed may now fail, especially with passenger crowding to one side and vehicle-deck free surface. The limiting GM of 1.20 m may have come from the original worst damage case, but the new worst case must be identified, not assumed.
The yard must run the full required damage-case set with the new lightship KG and maximum wash-water tank free surface, and the results must be approved by class. An inclining experiment is required after the modification to establish actual lightship displacement, KG and free surface; a calculation alone is not acceptable for a major topside change. The surveyor should also check that the loading manual, stability instrument and passenger evacuation procedures reflect the new condition. Signing before the inclining experiment would put the surveyor's verification in the wrong order.
3. Recommend a technically viable route to compliance and describe the approval and surveyor sign-off process.
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The owner needs to recover at least about 0.073 m of effective GM with margin for calculation uncertainty. Options should be compared with draught, load-line, power and operational penalties.
One viable combination: add about 100 t of fixed ballast at VCG 1.0 m. New KG = (54,330 + 100) / 6,640 = 8.197 m; solid GM = 1.283 m; effective GM = 1.283 − (300/6,640) = 1.238 m. This passes, but the extra 100 t increases draught and must be checked against the load line and route limits. Relocating the 30 t battery from 3.0 m to 1.0 m saves only 60 t·m, about 0.009 m of KG, so it is useful but not sufficient alone. Lowering both towers by 5 m saves 550 t·m, giving roughly 1.21 m effective GM - marginal and sensitive to the free-surface assumption. Subdividing or shrinking the wash-water tanks to cut the 300 t·m free-surface moment is also effective and should be pursued.
The approval path is: class-approved damage-stability recalculation, inclining experiment after the modification, updated stability booklet and loading manual, verification of damage control and evacuation arrangements, and reissue or endorsement of the Passenger Ship Safety Certificate. The surveyor signs only after the inclining results and class approval are in hand; the yard's programme pressure is not a technical justification.