Kuryk Loading: Reading Roll Period and Free Surface Before Sailing
A 4,200 t displacement coaster (L 88 m, B 22 m) is completing loading at Kuryk. The plan places 1,150 t of steel coils in the lower hold, stowed with axes fore-and-aft, chocked and lashed, plus 380 t of bagged ferro-alloy on deck. Alongside in sheltered water the chief officer times a roll period of 33 seconds. Soundings show No.2 double-bottom tank (10 m long × 8 m wide) about half full and No.3 (same dimensions) about one-third full; he is unsure whether to top them up or empty them. The ship has a 3° list to starboard and the starboard deck lashings are visibly slack. From the stability booklet: KM 8.10 m, KG (solid, including cargo) 7.60 m, free surface moment 437 t·m for each slack tank, present draft 6.0 m. The forecast for the 20-hour passage is north-west 25–30 kn with 2.5 m seas, and the charterer is pressing for an immediate departure.
1. From the figures given, calculate the solid and corrected GM, cross-check with the roll period, and decide whether the ship may sail as loaded.
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Solid GM = KM − KG = 8.10 − 7.60 = 0.50 m.
Free surface correction per tank = 437 / 4,200 = 0.104 m; two slack tanks = 0.208 m.
Corrected (fluid) GM = 0.50 − 0.208 = 0.29 m. Roughly 42% of the ship's stability has been eaten by two part-filled tanks - the tanks, not the cargo, are the problem.
Cross-check with the measured roll period: GM ≈ (0.8B/T)² = (0.8 × 22 / 33)² = (0.533)² = 0.284 m, which agrees with 0.29 m to within reading error. That agreement is valuable: it says the booklet KG, the tank soundings and the roll observation are all consistent, so the number can be trusted.
Against the IS Code 2008 Part A criterion of initial GM ≥ 0.15 m the ship formally passes, but that criterion is a floor, not a passage plan. With 380 t on deck, 25–30 kn forecast, a 3° list and a GZ curve still unverified, there is no margin. Do not sail as loaded. Correct the tanks and the list, verify the GZ curve, then sail.
2. Compare topping up against emptying the two tanks, and state the loading plan you would give.
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Both tanks must end up either completely full or completely empty - a slack tank costs its full free surface moment whatever the level.
- Top both up: each tank holds about 10 × 8 × 1.2 m × 1.025 ≈ 98 t, so ~196 t of low ballast is added. This removes the whole 0.208 m free surface correction and adds weight below the centre of gravity, lifting GM to roughly 0.70 m or better. Cost: draft increases by perhaps 0.15 m, which must be checked against the berth depth, the channel UKC at Kuryk and the load line marks.
- Empty both: also removes the free surface, but removes low weight and slightly raises KG; the same GM gain is not achieved and pumping oily ballast ashore creates its own problem.
- Fill one, leave one slack: worst option - half the benefit for the same pumping effort.
Plan: first find the source of the 3° list - check for an off-centre coil or an uneven bagged stow, and shift it rather than using ballast to hide it. Then fill the port-side tank(s) first in steps, checking list after each, and top up both tanks completely with the ship upright. Re-time the roll period and re-run the departure condition in the booklet. Correct the slack starboard lashings and re-tension before departure.
3. What stability and cargo checks must be completed before the pilot is disembarked, and what would make you refuse departure?
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Stability: the departure and arrival conditions in the booklet must be recomputed with the tanks full, showing initial GM ≥ 0.15 m; area under the GZ curve to 30° ≥ 0.055 m·rad and to 40° ≥ 0.09 m·rad; area between 30° and 40° ≥ 0.03 m·rad; GZ at 30° ≥ 0.20 m; maximum GZ occurring at not less than 25°. The arrival condition matters because fuel and water will have been consumed and the tanks may not stay full - plan the voyage with both tanks pressed up throughout, or accept a known slack tank deliberately and account for it. Free surface in a tank being consumed must be in the calculation, not discovered at sea.
Cargo: coils stowed with axes fore-and-aft, chocked, no point loading on the tank top beyond the allowed limit, lashings to the CSS Code arrangement and re-tensioned; deck cargo checked against the securing manual with adequate MSL and no slack on the starboard side; deck load limits and hatch cover strength verified; stability booklet GZ curve checked against deck cargo windage and water on deck.
Refuse departure if: corrected GM falls near or below 0.15 m; the GZ criteria are not met; the list repeats itself after ballasting (an unexplained list is a stow or flooding problem, not a trim problem); lashing cannot be made good; or the vessel would sail with a slack tank that the arrival condition cannot tolerate. Then inform the master in writing, and let him inform the company and charterer - the commercial pressure is not a stability input.