Marginal GM from slack double-bottom tanks before sailing into a rising wind
MV Selby Progress is loading steel coils at Aldenham. Displacement is 8,500 t, KM is 7.60 m and the loading computer gives KG 7.05 m, a GM of 0.55 m. For trim, the chief officer has left No.3 port and No.3 starboard double-bottom tanks slack, each with a free surface moment of 1,240 t·m; both tanks are on the centreline. The ship is due to sail at 1800 with a forecast of SW force 6 and a 2 m swell. The roll period measured during the afternoon has lengthened from 17 s to 24 s. At 1700 the second officer reports a steady 3° list to port; the tank soundings are unchanged and no weights have been shifted. The intact stability criteria of the IS Code 2008 apply. Ballast is available in the forepeak and in No.1 double bottom.
1. Calculate the effective GM and assess the ship against the applicable criteria. Would you sail?
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Free surface correction = total free surface moment ÷ displacement = (1,240 + 1,240) ÷ 8,500 = 2,480 ÷ 8,500 = 0.29 m.
Effective GM = 0.55 − 0.29 = 0.26 m.
The IS Code 2008 Part A criteria require GM₀ of at least 0.15 m, an area under the GZ curve to 30° of at least 0.055 m·rad, GZ maximum of at least 0.20 m occurring at 25° or more (preferably 30° or more), an area to 40° of at least 0.09 m·rad, an area between 30° and 40° of at least 0.03 m·rad, and a maximum righting lever at an angle of at least 25°. GM alone does not prove compliance; the GZ curve must be checked against all of them.
At 0.26 m the ship passes the GM minimum on paper but with almost no margin, and the lengthening roll period from 17 s to 24 s is independent evidence of a small effective GM. With SW 6 and a 2 m swell forecast, I would not sail as loaded. The correct decision is to press up or top up the two slack tanks one at a time, which removes the free surface entirely, then re-run the loading computer and re-check the GZ curve and the draught and UKC before departure.
2. The ship has a steady 3° list to port, unchanged soundings and a long roll period. Distinguish between a list and an angle of loll, and say how you would test which you have.
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A list is caused by G being off the centreline: the ship is stable, the righting lever curve still starts at the origin with positive slope, and the ship rolls at a normal period. A weight added on the low side increases the list, and transferring weight to the high side reduces it. An angle of loll is caused by negative GM: G is on the centreline but above M, the ship heels until the GZ curve crosses zero again, and she floats at that angle with zero righting moment. The roll is slow and sluggish, the ship hangs at the end of the roll, and she may flop suddenly from one side to the other.
Tests in this case: the long roll period points to loll, not list, and the absence of any weight transfer or change in soundings argues against an off-centre weight. Check the loading computer's GM and the GZ curve, and look at the sounding sequence. A small known weight shifted across the deck will move a listed ship predictably but will move a loll unpredictably or not at all. If the ship is at loll, ballasting into the low side makes it worse; the remedy is to lower G centrally by filling double-bottom tanks on both sides or pressing up slack tanks, and to remove free surfaces. If it is a list, correct it by transferring ballast to the high side in small steps.
3. Set out the corrective actions in sequence, with the risks at each step.
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- Stop cargo work and tell the terminal the ship will not sail on schedule. Close and secure everything, and confirm all adjacent tanks and the double-bottom are intact; a leak into a slack tank can explain both the list and the free surface.
- Press up or top up the two No.3 tanks one at a time, not simultaneously. Each filling removes free surface, so the effective GM improves with each step, but a partially filled tank on one side is itself an off-centre weight and will increase the list while filling. Complete one tank before starting the other.
- After each step, take soundings, read the list and re-run the loading computer. Watch the low-side draught and the deck edge immersion, and check the freeboard and the UKC against the berth depth and the channel.
- Correct the residual list with small transfers in double-bottom tanks, keeping the tanks either full or empty. Avoid leaving a tank slack at sea.
- Verify the final condition against all the IS Code criteria on the GZ curve, not just GM; check the draught and trim for the passage, the UKC, and the lashing of the coils. Only then sail.
Principal risks: over-ballasting and losing deadweight; pressing up the wrong tank and increasing the list; and treating a loll as a list and worsening it.