GM and Free Surface on a Timber-Laden Coaster
M/V Fjordlast, 3,200 GT, is completing loading sawn timber in Kristiansund for a UK port. From the loading computer: displacement 4,600 t, KM 6.80 m, KG 6.35 m (solid), giving GM 0.45 m. For trimming, No. 2 double-bottom tank (12 m long × 7 m wide × 1.6 m deep) is 60% full of ballast and No. 3 (same dimensions) is 45% full; neither tank has wash bulkheads. The deck cargo is 380 t of timber, stowed 2.2 m high, with lashings and uprights checked. The forecast for the passage is SW 8–9 with 6 m seas. The load line is not exceeded, the ship floats at her marks, but she has a 3° list to starboard. Ballast density is 1.025 t/m³.
1. Calculate the free surface correction and the fluid GM, and assess the condition against the IS Code 2008 intact stability criteria.
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Free surface correction.
- Transverse moment of inertia of one tank: i = l·b³/12 = 12 × 7³/12 = 343 m⁴.
- Free surface moment per tank: FSM = ρ·i = 1.025 × 343 = 351.6 t·m. Two slack tanks: 703.2 t·m.
- FSC = ΣFSM/Δ = 703.2 / 4,600 = 0.153 m.
- Fluid GM = 0.45 − 0.15 = 0.30 m.
Assessment. The IS Code 2008 minimum initial GM of 0.15 m is met, but only just - the criterion is a floor, not a safe target. With 380 t of timber 2.2 m high on deck the GZ curve will be flat: maximum GZ will be small and will occur at a large angle, and the area criteria (0.055 m·rad to 30°, 0.09 m·rad to 40°, 0.03 m·rad between 30° and 40°) must be checked on the loading computer's GZ curve for this exact condition, with free surface included. Timber absorbs water and, in a Norwegian November, ice can accrete on the stow, both raising KG. A 0.30 m fluid GM leaves no margin for that, so the condition should be treated as unacceptable for the forecast.
2. Explain how you would remove the 3° starboard list with the least additional free surface, and what you would check after each step.
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Correcting the list.
- The 3° list is a transverse weight moment, not a stability failure by itself, but with GM 0.30 m it takes very little to produce it.
- Do not fill a starboard (low-side) tank to correct a starboard list; that increases it and adds free surface.
- Fill one port-side double-bottom tank completely - No. 2 port first, being nearer the centre of flotation and adding least trim - then recheck the list on the inclinometer and the loading computer.
- If still listed, fill No. 3 port in stages, but press each tank up to the top (or empty it completely). The two slack tanks were the source of the 0.153 m FSC; the aim is to finish with no slack tanks except one small tank used for fine adjustment.
- After each step check: fluid GM, list, trim, draughts and freeboard, load line, and the GZ curve. Record soundings.
- Watch the overflow and tank vents, and never press up a tank without checking the air pipe and high-level alarm. Correcting list by ballast takes time; do it alongside, not after departure.
3. The forecast is SW 8–9. State your decision on sailing and the measures you would require before departure.
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Decision: do not sail as loaded into SW 8–9. Before departure I would require:
- All double-bottom tanks either full or empty, so the FSC is reduced to near zero, and the loading computer re-run with the final condition.
- The GZ curve verified against all IS Code 2008 criteria, including water absorbed by the timber and a realistic ice allowance for the season.
- If the GZ curve still fails or is marginal, either reduce or lower the deck cargo or add low ballast to raise GM - checking that the load line, freeboard and timber load line (if applicable) are still satisfied and that the extra draught still gives adequate UKC for the passage.
- Cargo securing checked against the CSS Code and timber deck cargo provisions: compact stow, uprights, lashings, no interference with freeing ports or crew access.
- The company informed of the delay and the reason. The master's authority on seaworthiness is not overridden by the sailing schedule.