Planning the voyage condition: departure, arrival and the worst point
1The lesson in plain words
Plan departure, intermediate and arrival conditions: stability and hull stresses must stay within limits throughout the voyage.
Plan the voyage, not the berth
As chief mate you plan the cargo with stability and strength for every stage: departure, any intermediate point and arrival. Fuel, water and stores are used up on passage. Most of that weight sits low, so using it raises G, while part-empty tanks add free surface moments (FSM). The booklet’s criteria apply to every one of these conditions.
The worst condition
The arrival condition is often the worst: least fuel, most slack tanks, perhaps ice or water-soaked deck cargo. On some ships an intermediate point is worse, for example when a group of tanks is half full. The 2008 IS Code’s standard conditions include arrival with 10 % stores and fuel. At every stage, compare GM or KG with the approved limit on the booklet’s stated free-surface basis.
Consumables plan
Estimate daily consumption and the tanks it will come from, in the order the engineers will use them. Include fresh water production and any ballast changes. A fuel-consumption plan that empties tanks in an agreed sequence keeps free surface small; drawing from many tanks at once makes it large. Agree the order with the chief engineer.
Margins
Leave margins for weather delays, extra fuel and water absorbed by deck cargo. Plan any ballast you may need to take on passage, and check that it doesn’t overstress the hull. The seagoing shear force (SF) and bending moment (BM) limits apply at arrival too. Record each calculated condition.
2Step by step
Worked example: from departure to arrival
- 1
Departure: Δ 24,000 t, KG 8.20 m, KM 9.30 m; the checked free surface correction (FSC) is 0.050 m.
GM fluid = 9.30 − 8.20 − 0.050 = 1.05 m.
- 2
Passage of 12 days at 38 t/day: 456 t of fuel and 120 t of fresh water used from low tanks.
Removing low weights raises G.
- 3
Arrival Δ = 24,000 − 456 − 120 = 23,424 t. The checked weight-change calculation gives KG 8.380 m.
The arrival condition has less displacement and a higher KG.
- 4
The checked arrival tank calculation gives FSC 0.090 m.
More tanks are slack at arrival, so the correction has increased.
- 5
Read arrival KM 9.36 m from the hydrostatic tables at 23,424 t.
Use the arrival draught and trim, with their applicable booklet limits.
- 6
Arrival GM fluid = 9.36 − 8.380 − 0.090 = 0.890 m.
Compare it with the booklet minimum at this draught.
3Key data
- Worked free surface corrections
- Departure FSC 0.050 m; arrival FSC 0.090 m
- GM fluid
- KM − KG − FSC
- IS Code standard condition
- Arrival with 10 % stores and fuel
- Worked example
- GM fluid 1.05 m at departure → 0.89 m at arrival
4Key points
- 1
Check every stage of the voyage, not only departure.
- 2
Using low fuel and water raises G; slack tanks add free surface.
- 3
Arrival is often the worst condition; sometimes an intermediate point is.
- 4
Agree the fuel-consumption and water order with the chief engineer.
5Frequent mistakes
Wrong: If she leaves with good GM, she will arrive with good GM.
Right: Here GM fluid falls from 1.05 m to 0.89 m. Calculate the arrival and any worse intermediate condition.
Wrong: Use the departure free surface correction at arrival.
Right: Here the checked correction rises from 0.050 m to 0.090 m. Reusing the departure value would overstate arrival GM by 0.040 m.
6Practice case & exam keys
The booklet minimum free-surface-corrected GM at the arrival draught is 0.95 m. Your plan gives 0.89 m GM fluid at arrival. What can you change before sailing?
Show the answer
Don’t sail on this plan. Consider changing the fuel-tank order, keeping fewer tanks slack, keeping double bottom tanks full, adding low ballast before the margin is lost, or lowering heavy cargo. Check each change. Recalculate every stage against its own approved minimum GM or maximum KG at that draught and trim, and recheck strength limits.
Exam keys
- GM fluid = KM − KG − the checked free surface correction at that stage.
- Using low weights raises G; slack tanks add FSM.
- IS Code: check arrival with 10 % stores and fuel.
- The worst condition governs the plan.
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