Before departure: stability, strength and the worst condition
1The lesson in plain words
Before you sail, check the actual condition, not the plan, and the worst point of the voyage: stability, strength, draught, view ahead and records.
The departure condition
Calculate the final condition from actual figures. Check draughts against voyage load-line limits; GM, corrected for free surface, against the booklet’s minimum; and stability against the approved limiting KG/GM curve or applicable GZ (righting-lever) criteria of the International Code on Intact Stability (IS Code). Check shear forces and bending moments using the limits approved for sea passage.
The worst condition
Stability changes on passage. Fuel is burnt and fresh water used, often from low tanks, so KG usually rises and more tanks go slack. Timber deck cargo can absorb water and ice can build up on deck and superstructure: weight added high. Calculate the arrival condition and any worse point in between. The ship must meet the criteria throughout the voyage, not only when she sails.
Other checks
Check propeller and rudder immersion and suitable trim. From the conning position, SOLAS V/22 requires the sea surface visible within two ship lengths or 500 m forward of the bow, whichever is less, within 10° either side. Check air draught under bridges and under-keel clearance allowing for squat, the sinkage and change of trim caused by moving through water. Secure cargo; set required hatches and watertight and weathertight closures for sea. Keep essential ventilation and tank vents working.
Records
Keep the signed departure stability and strength printout. Record draughts forward, aft and amidships, freeboard and water density in the log book as your flag requires. Port State Control inspects these records, and after an accident they are the first thing investigators ask for.
If it fails
Fix it before you sail. For deficient stability, lower high weights or reduce free surface; removing low weights can worsen GM. Adding low ballast may help, but check its free surface, load line and hull stresses. For overload, reduce weight. Recheck stability, draught, trim and strength at every affected stage. The master authorises sailing only after the revised plan passes; give honest figures.
2Step by step
Departure checklist
- 1
Enter actual cargo, tank soundings and densities. Compare calculated draughts with observed draughts and resolve any unexplained difference.
The plan is not the ship; the check must use what is really aboard.
- 2
Check draughts against the load line for the zone and every zone on the voyage.
The ship must not be overloaded at any point of the passage.
- 3
Check corrected GM and the approved stability limits at departure, arrival and the worst intermediate stage.
Consumption and slack tanks usually make the arrival condition worse.
- 4
Check shear forces and bending moments against the seagoing limits at those stages.
Use the approved seagoing limits, which account for wave loads.
- 5
Check trim, propeller immersion, view ahead, air draught and under-keel clearance allowing for squat.
A ship can pass stability and still be unsafe to navigate.
- 6
Sign and retain the calculation; log draughts, freeboard and density as your flag and company require.
These records let the master and inspectors verify the departure checks.
3Key points
- 1
Check actual figures: load line, free-surface-corrected GM, approved stability limits, shear forces and bending moments.
- 2
The arrival or a mid-voyage point is often worse than departure: calculate it.
- 3
Burnt fuel from low tanks, slack tanks, wet timber and ice all raise effective KG.
- 4
SOLAS V/22: sea obscuration ≤2 ship lengths or 500 m forward of the bow, whichever is less, within ±10° ahead.
- 5
Sign and keep the printout; log draughts, freeboard and density.
4Frequent mistakes
Wrong: The departure condition passes, so the voyage is covered.
Right: Calculate arrival and any worse point. Burnt fuel, slack tanks, water absorption and ice can take a passing ship below the criteria.
Wrong: Harbour stress limits are fine for the sea passage.
Right: At sea, use the loading manual’s approved seagoing limits; do not substitute harbour limits.
5Practice case & exam keys
Your departure condition meets every criterion. The arrival condition, after 9 days of fuel burnt from double-bottom tanks and three tanks slack, falls just below the required GM. The ship is ready to sail and the pilot is aboard. What do you do?
Show the answer
Tell the master: the ship does not meet the criteria throughout the voyage, so as calculated she must not sail. Fixes include ballasting a low tank on passage, planned and calculated, or taking ballast low now, drawing fuel so fewer tanks are slack, or adjusting cargo. Recalculate the corrected plan for every stage, record it and then sail.
Exam keys
- Criteria must be met throughout the voyage: departure, arrival and worst point.
- Use seagoing shear force and bending moment limits at sea; harbour limits only in port.
- SOLAS V/22: sea obscuration ≤2 ship lengths or 500 m forward of the bow, whichever is less, within ±10° ahead.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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