Stability and Free Surface After a Ballast Transfer
A coastal vessel is alongside and preparing to sail. During ballast adjustment, one centre tank has been partly filled to about half capacity and two wing tanks are slack. The chief officer reports that the vessel feels tender and that the list is small but the righting feel is soft. The calculated metacentric height before ballasting was 0.45 m; the chief officer's quick estimate after ballasting is about 0.20 m, but he is not certain the free surface correction was applied. The weather forecast is for a moderate beam sea on the passage. The master asks for your engineering view before departure.
1. Explain why the vessel feels tender and what the free surface effect has to do with it.
Read the worked answer
A partly filled tank allows the liquid surface to remain horizontal as the vessel heels, so the liquid shifts to the low side. That shift moves the vessel's centre of gravity toward the low side and reduces the righting lever for a given angle of heel. The effect is accounted for as a reduction in metacentric height, called the free surface correction.
The more slack tanks, the larger the loss. A centre tank at half capacity and two slack wing tanks give significant free surface. If the chief officer's estimate of 0.20 m did not include the correction, the true effective metacentric height is lower still, which matches the tender feel and soft righting.
A small list with soft righting is consistent with reduced stability rather than a large off-centre weight.
2. What would you advise before departure, and how would you verify the stability condition?
Read the worked answer
Advice before departure:
- Do not sail on an uncertain stability calculation. The chief officer should redo the calculation with the free surface correction properly applied, using the actual tank contents.
- Reduce free surface by pressing up or emptying slack tanks where the ballast plan allows, so that as few tanks as possible are slack.
- Check the final list and trim, and confirm the vessel's righting feel after adjustment.
- Compare the result with the vessel's minimum required metacentric height for the intended condition and route.
Verification:
- Reconcile tank soundings with the calculation.
- Have the master review and approve the final loading and ballast condition.
- If the corrected metacentric height is marginal for the forecast beam sea, adjust ballast further or delay sailing.
My engineering role is to make sure the machinery and ballast systems can achieve the plan and to support the master's decision with clear information.
3. If the vessel must sail, what operational precautions would you recommend for the passage?
Read the worked answer
If the corrected stability is acceptable but not generous:
- Keep slack tanks to a minimum and avoid transferring ballast during the passage unless necessary; if transfer is needed, do it in a controlled way with the chief officer monitoring.
- Avoid sudden rudder and engine movements that induce large heel, especially in a beam sea.
- Consider altering course or speed to reduce rolling if the motion becomes uncomfortable or heavy.
- Secure machinery and stores against movement.
- Monitor the vessel's behaviour and report any change to the master.
If the corrected metacentric height is below the required minimum, the professional advice is not to sail until the condition is corrected. The master makes the final decision, but the engineering and stability information provided should be unambiguous.