So your GM is good, your GZ areas exceed the required minimums, you’re all set for a safe passage, right?
Not necessarily. Your calculations may become invalid once you cast-off because stability is a moving target.
There are dynamics at work on your vessel, a mechanism that depletes stability reserves in a dynamic process whereby energy is gained from the wind and waves.
A ship underway at sea has energy on all six axes of motion and there are energy exchanges between these axes. The 6 axes are pitch, roll, heave, sway, surge, yaw.
For example energy on the yaw axis can build until directional control is lost, rudders and thrusters become ineffective.
In following and quartering seas yaw instability can result in rapid energy transfer into the roll axis.
Excess energy on the roll axis can result in cargo shift, flooding, injuries, drowning, broaching and capsize.
A capsized hull can be resuscitated or salvaged, but dead crew cannot be.
Combine these dynamics with a novice at the helm, deteriorating weather and we can expect unpleasant consequences.
Owners worry over the abilities of new inexperienced crew.
An alternative is a sensor which displays depleted stability data to enable ship’s officers to make the best decisions to recover lost stability reserves. An example is the Stabilicube, currently installed in a number of anchor handlers, OSVs and tugs, which uses a bargraph display as the voyage progresses and the helmsman and OOW can track trends in energy transfers by reviewing the energy history over the past hour. When the total energy of the ship and wave system has been increasing, he keeps a more frequent eye on the height of the red bar.
Peter Ebbutt of Ocean Motion, which makes the Stabilicube sensor says: “Simulators can certainly help but there will always be that first time when the helmsman is challenged in deteriorating conditions or when he suffers impaired judgement from fatigue. A growing number of owners are aiding their novice crews by giving them a sensor that displays the depleted stability after wave impacts and resonant rolling. The helmsman then sees regained stability when he makes the best speed and heading changes. Ditching cargo is not an option”.
He gives the example of a recent March 2012 Pacific Cyclone onboard a brand new Offshore Supply Vessel, OSV: “The helmsman altered heading and speed to keep the energy transfer levels low for the duration of the storm’s 130knot winds and 10 Metre seas. Records from the Stabilcube showed just 60% righting energy consumed at the height of the storm, thus avoiding the ultimate loss of life, cargo and vessel. The vessel and crew survived unscathed because they made the right speed and heading choices”.