FROM THE CAPTAIN'S WINDOW
Stories on Maritime Leadership
GZ or GM?
Most marine officers and naval architects will tell you that the righting-lever curve, GZ is a better measure of the ship’s stability than the metacentric-height (GM) value. I agree, and will narrate a personal experience which illustrates the point beautifully.
I was investigating an accident where a first-generation container ship had developed a severe heel during its voyage. The heel had resulted in damage to some containers in the cargo hold, especially where there were no cell-guides. There were several other contributing factors but I will focus on one very interesting lesson I learnt that day.
The International Maritime Organization Intact-Stability (IS) Code requires that the area under the righting lever curve (GZ curve) shall not be less than 0.055 metre-radians up to φ= 30° angle of heel and not less than 0.09 metre-radians up to φ= 40° or the angle of down-flooding φf if this angle is less than 40°. The initial metacentric height GM shall not be less than 0.15 m
What was interesting in this case was that the ship was complying with the above criteria but still behaved as if in an angle of loll. The ship first listed about 16 degrees on the port side and when the wind direction changed, flipped over the same angle to the starboard side. It was quite puzzling at first why the ship with a positive GM behaved in such a way, but looking at the GZ curve, things became clearer.
Picture of the GZ curve during the incident
The righting lever (GZ) values were very low until the angle of 16° which made a minor heeling moment lead to a much greater angle of heel. I am sure you have experienced this on ships with relatively low GM which just seem to tilt so uncomfortably by wind or even when tugs are pulling the vessel on the side. It is all about low righting lever which is not good.
An ideal curve looks something like the below.
The IS Code contains specific stability criteria for container ships of over 100 metres length; the area under the righting lever curve (GZ curve) should not be less than 0.009/C metre-radians up to φ= 30° angle of heel, and not less than 0.016/C metre-radians up to φ= 40° or the angle of flooding φf. The maximum righting lever GZ should be at least 0.042/Cm. The total area under the righting lever curve (GZ curve) up to the angle of flooding φf should not be less than 0.029/C metre-radians.
I would recommend Masters and Mates to have a look at the righting lever (GZ) curve to properly assess the stability at various legs of their voyage. It is indeed a better measure of ship stability.
Another lesson learnt for me was that on ship, or in life, do not rely on just one source of information. Cross-check and calibrate. It is better to be safe than sorry!
I am also attaching some good video links on the topic of righting-lever curves: