If the name of this article suggests that it has something to do with the strength of the ship structure, then leave your presumptions at the door. But rinsing up the basic concepts of stability would reap a better understanding of what tender and stiff ships are all about. Form the designer's point of view, is that, deciding on the suitable metacentric height (GM) of the ship is a very important factor in setting the stability and comfort standards of the ship. Why? Well, this has everything to do with the behaviour and response of the ship in rolling motion.
A ship having very high center of gravity (CG) will end up having a low metacentric height (GM), resulting in a reduced righting lever (GZ). Which means:
- More force is required for the heeling ship to return to the upright position.
- If you look into the formula for the roll period of a ship (below), the roll period will be high for a ship with low GM.
The rolling pattern of this ship will be sluggish. It will take longer time to roll back to an upright position, rendering the ship to be comfortable. Such kind of a vessel will be a tender ship. Cruise ships and ocean liners (ships that are designed for more comfort) are designed to be slightly on the tender side, with roll periods around 10 to 12 seconds. At the same time, the designer maintains a minimum required GM to provide the ship with stability. A ship too tender (very low or zero metacentric height), would be unstable. A tender ship will also have more probability of capsizing in case of large weight shifts or high speed turns and strong beam winds. So optimum level of safety and comfort is attained midway between a very high and very low metacentric height. This comes primarily from experience and feedback from existing ships. The Korean ferry MV Sewol (article) is now said to have capsized due to an overloading. Its CG was actually higher than what it was designed for, which resulted in a very low GM, resulting in the rapid capsize. This is would not have happened, had the Captain made a check on the tenderness of the ship that day.
On the other hand, a ship having very high metacentric height (GM) would show the following behaviour:
- A stiff ship will tend to respond to the wave profile more rapidly, tending to assume the slope of the passing wave.
- So, even though a stiff ship will develop rolling moment easily in a passing wave, it will also require less force to return to an upright position, rendering the ship more stable.
- Also, the time period of the rolls would be shorter.
In many cases, ships were found to be too tender after trials. Redesigning and rebuilding is an uneconomical and meaningless option to think about. So in such cases, permanent ballasting often proved to be useful, wherein one or more void tank spaces in the lower compartments of the hull were filled with calculated amounts iron ore or similar high specific weight material that resulted in lowering the center of gravity by required levels to attain the required metacentric height. For ships that were on the stiffer side, motion dampeners like stabilizer fins and anti-roll tanks were incorporated.
A lot of experience goes into designing a ship for its required metacentric height according to stability standards and at the same time, keep the roll periods in favourable limits. Still doubting the importance of these terms? Imagine you own two cruise ships. One that can capsize faster than your childhood paper boats and the other that tosses your passengers around with every passing wave! LSD
Article By: Soumya Chakraborty