Which ONE or MORE of the following methods of shifting weights can be used to improve the transverse stability of a rectangular pontoon?
Note: CG denotes the vertical centre of gravity of the pontoon.
Note: CG denotes the vertical centre of gravity of the pontoon.
Show Hint
- From below the CG to above the CG
- From above the CG to below the CG
- From port to starboard at the same height from the keel
- From forward to aft at the same height from the keel
The Correct Option is B
Solution and Explanation
Transverse stability of a floating body is measured by its metacentric height, \(GM = KB + BM - KG\). Since KB and BM stay the same for a given draft and waterplane shape, the only way a vertical weight shift changes GM is by changing KG, the height of the centre of gravity above the keel.
- From below the CG to above the CG: wrong. Moving weight upward raises the overall KG, which reduces GM and makes the pontoon less stable, the opposite of what is asked.
- From above the CG to below the CG: correct. Moving weight downward lowers the overall KG, which directly increases GM and improves transverse stability.
- From port to starboard at the same height from the keel: wrong. This is a purely transverse shift at constant height, it moves the centre of gravity sideways and causes a heel, it does not change KG or GM.
- From forward to aft at the same height from the keel: wrong. This is a longitudinal shift, it affects trim, not transverse stability, and it does not change KG either.
Only lowering the centre of gravity, option B, genuinely raises GM and improves transverse stability, so B alone is correct.
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