Question

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.

• A. From below the CG to above the CG
• B. From above the CG to below the CG
• C. From port to starboard at the same height from the keel
• D. From forward to aft at the same height from the keel

Hint:

Ask which shift actually lowers the vertical centre of gravity, since GM depends only on KG among these choices.

Answer 1

The Correct Option is B

Step 1: Write down the GM relation.
$GM = KB + BM - KG$, where KB and BM are fixed by the hull shape and the draft.
Only KG changes when weight is shifted inside the pontoon, so only vertical shifts matter for GM.

Step 2: Test each shift against this rule.
Moving weight from below the CG to above the CG raises KG, so GM falls, ruling out option A.
Moving weight from above the CG to below the CG lowers KG, so GM rises, which is exactly what improves stability.
The port to starboard and forward to aft shifts happen at a fixed height above the keel, so KG does not change at all in either case, ruling out C and D.

Final Answer:
Only a downward shift of weight, from above the CG to below the CG, raises GM.
The correct option is B.