Stability Practice Questions

1. If a vessel's KM is 22 ft and her KG is 19 ft, her metacentric height (GM) is:

• A.41 ft
• B.1.16 ft
• C.3 ft✓
• D.19 ft

Why: GM = KM − KG = 22 − 19 = 3 ft. A positive GM means the metacenter is above the center of gravity, so the vessel is initially stable.

2. For small angles, the righting arm GZ ≈ GM × sin(heel). With GM = 2 ft and a heel of 10°, GZ is approximately:

• A.0.35 ft✓
• B.1.0 ft
• C.0.20 ft
• D.2.0 ft

Why: GZ ≈ GM × sin θ = 2 × sin 10° = 2 × 0.174 ≈ 0.35 ft. The righting arm is the horizontal distance between the lines of buoyancy and gravity.

3. A vessel with a very LARGE GM will be:

• A.Unaffected in her rolling
• B.Tender, with a slow easy roll
• C.Stiff, with a short, snappy roll✓
• D.Unstable and prone to loll

Why: A large GM makes a vessel 'stiff' — strong righting moments give a short, quick (snappy) roll. A small GM makes her 'tender' with a slow, easy roll.

4. A vessel with a small NEGATIVE GM will:

• A.Loll — heel to an angle and hang there or roll to the other side✓
• B.Have an unusually large righting arm
• C.Be the most stable condition
• D.Sit perfectly upright

Why: A negative GM means the upright position is unstable; the vessel develops an angle of loll, hanging heeled to one side (and may flop to the other), rather than capsizing outright.

5. To improve the stability of a tender vessel, you should:

• A.Increase the free surface
• B.Add weight as high as possible
• C.Lower the center of gravity — add weight low or remove weight high✓
• D.Slack off all the tanks

Why: Lowering KG (ballasting low, striking weight down, or pressing up slack tanks to kill free surface) raises GM and makes a tender vessel stiffer and more stable.