Slack tanks, or partially filled tanks, significantly impact a ship's stability due to the free surface effect

• When tanks are partially filled (slack), the liquid inside moves freely as the ship heels.
• This movement shifts the centre of gravity (G) laterally towards the heeling side.
• The righting lever (GZ) decreases, resulting in reduced metacentric height (GM) and overall stability.
• The virtual loss of GM is proportional to the breadth of the tank and the height of the free surface.

Consequences of Slack Tanks:

• Increased angle of heel.
• Reduced stability, making the ship tender and more prone to capsizing.

To Minimize the Effect:

• Avoid slack tanks where possible; either fill tanks completely or empty them.
• Design tanks with longitudinal divisions or swash bulkheads to limit liquid movement.
• Use sluice valves in tanks to control liquid transfer.
• Prioritize filling smaller tanks at the ship's bottom to lower G and improve stability.

Ice accumulating on the superstructure adds mass high up on the ship. This raises the centre of gravity (G), decreasing the metacentric height (GM). A lower GM reduces stability, making the ship more tender (more easily rolled) and increasing the period of roll. The ship becomes more susceptible to capsizing.

Wind:

• High freeboard or tall superstructures increase windage, leading to a greater rolling effect.
• Rolling caused by wind reduces stability, especially if the ship remains heeled for a prolonged period.

Waves:

• Large waves, especially when the ship is on the crest, can cause a significant loss of stability due to reduced underwater buoyant volume.
• The ship may develop excessive heeling or capsizing tendencies.
• Long ships are more vulnerable to wave action due to greater surface exposure, further reducing stability.

• Water Absorption by Deck Cargo:
  • Certain types of deck cargo, such as timber or other absorbent materials, can take up water during a voyage.
  • This absorbed water increases the weight of the cargo, raising the centre of gravity (G) of the ship.
  • As G moves higher, GM reduces, leading to decreased stability.
  • Retention of Water on Deck:
    • Water that accumulates on the deck, such as from heavy rain or seawater, adds additional weight to the ship's upper structure.
    • This shifts G upward, reducing GM and stability.
    • Retained water may also cause a list if it collects asymmetrically, further impacting the ship's balance.
    • Consequences:
      • Increased risk of capsizing in rough seas.
      • Increased rolling and reduced righting ability.
      • Mitigation Measures:
        1. Ensure proper drainage systems are in place to quickly remove water from the deck.
        2. Regularly monitor and secure deck cargo to prevent excessive water absorption.