How ship's Center of Gravity (G) moves?

Summary

This video explains the movement of a ship's center of gravity (G) and its effects on stability parameters, such as metacentric height (GM) and vertical distance from the keel (kg). It covers various scenarios, including shifting, loading, and discharging weights, as well as filling ballast tanks. The position of G relative to the metacenter (M) determines the ship's stability. The video emphasizes the importance of understanding these changes to ensure safe navigation and compliance with regulations, offering insights into how each action impacts the ship’s stability.

Takeaways

• 🚢 The ship’s center of gravity (G) is the point where the total weight of the vessel acts vertically downward, expressed as its height above the keel (KG).
• ⚖️ The metacentric height (GM) is the vertical distance between the ship’s center of gravity (G) and the metacenter (M), and it determines the ship’s initial stability.
• 📉 If the center of gravity (G) is below the metacenter (M), the ship is stable; if G is above M, the ship becomes unstable and prone to capsizing.
• 🧮 The position of G is crucial in calculating and maintaining a ship’s stability, safety, and compliance with international maritime regulations.
• 📦 When a weight is shifted upward (e.g., from lower hold to main deck), G moves upward, increasing KG and decreasing GM—reducing stability.
• ⚓ When a weight is shifted downward (e.g., from main deck to lower hold), G moves downward, decreasing KG and increasing GM—enhancing stability.
• 💧 Loading ballast water into higher tanks (like wing tanks) raises G, increasing KG and reducing GM, which lowers stability.
• 🌊 Filling ballast water in lower tanks (like double bottom tanks) lowers G, decreasing KG and increasing GM, which improves stability.
• 🚮 Discharging or removing weight from higher positions causes G to move downward, resulting in a lower KG and higher GM—thus improving stability.
• 🪣 Pumping out ballast water from lower tanks causes G to move upward, increasing KG and decreasing GM, making the ship less stable.
• 📘 The movement of G is always parallel and in the same direction as the movement of the weight being added, removed, or shifted.
• 📊 Determining the KG or vertical center of gravity (VCG) of liquids in tanks can be done using the ship’s sounding table.

Q & A

What is the ship's center of gravity (G)?

The ship's center of gravity, denoted by the letter G, is the point at which the total weight force of the ship is considered to act vertically downward. It represents the average position of all the weights on board.

What does KG represent in ship stability?

KG represents the vertical distance between the ship’s keel and its center of gravity (G). It is a key parameter used in stability calculations to determine how the weight is distributed vertically within the ship.

What is GM or metacentric height?

GM, or metacentric height, is the vertical distance between the ship's center of gravity (G) and its metacenter (M). It is a measure of the ship’s initial stability and its ability to resist rolling and capsizing.

How does shifting a weight upward affect KG and GM?

When a weight is shifted upward, the ship’s center of gravity (G) moves upward as well, resulting in an increase in KG and a decrease in GM, making the ship less stable.

How does shifting a weight downward affect KG and GM?

When a weight is shifted downward, the ship’s center of gravity moves downward, which decreases KG and increases GM, thereby improving the ship’s stability.

What happens to the ship’s center of gravity when a weight is loaded on the main deck?

When a weight is loaded on the main deck, the ship’s center of gravity moves directly toward the center of gravity of the added weight. This causes an increase in KG and a decrease in GM.

What is the effect of loading weight in the lower hold on ship stability?

Loading a weight in the lower hold moves the ship’s center of gravity downward, decreasing KG and increasing GM, which enhances the ship’s stability.

How does filling ballast water in the wing tank affect the ship’s stability?

Filling ballast water in the wing tank causes the ship’s center of gravity to move upward toward the weight’s center of gravity, resulting in an increase in KG and a decrease in GM, slightly reducing stability.

What is the effect of filling ballast water in the double bottom tank?

When ballast water is added to the double bottom tank, the ship’s center of gravity moves downward, leading to a decrease in KG and an increase in GM, which improves stability.

What happens when a weight is discharged from the main deck?

When a weight is discharged from the main deck, the ship’s center of gravity moves away from the center of gravity of the discharged weight. This causes KG to decrease and GM to increase, improving stability.

How does pumping out ballast water from the double bottom tank affect KG and GM?

Pumping out ballast water from the double bottom tank causes the ship’s center of gravity to move upward, leading to an increase in KG and a decrease in GM, thereby reducing stability.

Why is understanding the movement of a ship’s center of gravity important?

Understanding the movement of the ship’s center of gravity is crucial for ensuring vessel stability, safety, and compliance with international regulations. It helps in preventing capsizing and maintaining proper balance during loading or unloading operations.

What is the relationship between G and M in determining stability?

If the ship’s center of gravity (G) is below the metacenter (M), the ship is stable. If G is above M, the ship becomes unstable and is more prone to capsizing.

What is the role of inclining experiments in ship stability?

Inclining experiments are conducted when the ship is in a lightweight condition to determine the exact position of the center of gravity (G) and to calculate stability parameters such as KG and GM accurately.