Naval Arch 04 - Effects of Loading on Stability

Summary

In this lecture, Laura Alfred explores how dynamic loading conditions affect a ship's stability. She explains how high deck cargo, ice accumulation, and partially filled tanks can raise the center of gravity and reduce metacentric height (GM), lowering stability. A key focus is the free surface effect, where liquid sloshing in tanks shifts the virtual center of gravity, further decreasing GM. Alfred highlights how splitting wide tanks with longitudinal bulkheads mitigates this effect, improving stability. She also discusses the concept of list caused by off-center loading and emphasizes designing ships to handle these variable conditions safely throughout a voyage.

Takeaways

• ⚓ Ship stability changes during a voyage due to consumption of fuel and water, ice accumulation, and offloading or loading cargo.
• 🧊 Ice on the deck or heavy cargo placed high increases the center of gravity (KG) and decreases GM, reducing initial stability.
• 🚢 Emptying tanks at the bottom of the hull raises KG and lowers GM, making the ship less stable.
• 💧 Partially filled tanks create a liquid free surface effect, allowing liquid to slosh and shift the center of gravity, further reducing stability.
• 📏 Free surface correction is the virtual shift in the center of gravity caused by liquid movement in partially filled tanks.
• 🔧 Splitting wide tanks into two longitudinal tanks reduces liquid sloshing, resulting in a smaller free surface correction and higher corrected GM.
• 📉 Corrected GM can be calculated by subtracting the sum of all free surface corrections from the original GM.
• ⚖️ Excessive slack tanks can lead to corrected GM values less than zero, creating dangerous stability conditions.
• ↔️ Off-center cargo or liquid sloshing can cause a lateral shift of the center of gravity, producing a list or nonzero angle of heel.
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• 📊 Ships should be designed to minimize list and maintain stability under various operational conditions, while the most stable condition is when cargo is fully secured and tanks are properly filled.
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• 💡 Proper ship design must account for dynamic changes in KG, GM, free surface effects, and loading conditions to ensure safety during operations.

Q & A

What are some factors that can affect a ship's stability during a voyage?

Factors include ice accumulation on the deck, heavy cargo on deck, offloading or emptying tanks in the hull, and partially filled tanks that create a liquid free surface.

How does adding weight high on the ship, such as ice or cargo on the deck, affect stability?

Adding weight high on the ship increases the center of gravity (KG), which decreases the metacentric height (GM), reducing the ship's initial stability.

What is the 'liquid free surface' effect and why is it important?

The liquid free surface effect occurs when a tank is partially filled with liquid, allowing it to slosh as the ship heels. This movement shifts the center of gravity laterally and effectively reduces GM, making the ship less stable.

What is the virtual shift of the center of gravity?

The virtual shift of the center of gravity, also called the free surface correction, is the lateral movement of the ship’s center of gravity caused by the sloshing of liquid in partially filled tanks.

How can longitudinal bulkheads mitigate the free surface effect?

Longitudinal bulkheads divide wide tanks into narrower compartments, limiting lateral movement of liquid. This reduces the virtual shift of the center of gravity and results in a higher corrected GM, improving stability.

How is the corrected GM of a ship calculated when considering free surface effects?

Corrected GM is calculated by subtracting the sum of all free surface corrections from the original GM: GM_corrected = GM_original - Σ(free surface corrections).

What is meant by 'list' on a ship, and what causes it?

List is a nonzero angle of heel caused by off-center loading, such as heavy cargo on one side or liquid shifting in wide tanks. It reduces the magnitude of the righting arm (GZ) and affects stability.

Why is minimizing wide slack tanks important for ship stability?

Wide slack tanks allow more lateral movement of liquid, increasing the free surface effect and reducing GM. Narrower or split tanks reduce this effect and enhance stability.

What happens to the center of gravity and GM when low hull tanks are emptied?

Emptying low hull tanks removes weight from the bottom of the ship, causing the center of gravity (KG) to rise, which decreases GM and reduces initial stability.

What are the recommended practices for maintaining ship stability under varying loading conditions?

Best practices include fully securing cargo, minimizing slack tanks and wide tanks, using longitudinal bulkheads, monitoring GM, and avoiding list during operational conditions.

How does offloading or loading cargo affect ship stability temporarily?

During cargo operations, uneven or off-center loading can temporarily create a list and reduce the righting arm (GZ), which is why careful planning and securing of cargo are essential.

Why is understanding free surface corrections critical in ship design?

Because multiple slack tanks can cumulatively reduce GM to unsafe levels. Designers must account for free surface effects to ensure sufficient stability under real operational conditions.