Aux 2 Unit 16 Q7

In the context of a vessel’s hull, racking refers to the distortion or deformation of the ship’s structure due to lateral forces, primarily caused by rolling in heavy seas. These forces tend to move the ship’s structure sideways, causing the vertical sides of the ship, including bulkheads and frames, to twist or deform in a parallelogram-like shape. This can impose significant stress on the structural integrity of the ship.

Key Points About Racking:

1. Lateral Forces: Racking occurs when the vessel is subjected to lateral forces, such as those from waves striking the side of the ship, causing a sideways motion or rolling.
2. Deformation: The main structural effect of racking is that it causes the vessel’s frame to twist or distort, especially in the transverse (side-to-side) direction. The vertical frames and bulkheads bend out of their normal right-angle positions, creating a racking stress.
3. Structural Resistance: To resist racking, ships are reinforced with a network of structural components, including:
   • Transverse Bulkheads: These act like walls running across the width of the vessel and help distribute racking loads.
   • Side Frames and Brackets: Strong frames and brackets at the ship’s sides provide extra rigidity to reduce racking.
   • Diagonals or Web Frames: These diagonal supports are often added to stiffen the structure and resist racking forces.
4. Importance of Control: If left unchecked, racking forces can lead to structural fatigue, cracks, and even compromise the ship’s watertight integrity over time.

In summary, racking refers to the sideways distortion of a ship’s hull structure due to lateral forces, and it is resisted by the ship’s internal framework, including bulkheads, side frames, and diagonal bracing. Proper design and structural reinforcement are essential to mitigate racking and maintain the vessel’s integrity during operation.

Racking in a vessel’s hull occurs when lateral forces, such as those generated by rolling in heavy seas, cause distortion or deformation of the ship’s transverse structure. These forces push and pull the sides of the vessel, leading to a twisting or shearing effect on the vertical frames and bulkheads. Here’s a more detailed explanation of how racking occurs:

1. Lateral Wave Impact

• Cause: Racking primarily occurs when a vessel is subjected to side forces from waves, particularly when the vessel rolls due to wave action. This lateral pressure from the sea creates uneven forces along the sides of the hull.
• Effect: These forces push the port (left) side of the ship in one direction and the starboard (right) side in the opposite direction, causing a twisting effect on the ship’s structure.

2. Rolling Motion

• Cause: The rolling motion of the vessel, which occurs when waves strike the sides, amplifies the lateral forces acting on the hull. The ship leans or tilts from side to side, increasing the pressure on its transverse structure.
• Effect: The rolling motion causes the vertical sides of the ship, including the frames and bulkheads, to bend or shift, putting stress on the transverse structural members. This results in deformation where the vertical structures, such as bulkheads and frames, no longer remain perpendicular to the deck and keel but are skewed into a parallelogram shape.

3. Distortion of the Ship’s Structure

• Cause: As the rolling and wave impact continues, the repeated application of lateral forces distorts the transverse framework of the ship. The side plating, bulkheads, and frames experience stress, leading to a shearing effect.
• Effect: The vertical components of the ship’s structure, such as bulkheads and side frames, twist or deform, no longer maintaining their right-angled position relative to the deck and bottom. This distortion is the characteristic effect of racking.

4. Localized Weakness

• Cause: Certain parts of the ship’s hull, particularly large open spaces such as cargo holds or passenger areas, may experience more severe racking forces because they lack the rigidity that smaller, confined spaces provide.
• Effect: These large open spaces can be more susceptible to deformation if not adequately supported with bulkheads or bracing. The lack of structural support can make the hull more vulnerable to racking stress.

5. Structural Stresses and Fatigue

• Cause: Continuous exposure to racking forces, especially in rough sea conditions, can lead to the accumulation of structural fatigue in the hull over time. These stresses may cause the ship’s framework to weaken.
• Effect: If not properly managed, racking can lead to cracks in the hull or bulkheads, distortion of the ship’s frames, and in extreme cases, structural failure.

Prevention of Racking

To prevent or reduce racking, shipbuilders incorporate several structural elements:

• Transverse Bulkheads: These provide rigidity and help resist the lateral movement of the hull.
• Diagonal Bracing or Web Frames: Additional reinforcement helps to stiffen the structure and maintain the ship’s integrity against racking forces.
• Strong Side Frames: The frames and brackets along the sides of the hull provide additional support to resist racking forces.

In summary, racking occurs when lateral forces, such as wave impacts and rolling, cause the vessel’s transverse structure (frames, bulkheads, and side plating) to distort, creating a twisting or shearing effect. This stress can lead to structural fatigue and eventual failure if not properly controlled with appropriate design features.

To resist racking in a vessel’s hull, the ship’s structure is designed with specific elements that provide lateral stability and prevent deformation caused by lateral forces. These structures are crucial for maintaining the hull’s shape and ensuring its structural integrity when the vessel is subjected to rolling or wave impacts. The key structures that resist racking include:

1. Transverse Bulkheads

• Function: Transverse bulkheads act as vertical walls running across the width of the ship. These bulkheads divide the hull into compartments and provide rigidity, helping to prevent the sides of the vessel from twisting or deforming under lateral forces.
• Role in Racking Resistance: They distribute racking forces across a wider area, preventing the hull from distorting. The bulkheads also limit the size of large open spaces, reducing the potential for significant lateral movement.

2. Web Frames (Deep Frames)

• Function: Web frames, also known as deep frames, are large, stiffened frames that run transversely across the hull. They are designed to provide additional support to the ship’s side structure and resist lateral forces.
• Role in Racking Resistance: Web frames offer greater strength and rigidity compared to standard frames. They help maintain the alignment of the hull structure, preventing it from twisting or skewing under pressure.

3. Diagonal Bracing

• Function: Diagonal bracing consists of angular stiffeners or braces that are installed at an angle, often between frames or bulkheads, to provide additional resistance against deformation.
• Role in Racking Resistance: Diagonal braces help counteract the shear stresses caused by racking, reinforcing the hull against twisting by spreading the lateral forces more evenly through the ship’s structure. They are particularly useful in resisting the parallelogram-like deformation associated with racking.

4. Side Frames

• Function: Side frames are the vertical members of the ship’s structural framework that run transversely along the side plating of the hull. They provide rigidity and support to the hull.
• Role in Racking Resistance: These frames resist the bending and distortion of the hull’s sides when lateral forces act on the vessel. Strong side frames help maintain the integrity of the vessel’s transverse structure, reducing the chances of racking.

5. Decks and Stringers

• Function: Decks are horizontal structural members that provide transverse strength to the ship’s hull, while stringers are longitudinal stiffeners attached to the hull’s side plating.
• Role in Racking Resistance: Decks act as horizontal diaphragms that help distribute racking forces. They provide rigidity and prevent excessive movement or deformation of the hull. Stringers, placed longitudinally along the sides, add further resistance to lateral forces by stiffening the side plating.

6. Strong Beam Knees (Brackets)

• Function: Beam knees or brackets are angled supports used to reinforce the connection points between decks and bulkheads or frames.
• Role in Racking Resistance: By strengthening these connection points, beam knees provide additional stiffness to the overall structure, reducing the chance of racking deformation at critical junctions.

7. Double Bottom Structure

• Function: The double bottom structure provides additional strength at the bottom of the vessel by creating a second layer of hull plating with transverse floors and longitudinal girders.
• Role in Racking Resistance: The double bottom stiffens the bottom of the vessel and resists the racking forces that could otherwise cause deformation at the base of the hull. It also provides a solid foundation for the transverse bulkheads and side frames.

8. Sheer Strakes

• Function: Sheer strakes are the plates located near the top of the ship’s hull, running longitudinally along the edge of the deck.
• Role in Racking Resistance: These strakes add strength to the hull’s upper edge, preventing the top of the side plating from deforming under lateral loads. This structural element plays a crucial role in resisting racking forces near the deck level.

In summary, racking is resisted by a combination of transverse bulkheads, web frames, diagonal bracing, side frames, decks, beam knees, double bottom structure, and sheer strakes. These elements work together to provide rigidity and lateral strength to the vessel, minimizing the deformation caused by lateral forces, particularly during rolling in heavy seas.