Why the first touch during the dry docking is at the aft perpendicular?

 The first touch during dry docking typically occurs at the aft perpendicular (the point at the stern where the keel touches the blocks). This happens for several critical reasons related to the ship’s structure, weight distribution, and stability during the dry docking process.

 

Why the first touch during the dry docking is at the aft perpendicular?



Reasons for First Touch at the Aft Perpendicular

  1. Structural Strength at the Stern:

    • The aft region of the ship, near the stern, is one of the strongest parts of the hull, as it often contains reinforced structures such as the engine room, propeller shaft, and rudder.
    • This area can better handle the initial concentrated forces when the ship begins to rest on the blocks.
  2. Weight Distribution:

    • Ships are designed with a trim by the stern, meaning they sit slightly deeper at the stern than at the bow. This natural trim causes the aft end to make contact with the blocks first as the water level is lowered.
    • The stern is typically heavier due to the machinery, fuel tanks, and other equipment located there, making it logical for this end to settle first.
  3. Controlled Stability:

    • Allowing the stern to settle first ensures the ship remains stable during the dry docking process. If another part of the ship (e.g., the bow) touched first, it could cause the ship to pitch or become unstable.
  4. Gradual Transition of Support:

    • By starting at the aft perpendicular, the ship gradually transfers its weight to the blocks as the water level decreases. This minimizes sudden stresses on the hull and ensures even weight distribution over time.
  5. Alignment of the Keel Blocks:

    • The keel blocks are designed to support the ship along its length, starting from the aft perpendicular. Ensuring the stern is aligned first helps guide the rest of the ship into its proper position.
  6. Protection of Propeller and Rudder:

    • During dry docking, it’s essential to avoid any sudden or uneven contact that could damage sensitive components like the propeller or rudder. Allowing the stern to touch first ensures these parts are safely aligned and supported before the rest of the ship settles.

The first touch at the aft perpendicular is a deliberate and well-engineered step in the dry docking process. It takes advantage of the ship’s natural trim, structural strength, and weight distribution to ensure stability, minimize stress on the hull, and protect critical components like the propeller and rudder. This approach ensures a safe and controlled transition from buoyancy to block support

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