SHIP CONSTRUCTION Group C Basujit Chakravarty Harsh Thakkar

SHIP CONSTRUCTION Group C Basujit Chakravarty Harsh Thakkar Vimal Raj Menon Shantanu Mishra

Explain, with the help of diagrams the effect of dynamic forces on the structure of the ship.

DEGREES OF FREEDOM Six ship motions are illustrated in the diagram. When these motions are large then very large forces may be generated. These forces are often of a local nature, but are liable to cause the structure to vibrate and thus transmit stresses to other parts of the structure.

DYNAMIC FORCES Dynamic Forces result from • The ship’s motion in the sea • The action of the wind and waves • The effects of operating machinery Effects of Dynamic Forces • Sagging • Hogging • Racking • Torsion • Pounding or Slamming • Panting

SAGGING In a heavy seaway, a ship may be supported at the ends by the crests of waves while the middle remains unsupported. If the wave trough is now considered at midships then the buoyancy in this region will be reduced. With the wave crest positioned at the ends of the ship, the buoyancy here will be increased. This loading condition will result in a bending moment which will make the ship to sag.

HOGGING If the wave crest is considered at mid-ships then the buoyancy in this region will be increased. With the wave trough positioned at the ends of the ship, the buoyancy here will be reduced. This loading condition will result in a significantly increased bending moment, which will cause the ship to hog. This will be an extreme condition giving the maximum bending moment that can occur in the ship’s structure.

RACKING When a ship is rolling, the accelerations on the ship’s structure are liable to cause distortion in the transverse direction. The deck tends to move laterally relative to the bottom structure, and the shell on one side to move vertically relative to the other side. This type of deformation is referred to as ‘Racking’. Transverse bulkheads primarily resist such transverse deformation, the side frames contribution being insignificant, provided the transverse bulkheads are at their usual regular spacing. Its greatest effect is felt when the ship is in the light or ballast condition. The brackets and beam knees joining horizontal and vertical items of structure are used to resist this distortion.

TORSION A ship traversing a wave train at an angle will be subject to a twisting moment (torque) and the structure in ‘torsion’ The greatest effect occurs with decks having large openings. In some ships, a heavy torsion box girder including the upper deck is provided at the topsides to accommodate the torsional stresses.

POUNDING OR SLAMMING Heavy pitching assisted by heaving as the whole ship is lifted in a seaway may subject the forepart to severe impact from the sea. The greatest effect is experienced in the lightship condition. To compensate for this, the bottom over 30% forward is additionally strengthened in ships exceeding 65 m in length and in which the minimum draught forward is less than 0. 045 L in any operating condition.

PANTING This is a stress which occurs at the ends of a ship due to variations in water pressure on the shell plating as the ship pitches in a seaway. This tends to create and in-and-out movement of the shell plating, known as panting. The effect is accentuated at the bow when making headway. Additional stiffening is provided in the form of panting beams and stringers.