ECSU/ /NASC /July 2007 /RB Ship Construction: Forces on Hull 1 To Be A World Class Maritime Academy • Learning Objective: Understand the various static and dynamic forces acting on the ship structure • Specific Objectives: • Describe the static forces acting in the structure. • Describe the dynamical forces acting on the structure. • Explain the imbalance of weight and buoyancy along the length of a ship. • Describe the conditions of hogging and sagging. • Describe the stresses in the top and bottom plating in objective 26.4. • Sketch and interpret a typical weight curve. • Sketch and interpret a typical: a. load curve b. shear force diagram c. bending moment diagram. • Sketch and interpret typical buoyancy curves when in: a. still water, b. a wave crest amidships and c. a wave trough amidships
• Learning Objective: Understand the various static and dynamic forces acting on the ship structure
• Specific Objectives: • Describe the static forces acting in the structure. • Describe the dynamical forces acting on the structure. • Explain the imbalance of weight and buoyancy along the
length of a ship. • Describe the conditions of hogging and sagging. • Describe the stresses in the top and bottom plating in
objective 26.4. • Sketch and interpret a typical weight curve. • Sketch and interpret a typical:
a. load curve b. shear force diagram c. bending moment diagram. • Sketch and interpret typical buoyancy curves when in:
a. still water, b. a wave crest amidships and c. a wave trough amidships
Understand some dynamic forces due to water pressure load on the ship’s hull. Understand and state which parts of the ship is affected by the dynamic forces.
Water pressure acts perpendicular to the surface and increases with depth. The effect of water pressure is to push in the ship’s sides and push up the ship’s bottom.
Liquid pressure acts on ship tank and will increase as the free surface area of the tank increases. The effect of liquid pressure is to push up in the ship’s tank structures during sailing.
When a ship is rolling the accelerations on the ship’s structure are liable to cause distortion in transverse section. The greatest effect is under light ship conditions.
The cyclic rise and fall of the waterline at the bow and Stern creates an alternating pressure
change against The hull.
Panting
This causes the bow and stern plating to flex in And out and can lead to fatigue. Frame spacing at the Ends of the hull is reduced and additional longitudinal stringers are built into the structure to resist panting.
When the hull is pitched head down, the normal fine entry of the forward water plane is 'blunted‘ by the waterline rising up the bow, particularly if the hull has a large flare. This is a much less suitable shape for displacing water ahead of the vessel and, if the ship is being driven hard, water will not be able to move out of the way fast enough. The ship will slow down, shudder and shake, almost as if it has run into a solid wall.
This occurs when the bottom plates at the bow and stern are lifted out of the water and then re-immersed too rapidly for the water to move out of the way. Slamming tends to 'corrugate' bottom plating at the fore and aft ends.
