SHIP CONSTRUCTION

Sheer

• It is the rise of a ship’s deck from fore and aft.
• It adds buoyancy to the ends where it is most needed.

Camber

• It is the curvature of decks in the transverse direction.
• It helps to shed water from the deck & add to its longitudinal strength.

Rise of floor

• It is the rise of the bottom shell plating above the horizontal baseline, measured at the ship’s side.
• It allow liquid in the double bottom to drain to the centerline.

Flare

• It is the outward curvature of the hull above the waterline in the forward end of the ship.
• It increases buoyancy & provides a wider forecastle deck, allowing anchors to drop clear of shell plating.

  

Deck line

• It is a line from where the freeboard is measured.
• It is drawn at midship, where the deck plate meets the side shell plate.
• It is of size: 300 mm by 25mm

Margin line

• It is a line drawn at least 76 mm below the upper surface of the bulkhead deck at the ship’s side, as agreed by the SOLAS Convention 1929.
• It is the highest permissible location on the side of the vessel in case of sinking, heel, or trim.
• It is a margin of safety line or a line of safety.

Keel rake

• It is the inclination of the keel line from the horizontal.

Stem rake

• It is the inclination of the stem line from the vertical.

Transom floor

• It is the floor at the head of the rudderpost that supports the framework of the stern.
• It must have the same depth as the floors in the double-bottom tanks.

The stresses on the ship structure are divided into two categories:

1. Global stress 
2. Local stress

=> Global Stress

(a) Hogging & Sagging

These are longitudinal bending stresses, which may occur when the ship is in a seaway or may be caused during loading.

• When the ship is loaded with too much weight in the ends (fore & aft), it may cause her to hog.
• When the ship is loaded with too much weight amidships, it may cause her to sag.

(b) Shearing stress

This is also a longitudinal stress caused by the difference in weight and buoyancy distribution.

(c) Racking

• Ship may be racked by wave action or by rolling in the seaway.
• The stress comes mainly on the corners of the ship.
• Transverse bulkheads & web frames provide very good resistance to the ship.

(d) Torsion

• When a hull is subject to a twisting moment (torque), then the structure is said to be in torsion.
• This effect mostly occurs with decks having large openings. 

(e) Water pressure

• It tends to push the sides and bottom of the ship.
• It is resisted by bulkheads and by frames & floors.

=> Local Stress

(a) Panting

• Panting is the in & out motion of the plating in the bows of the ship and is caused by unequal water pressure as the bow passes through successive waves.

(b) Pounding 

• When the ship is pitching, her bows even lift clear of the water and come down heavily.
• 25% of the ship’s length from the forward is known as the pounding region.

Garboard Strake

• The plates arranged in fore and aft lines around the hull are called strakes.
• For identification, the first strake, which is lettered and is adjacent to the keel, is known as the “garboard strake.”
• The garboard strake is Strake ‘A’

Sheer Strake

• It is the uppermost strake of shell plating on either side, next to upper deck.
• The sheer strake is strake ‘J’
• The upperedge of the sheer strake is kept smooth & free of any welded fitting or connection.
• Where the sheer strake is rounded, the radius should not be less than 15 times the thickness of the plate.

At the end of the ship, particularly at the bow, the width of the strake decreases, and it is often desirable to merge two strakes into one. This is done by means of a stealer plate.

Note: It always takes the name of the lower strakes that run together.

• A panting stringer is a horizontal, deep-web structural member used to reinforce the bow of a ship, specifically to counter the effects of panting loads. 
• To counteract this movement and strengthen the structure, panting stringers are installed. These are typically horizontal, deep-web members that run along the inside of the hull, parallel to the keel. 
• Location: Panting stringers are often found in the forepeak (the pointed front part of the ship) and the aft peak (the pointed rear part of the ship). 

• It is a longitudinal plate, like a fin, fitted externally in pairs (one for each side of the ship).
• It is often in a V shape, welded along the length of the ship.
• It does not have any component inside the hull that would affect cargo or machinery space.
• Bilge keel increases hydrodynamic resistance to rolling, making the ship roll less.
• It has the disadvantage of affecting the forward motion.

Floors: It is found only in the double bottom tank and runs transversely.

Beams: It is found under the deck and runs transversely.

Girder: It is found under the deck and under the bottom plating and runs forward and aft.

Coffin plate: It is used to connect the stern frames to the flat plate keel.

SOLAS II-1/12

COLLISION BULKHEAD

• Every ship shall be fitted with a collision bulkhead at the forward end.
• The collision bulkhead shall be watertight up to the bulkhead deck on a passenger ship.
• The collision bulkhead shall be watertight up to the freeboard deck on a cargo ship.
• The bulkhead shall be located:
  ➤ not less than 5% of the length or 10 m from the forward perpendicular, whichever is less.
  ➤ not more than 8% of the length or 5% of length + 3 m from the forward perpendicular, whichever is greater.
• The ship should be able to survive any damage forward of the collision bulkhead.
• No doors, manholes, access openings, or ventilation ducts are allowed to be fitted on the collision bulkhead.
• The collision bulkhead may be pierced by not more than one pipe to deal with liquid in the forepeak tank.
• No flanges are allowed at the collision bulkhead, & the penetration piece should be welded to the collision bulkhead with a collar plate.
• A valve should be provided that should be able to be remotely operated from above the bulkhead deck of a passenger ship and the freeboard deck of a cargo ship.
• The valve should be normally closed. Any failure will make the valve close automatically.

• It is a two-dimensional drawing of the three-dimensional surface of the ship’s hull.
• It is very useful for marking the location of hull damage by identifying the strake letter & number.
• For the keel strake, it starts from aft (by frame number zero) & goes towards the bow.
• The garboard strake, above the keel strake, is lettered like A1, A2, A3… from aft to bow.
• In aft numbering starts from rudder stock.
• The strakes behind the rudder stock are marked as a negative number.
• The numbering goes till sheer strake, from bottom to top.

• The frame numbering starts from number “0” from the forward perpendicular & goes till the aft perpendicular.
• The numbering forward of forward perpendicular is A, B, C, D… from FP to bow.
• The numbering aft of after perpendicular is AA, BB, CC… from AP to aft.

Scantling is the collective dimensions of the framing to which plates are attached to form a hull.