Definitions of hull elements

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4. Construction

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4.1. Definitions of hull elements

  • Keel: The keel is a member, or series of members, running longitudinally that forms the structural base of a ship. The keel always corresponds to a ship's centreline. It is a major component in providing longitudinal strength and efficiently distributes local stresses when the ship is dry docked. There are two types of keels used to build ships of a certain size, the flat keel and the duct keel.

Flat keel

Flat keel

Duct keel

Duct keel

  • Girders: A girder is a longitudinal member used in the construction of the bottom of a ship. They can be solid or not and can be placed above the keel (centre girder) or spaced equal distances from it (side girders). They can be continuous or divided by floor sections (intercostal side girders). The centre girder is always one continuous piece and must be fastened to the keel with a continuous weld. Girders must extend as far as possible from the forward to the aft end of a ship.
  • Floors: These are made up of cross members that are mounted perpendicular to the keel and girders. There are three main types of floor: solid, plate and bracket.

Plate floor

Plate floor

Solid floor

Solid floor

Bracket or open floor

Bracket or open floor

  • Frames: These are vertical members that make up the framing of the vertical part of the hull. Frame type and spacing vary considerably depending on the ship's construction.

Shell framing

Shell framing

  • Deck beams: These are transverse members that connect the top ends of the frames, forming the transverse framing for the deck.

Longitudinal framing, deck and shell

Longitudinal framing, deck and shell

  • Deck girders: These are longitudinal members that combine with the beams to form the longitudinal framing of the deck.
  • Longitudinals: A very general term to identify any small longitudinal member that can be used for several purposes. This term is used more specifically in longitudinal framing.
  • Web frames: Oversized members that replace a frame at certain locations on a ship.
  • Bracket: A general term that identifies any part used to connect two members.
  • Beam knee: Bracket located at the end of deck beams that connect the beam and frame to the shell plating.
  • Pillar: Vertical member inside a ship that connects the deck to the ship's bottom, where it is installed between two tweendecks, especially around hatches. They are quite bulky and complicate cargo handling inside holds.
  • Plating: The plating of a hull is the series of plates that form the watertight shell of the hull. There is bottom plating, deck plating and side shell plating.
  • Bilge plating: Longitudinal plating that connects the side shell plating to the bottom plating.
  • Tank top: Watertight series of plates attached to a ship's bottom framework.
  • Double bottom: The double bottom is the watertight space between the bottom plating and the tank top. Its height varies according to the size and type of ship, but it is generally between 0.75 and 1.5 metres. A double bottom is divided into several watertight compartments by watertight floors and girders. These compartments can be used to store fuel, oil and ballast water. They are often used to adjust a ship's list and trim.

A double bottom maintains a ship's watertight integrity when the bottom is damaged. The tank top greatly increases a ship's longitudinal strength and forms a platform to carry the ship's cargo and machinery.

Transversely framed double bottom

Transversely framed double bottom

Longitudinally framed double bottom

Longitudinally framed double bottom

4.2. Types of construction

4.2.1. Transverse framing

Transverse framing is used primarily for ships less than 120 metres in length. The floors, frames and beams form rings spaced closely together. Longitudinal strength is provided by the keel, centre girder, side girders, deck girders, the entire bottom, deck and side shell plating, and the tank top. Transverse framing ensures good cross sectional strength to handle overall stresses, vertical loads, rolling and dry docking. However, on very long ships, sheer stresses can cause deformations between the rings.

4.2.2. Longitudinal framing

Longitudinal framing is mandatory for very large ships, oil tankers and bulk-ore carriers. The rings are formed of floors, deck beams and web frames that replace the frames. These rings are farther apart than in transverse framing. The longitudinal reinforcement members are deck girders, girders, the keel and a large number of deck, bottom and side longitudinals. The longitudinals are slender but there are very many of them.

4.2.3. Mixed framing

Mixed framing combines longitudinal and transverse framing. One type of framing is used in one part of the ship and the other type is used in another part. The most common combination is longitudinal framing for the bottoms and the deck, and transverse framing for the sides.

4.3. Stresses and constraints on ship structure

4.3.1. Static stresses and constraints

These stresses are measured when the ship is not under way. They are often caused by a poor longitudinal distribution of mass. Even if the ship's total weight is balanced by the total force of buoyancy, these forces may not be distributed evenly along the full length of the ship.

  • Hogging: If the forces of buoyancy are concentrated around the section amidships and the ends are loaded, the ship will tend to move downwards at the bow and stern while the section amidships will tend to move upwards. In this situation, the deck's structural members are being subjected to tensile stress while the bottom structure is under compressive stress. This phenomenon can be compared to a beam supported in the centre and loaded with weights on the ends.

Hogging

Hogging

  • Sagging: If the forces of buoyancy are concentrated under the bow and stern of the ship and the section amidships is loaded, the ship will tend to move upwards at the ends and trough amidships. In this situation, the deck's structural members are under compressive stress while the bottom structure is being subjected to tensile stress. This phenomenon can be compared to a beam that is supported at both ends and loaded with weights in the middle.

Sagging

Hogging and sagging can be amplified by the movement of waves passing along the hull. A crest of waves at each end of a ship combined with a trough amidships will amplify sagging, while a crest amidships combined with a trough at both ends will amplify hogging.

The stresses caused by these situations can be calculated using the load curves table, the stress and sheer curves table, and the bending moments table. Manual or electronic calculators also exist to find the value of the stresses on the hull. The maximum permissible stress values can be found in the ship's stability book.

4.3.2. Dynamic stresses and constraints

When a ship is under way, some situations create additional stresses. They are caused primarily by the effect of waves on the hull in rough seas. Two of these are pounding and panting.

  • Pounding: When a ship sails in heavy seas, it pitches. It can happen that the bow rises over the crest of a wave and emerges completely out of the water. When the bow comes back down on the water, it can be subjected to a major impact, which is pounding. The hull plating at the bow end of the ship must be reinforced to avoid bending of the plating. This stress can also occur at the ship's stern, but to a lesser degree.
  • Panting: When waves hit the bow and stern of a ship, they create variations in pressure that tend to push the plating in and out. This is panting. The framing at the ship's ends must be reinforced to prevent exaggerated movement of the hull plating.

4.4. Watertight bulkheads

A watertight bulkhead is a transverse bulkhead mounted on the tank top and it must extend right to the uppermost continuous deck.

Watertight bulkheads are installed to:

  • Divide the ship into watertight compartments and thereby limit flooding if the hull plating is damaged;
  • Improve the transverse strength of the structure;
  • Prevent distortion of the hull plating;
  • Support the deck girders and longitudinals;
  • Rigidly attach the tank top to the upper deck;
  • Greatly slow the spread of fire.

The number and location of watertight bulkheads on a ship depend on the length and type of ship and the location of the machinery space. The SOLAS Convention determines the number and location of these bulkheads. But in general, there is a watertight bulkhead (collision bulkhead) at the bow that should be located between 0.05L and 0.075L (L = length between perpendiculars of a ship), a watertight bulkhead at the stern that should form a watertight aft compartment (after peak) that encloses the stern tube, and a watertight bulkhead at each end of the machinery space (where the aft bulkhead may be the after-peak bulkhead).

All members that pass through a watertight bulkhead, such as ventilation ducts, piping and electric wiring, must be mounted so as to maintain the watertight integrity of the bulkhead. That is why remote controlled stopcocks are generally found on certain pipes that pass through watertight bulkheads.

4.5. Watertight doors

In some situations, it is necessary to pierce bulkheads to allow crew or passengers through. In this case, a sliding watertight door is installed. An example of this situation is the watertight door that is found on some ships between the machinery space and the shaft tunnel. Liners have many of these doors that allow passengers to go between the different sections of the ship. These watertight doors are usually hydraulically activated. Local control stations must be located on either side of the door. In addition, a remote control station (generally located in the wheelhouse) must be placed outside both compartments separated by the watertight bulkhead.

Chapter II-1, Regulation 15 of the SOLAS Convention governs the installation and operating requirements for these doors.

Extract, Regulation 15,

7.1.6: [A watertight door] shall be provided with an audible alarm, distinct from any other alarm in the area, which will sound whenever the door is closed remotely by power and which shall sound for at least 5 s but no more than 10 s before the door begins to move and shall continue sounding until the door is completely closed. In the case of remote hand operation it is sufficient for the audible alarm to sound only when the door is moving. Additionally, in passenger areas and areas of high ambient noise the Administration may require the audible alarm to be supplemented by an intermittent visual signal at the door; and

7.1.7: shall have an approximately uniform rate of closure under power. The closure time, from the time the door begins to move to the time it reaches the completely closed position, shall in no case be less than 20 s or more than 40 s with the ship in the upright position.

Watertight bulkhead

Watertight bulkhead

Watertight door operating mechanism

Watertight door operating mechanism

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