Ships Technology Flashcards
What are docking strains?
Temporary static forces that are imposed on the ships structure during a docking period. As it’s no longer supported by water, the ships hull tends to settle in a state of hog when on the dock blocks.
Define ‘Hogging’
In this condition, the ship is supported midships by the wave crest whilst the ends are unsupported.
Define ‘Sagging’
In this condition, the hull is being supported at each end by the crests of adjacent waves whilst the midships section is unsupported.
Define ‘Racking’
When ships are subject to external transverse forces, such as wind and waves, one side of the vessel will tend to change the section so that the sides are no longer perpendicular to the ships’ bottom.
What is ‘Grade D’ Steel?
Mild steel with guaranteed toughness. Used in the main ship structure except in highly stressed areas or where crack arresting properties are required.
What is ‘Grade B’ Steel?
Tougher Steel, used in the more highly stressed areas of the ships structure, or where crack arresting properties are required.
What is ‘Grade BX’ Steel?
An improved form of ‘B Grade’, over 18mm thick and used in heavy box sections such as the vertical keel.
List 6 Advantages of Steel:
Weldable
Strong
Relatively cheap
Readily available
Ductile
Can be hot or cold rolled
List 4 Disadvantages of Steel:
Corrodes
It’s heavy
Magnetic
Subject to failure
Describe an advantage of Aluminium and where it’s therefore used.
Good strength to weight ratio. 1/3 the destiny of steel, but only half the strength. Therefore, it is used in areas of the ship where weight is an important consideration, such as in the superstructure.
Explain a drawback of using Aluminium.
In the event of a fire, Aluminium loses its strength at about 250°C and melts at about 650°C. Therefore, it’s structural uses are severely limited and can’t be used for primary structure, ladders or deckplates.
Describe the three categories of ships structure:
Primary - contributes to main structural strength and shape.
Secondary - Does not primarily contribute to structural strength but does contribute to watertight integrity and supports local loads.
Minor - structure that contributes to neither structural strength nor watertight integrity.
How can local strains and stresses be prevented?
By ensuring that the structure is arranged so that no discontinuities of strength occur.
In what ways are RN ships different to merchant ships?
The layout of compartments is different in order in incorporate items such as firefighting equipment, weapons, ammunition etc. A higher standard of watertight subdivision is also required.
How are superstructure blocks supported?
At each end by transverse bulkheads underneath.
Describe the keel:
Both the flat and vertical keel run continuously from fwd to aft and vary in scantling along their lengths. The flat keels max strength is at midships to resist hogging and sagging.
The vertical keel has its max strength at the after cut-up to resist the forces encountered when docking.
Where access holes are cut into the vertical keel, a rider plate is always fitted to compensate for the loss of strength.
Describe ‘double bottoms’.
Consist of two watertight bottoms with a void space between then.
They are strongly constructed and withstand the upward pressure of the sea, in addition to hogging and sagging. They provide storage space for fuel, oil, fresh water and ballast and can withstand considerable bottom damage caused by grounding or underwater blasts.
List 6 Advantages of ‘double bottoms’.
Provides additional protection.
Contributes to longitudinal strength.
Assists with watertight subdivision.
Form tanks for the storage of liquids.
Act as a second hull.
Available for counter flooding.
Describe the inner bottom:
The watertight deck fitted on top of the plate longitudinal and transverse frames to form the double bottom.
It helps to maintain the WT integrity of the hull in the event of damage to the outer bottom and contributes to the longitudinal strength of the hull.
Explain the purpose of watertight subdivision.
To divide the boyant volume of the ship into smaller compartments and preserve the largest amount of buoyancy in the event of ships hull being breached. Therefore, the boyant volume under and above the waterline is divided into watertight boxes.
