Answer Construction Question 19
Question 19: Give a reasoned opinion as to the validity of the following assertions concerning ship structure:
A. Crack propagation in propeller shaft ‘A’ brackets or spectacles frames is indicative of inadequate scantlings and strength;
B. The adequate provision of freeing ports is as critical to seaworthiness as watertight integrity.
Answer:
A. The statement "Crack propagation in propeller shaft 'A' brackets or
spectacles frames is indicative of inadequate scantlings and strength;"
is incorrect.
Scantlings are based on "rules" (e g. Lloyd's rule) developed from theoretical calculations. tests and Past design proven in service. They are unlikely to be inadequate unless the design is completely new. With past designs, evidence of crack initiation and propagation problems would have caused modification of design.
More likely causes would be:
Poor workmanship, fabrications having high residual stresses.
Poor welds having cracks within, from which a major crack would propagate.
Incorrect heat treatment causing brittle hard spots.
Faulty material, impurities, laminations, cracks, low toughness.
Working environment - very low temperature, corrosion, vibrations.
Therefore, it can be said that crack propagation in propeller shaft 'A' bracket/spectacle frame is not indicative of inadequate scantling and strength.
B. The statement "The adequate provision of freeing ports is as critical to sea worthiness as water tight Integrity." is correct
A bulwark is an extension of side shell plating about upper deck and is a safety barrier for personnel. But, bulwark may present a problem, the retention of large volume of green seas taken on board open deck areas. Retention of water by bulwark for sometimes may greatly decrease stability, endangering the ship (may result in capsizing) - Therefore, freeing ports in bulwark are provided to allow the water to be rapidly unshipped. If adequate scuppers are not provided: - It would prevent complete drainage of water - Promote corrosion, which would ultimately result in failure of hull material due to decrease in strength. Hence freeing ports and scuppers are as critical to seaworthiness as water tight integrity.
Scantlings are based on "rules" (e g. Lloyd's rule) developed from theoretical calculations. tests and Past design proven in service. They are unlikely to be inadequate unless the design is completely new. With past designs, evidence of crack initiation and propagation problems would have caused modification of design.
More likely causes would be:
Poor workmanship, fabrications having high residual stresses.
Poor welds having cracks within, from which a major crack would propagate.
Incorrect heat treatment causing brittle hard spots.
Faulty material, impurities, laminations, cracks, low toughness.
Working environment - very low temperature, corrosion, vibrations.
Therefore, it can be said that crack propagation in propeller shaft 'A' bracket/spectacle frame is not indicative of inadequate scantling and strength.
B. The statement "The adequate provision of freeing ports is as critical to sea worthiness as water tight Integrity." is correct
A bulwark is an extension of side shell plating about upper deck and is a safety barrier for personnel. But, bulwark may present a problem, the retention of large volume of green seas taken on board open deck areas. Retention of water by bulwark for sometimes may greatly decrease stability, endangering the ship (may result in capsizing) - Therefore, freeing ports in bulwark are provided to allow the water to be rapidly unshipped. If adequate scuppers are not provided: - It would prevent complete drainage of water - Promote corrosion, which would ultimately result in failure of hull material due to decrease in strength. Hence freeing ports and scuppers are as critical to seaworthiness as water tight integrity.
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