Page 61 - Ship Construction.DJ Eyres 6Ed

Ch06-H8070.fm Page 50 Wednesday, October 18, 2006 8:03 AM

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Aluminium Alloy

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งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
There are three advantages which aluminium alloys have over mild steel in
the construction of ships. Firstly aluminium is lighter than mild steel
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(approximate weight being aluminium 2.723 tonnes/m , mild steel 7.84
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tonnes/m ), and with an aluminium structure it has been suggested that up
to 60 per cent of the weight of a steel structure may be saved. This is in fact
the principal advantage as far as merchant ships are concerned, the other
two advantages of aluminium being a high resistance to corrosion and its
non-magnetic properties. The non-magnetic properties can have advantages in
warships and locally in way of the magnetic compass, but they are generally
of little importance in merchant vessels. Good corrosion properties can be
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utilised, but correct maintenance procedures and careful insulation from
the adjoining steel structure are necessary. A major disadvantage of the use
of aluminium alloys is their higher initial and fabrication costs. The higher
costs must be offset by an increased earning capacity of the vessel, resulting
from a reduced lightship weight or increased passenger accommodation on
the same ship dimensions. Experience with large passenger liners on the
North Atlantic service has indicated that maintenance costs of aluminium
alloy structures can be higher for this type of ship and service.
A significant number of larger ships have been fitted with superstructures
of aluminium alloy and, apart from the resulting reduction in displacement,
benefits have been obtained in improving the transverse stability. Since the
reduced weight of superstructure is at a position above the ship’s centre of
gravity this ensures a lower centre of gravity than that obtained with a
comparable steel structure. For example on the Queen Elizabeth 2 with a
limited beam to transit the Panama Canal the top five decks constructed of
aluminium alloy enabled the ship to support one more deck than would
have been possible with an all steel construction.
Only in those vessels having a fairly high speed and hence power, also
ships where the deadweight/lightweight ratio is low, are appreciable savings
to be expected. Such ships are moderate – and high – speed passenger liners
having a low deadweight. It is interesting to note however that the Queen
Mary 2, not having a beam limitation, the owners decided to avoid aluminium
alloy as far as possible to ensure ease of maintenance over a life cycle of
40 years. A very small number of cargo liners have been fitted with an