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Background:
QinetiQ has recently completed sea
trials of a new composite propeller in Falmouth Bay, UK.
With a 2.9 metre diameter it is the world’s largest
propeller. However, despite its immense proportions it
weighs significantly less than an equivalent traditional
ship’s propeller made from nickel aluminium bronze
(NAB). The reason - new lightweight composite materials. |
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Composite
Propeller Construction: |
The 2.9m
diameter composite propeller on the RV Triton triple
hull warship. |
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propeller has five composite blades bolted and bonded to
a NAB hub. The materials used within the composite are
commercially available, and it is the development of the
right mix of fibers, resin and laminate lay-up that
provide the desired mechanical and environmental
performance for marine application. The extensive
development trials included durability testing in the
marine environment, and water uptake and fouling tests.
The effects of cavitation, impact and environmental
fatigue were also studied. |
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The
Vessel that The Composite Propeller was Tested On: |
The
propeller, designed for QinetiQ’s trimaran warship
prototype, RV Triton, was developed to explore the
application of composite materials for marine
propulsion. RV Triton is the world’s largest motor
powered triple-hull vessel.
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Optimizing Properties of the Composite Propeller: |
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Catherine
Kane, Technology Chief of Marine Structural Concepts at
QinetiQ, described the optimization process of the
materials used. ‘The fundamental mechanical properties
required in this application include stiffness, strength
and fatigue performance. The structure was optimized to
be stiffest along the length of the blade (i.e. in the
radial direction of the propeller) and strong enough to
have a significant factor of safety upon the design
load. On a materials basis, the composite was about half
as stiff as NAB but had a similar strength (see figure
2). On first inspection, one might have concerns about
the stiffness, but the specific stiffness of these
materials makes them very attractive. Structural
stiffness was regained through improved design of the
propeller itself. Additionally, the fatigue performance
is largely dependent on the performance of the metal
insert at the root. During tests, failure has been
initiated by flaws in the NAB while the composite has
remained undamaged.’ |
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Figure 2:
Stress-strain graph comparing nickel aluminum bronze and
the new composite material. |
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Improved
Cavitation Performance of the Composite Propeller: |
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A
reduction in the perennial problem of cavitation damage
pitting caused by pressure gradients in water forming
bubbles of vapor on the surface of the propeller, which
then collapse and cause repetitive localized stress was
a priority for the design. Theoretical models give a
cavitation inception speed of 30% higher for the
composite propeller design, compared with the original
NAB propeller. This was found to be a result of the
improved shape, rather than the use of new materials -
though the combination provides a number of benefits,
explains Colin Podmore, Project Manager. ‘The use of the
lighter composite materials meant that the blades could
be thicker without significantly adding to the weight of
the propeller. Thicker blades offer the potential for
improved cavitation performance, so reducing vibration
and underwater signatures.’ |
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Improved Life Expectancy of the Composite Propeller: |
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composite propeller is expected to last for the lifetime
of the vessel - 25 years. In contrast, an NAB propeller
would be expected to suffer cavitation erosion and
corrosion and need to be replaced periodically.
Additional trials need to be carried out to address the
durability issues and fully validate the long-term
performance. |
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Expanded Applications for the Composite Propeller: |
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The
composite propeller has aroused the interest of a number
of MoD and defense companies. ‘It is hoped that we will
be able to grow interest in the commercial sector as
well,’ said Kane. |
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Summary |
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Qinetiq are now looking at ways of exploiting their
composite materials, and it is expected that it will
find a natural home among other marine applications,
such as secondary structures on ships and submarines. It
could also provide cost savings. ‘The processing
techniques allow for the production of complex shapes
and so it is probable that unit production costs can be
reduced if sufficient numbers are required, by reducing
the postproduction machining and joining that is
commonly needed for metallic components,’ said Kane. |
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