The
aft propeller recovers the swirl energy left
in the slipstream by the first propeller
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There
are basically two physical features behind the
efficiency gain offered by a propulsion unit
with contra-rotating propellers:
- First,
and most well known, is the aft propeller
recovering the swirl energy left behind by
the forward propeller
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The
power is split between two propellers and gear
sets |
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Second is the splitting of the power not only
between two propellers but also between two
gear-sets. This makes it possible to reduce
the propeller RPM and have two large propellers
with light load and low speed of rotation.
A combination that yields a very high propeller
efficiency. Additionally, also the frictional
losses are reduced because of the low RPM.
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The pod behind the pulling propeller creates
a beneficial pressure wave in front of it
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A propulsion unit with a pulling propeller offers
an undisturbed flow to the propeller. This means:
- Less
noise and vibrations due to better cavitation
behaviour and lower pressure pulses
- Tip
clearance can be reduced enabling a larger
propeller to be used and thus higher efficiency
-
The presence of the pod behind the propeller
offers an additional gain: The pod geometry
may be designed to create a pressure wave
in front of it, which acts like an additional
wake for the forward propeller. The increase
in propeller thrust due to this unique interaction
between pod and pulling propeller practically
cancels the pod drag.
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