Page 184 - Technology Roadmap Transportation

TECHNOLOGY VISION 2035

FIG 4.7: POST SWIRL DEVICE[7] direction, with the aft propeller recovering
the rotational energy in the slipstream from
the forward propeller. Theoretically about 10-
12% improvement in propulsive efficiency is
achievable. However, there are mechanical
difficulties of rotating shafts at different speeds in
opposite directions, one inside the other, and the
bearing and lubrication problems that arise as a
consequence. Some solutions have been found,
the best of which appears to be mounting a
pod on the rudder horn separate from the main
propeller, but there are very few ships in service
with this type of propulsion and it is not yet a
mature technology to be applied to large full form
ships. The complexity is quite high with cost and
risk but with promise.
rotate (so without bearings) and can reduce
the rotational losses generated by the propeller.
Rotational losses are generally between 5~7%, so
the potential saving is attractive and therefore the
technology is promising for future development.

4.2.6 Podded Propellers
Podded propellers have higher efficiencies because
the propellers operate in faster, less disturbed
water outside the boundary layer. There is also a
saving resulting from being able to reshape the aft
end of the hull and reduce the overall resistance
of the ship. Pods also have greater flexibility for the
power train and provide efficient manoeuvrability FIG 4.9: COAXIAL CONTRA-ROTATING
in port. However, there have been some high- PROPELLERS[9]
profile bearing problems reported following the 4.2.8 Design and optimization with computational
application of pods to the cruise ship market and it
is still an evolving technology. fluid dynamics (CFD)
Traditionally hull forms and propeller designs have
been optimized by model testing which was done
relatively late in the design process and changes
required if any were very costly. Presently, the use
of CFD speeds up that process. CFD is based on
numerical methods and algorithms and is used
to analyse and solve complex problems involving
fluids and gases. It is regarded as a reliable and
accurate tool for calculation of the hull resistance
of a ship at a certain speed in order to find
the optimum design in the early design phase.
In that phase many important overall design
considerations and decisions are made and the
propulsion system is selected. Use of advanced
CFD technology can solve a long list of complex
problems e.g. resistance, noise from propellers,
and investigate the interaction between propeller,
hull and rudder. The impact of waves on the hull,
superstructures or offshore structures can also
FIG 4.8: PODDED PROPELLERS[8] be investigated. Therefore, use of advanced CFD
technology in the early design phase where it
4.2.7 Coaxial contra-rotating propellers
In a contra-rotating configuration two propellers is easier and more cost effective, may prove to
face each other, rotating in the opposite be of great help and should be encouraged for

182 WATERWAYS