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Feature 8
occurred at steering angles. Tests with
flow blockers that prevented flow through
the vertical gaps were also made and it was
found that these even further improved
the situation, especially in the pintle area.
The inception of the rudder tip vortex
cavitation was significantly improved by
the new rudder sole design.
In conclusion to the rudder optimisation
studies the decision was taken to adopt
the NACA662 section with a twisted
leading edge below the shaftline and
rounded rudder sole. The final design
also adopted the recommendations with
respect the rudder horn trailing edge and
the rudder blade leading edge in the ‘gap’
areas, namely; maintaining a 25mm gap
between the horn and the rudder blade
and removal of sharp edges that would
increase the likelihood of cavitation. The
rudder horn and rudder have a toe out
of 2degs for the purposes of propulsive
efficiency. The scissor plates were not
CAD pictures of the optimised rudder design with details in pintle area and gaps. adopted following the cavitation tunnels
tests as supported by advice from LRS.
This was primarily due to the cavitating
iterative process is continued until the rudder cavitation test vortex that the leading edge of the rudder
propeller forces and the flow velocities Two new rudder models were blade produced, being accentuated by the
have converged. The RANS calculation manufactured for a cavitation test to scissor plate design.
was done in a rectangular shaped verify the rudder designs. Both rudders During the cavitation tunnel tests,
volume and the influence from the were of the same design with the experiments were conducted using ‘flow
hull wake was neglected. The only hull exception of the twisted leading edge blockers’, to prevent flow from passing
effect taken into account was a no-slip applied to one of the rudder models. through the gaps around the pintle
boundary condition at the box surface of The tests were made using the complete housing. The results of these experiments
the box. hull model with shaft lines, bracket arms showed significant improvements in
The RANS solver uses an overlapping and “final-design” propellers. The tests relation to the cavitation produce by
grid technique, which means that were performed at maximum power and the rudder blade leading edge in this
computational meshes for propeller two draughts, which were judged to be region. At this stage it was known that
forces, rudder geometry and surrounding worst case conditions for the rudders. the rudder and rudder horn would have
water can be made separately. This also The rudder angles were set from 0deg to a Limpetite rubber coating to provide
means that the rudder mesh can be =/- 8deg rudder angle and cavitation was protection against corrosion, erosion,
rotated easily or translated for simulations observed and documented on both side cavitation, and abrasion. This coating
of rudder angles and different rudder of the rudders as well as in the gaps and system is widely used on the underwater
positions. pintle areas. appendages of Royal Navy vessels and is
No cavitation is included in the The tests verified the improvements seen therefore considered suitable mitigation
mathematical model, but the white area in the CFD calculations, namely that the against in-service erosion caused by
represents an area where the pressure NACA662 profile has a better cavitation cavitation. WT
is lower than the vapour pressure of performance than the NACA00 profile.
the water and cavitation would occur. The NACA662 profile significantly * This article is based on an edited version
The differences in pressure distribution reduced the amount of cavitation on of a paper presented at the Warship 2009
between the different profiles are the rudder blade, and offers a number conference in the UK in June by G Grunditz,
significant and the effect of the twist of further improvements. In addition, Rolls-Royce Hydrodynamic Research
is clearly seen. The right most rudder significant improvements were seen in Centre, Sweden; J Fisher, ACA/Thales
shown in the figure with the twisted the gaps and the pintle area, even though Naval Ltd Steering Gear Lead Engineer,
NACA662 profile was later selected as cavitation in the horizontal gap between UK; B Thorp, Rolls-Royce Naval Design
the final design of the optimised rudder. the scissor plates and the pintle still Manager, UK.
Warship Technology October 2009 47
p40-47_WT Propulsors-QE OCT09.indd 47 17/09/2009 17:03:07
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