Feature 8 | ProPulsors
surface is caused by an inclined flow onto
Pictures showing
the rudder profile. The inclined flow is
calculated rudder
caused by the rudder blade angle and by
blade pressure
the rotation of the propeller slipstream.
distribution
The angle of attack caused by the propeller
and predicted
slipstream rotation at the location of the
cavitation extension
rudder is in the range of -12 to +8deg for
at 2deg rudder
this particular vessel. A rudder of this
angle. From left
type operates within +/- 5deg rudder
NACA00, NACA662
angle during normal transit operation and
and NACA 662
needs to be free from erosive cavitation in
twisted.
this range.
In order to reduce the amount of
cavitation on the rudder blade surface, an
alternative rudder profile was required.
The NACA00 profile is known to have of the rudder tip that could cause erosion most cases is located down stream of an
a pronounced pressure drop causing and increased noise. A normal way of operating propeller. The flow field behind
cavitation along its leading edge when solving cavitation on the sole and rudder the propeller has to be taken into account
exposed to an inclined flow. Therefore tip vortices is to make a sole of the rudder and the rudder will also affect the loading
two additional profiles were compared that is rounded in all directions. This was on the propeller. Different methods were
with the NACA00 profile, concentrating done by rotating the bottom profile shape used to analyse the rudders cavitation
on pressure distribution both in 2D and of the rudder 180degs. performance and the propeller-rudder
3D. The additional profiles studied were When the rudder blade is rotated there interaction effects.
the NACA662 profile and the HSVA is an increased risk of cavitation in the The simplest method used was a 2D
MP-71 profile. Both these profiles have horizontal gap between the rudder blade potential flow method for calculation
their maximum thickness further aft and the rudder horn. Adding scissor of the pressure distribution along a 2D
in comparison to the NACA00 profile, or splitter plates to the rudder above wing section exposed to an inclined
which normally is favourable for and below the horizontal gap is a good flow. The flow velocity and rotational
cavitation performance. Following this way of reducing both the amount of angle in the propeller slipstream was
comparison and results on the analysis cavitation in this area and the effect of it. calculated by 3D computations and
the NACA662 profile was selected for the Any cavitation in the horizontal gap will empirically by momentum theory. The
optimised rudder design. erode the plates, which can be replaced, maximum velocities were corrected for
In order to further improve the and not the rudder plating. The scissor hull influence and turbulence mixing. The
cavitation performance of the rudder plates will also have a straightening effect 2D pressure distributions were calculated
a twisted leading edge was introduced. on the flow creating a more horizontal for one section below and one above the
The twist is a local modification of the flow in the pintle area. The scissor plates propeller axis.
profile’s leading edge, which adapts the ends in a spoiler edge on the pintle. The The most advanced method used for
rudder profile to the propeller slipstream purpose of the spoiler is to separate the analysing propeller rudder interaction
rotation. It was decided, due to flow and any cavitation from the rudder was a combined potential flow and
manufacturing reasons, that the rudder surface, so that it implodes in the water RANS CFD. These calculations take the
horn should remain straight and the twist instead of on the rudder surface. 3D rudder geometry into account and
should only be applied to the lower part The main philosophy applied to the the propeller is represented by a body
of the rudder. This also corresponded interface between the rudder horn and force distribution in the computational
well with the location of the cavitation the rudder blade was to reduce the size domain. A vortex lattice potential flow
observed in the cavitation test. In order of the gaps as much as possible and if method calculates the propeller force
to confirm the effects of the twist, two needed, block the gaps with flow blockers distribution in 3 dimensions and feeds
similar rudder designs were produced, to prevent flow through the gap causing the results into a RANS solver as a body
one with twist and one without twist. cavitation. force distribution. The viscous flow field
The second target of the rudder due to this body force distribution and
optimisation study was to improve the Analysis of rudder design the free stream velocity is then solved
rudder sole design. The original rudder When designing propellers and rudders, within the computational domain.
had a rounded leading edge corner at quick and reliable tools for analysis of At the start of next iteration the
the bottom of the rudder that prevented the propeller as well as the rudder are velocities at a wake plane in front of
any cavitation on the sole but had no crucial. The difficulty with predicting the propeller are exported to the vortex
rounding in the transverse direction. This the pressure distribution over a rudder lattice method and a new body force
caused an early inception of a cavitating blade is not due to the rudder design, distribution is calculated and submitted
tip vortex starting from the leading edge but due to the fact that the rudder in back into the RANS solver. This
46 Warship Technology October 2009
p40-47_WT Propulsors-QE
OCT09.indd 46 17/09/2009 17:03:06
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