Feature 8
puts them on the blade surface. The BEM showed levels that were somewhat larger
gives a better representation of the blade than for the “near-design” propeller,
geometry but the calculations require but were still well inline with the
increased computational time to resolve. previously predicted levels for the
The figure shows predicted blade pressure “final-design”. No erosive cavitation was
distributions and cavitation extension at observed in any of the conditions tested,
the same blade angle and condition as hence the ‘final-design’ propeller was
seen in the figure. accepted by the ACA at model scale to
For predicting propeller-induced proceed with manufacture.
pressure pulses a potential code that takes
the unsteady propeller force distribution rudder design
calculated by the vortex lattice method rudder cavitation observed on the original The development of the propeller design
as input and calculates the pressure rudder design at 5deg rudder angle during concluded with the cavitation tunnels
fluctuations on panels distributed over the cavitation test of the “final-design” tests of the “final design” at SSPA during
a hull surfaces. The panel distribution is propellers. November 2006. The cavitation model
made in any general grid generation tool included the initial semi-spade rudder
and is given as input. From this calculation design proposed by the ACA based on the
a pressure distribution on the surface is were performed with 220mm diameter NACA00 section with a thickness to chord
received together with amplitudes of the propeller scale models, which were large ratio of 18%. During these tests it was
three first harmonics at each panel. enough for the intended testing, mounted observed that erosive sheet cavitation was
to a hull model fully equipped with visible on the rudder surface, particularly
Cavitation test rudders, shaft lines and bracket arms. in the pintle bearing area and in the gap
Because of the simplifications made in the Tests were performed at six different between the rudder horn and the rudder
potential flow theories and the unsuitability operating conditions, four for cavitation blade. In the normal operating range of the
of more advanced computational fluid observations and further two for noise rudder (+/-5degrees), tip vortex cavitation
dynamics (CFD) methods for analysing measurements using hydrophones was seen starting from the leading edge of
and verifying propeller designs, cavitation mounted in the tunnel. the rudder tip.
tunnel tests are still required. The aim of The observations made during the S u b s e q u e n t l y R o l l s - R o y c e
such a cavitation test is to observe the cavitation testing showed a limited Hydrodynamic Research Centre was
cavitation occurring on the propeller amount of suction side cavitation contracted by the ACA to carry out a
when it is operating in the correct flow and occurring when the blade passes through programme of hydrodynamic optimisation
loading conditions behind the ships hull. the wake peak and rotated downward on of the semi-spade rudder design.
The amount of cavitation, its appearance, the inner side of the shaft. The amount Following the observations made
extension and behaviour on the propeller of cavitation was slightly increased during the cavitation tunnel testing
blades, the rudders and the shaft line are compared to the “near-design” propeller of the “final-design” propellers the
observed and documented by means of due to the increased wake peak, but requirements/targets for the rudder design
sketches, photos and video recordings for showed a stable and harmless behaviour. optimisation were: reduce the amount of
a number of operating conditions. Further, The blade root showed a very small cavitation occurring on the rudder blade
pressure pulse fluctuations were measured amount of cavitation on both the pressure surface; improve the rudder sole design
by pressure transducers mounted in the and the suction side, which indicated to prevent sole cavitation or rudder tip
hull model above one of the propellers. that the root design was well balanced vortex cavitation; improve the design of
The “final-design” propellers were and it was judged that no further pintle area and gaps between the horn
tested in the cavitation tunnel at SSPA in modification would improve this situation. and rudder blade to avoid or reduce the
Gothenburg in November 2006. The tests The pressure pulse measurements cavitation in these areas.
The size and the thickness of the rudder
were fixed and the above modifications
semi-spade rudder
were made purely to improve the
Number of rudders - 2 hydrodynamic behaviour of the rudder.
Some of the rudder main dimensions
rudder height H 7500mm
are given in the table. The dimensions of
the rudder were defined by the ACA.
Thickness of ratio root T/C rooT 0.18
The main aim of the rudder optimisation
Thickness of ratio tip T/C TIP 0.18
study was to reduce the amount of
Profile type NACA662
cavitation occurring on the rudder blade
surface that would cause erosion damage
rudders main dimensions
in-service. The cavitation on the rudder
Warship Technology October 2009 45
p40-47_WT Propulsors-QE OCT09.indd 45 17/09/2009 17:03:06
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