Feature 8
the recent years as it exceeds the normal could not be gained by moving to a
limit values for these parameters. more conventional monoblock propeller
design. In addition, the ability to fit
Propulsion concept design brake-blades (zero axial thrust) was an
The high level of power to be delivered important factor in the commissioning
into the water, together with strict programme as it allows engine trials
requirements on efficiency, cavitation free alongside, hence de-risking the IEP
speed, hull-induced vibrations and noise System in the basin prior to sea trials.
targets makes the design considerations The Rolls-Royce ABP or Adjustable
of the hull and the propellers and their Bolted Propeller is a type of fixed-pitch
interaction crucial. propeller that is built up by separate
The key starting point is the selection of blades, which are bolted on to a hub body.
the number of propellers. Compared to a What is special with the ABP is that the
triple screw configuration, two propellers blades are bolted from inside the hub and
give a high total propulsive efficiency, but the bolt holes in the hub body are slotted
at the same time a large power density to enable a blade pitch adjustment over
on each propeller. To ensure reasonable a range of 2.5degrees. This means that
propeller loading the propeller diameter the upper blade flange surface is smooth
has to be increased, therefore the hull-to- without any disturbing blade bolt heads
tip clearance is reduced. Increasing the and the propeller pitch can be adjusted
number of propellers reduces the power to perfectly match the rpm of the engine
on each propeller, but the volume required or be compensated for an increased
to accommodate additional shaftlines displacement during the vessels life time.
together with increased mechanical Other benefits with the ABP compared
complexity leads to much higher initial to a monoblock fixed-pitch propeller are
and through-life costs. Therefore, the possibility to have spare blades, easier
the smallest number of propellers is
The HY23 (above) and HY24 (below) wake
logistics, better manufacturing accuracy
preferable both from total propulsive
fields.
with milled bladed instead of grinded
efficiency, mechanical complexity and blades. Further it is possible to apply
cost point of view. high-strength stainless steel blades that
The ACA defined the following key scale measurements an initial propeller are harder, have higher statical strength
design parameters to Rolls-Royce: aft design was made at the Rolls-Royce and holds the surface roughness finer with
hull lines; resistance and powering data; Hydrodynamic Research Centre. This time in operation. The ABP also allows a
prime mover characteristics; wake field; design, called the “near-design” propeller, new set of blades, with a new blade design
shaft speed; maximum propeller diameter was tested in the cavitation tunnel at adapted to a new operating profile, to
of 6.7m; number of blades; direction of SSPA in Gothenburg in August 2004. be under-water mounted without any
rotation (inward over the top); pressure The cavitation was observed at a number dry-docking of the vessel.
pulse limits; open water efficiency target; of different operating conditions and
and maximum skew angle. pressure pulses and noise levels were Main requirements
Due to the high thrust loading for recorded. The results concluded that the The first propeller design, the
a twin-screw configuration the ACA risk of delivering the required power via “near-design”, was made to support the
embarked on a de-risking programme. a twin-screw arrangement was acceptable ACA de-risking of the twin-screw HY23
A twin-screw hull form, called HY23, to the ACA. hull form. The results from the model
was model tested with stock propellers to testing showed very good performance
assess the performance and to determine Propeller design of this “near-design”. The amount of
the wake field. Based on these model The ACA selected the Rolls-Royce suction side cavitation was moderate
Adjustable Bolted Propeller (ABP) for and no face or root cavitation occurred.
the QE class. The ability to re-pitch or The pressure pulse levels and noise levels
Main DiMEnsions possibly change the propeller blades were at expected and acceptable values.
Queen Elizabeth class to a different design during the vessels The requirements of the “final-design”
Displacement ..........................65,000tonnes life-time was a significant factor. propeller were therefore based on the
Length ...................................................280m Hydrodynamic considerations further test results of the “near-design”. It was
Beam, max .............................................70m supporting the decision were that the envisaged that the performance of the
Draught, max .........................................11m A-bracket bossing ahead of the propeller “final-design” propeller would be at least as
Complement.......1450 (including air crew) could not be reduced in size (diameter), good as the “near-design” i.e, the efficiency
meaning that improvements in efficiency would not be lower; the pressure pulses
Warship Technology October 2009 41
p40-47_WT Propulsors-QE OCT09.indd 41 17/09/2009 17:03:04
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