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Correspondence - Propulsors
QE class propeller & rudder design
Warship Technology has received the following letter from Dr Graham Patience, Managing Director
of stone Marine Propulsion, regarding the article in the October 2009 issue of Warship Technology
‘Propeller and rudder designs for the QE class carriers’ and the use on the Queen Elizabeth (QE) class
of built-up propellers.
sir,
I have read with some interest the feature in the October edition on the propeller and rudder design
for the QE class carriers, which I understand to be based upon a paper presented at the June 2009
Warship Conference in the UK.
My interest is in the fact that, like the Type 45 propellers, the new aircraft carrier is to be fitted with
built-up propellers. This is a subject upon which I have previously expressed my views and my opinion
has not changed. If anything, it has hardened. The utilisation of this out-dated concept on what will
be the future capital ships of the Royal Navy appears to confirm that the Navy is condemned to this
type of propulsion for the foreseeable future – a prospect which no doubt will be viewed with some
commercial satisfaction by those who promote it, but not by any self-respecting propeller specialist.
We have already been subjected to the unconvincing justification of built-up propellers
as reducing through-life costs, despite the necessity for continuous and regular monitoring –
inspection and replacement – of blades, hubs, holes and bolts, all of which add risk and cost
to what should be kept as a simple product. But the subject article goes even further, making
some effort to justify the adoption of built-up propellers on the grounds of slotting, pitch
compensation, manufacturing accuracy and the adoption of stainless steels – none of which
stand up to proper scrutiny.
As a critical component of any ship, the propeller has to convert propulsive power into thrust to
provide the required mobility. In doing so it is subject to cyclic forces and hence susceptible to
fatigue. It has to withstand erosive and corrosive attack; operate for 24 hours a day as required;
occupies a vulnerable position and because of difficulty of access, it is subject to long intervals
between servicing. Yet it is expected to operate efficiently and reliably with acceptable noise and
vibration properties. The practical engineering approach to these conflicting requirements must
be the knowledgeable application of simplicity – not to unnecessarily complicate the issue with
an increasing number of smaller components and over-sophisticated attachment systems. Such
a concept appears even more essential for the military configuration unless there are quite clear
military advantages to be gained – which for built-up propellers there are not.
In reading the paper I was immediately struck by the comparison between military and
commercial design practices. The authors point out that the operating requirements for the new
carriers result in a design that must be recognised as challenging. For a military application this may
indeed be the case but they fail to point out that in the commercial world, such challenges have
become commonplace. So much so that the commercial designer now copes with single-shaft
transmissions of up to 80MW – twice the level the authors find challenging – and has in fact provided
solutions for this power level that have proven eminently successful in service.
The modern large containership is rarely configured with a shaft power as low as the 40MW
of the new carriers and, whilst commercial design objectives do not place as high an emphasis
on cavitation inception as in the military world, the requirements for acceptable cavitation and
excitation behaviour at the highest efficiency are no less stringent.
The commercial design problem is exacerbated by the fact that the ships are single screw – the
propeller does not enjoy the predominantly uniform open water characteristics of twin-screw flow. It
has to operate within the boundary layer of the hull with its far greater variations in wake. The reader
may draw his own conclusions as to which is the more challenging design case.
But what really struck me was the comparison to be made between the new class of carriers and
its namesake in the commercial world – the QE2. The similarities of the design requirements are
close. QE2 was initially configured at 55,000shp per shaft (40.6MW) on a twin-screw arrangement. It
too had a high ship speed and, as a high profile passenger vessel, strict performance requirements.
But because of different shaft revolutions, the QE2 propellers had a smaller diameter of 19ft (5.79m).
8 Warship Technology March 2010
WT_Mar-2010_p6-7-8-10-11-12-13.indd 8 23/02/2010 15:51:27
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