NewS
Correspondence - Propulsors
Continued ...
This, in the author’s terms, increased its design
difficulty from a Density Factor of 1112 for the
QE carriers to a Factor of 1540 for the QE2, some
40% higher.
The QE2 propellers were designed in the 1960s,
nearly 45 years ago. They were designed against
the background of computers limited to 1k (yes,
just one); no wake surveys; cavitation tests were
in open water; there was no CFD [computational
fluid dynamics]; finite element techniques were
just a dream; and large numbers of blade design
alternatives were out of the question. As far as
analysis went, the only patches we had were
those on the elbows of our jackets.
Despite all that we got it right. The propellers
– they were fixed-pitch monobloc propellers –
were highly successful. They operated day in
and day out for nearly 17 years, providing the
required performance with acceptable noise
and vibration until the ship was re-engined and
fitted with CPPs [controllable pitch propellers]
The QE2 propellers, not long after they had been
and vane wheels. It is worth pointing out that
fitted.
the latter demonstrated a good example of
the consequences of added complication and
risk since it is a matter of record that the vane
wheels failed during the sea trials; the blade design had to be changed to resolve a tip vortex
problem; and root erosion was a severe problem for even single Atlantic crossings.
Yet, other than polishing and dressing out the occasional minor edge damage, the original fixed-
pitch propellers didn’t need any major maintenance. They didn’t need any pitch adjustment or any
other of the so-called benefits of the built-up propeller. They just worked, and worked very well – a
testament to the knowledge and judgement and experience of the designers of the day.
That is not to denigrate modern design methodology. The development of computers and
mathematical treatments of what is a very complex flow interaction are most welcome and
nowadays readily available to the designer. Rather, it gives support to the practice of design
by engineers with a more complete understanding of their product and its operation than that
provided by the generation of numbers extruded from a ‘black box’ which have little meaning
without the experience and understanding required for their interpretation and application.
For interest I include a picture of the QE2 propellers, which was taken not long after they had
been fitted. Perhaps the modern military designer would look at this and, in the context of the new
QE class, draw attention to the low level of skew and the high number of blades – and pronounce
that in accordance with the modern doctrine they would have been much better if they had been
designed as built-up.
Well, would they? No, of course not. Built-up propellers were killed off in the 1930s. They were
overtaken by technology and replaced by the more efficient, simpler, cheaper, lower maintenance,
lower risk, fixed-pitch propeller. The present resurgence in built-up propellers should be recognised
for what it is – a needless over-complication – and as a consequence would be better buried
alongside its predecessor.
Yours faithfully
Dr Graham Patience
Managing Director
stone Marine Propulsion Ltd
10
Warship Technology March 2010
WT_Mar-2010_p6-7-8-10-11-12-13.indd 10 23/02/2010 15:51:34
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