Feature 8 | ProPulsors
and required sea state to be designed for. Above: Blade pressure distribution and
In order to investigate the influence of the cavitation extension on the “final-design”
design point over the 30-year design life, predicted by a potential flow boundary
it was decided that four similar propeller element method (BEM).
designs, with four different design points,
should be compared. The procedure to Below: Distribution of max 1st order
produce the “final design” propeller blades propeller-induced hull pressure amplitude
was a follows: on the QE vessels.
•
Define the four design points/conditions
of interest, covering the range from
start-of-life draught trial condition
(clean hull) to end-of-life draught with differences in wake field and to optimise
20% fouling and some further wave the performance can be summarised as
added resistance; follows: chord length increased between
•
Find the optimal blade design for 0.80R – 0.95R, increased between 0.27R
one design point in the middle of the – 0.40R, EAR increased by 2.3%, tip load
range; reduced, blade weight increased by 2.9%.
•
Produce this design and determine the
pitch and camber distributions at the Analysis
four design points using the same: To be able to calculate and investigate a
- thickness and chord length large number of blade design alternatives
distribution a fast but also accurate tool is required
- load, rake and skew distribution that handles a non-homogenous inflow
- blade root design; amount of suction side sheet cavitation and predict blade pressure distribution
•
Analyse influence of the design point in and lower pressure pulse levels. and cavitation extension during the
terms of: During the design phase of the propeller rotation. These requirements
- efficiency in wake field “final-design” propeller, special attention necessitate the use of potential flow
- cavity extension/volume was paid to the blade root design, which methods prior to viscous RANS or LES
- circulation distribution is often a problem area on twin-screw methods. At Rolls-Royce Hydrodynamic
- pressure pulses levels; high-speed vessels. The lower rotational Research Centre a number of potential
•
Select the preferred design point for the velocity on the inner blade radii gives flow methods were used for analysing
“final-design” propeller; and a relatively large variation in loading propeller designs.
•
Optimis e and pro duce the caused by the wake field than on the outer Unsteady blade pressure distribution
“final-design”. radii. This in combination with the high and cavitation extension is routinely
vessel speed gives a significant risk of calculated for all propeller designs using
The four design points chosen can be cavitation on the blade root, which has a potential flow code which is based on a
described by the following: start-of-life to be controlled by a very careful blade potential vortex lattice theory and takes
draught, trial/clean hull condition; mid-life root design. The “near-design” propeller the propeller geometry and the 3D wake
draught, service/with fouling condition; was free from any root cavitation, but as field as input. The results are unsteady
end-of-life draught, service/with fouling mentioned above the wake field on the blade and propeller forces, blade pressure
condition; end-of-life draught, fouling inner radii had become worse due to the distributions and cavitation extension
with waves. modifications to the hull appendages. predictions. The potential flow theory
Generally, for propeller designs it can be Compared to the “near-design” presumes that the flow is inviscid and
said that a more loaded design point gives propeller, the modifications incorporated irrotational, which means that viscous
less suction side cavitation at high load to the “final-design” to address the effects such as separation and skin friction
and higher risk for pressure side cavitation is neglected.
at low loaded conditions. Based on the For more advanced propeller designs
study presented and recommendations by as for the QE class, another potential
Rolls-Royce the ACA decided that design flow code is used. This code is based
point no 3 was preferred and should be used on a potential flow boundary element
as the basis for the “final-design”. Design method (BEM) that instead of putting
point no 3 is a relatively heavily loaded singularities (vortices, sources and sinks)
condition at end-of-life draught that gives on the mean line of the blade profiles
a blade design with larger cambers on the
blade profiles as opposed to that given by
a more lightly loaded design point. Larger Picture from SSPA cavitation test of “final-
cambers are preferable as it gives a small design” propellers.
44 Warship Technology October 2009
p40-47_WT Propulsors-QE OCT09.indd 44 17/09/2009 17:03:05
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