Media Technical Brief Hydrodynamics and Ship - Rolls

Media Technical Brief
Hydrodynamics and Ship
Hydrodynamics and ship efficiency
Hydrodynamics is at the heart of Rolls-Royce marine activities. A large number of products in the portfolio
involve moving fluid, therefore a large knowledge base has been built up in propeller design, a number of
azimuth thruster designs, tunnel thrusters and waterjets.
Advanced Computational Fluid Dynamics (CFD) together with tank testing have become extremely important
in ship design focussing on overall vessel efficiency to reduce fuel costs and emissions to meet Energy
Efficiency Design Index (EEDI) targets.
The complex interaction between the hull and rudder has to be fully understood and the hydrodynamic
performance of the propulsor be defined for good performance to be achieved with the minimum noise and
Most people would say that good performance combines good efficiency with low fuel consumption. But for
ice breakers, blade strength and reliability could be top and for cruise vessels and yachts it’s normally
onboard noise levels. For naval vessels, that need to operate silently at high speeds, it’s cavitation inception
speed - the speed at which they can operate before the propellers produce any cavitation and therefore
radiated noise, so making them much easier to detect.
Cavitation performance and noise levels are normally contradictory to high efficiency. High propeller efficiency
often results in high pressure pulses and a lot of cavitation with the resulting high risk of erosion with high
noise levels. As no ship owner wants a propeller that initially has very good efficiency, but after a short time
experiences cavitation erosion or high vibration levels, two types of comparative model testing is the best way
Cavitation comes in many forms, the main ones being; suction side sheet, blade root, tip vortex, hub vortex
and propeller hull vortex cavitation. The balance between efficiency and cavitation is just one of the balancing
acts for the designer, several operating conditions of the propeller must also be considered.
To be a winner in the efficiency model testing stakes, the target is to achieve the lowest possible power
consumption in the towing tank test at model scale. But this can mean other factors like vibrations on board,
radiated noise levels, risk of corrosive cavitation, and fuel consumption at full scale, with the vessel exposed
to different drafts, trim angles and winds are not evaluated as fully as they should be.
First is a towing tank test (resistance and propulsion) where power consumption is measured. It should be
followed by a cavitation test, where cavitation performance and pressure pulses can be validated.
The standard test is designed to study fully developed cavitation, to determine if it is erosive and the noise it
generates. As there is a step-change in the noise level a non cavitating propeller produces and its cavitating
counterpart, it is necessary to find the cavitation inception speed or CIS. This is the point where cavitation is
barely visible. But the change in noise level happens quickly, as soon as cavitation starts, so even a small
amount gives rise to a significant increase noise.
Identifying CIS requires a special test to find incipient cavitation and starts with the propeller completely free
of any type of cavitation. By varying different operational parameters the inception point of the various types of
cavitation can be determined and mapped. For each type of cavitation the points are combined to form limit
curves, then another curve representing the operation of the vessel is overlaid. The intersection point between
this curve and the propeller cavitation curves identifies the inception speeds.
The Rolls-Royce Hydrodynamic Research Centre
Gaining a deep understanding of hydrodynamic issues requires a lot of experience and historical data on
which to draw. The Rolls-Royce Hydrodynamic Research Centre has been undertaking detailed
hydrodynamic studies of propulsors for over 70 years. Over 1,500 propeller designs have been model tested
to date, and around 40 are undertaken each year.
The Rolls-Royce Hydrodynamic Research Centre, is located in Kristinehamn, Sweden. The site was chosen
by KaMeWa as the location for their, and Scandinavia’s, first cavitation tunnel in 1942. The facility moved to
its current site in November 1970. The site has two cavitation tunnels, a conventional one with a volume of
117 m3 and another with free water surface, containing nearly 400 m3 of water. A free surface is required to
test surface piercing propellers.
The conventional tunnel is focused on studies of the propulsor itself in an undisturbed homogenous inflow, i.e.
propeller open water tests or water jet pump loop tests where the characteristics of the propulsor is mapped.
In the larger free surface tunnel, the propulsors are tested in their “real” environment, the propeller behind the
hull or the water jet pump together with inlet and steering/reversing unit. The test set-up used for water jets
where efficiency and head rise of the pump unit is measured in the conventional tunnel and propulsive
efficiency and cavitation performance of the complete water jet unit in the free surface tunnel is a setup that
few other institutes in the world can match and that has made a strong contribution to the development of the
water jets performance.
The Centre has a wider role than cavitation tests of propeller and waterjets, with an objective to provide
hydrodynamic support throughout the Rolls-Royce Group. The work carried out at the laboratory can be
divided into five different areas:
Hydrodynamic design,
Hydrodynamic analysis of performance,
Sales support,
Product development (R&D) and;
Long term strategic research (R&T).
Over the last decade CFD and numerical methods have advanced at a rapid rate and brought further
improvements. Using this bank of hydrodynamic and CFD data, a design system, ‘Propcalc,’ has been
developed, automating and integrating optimisation and CFD analysis tools into the Rolls-Royce design
process. This makes it possible for a designer to arrive at a preliminary blade design very quickly.
Rolls-Royce has worked with designers from Fjellstrand AS on the design of an innovative battery powered
catamaran ferry. CFD analysis was used to minimise drag, study propulsion / hull interaction and develop an
Azipull thruster with a fully feathering propeller.
For further information including pictures, please contact:
Craig Taylor
Head of Communications – Marine
m: +44 (0) 7807 969 426