MARITIME
MARINE
PROPULSION
“Rim-driven” electrical thruster
CLIENT
British marine propulsion company
CHALLENGE
Develop a high performance rim-driven thruster
SUCCESS
20% increase in thrust
15% increase in propulsive efficiency
01
CHALLENGE
In traditional marine propulsion systems, an engine is connected by a shaft line to a propeller immersed in water. The increasing use of electric motors in the nautical sector has led to new configurations, called “rim-driven”, in which the rotor of the electric motor is mounted on a ring attached to the vane, while the stator windings sit on a coaxial nacelle.
The company had a well-tested methodology for traditional propeller design: an analytical method performed a preliminary sizing of the propellers, and modifications were then made to the geometry following indications from CFD simulations.
The rim-driven thrusters designed with this method, however, were not characterised by very high performance — the differences in construction compared to traditional thrusters required a change in the design methodology.
02
SOLUTION
A rim-driven thruster has much more affinity with an axial pump than with a propeller, so we were able to put our expertise in axial pump modelling and simulation into play.
Having previously developed the MPROP software for axial pump simulation, we proposed a new design approach: the through-flow method, widely used in aeronautical turbine design. Its advantages:
01
it simulates the flow field as a function of the radial coordinate. Unlike traditional analytical methods, which only calculate at the mean line of the duct, it allows accurate optimisation of the geometry
02
coupled with an optimisation algorithm, it obtains an efficient geometry in a short time. Fluid dynamic simulations give accurate results but take far longer and carry significant licensing costs
Since MPROP tests each pump configuration in a few seconds, the optimizer was able to explore different geometries until the optimal one was reached, in a total calculation time of under five minutes — where testing a single configuration with CFD typically takes several hours. MPROP turned out to be the best trade-off between design speed and accuracy of results.
03
RESULTS
The optimised geometry calculated with MPROP produced significant improvements over the configuration the company had initially developed:
01
a 20% increase in thrust
02
a 15% increase in propulsive efficiency
Before the prototype was made, these values were verified by a single CFD analysis, which confirmed the accuracy of the design calculation — differences of a few percentage points. The improvement was further confirmed by the experimental tests carried out in collaboration with the University of Southampton.
The results are a concrete example of the technology transfer typical of our R&D philosophy: experience gained in industrial pumps, combined with a method borrowed from the aeronautical sector, allowed technological progress in marine propulsion.
