MARIN measurement signal SOLAS test.
propeller loads can be measured with the same high accuracy as MARIN’s standard thrust and torque sensors. Finally, a light weight aluminium propeller ensures minimum influence from the propeller mass on the measurement signals.
Model tests With this newly developed test set-up the following test series were carried out:
• Open water tests for several steering angles, including those above 90 deg
• Pod-pod interaction tests at zero speed
• Zigzag, Crash-stop and SOLAS tests with a free sailing model.
The open water tests – with such
have also been carried out at the MARIN basin in Te Netherlands to measure the loads on the propellers and pod units of vessels operating in various conditions. Te study, which forms part of one of
the Cooperative Research of Ships (CRS) working groups, has brought together a group of companies with a common interest in non-competitive research into hydrodynamics and related issues in terms of design and operation. Loads on Pods is one of the CRS projects
and the working group has been tasked with developing a mathematical model to enable its users, such as shipyards and classification societies, to determine the loads on pods for various operational conditions, at the design stage. For the development and validation of the model, an extensive series of CFD calculations and an innovative series of complex model tests were carried out to measure the loads on the propeller and the pod unit. MARIN was asked by the working group
to create a specifically-designed model test set-up; but to determine the loads on the pod slewing and propeller shaft bearings, it had to be capable of measuring six components (three forces and three moments) at the pod steering axis and at the propeller shaſt. Te system also had to be capable of measuring the average forces and moments, as well as the unsteady forces and moments up to the first blade frequency. In addition, precise azimuthing
The Naval Architect July/August 2009
angles and negligible mechanical vibrations were needed.
New test set-up Core elements of the set-up were the model-scale electric pod drive system developed some years ago and the well-proven technique of measuring pod unit forces and moments by a six-component balance. Just as at full scale, the propeller is directly driven by an electric pod motor, thus eliminating additional gearboxes, couplings and shaſt lines. Another advantage is that the position
of the turning axis of the pod can be kept the same as at full scale, which means that the forces of the pod during steering conditions are correctly modelled. Cooling of the motor is provided by an aluminium housing which ensures good heat transfer to the water. For this particular test set-up, an even smaller motor was developed that delivers ample power to drive the propeller within the available pod geometry. The pod unit balance has been developed with a high natural frequency, providing a clear view on the hydrodynamic forces with minimum distortion by vibrations. A new, zero backlash drive system was
added to deliver precise azimuth steering during the test runs. Te main innovation, however, is the application of a newly developed balance in the propeller shaſt between the motor seal and the propeller hub. Now the full six components of the
large steering angles – provided unique information on the forces and bending moments on the propeller shaſt and on the total pod unit. Particularly special, was the information derived for propellers operating in oblique flow conditions above the common limit of 35deg. Results that have been derived are being used for the validation of the mathematical model. Pod-pod interaction tests were carried
out to determine thrust breakdown of the propeller due to the wash of the other propeller. Apart from the thrust breakdown, it is known from full-scale experience that these situations can lead to undesirably high loadings of the bearings. Tese tests were the first ever to be carried out that did not only provide data on the propeller thrust but also on the side forces and bending moments of the propeller in these harsh conditions. Tests with the free sailing model
provided unique information on the unsteady propeller shaft forces. With this information insight has been gained on the loading distribution between the propeller and the pod housing and their contribution to the resulting side force of the pod unit while steering. With MARIN’s new model test set-up, a unique data set was successfully obtained on the steady and unsteady loads on pods. Te newly developed model test set-up elements have proven their merits and are more than ready to be applied to challenging projects in the future. NA
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