(2005) Oosterhuis - Model-scale podded propellers for maritime research

G. Oosterhuis, Model-scale podded propellers for maritime research, Doctoral degree 12-10-2006; Department of Applied Physics

Model-scale podded propellers for maritime research
Gerrit Oosterhuis [doctoral thesis, Eindhoven : Technische Universiteit Eindhoven], H.C.W. Beijerinck, Tom J.C. van Terwisga [Promotor], J. Tukker [co-promotor]

This project was financially supported by the Maritime Research Institute Netherlands (MARIN), Wageningen, NL and the Stan Ackermans Institute of the Eindhoven University of Technology, Eindhoven, NL.

Summary
Podded propellers constitute a major new development in ship design, especially for cruise liners but also increasingly for freight ships. A podded propeller (pod) consists of an electric motor which directly drives the propeller. This motor unit is located inside a gondola that is hanging from a strut. This strut has a rotating connection to the ship. Advantages with respect to conventional propulsion are an increase in fuel efficiency (up to 10%), better manoeuvrability and low vibrations. However, recent service breakdowns have slowed down the market. The availability of highly accurate model-scale measurements may help to discover the causes of failure in these 23 MW devices.
The Maritime Research Institute Netherlands (MARIN) provides the maritime industry with performance predictions and design consultancy. Part of this is based on model-scale tests performed in large water basins, so-called ‘towing tanks’. The ability to perform highly accurate model-scale experiments with pods is a major challenge for MARIN. As yet, all model-scale experiments with pods have been performed with electric motors external to the pod, using bevel gears to drive the propeller axis. This set-up limits the geometric modelling flexibility. Also, it introduces vibrations, which make it difficult to measure dynamic loads which are essential for analysing manoeuvring tests. Further, the sixcomponent load balance that measures the propulsive force of the pod-unit does not deliver the <1% accuracy that is required. The goal of the project was to solve the existing technology problems and to design a new pod model. With this new pod model, new testing opportunities will be created. A successful new design will generate a new impulse to pod research as well as new market opportunities for MARIN.
The key to reaching this goal is to design a pod model with the powering motor housed inside the gondola. Hydrodynamic scaling laws in combination with flexibility requirements demand a very high power density from the drive motor; up to 1:6MW=m3, which is about 2-3 times larger than the full scale power density. This makes the search for and the design of the
drive motor a challenging task.Of the available choices – a hydraulic or an electric motor – a hydraulic motor has the
highest power density. However, hydraulic solutions have many negative side effects like: low user-friendliness and disturbance of the pod-unit force measurements. Most electric motors lack power density when used according to supplier specifications, which is the reason that currently no solutions are available in the field. The key step is to utilise the towing tank water as efficient cooling medium while accepting a reduced lifetime. With these boundary conditions, the supplier specifications can ’thoughtfully’ be overruled. For an electric motor, this approach leads to an increase in power density by a factor of 2-4, so that sufficient shaft power can be achieved.

In hydrodynamic research on pods, several standard types of experiment on model scale exist which all imply different requirements for the pod model. As a result, no single generic solution exists Therefore, a toolbox has been defined which provides direct propeller drive solutions, with the propeller drive motor located inside the pod gondola. The toolbox covers
over 80% of all model-scale experiments with pods at MARIN. A direct propeller drive is the core of solving problems with the existing devices.
In this project, the two most relevant tools for MARIN have been designed and tested successfully. First, a small scale (1:30) basic pod model has been realised, which is especially suited for manoeuvring and sea-keeping tests. Here, the drive motor is an electric motor with a gear box, which was optimised to deliver 3.5 times its supplier-rated power. No measurements at the propeller shaft are possible. However, the measured motor current can be used to provide a measure for the motor torque with an accuracy of 1% up to a measurement frequency of 10 Hz. This measurement frequency is sufficient for manoeuvring and sea-keeping tests Second, a direct propeller drive has been realised for large-scale (1:20) powering experiments. This design path yielded a more complex, large-scale pod model which is equipped for most regular powering optimisation tests. The drive motor was purchased as a separate rotor-stator package and optimised through water cooling to deliver a torque up to 17 Nm, which is about 3 times the supplier-rated limit and 50% higher than required. Using separate motor parts allowed full integration of the direct-drive motor and the force sensor in the propeller shaft.
The six-component force balance that is used to determine the propulsive forces of the pod-unit was found to lack accuracy, with typical errors of 2-5%, depending on the loading case. The existing balance is also not suitable for dynamic measurements as of its low resonance frequency of about 10 Hz, where 80 Hz is required. Therefore, first the correct implementation of underlying theory in calibration routines and data processing has been reviewed and updated. Also, a new six-component, pod-unit force balance has been designed, to fit the state-of-the-art accuracy requirements. Measurement errors could be be reduced to 1.5% for complex combined loads. Finally, dynamic pod-unit force measurements during manoeuvring have been proven feasible up to a measurement frequency of 40 Hz, which equals a typical first blade harmonic.
The current results also form the starting point for further design efforts. First, the developed concepts have to be implemented in the operational towing tank processes. Further, the next major challenge is to further improve the data quality of dynamic pod-unit force measurements during manoeuvring.
The design of an innovated pod model has removed a major obstacle on the way to new model-scale testing opportunities. Existing test services will improve when the current concepts are implemented. This will consolidate MARIN’s leading position in model-scale research on pods. Furthermore, with the developed tools MARIN is prepared for future developments, thus capable to stay ahead of global competitors.
Finally, the innovated pod model provides the maritime community with a range of opportunities to gain deeper knowledge on podded propulsion. This knowledge will help to strengthen the position of pods in the maritime market. In this way, podded propellers can continue to grow, offering their benefits to the maritime industry and the global society.

KEYWORDS
dissertations the and tu/e; mechanics: dissertations. heat: dissertations; ship propulsion. ship performance; propellers; digital full-text; ship propellers; ship propulsion; electric propulsion; model research; force measurement; sensor technology

Source :Universiteitsdrukkerij Technische Universiteit Eindhoven

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