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but - if correctly tackled - it is possible to achieve high measurement accuracy. In addition, the necessity to have full control of the image quality led to the development of a remote control system that enabled the optics to be adjusted from on board the vessel.


Exceeding expectations The resulting measurement setup includes miniaturised, artificial lights, which were custom-made by MARIN, to achieve the required illumination power, and two cameras enclosed in underwater housings. A sturdy measurement system was developed to make sure that the integrity of all the components is maintained for the duration of the measurement campaign. For example, the camera housings and supports were designed and installed with extreme care to minimise vibrations despite the strong flow present, as they would directly affect the measurement quality.


Composite propeller at full speed. Colour bands indicate the measured deformations


environments. DIC is a full-field, image analysis method that uses the image data collected by a stereo-camera system to measure displacements and deformations of objects in the three dimensional space. This method was successfully applied at model scale in the cavitation tunnel in uniform and non-uniform flow and in behind conditions in the towing tank (see MARIN Report 121, pages 16-17). The next challenge is to turn it into full-scale measurements on a sailing vessel.


MARIN was tasked to design a system able to apply DIC measurements to a flexible, composite propeller at full scale and carry out these measurements onboard. The propeller was designed by Delft University of Technology, Solico and Wärtsilä and installed on one of the Royal Netherlands


Navy’s diving support vessels after the application of a random speckle pattern on the surface of the blades to enable DIC measurements. The propeller diameter is one metre and the laminate lay-up of the blades was optimised to achieve high flexibility, while maintaining the required strength and fatigue performances. In particular, the design of the blade-hub connection was especially challenging due to the interface between bronze and composite material.


Application at full scale Applying DIC to a sailing vessel presents several challenges including the high rotational speed of the propeller, underwater visibility, cavitation, scarcity of light and vibrations among others. All these challenges can easily compromise the quality of the results


The quality of the results exceeded all expectations: measurements of the propeller blade deformations were delivered with an accuracy comparable to the tests in the cavitation tunnel. In addition, observations of the propeller cavitation pattern were achieved with superb image quality. These results are the outcome of careful design and assessment of the risks of an innovative measurement system and - last but not least - the fruitful and effective collaboration between all the partners of the Greenprop project. Going forward, these measurements are valuable to validate predictions used to design flexible propellers and to facilitate improvements in ship performance.


report


17


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