The registration of the dynamic waterline on a ship model in a towing tank has been challenging the ship hydromechanics community for some time now. One of the methods to investigate squat, freeboard and bow wave dynamics with passing ships and in waves is the use of images of the ship’s side to determine the instantaneous waterline.
Two problems arise whilst using this method; large amounts of image data are acquired especially when filming a complete run, which is necessary to investigate non-stationary conditions. A second problem is poor image quality due to reflections and glare due to lighting sources. This will greatly increase the effort needed to determine the instantaneous waterline on the image. Currently this is often done manually
To perform an efficient analysis of the waterline and wetted surface, a high degree of automation has to be introduced due to the large amounts of image data. This requires a high level of conditioning to minimise reflections on the image in order to create a simple and fast processing algorithm.
To minimise glare and shadows, the models have to be light-emitting, which can be achieved in three ways:
- phosphorescent coating which is charged prior to the test,
- a fluorescent coating which is activated by using ultra-violet lighting,
- or a radioactive coating using a special camera.
For implementation in the Towing tank for manoeuvres in shallow water (co-operation Flanders Hydraulics Research – Ghent University) in Antwerp, Belgium, the radioactive coating was rejected due to obvious objections of safety and practicability. Also the phosphorescent coating was dismissed because of practicability issues of the charging of the coating above and under the water surface.
Eventually, a ship model has been covered with a semi-transparent fluorescent coating. A projected Cartesian mesh, painted on the mode,. Is used to correlate the warped image with a projected side view of the ship’s hull. In this way, the analysis stays two-dimensional and no stereovision systems have to be used.
The test setup consists of four black light tubes in two waterproof housings and a digital camera controlled via a laptop computer. The UV light emitted by the black lights is reflected by the fluorescent coating as visible light which is captured with the digital camera. The processing algorithm is in development at the Image Processing and Interpretation Division of Ghent University.
A definitive setup is in development. The current state of the research and the design considerations of the software and hardware will be described in this paper.