Here´s the new swimming platform.
We hope this one will give more useful results.

The platform is stabilized by two cuboid neodymium magnets, one under the platform and one under the water tank. The magnets keep the platform aligned in the center of the water tank while the cuboid shape works like a weak torsion spring, aligning the rotation angle to a defined state.

The data acquisition will be made automatically via optical tracking of two LEDs mounted in the edges of the platform.

Many thanks for your comments and suggestions - we hope to provide some results for you soon.