Two years ago, I had the pleasure of being part of an extensive study at Chalmers University of Technology on the performance of 3 different 41′ racing hulls designed by us, mysteriously called A, B and C. The tests were made in different wave lengths, using CFD. Each test run of each of the hulls in each wave scenario required 28 fast PCs hooked up in a cluster, running for up to 24 hours.
The output from these files has a good accuracy, as checked against previously known results from other tests. For each simulation a series of images was produced; shown below is an example, in this case hull C running into a head sea with a wave length equal to the hull length. Notice the curtain of water flowing from the freeboard forward as the bow has just launched out of the wave peak, indicating the high precision achieved. Not so many years ago, simulations like these would have been technically impossible to perform, but even today, they are rare indeed.
The simulations were produced at a rate of 25 per second, each showing data readings of pitch, heave, wave encounter, and drag. The images were then spliced together, creating a film of each wave scenario – here is one example:
These tests were both revealing and confusing – we did learn which kind of hull shape offered the lowest drag in different wave patterns, but at the same time the amount of data gathered was so huge, and so new, that until now, we have spent countless hours evaluating the results. It is possible to interpret some phenomena in a number of ways and we have sometimes found ourselves in new territory.
After some deliberation we came to the conclusion that it was necessary to validate our methods against other standard tests. Therefore, a series of tank tests were performed on 7-8 February 2017 at SSPA, using the 260 m long test tank, with perfectly calibrated wave generators. This is a view of the test setup, taken from the 23 ton test carriage holding the model:
Closest in the image above are Professor Lars Larsson, the hydrodynamics wizard at Chalmers and the driving force behind both studies (standing) and Matz Brown, in charge of yacht testing at SSPA (sitting). The model is attached below the carriage which is running along the tank, in this case, at a speed corresponding to 7,5 knots.
Seen at different speeds and in different wave scenarios, this is what the tests looked like:
Both he CFD study and the results from tank testing will be shown later under the ‘EVOLUTION’ tab, along with other studies.