This Is Only A Test

The 6.50 Minis are truly trippy little things, and it is nice to see the class starting to have some momentum here in the States. Here is a Mini project from designer Chris Cochran, who is a Naval Architect & Marine Engineer with Morrelli & Melvin Design. Enjoy.

The Mini 6.50 model test project was started in the Fall of 2003, when a friend asked if I could design a Mini, possibly to build in the future. Naturally, I took the idea and ran with it, and decided to try and somehow get college credit in the process, towards my Masters degree in Naval Architecture and Marine Engineering, at the University of Michigan. After brainstorming with a professor, I decided to build two 1/3 scale models of different Mini 6.50 hull, and test them in our towing tank. At 1/3 scale, the models could be tested at speeds over 15 ft/sec, which correspond to about 15 kts at full scale, i.e. planing speeds. The two hulls vary by waterline beam, volume displacement and form stability - "Paris" is the skinny one, with narrow waterlines, flare and a fine entry forward, while "Nicole" is closer to the typical open hull, with a wider waterline beam, flatter underbody and fullness forward.Paris, the 7.1 model Mini 6.50 seen in the pictures, was built over Christmas break, at VanDam Woodcraft in Boyne City, MI, and was finished and tested at the University of Michigan's Marine Hydrodynamics Laboratory. The hull was constructed using the cedar strip method, with epoxy and cedar, and the keel strut and bulb were built of various woods, reinforced with stainless steel. The model was then fitted to equip the yacht dynamometer, a testing apparatus that can measure sideforce, roll moment, yaw moment and drag.

Testing was begun in March, and later wrapped up in May, with the help of many fellow U of M naval architecture students, sailing team friends and lab technicians. By working nights and weekends, we were able to test Paris in a huge matrix of sailing conditions, including displacement, pre-planing, and planing speeds, at various combinations of heel, yaw and trim. Testing was done for the bare-hull only, and later with a strut and bulb, both straight and canted. The data was compiled and regressed, and we were then able to look at trends, such as how resistance, sideforce, roll moment and yaw moment change with speed, heel, yaw, etc… Most importantly, we were able to numerically see what happens to resistance and other forces at planing speeds, with and without a canted keel. Unfortunately, the semester ended before I was able to finish building and test Nicole, so the two designs could not be compared, as I hoped.

The testing experience proved to be invaluable, as we were able to observe certain phenomena, not easily predicted by CFD. The results will help in understanding and optimizing the performance capabilities of the Mini 6.50 design, as well as any similar planing boats, and boats with canting keels. Lastly, the project provided an excellent "hands on" approach to naval architecture - not an easy task at the university level, with the advent of computer design and optimization. There are no immediate plans to build the full scale design, but I would like to see how the hull would perform amongst the existing designs. If you have any questions about the design, tank testing, or the naval architecture program at University of Michigan, email me and thanks very much.

10/15/05