How small can a towing tank be?

Todd that is a great idea. I suggest you contact the Northwest School Of Boatbuilding in Port Hadlock. I would bet they would be really helpful and might even send some of their students over to help you out. Next time I go to Port Townsend, if I get the chance, I may stop by and see how it's coming on. Even a small tank and model is really a great way to demonstrate Archimedes Principle and the basics of buoyancy and stability.

 

Did you ever hear of the Prandtl (Prof. of Aero- and Hydrodynamics) Test Tank ? http://www.dlr.de/en/desktopdefault...d-26563/gallery-1/gallery_read-Image.1.17454/

http://www.dlr.de/schoollab/desktopdefault.aspx/tabid-4964/14529_read-13948/ and http://www.dlr.de/schoollab/Portaldata/24/Resources/dokumente/hb/prandtl-kanal.pdf
http://www.google.de/imgres?imgurl=...0sU6HVAcrrswaG8oDYBg&ved=0CEQQ9QEwBA&dur=1443

 

I went to see Todd a couple of days ago and now have a better understanding of what he would like to do, and what he can do. We discussed a number of different ideas, not just boat related

The Prandtl test tank seems a great idea that would suit his school and students

Richard Woods

 

that is a great way to do it. Simple design and easy to build. But I would dump the paddle wheel, instead put a propeller in the underside, in the return duct, driven by a shaft that is powered by an electric motor (that is outside the tank). You can get a fairly inexpensive plastic prop designed for electric trolling motors at any marine supply store. with that set up you get less turbulence and less work, and you can hold a steady state flow and do comparisons between hull designs. that can be made fairly large without too much cost.

 

......

 

Yes, of course. One can make it better, bigger, longer, wider with instruments, a magic stick inserted in the model combined with a computer and filters to avoid turbulences. The Prandtl water channel is something like a minimum. Built more than 100 Years ago (1904) it helped to develop the theories of turbulent fluids and boudary layer. And the idea, to have a small test channel in the cellar/basement in my house to test models of hulls or profiles seems to be very attractive to me.

 

Thanks for the ideas. I really like the idea of the Prandtl tank, I had actually been thinking of how an "Endless Swim" lap pool works and this is nearly the same idea. I teach science and woods and I could imagine using it at all levels to just show the basics. As Uli suggested using milk to show flow could add to it. My kids build water rockets when we study Newtons laws and many of them have a very Aristotlian view of turbulence, they think it turns around an object and "pushes" it.

Richard Woods helped me clarify my thinking too. And this is something I can have some of my less motivated shop students build for me since they can't seem to find projects of their own to build

Todd

 

Great! Will you test hulls for your clients, making scientific research for the students you are giving lectures at University, try to get answers for yourself? Would be nice to be your neighbour and discuss the results with you.

And a last question: Do you plan to make windows with glass at the bottom and the sides to watch the flow, the whorls the vortices and eddies and the flow around the hulls making them visible in the water by mixing chemicals (or sesame/linseed) into the flow and filming them with a camera running with the hull?

 

All the above

No. Too many earthquakes on a regular basis, so everything is "simply supported" as such and not 100% fixed; to allow movement during a quake. As such I've compromised to save cost and construction, i.e. to minimise all possible sources of leaks during a quake!

 

Model size is the real driver with a tow tank. You have to avoid shallow water effects (based of depth Froude number) and blockage effects (base on model depth Froude number and tank cross sectional area). Remember that errors scale to the cube of the scale factor .

Also laminar flow is a huge problem with models under 5 ft LOA and moving at less than about 3 feet per second even with turbulent stimulation