# How small can a towing tank be?

Discussion in 'Hydrodynamics and Aerodynamics' started by Todd Miller, Mar 17, 2014.

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### Todd MillerNew Member

Hello, I teach physics and wooden boat building at a rural high school in Washington state. I am thinking of combining the two disciplines somewhat and I was wondering if kids could learn much from small models in a small towing tank. I was thinking of a tank, something about 2' wide, 1' deep and 24 feet long. It would be more qualitative than quantitative measuring for resistance mostly and to video tape wave formation. Any advice would be appreciated.

Thanks

Todd Miller
Chinacum High School

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### PetrosSenior Member

what a great idea for a high school class! depends on how big the model, but I would think 3 ft wide might be better so you do not have as much interference from the side walls (reflected waves off the side walls). If you have some school grounds to use you can cut a trench and line it with plastic as your tank.

more complicated, but perhaps less work might be to make a stationary tank where you pump the water to the front, perhaps six or 8 ft long. than the boat is stationary and the water moving, where you can stand there and watch the model on the moving water. You can also direly measure the drag with a simple spring or digital scale at the front of the tank with a string attached to the model. it would be easy to observe the scale and than you drop more weight on the model and you can watch the drag increase as it sinks deeper into the water.

You have large drains at the back of the tank with the moving water going into a large pump, and than have it discharge at the front of the tank in a spreader you would get a steady flow in the center section. You would have to experiment with some means of making the flow calm and even at the front (guide vanes, etc) so you can have a fairly uniform flow over the center section. It would also take a fairly large water pump to circulate enough water to get some decent flow in the test section. It should not be difficult, I used such devices in engineering school that were much larger, but pretty simple to build.

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### Wayne GrabowSenior Member

Are you sure you need a towing tank? When I was in college I used a swimming pool to tow models of various designs and compare their performance. The data seemed pretty accurate and repeatable. I was able to use exact towing forces over measured distances and with carefully recorded times to develop performance curves.

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### tspeerSenior Member

In the AYRS book, "Design for Fast Sailing," by Edmund Bruce, he describes the construction of a small tow tank like you suggest. He used a falling weight to tow his models, so they were subjected to constant force, not constant speed. He'd then time how long they took to make a run to compare the drag of different models.

If you really want to measure drag at constant speed, I think a whirling arm rig would be the way to go. That way you can use a round tank, like a wading pool, and you can run the model long enough to get to steady state.

When I was in college, they built a 40 ft tank into a new engineering building and I got it operational as part of my senior project. However, there really wasn't enough distance to get the model accelerated to speed and then run at constant speed for some distance. It would just get settled down when it had to decelerate before hitting the end. And my hand-made balance didn't work very well. I think the main thing I learned from the exercise was a lot of care & effort has to go into designing and implementing a tow tank, and the equipment they inherited from a decommissioned tank wasn't really suited for their tank.

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### Richard WoodsWoods Designs

I studied yacht design at Southampton, UK. The college built the towing tank and then built the rest of the campus round the tank. And that was a small tank for students. The tank you'll see used in Tony Marchaj's first book was at Southampton University and was built in the late 1950's from old oil drums and was very crude. That's why they built a better tank at my college.

The big problem with small tanks is that errors are huge, as loads are so low. To get sensible results you really need models at least 5ft long. Then, when you scale the speed, you will find you need a long tank,as you have to get up to a steady speed first, maybe a 100ft or so long tank. Side and bottom effects are also a problem, sensibly you need say 5ft deep and 10ft wide. It is, as others said, a swimming pool

I would suggest, since you are only doing comparative tests, that you do some testing at sea on a motorboat and using a long balancing pole. One model is towed on each side, with the central pole pivot on the boat. Then it is easy to see which model has more drag

Although I am UK based I am actually in the PNW right now and will be at Port Ludlow on my boat all this week. If you would like me to come in to see you for a chat about your options please let me know by pm or email

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### Todd MillerNew Member

Thanks for the responses. At this point I am really just in the pondering stage. If we do this it would be for next school year but I was actually considering having some of my shop kids who need a project this year building the tank for next year. A swimming pool or testing on a lake are not really possible in that the closest swimming pool is 12 miles away and getting permission, transportation and so on for at sea trials would be problematic for a class full of students.

I am trying to come up with some projects that would encourage my physics students, who all see themselves as college bound, to set foot in the woodshop and my woodshop students who typically don't see themselves as college material to see some engineering in action.

You all have given me a lot to think about.
Thanks.

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### Remmlingerengineer

During my time at university I used a circulating water channel to teach some aspects of fluid mechanics. The water channel was built by the local university workshop. We only bought an electro motor, some pipes, bearings and seals and welded the parts together. The commercial towing tanks are so large because of the high Reynolds number that must be achieved for realistic drag measurements in turbulent flow. In teaching one only wants to show the phenomenons and that is quite possible in laminar flow. The flow is qualitatively the same. A channel of 15 feet length is large enough. In a circulating water channel the model is at rest and the water flows past the model. The wave system is stationary and can easily be illuminated and filmed. I used (low fat) milk, flowing out of a hollow needle (cannula) to show streamlines along a body. One can even show the horseshoe vortex in front of an obstacle, that rests on a wall (like a house on the ground in a strong wind, or the intersection of the fin-keel of a yacht with the hull).
The students were always excited.
Uli

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### gonzoSenior Member

You can build a fairly long tank that can be easily dismantled. A wooden or pipe frame and plastic sheeting is all you need. It can either be towed or have water circulating.

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I think this is an excellent idea. The length is not really long enough for "decent" results. BUT, you're not after high quality results, rather the "experience" and blending theory with practice to get their creative juices going and a thirst for science etc. The more "intelligent" ones will come to realise this from their results too..perfect

FWIW, here is mine, I built this last year, its 20m long, by 2m wide by 1m deep.

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### DCockeySenior Member

Several considerations for a recirculating channel compared to a tow tank, which may not be important to Todd:

1) Recirculating channel will need higher power for same velocity.

2) Flat velocity profile entering the channel generally requires a screen or similar.

3) Boundary layers will grow along the channel walls.

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### Leo LazauskasSenior Member

Typically English. You even need to form a queue in your jacuzzis.
Is it still ice free?

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Hasnt been ice free for months
But...past 4-5days, no frost...so, maybe spring is finally on its way

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### Leo LazauskasSenior Member

I'll be interested in seeing the results of very simple cases, like a plane wave
travelling down the length of the tank and whether there is significant
contamination from the side walls.

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### idkfaSenior Member

Nice going Ad Hoc. How do the sides resisting the weight of the water?

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