Cheap Tow Testing

Discussion in 'Boat Design' started by Sean Herron, Mar 29, 2008.

  1. Sean Herron
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    Sean Herron Senior Member

    Hello...

    Aside from building a big doughnut shaped flume like tank and pumping fluid around the model vs. towing the thing - how about this...

    The cranberry farmers around here keep these irrigation ditches which have nice - high dykes - tension up a cable down its length and make a carriage that can be hauled in from one side - easy enough - but I have a question...

    Where to connect to the model in a free float state - I imagine a line through the line of thrust and intersecting at the LCG - but I am not sure...

    If the above is true I imagine the rig or truck to operate per below - does the 'towing angle' need to be relatively parallel to the LOT...

    Thanks in advance...

    SH.
     

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  2. marshmat
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    marshmat Senior Member

    I'm looking for scientific papers on this subject. There are a lot of different ideas going around that don't necessarily agree. What you're planning is a bit more sophisticated than the one I'm working on (which will be suspended off the side of a motor launch, 1920s-style, and used to study hull motion in various sea states but not for resistance).

    I'd suggest as a starting point that we generally want the forces on the model to be as similar as possible to the forces on the real thing. No doubt you're not interested in building a full, multi-axis yacht dynamometer ($$$!).

    As a starting approximation I would draw a properly scaled free-body diagram of the yacht in profile, including prop thrust at the proper angle. I'd suggest starting with the harness attached at the centre of gravity and parallel to the actual thrust direction, and see how the forces balance out from there. Once you have some idea how that seems to be working and can resolve or approximate some of the forces, the bridle/harness can be refined.
     
  3. Sean Herron
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    Sean Herron Senior Member

  4. tom28571
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    tom28571 Senior Member

    This pretty close to the way I did some towing tests. I rigged a towing rig made from an old Sunfish boom near the bow of a 15' skiff powered by an outboard. This rig allowed the model to run alongside the skiff about 10' out to the side and completely out of the waves generated by the towing boat. I did use a string bridle attached to the CG on either side of the model. It was necessary to attach a third string to the bow of he model to facilitate getting it started in the right direction without too much fiddling around. This string to the bow was loose and had no effect on behavior of the model after it got started in the tow. The tow line was not exactly in line with the thrust line of the propeller and varied somewhat but was at an angle that was fairly close to the thrust line.

    Being towed alongside the towing boat allowed close observation of the behavior of the model.

    I did measure the towing drag with a crude rig attached to a postal scale. I made no effort to relate the drag to full scale but only relative to other models and different displacements and hull configuration of the one model. The model was 4' long as this was recommended as the minimum length for getting good results.

    Most simple towing experiments I have seen in books or elsewhere have used a towing line attached to the bow of the model. The towing yoke attached to the CG allowed the model to move at will and eventually showed some bad behavior under different bottom configurations that would not have been properly observed if it were towed from the bow. With this rig I was able to tow to a scale speed of 33mph which was more than was needed for this model.

    There are several detail photos of the model at: http://www.bluejacketboats.com/Bluejacket 24 photos.htm Scroll to the bottom of the page.
     
  5. tom28571
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    tom28571 Senior Member

    I see Sean posted while I was writing. That rig is very similar to mine except for the towing attachment. I considered a rig like that but thought it more complicated and that it did not allow the model to move about as much as the simple string yoke attached to the CG. It would not have allowed some really bad behavior to be observed under certain conditions.

    Maybe using both attachment rigs separately would be more ideal.
     
  6. Sean Herron
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    Sean Herron Senior Member

    I Love This...

    Hello...

    Thanks all - but this is the game - and this is what keeps me going - for one - I am more thankful for same than you can imagine just now...

    All the GM's - all the DOE's - (Director of Engineering) - all the maths - and we still want to go back to models and just WATCH the bugger in its medium - or WATER...

    I love it - I have CFD proggies and other - but lets face it - we are humans - and as such - and with years of poorly digested information - like good cheeseburgers - real folks want to see that bugger running in water...:)

    This is all good...

    SH.
     
  7. marshmat
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    marshmat Senior Member

    Tom, Sean - This is proving to be an interesting discussion.

    I've seen your site, Tom; very inspiring. I hope you don't mind if I borrow some elements of your towing bridle design for the model I'm testing this summer ( http://www.boatdesign.net/forums/showthread.php?t=17597&page=8 ). Being hand-built from scrap in my basement it's not as sophisticated as the CNC-milled models or CFD simulations I used on the solar car, but it ought to tell me what I need to know.
    (Model and harness ~$200, FLUENT CFD and machine that can run it, $28000, the choice seems obvious)

    Just working through the dimensional analysis to figure out whether your results are meaningful is an interesting exercise in itself. I've been using EES to run parametric tables on my 1.66m model (10m boat) for estimating turbulence transition points, Reynolds and Froude numbers, surface tension problems, etc- the numbers can give a lot of insight into how the model will compare to the real thing.
     
  8. tom28571
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    tom28571 Senior Member

    The extra line to the bow was added after a disaster in one test run. I was making some runs to look at the effect of adding warp to the bottom of the hull. Being free to react in any manner, the model started to oscillate in yaw and as speed was increased, eventually swapped ends and destroyed the towing rig. The center line to the bow was added to allow some movement in yaw before tension on the center line put a limit on it. An important side effect is that the center line makes it much easier to get the model started without running over the tow line.

    I did not try to make any absolute numerical analysis of these test runs. Being an analog type engineer, I just tried to watch the behavior of the model and visualize what was going on to cause it to do whatever it was doing. This approach will not satisfy everyone but was the fun part of the testing for me.
     
  9. Sean Herron
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    Sean Herron Senior Member

  10. tom28571
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    tom28571 Senior Member

  11. Sean Herron
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    Sean Herron Senior Member

    Nice Site

    Hello...

    Nice boats - must be great to have a shop - I may need to look at renting some space...

    Did those people bring that narrow boat over - narrow boats are another one of my bad addictions - my wife is British - thank God - makes for great vacations...

    Walk 2 blocks and have another Guinness and a plate of grilled bangers and mash...

    I am still mentally fiddling with the idea of a flume tank...

    SH.
     

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  12. Sean Herron
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    Sean Herron Senior Member

    Water Slide Manufacturers

    Hello...

    Eureka - there is one just near where I work...

    Also - perhaps large diameter PVC sewer pipe...

    Pretty easy to make up some medite moulds and just do some mudular FRP...

    SH.
     
  13. messabout
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    messabout Senior Member

    During the tow tests as described by Tom, you can use a cheap and dirty drag measuring device. It consists of a fishing rod of some stiffness that you think appropriate. The rod is mounted along side an outrigger that is equipped with a yardstick or other linear measuring device. The deflection of the rod is a measure of the force applied to it and can be pretty accurate and repeatable. Calibrate the rod in the shop by using weights.

    Like others, I have needed to see, with my own eyes, how the model behaves. My experimentation has been mainly with displacement boats. I too have seen that some of my most inspired designs have had directional stability problems.

    When playing with models, one must select a day when the water and wave conditions are somewhere near scale. Otherwise we get impractical feedback.
     
  14. tom28571
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    tom28571 Senior Member

    I considered such a water tank facility for either moving water or moving boat but discarded it when the sheer volume and size needed for useful data is taken into count. Size of the boat, side and bottom distance effects at the boat calls for a big tank. The volume of water needed for a 4' boat is HEAVY. Generating useful waves is difficult, etc, etc, etc and etc.

    With very small models, a straight towing tank is fairly easy but what can you learn? Got your own Olympic sized swimming pool? The multi-speed towing and drag measuring apparatus is still complicated.

    Problems of towing alongside a skiff pales in comparison to building and using a tank. I think you can build or buy the skiff and motor and still be well below the costs and complexity of a tank. Can use the skiff for fishing afterwards too:D :D There is a reason there are not many of these and it costs a lot of $$$$ to use them. If such a facility is available, by all means use it.
     

  15. Sean Herron
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    Sean Herron Senior Member

    Tish Tish

    Hello...

    Tom - All of above is true - yet - I think you are getting argumentative - go out and hone some chisels man - wish that I had the need to do same...:)

    You are correct though - mine are and where - (past tense) - just random thoughts applied to the problem...

    Thing is - pulling models in a trench and then getting pied on Rye and scrunching a picnic - on the bank or in the farmers daughters favourite place - sounds very good to me...

    But then waking up - and thinking what the hell was this all for...

    Funny - how I look for the simple - and then get caught back up into the complex...

    SH.
     
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