tow tank/ hydrodynamic testing methiods

Discussion in 'Boat Design' started by nwahs, Sep 18, 2011.

  1. nwahs
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    nwahs Junior Member

    greetings

    im curious as to if any out there has worked, or tweeked craft hull designs by tow tank data, or done there own drag exsperiments to quantify testing hulls.
    any homebrew ideas are welcomed.

    thx shawn
     
  2. PAR
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    PAR Yacht Designer/Builder

    Is there something specific you're interested in, as several here have been involved in testing.
     
  3. nwahs
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    nwahs Junior Member

    specific drag design reduction for small paddle craft, surf ski, kayak, scull, ect.

    i get some, a little variation as to oppionions of best performing hull aspects- length , beam, rider mass placement ect and if i posed the question of rocker, symmetry, or hull waxes im sure i would get more good, but slightly varring info.

    i can chase my tail on this forever- i soon i will be forming some hull designs- and it would be very helpful from boat to boat if i can destinish real vs imangined atributes.

    (where im going with this- i want to test my own hull forms.) i was thinking of draging them loaded with 180# of dead weight (pretending to be me) behind another power boat and finding the force required (by springs i guess? or we use dynometers at work to test rigging spec) or to find resistance- weight of load but they could be used for drag effect i suspect. i worry head boat turblance will give me a huge flux.

    second idea is i could drop a line off stationary object in river current and git some readings- think it would work but every time i go to the same place river flow/ spped could vary, plus hard to test different speeds- and maybe be hard locally to find water speed at the 7 -10mph range i wish to test

    i have a small cat hull and i think maybei could use a trolling motor and actually drag the boat off the side or even infront off a beam slightly in front between the cat hull/ wake influence?

    anyways, i cant be the only person thinking of doing this? im looking for people thata have done this to give me ideas.

    whats going for my application, is im doing flat water, so im not concerned with rough water conditions flux, im 180 and can design no payload varation, its a play boat only and so i suspect craft to be aprox 25#. (so say 215 total). and finally i have a few hulls all reasdy i can use as data bases that even if i cant get true callibration, i can always have comparitive data.

    proublems, i have notice my gps seems to jump around alot on the lake- and i need a dynometer with sensitity in my scale--------

    anyways if anyone out there can give me ideas i would like to hear them

    thx shawn
     
  4. messabout
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    messabout Senior Member

    First, 7 to 10 mph is not a realistic speed for a small kayak or canoe that will weigh only 25 pounds. Think in terms of half those speeds unless you mean to build very long, very skinny boats with a very athletic paddler.

    I have amused myself at length with various towing/measuring gimmicks. In all cases I have arranged for the model to be at the side of the tow boat, well away from any wake or turbulence caused by the tow boat or its propulsion system.

    One of the homespun schemes for assesing drag is as follows. Rig up a cross beam that extends out several feet from the side of the tow boat. The outer end of the cross beam will have something like a yard stick attached in the fore/aft direction. The cross beam to have a clamp assembly somewhere inboard. The clamp will hold an ordinary fishing rod in the horizontal position, parallel to the cross beam. Tie the tow line to the rod tip and to the model. Now go at measured speeds and observe the deflection of the tip of the fishing rod by virtue of the yardsticks markings.

    Calibrate the fishing rod by fastening it solidly to a work bench. Observe and record rod deflection for various weights attached to a string at the rod tip. Tinker with different rods with different bending characteristics until you get one with appropriate stiffness. Make up a chart or graph that delivers the drag force over a range of speeds.

    Most outboard rigs will not go slowly enough to do this experiment. Use an electric trolling motor. Do not ignore wind influence on the test model. If you have wind, test in two directions, one directly compass opposite the other.

    This is a fun way to piss away a few hours on the water. In no case will it deliver definitive results worthy of a SNAME paper. It ain't bad for getting a general idea of drag comparisons between different designs.
     
  5. PAR
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    PAR Yacht Designer/Builder

    One home made resistance test I've used, is to employ a balanced beam with two models tethered, one at each end on a length of monofilament. The beam is towed at the appropriate S/L ratios desired with a fishing rod or small electric motor with a spool, the speed judged by the wave train and the model that lags behind on the beam has more resistance. I've done this in a pool and afloat and it's a good way to fine tune similar hull forms.
     
  6. nwahs
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    nwahs Junior Member

    wow a super thx, to both of ya.
    a delfection meter and ballance beam of sorts i see how either could work well.

    the ballance beam aproach could have a small sphere filled with weights to increase/ decrease displacement/ drag. when it is in ballance with the test boat i could measure the weight and compair to different hulls preavious test.

    i also like the deflection test as i think you could watch the deflection increase as you test over a range of increased speed.

    i picked the 7-10 range partly cuz im an optimist and wish to obtain thoses speeds ?someday??? ya i was gonna build apretty long skinny boat even though for me to claim the althetic thing might be a stretch. (currently it seems to be 3,4,5-thing) just looking at the water seems to git me to 3mph, mild stroking seems to do 4mph to just a hint over but is eaisly sustainable, and 5 mph is there but its fleeting and the next day ill feel it.
    - the funny thing is ive been useing a 10' waterline white water kayak with the rocker of a bannana, its hilarious you quit padddeling you get maybe 15' of glide!!!
    or course if you want to 180's its only a 1 stroke, 3/4 second deal. hydrodymanics pretty interesting..... thx again for the imput
     
  7. masalai
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    masalai masalai

    Measurement directly from the boat will not be steady enough to be useful... I tried motorising models of my boat, and measuring electrical draw - far to much variation... the best is mono-filament (a nice inherant elasticity and the tip displacement of a grp rod of equal diameter - not tapered - - around 3/8th of an inch?) attached as recommended above to a dingy to get useful relative data... to load up the test boat add weights inside the test boat after each successful run-set...

    The biggest influence in efficiency/performance I found to be "wave-making", - least is best, the rest is "eye"... Beyond that things become highly technical, complex and expensive...

    See the designs developed by Rick Willoughby, whose boat achieved a world record for man powered 24 hour distance or something like that...
     
  8. RThompson
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    RThompson Senior Member

    There was some testing done at Australian Maritime College (UTAS) a couple of years ago, maybe still going on, using hydrofoils towed on a lake with a converted (motorised) tornado catamaran ("the Davis Cat").
    Certainly there would have been many issues to solve to do it effectively, I'm guessing there would be some papers somewhere describing how it was done...

    Also, given all the problems with trying to directly quantify force/work on models; the comparative testing using a balanced beam should give you the answers you want -is this one faster than that one. I think the early towing tanks used a falling weight attached to tow line to generate a constant and repeatable towing force.

    FWIW - Basically drag can be described as made up of two components: friction drag plus wave making drag.
    Where friction (or viscous) drag is dependent on wetted surface area. Lowest wetted surface area = lowest friction drag. So the lowest surface area (and hence lowest friction drag) will come from semi-circular transverse hull sections. Although there may be other requirements that take a hull form away from semi circular sections.
    Wave making drag is dependent on immersed length. longer length = less wave making drag (for the same immersed volume).
    So, all other things being equal a long hull with semi circular sections is fastest. Generally! I emphasize that. Generally AND basically! Real world drag prediction is not clearly, accurately or absolutely understood by anyone (certainly not me) and there are a myriad of other considerations for designing a hull form.

    Rob
     
  9. nwahs
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    nwahs Junior Member

    ya thats what kinda been getting my to want to test- there seems to be a counterdiction involved

    the wave drag saying an infinatly thin long boat is best,
    and the friction drag saying we should all paddle hemispherical craft-

    there has to be an optimal form, well maybe not- oh a lose lose
     
  10. rwatson
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    rwatson Senior Member

    For small craft like you are suggesting, it would be just as easy and not much less expensive (if your time is valuable) to just build the full size craft.

    You have to go 1:5 scale for reasonable testing, and you could at least sell the least successful ones for the cost of materials once you have finished
     
  11. nwahs
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    nwahs Junior Member

    i thought about scaled test, but besides hull form ill do rider placement, and then paddler handeling test later and need a full size craft anyways (or monkey- just kiddin)
    so i was going to proubably just go full size, (foam is in the ball park of 50$ per hull volume) could maybe cut it in 1/2 with cheaper foam but its a little flimsy and i need a nice stiff hull.

    actually, since i am doing sit on top kayaks- the first boat or two may be soild foam blocks-
    closer to surf board constructuion than hollow walled craft. if i get a decent shape ill go make molds of it- down the road if i ever did find the prefect hull form i would do one in ceder.

    thanks for the if i do go with a model a 1:5 ratio is req- its new to me i did not know that- maybe i may do some might be a good waay to narrow things down alittle
     
  12. JRD
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    JRD Senior Member

    testing methods

    A book by Frank Bethwaite, "High Performance Sailing" describes how they used some of these methods to test and improve their racing dinghy designs in the 60s. They used a comination of balanced beam and scales, and offer comments on how they interpreted the results and some of the surprises they found as well. They also extended the discussion to performance testing in waves as well (with and against the waves) Not especially scientific with reference to hydrodynamic theory etc, but some very practical work that lead to many actual boats being built and on the water performance improvements.

    Some great thoughst and suggestions above, good luck
     
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  13. JRD
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    JRD Senior Member

    Thinking about your comment about rider placement, it occurs to me that this is more significant for a kayak than larger boats as the rider is by far the largest mass on the boat. The dynamics of paddling will have the COG continuously moving about which would create a rythmic occilation in trim. When you watch a paddler going flat out the shorter boats can be seen to snake from side to side, and trim up and down at the bow as the paddle force is applied. Its subtle of course, but logically this would set off draggy vortices down each side of the boat on each stroke. Would be interesting to compare a straight line tow test with one where the various dynamics are also applied.
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    There are some papers that might be of interest at:
    http://www.cyberiad.net/kayak.htm

    Leo.
     
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  15. DCockey
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    DCockey Senior Member

    That's one of the basic tradeoffs in boat design. Short and fat minimizes wetted area and friction drag. Long and skinny minimizes wave drag. The optimum for resistance is somewhere in the middle.

    For a non-planning boat the optimum for resistance falls depends on the speed. Slower and friction drag is more important. Faster and wave drag is more important.
     
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