The effects of a wavy keel

Discussion in 'Sailboats' started by Leo Lazauskas, Oct 5, 2010.

  1. Paul B

    Paul B Previous Member

    Exactly. The term seems to be undefined in the article. Is there one "wave" per meter, or per centimeter, of chord?

    Also, where on the foil is this "waviness"? Front 1/3, last 1/3, over the entire surface? That is important.

    The overall "experiment" outlined in the article is simply anecdotal. Even the claim in performance gain sounds suspect and undefined.
     
  2. tspeer
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    tspeer Senior Member

    The sailplane community has long used a wave gauge to measure the waviness of their wings. It consists of a machinist's dial gauge attached to a base with three rounded pins that ride along the surface. The pins form an isosceles triangle two inches long with the dial gauge mounted half way along the length. The gauge is slid along the surface and the maximum and minimum gauge readings are recorded. The difference between the maximum and minimum readings is the worst-case waviness of the surface.

    See Richard Johnson's Sailplane Performance Flight Test Methods.
     
  3. Paul B

    Paul B Previous Member

    Lowell North had a similar gauge back in the 1960s that he used on the bottom of his Star in the boatparks at major regattas. It was really a mindfake to psyche the rest of the fleet.

    This gauge will give you the amplitude of the "wave", but still leaves the other components to be considered.
     
  4. fng
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    fng Junior Member

    When looking at sailplanes as a comparision, you would be better looking at a modern yacht wing mast. Sailplane wings flex a lot and at rest the lower surface will show more waves than the top in flight it changes all the time. One thing to remember is that a modern day glider wing is as fair as you can get. for example the attached photo is a late 50's design ( not mine, but the same as mine ), it has a best glide ratio of 1 in 27, the latest gliders are in the upper 50's.
    Another thing to note is that with a wood fabric sailplane, the wings, although having waves root to tip, are reasonably fair leading edge to trailing edge.
    My thoughts with yacht foils is, fair fore and aft a must, and if you consider a yachts pitching moments you would want it fair top to bottom as well.
     

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  5. Tim B
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    Tim B Senior Member

    I'm a little at a loss to understand why we seem to be debating the absolute effect of waviness, when we know from empirical data that waviness increases drag and decreases lift. Therefore we should all aim at smooth and symmetrical sections. The rolling-ruler test and cardboard ( or plywood/aluminium) templates are an adequate measure for yachts.

    Sometimes we seem to spend a lot of energy proving the effect of something that we already know we wish to minimize. The question which should be posed is how do we minimize it?

    Tim B.
     
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  6. Paul B

    Paul B Previous Member

    I think the reason people are interested is because there is significant cost involved with minimizing "waviness".

    If you want to win yacht races then anything short of "perfection" is unacceptable.

    However, if you are a weekend daysailer or a cruiser it would be nice to have an idea about what the limits are where you are paying twice the price for a 2% gain or loss.


    If people want to be serious about this the output needs to be dimensionless.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed.

    However, to answer that, we need to know when does "roughness/waviness" start to have an affect on the performance and can this "minimum tolerance" be achieved by manufacturing?
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Apart from the economics of manufacturing high-precision wings, fins and keels, there are also some interesting applied mathematical issues.
    Do the "waves" in the surface give rise to significant vortices?
    How does the waviness affect boundary layer transition?
    How best to simulate waviness?
    Would similar techniques used in vibration analysis be applicable? etc etc.

    Cheers,
    Leo.
     
  9. Paul B

    Paul B Previous Member

    I looked at the link and using the dial gauge is not as easy as you might expect.

    If you have a perfectly fair foil (no "waviness") and you run the gauge shown in the article along the chord you will have movement in the gauge. This is due to the changes in curvature of the foil itself.

    Depending on the distance between your offset pins you will get more or less movement. So you would want to have the offset pins as close to the dial feeler as possible.

    In the end you would have to graph a theoretical baseline of what the dial should be doing based on your particular pin arrangement in your gauge, and the foil you are measuring. Then you would have to run the gauge over the surface and collect data to compare to the theoretical values at each point.
     
  10. tspeer
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    tspeer Senior Member

    True, which is why the distance between the pins needs to be part of the standard of waviness. Think of it like a high-pass filter. The surface shape can be divided into curvature due to the desired shape, plus waves that are unintended. The purpose of the gauge is to filter out the long wavelengths, and only measure the "noise".

    The distance between the pins determines the wavelength of interest. The gauge will not react very much to wavelengths much longer than twice the distance between the pins. For shorter wavelengths, the gauge will pick them up. Of course, extremely short wavelengths will be less than the resolution of the device.

    The gauge used by the Dallas soaring club is short enough (2"/5 cm) between the pins that there's not much deflection due to the curvature of the wing because the radius of curvature is much larger. If you make it a lot shorter, then there will be more wavelengths that fall outside of your definition of waviness - they'll be categorized as surface contour, instead.

    It's certainly not a perfect measurement, but it is practical and has been used successfully for a long time.
     
  11. Paul B

    Paul B Previous Member

    Do you know if the club has an established baseline for their foils that they use to compare with the readout on the dial?

    Would it not be easier to make steel templates that can be placed on the foil to check the "waviness"?

    Of course we are all born with highly calibrated gauges that can detect waviness in a foil probably better than the dial gauge. Sadly these tools aren't very good at giving a numerical readout...
     
  12. tspeer
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    tspeer Senior Member

    I don't know if they have a formal standard. You can look up the the flight test reports by Richard Johnson and see what his comments are. He frequently included plots of the dial readings vs distance, as well as comments about the quality of the surface.

    A template is best used to check the accuracy of the section shape. See Performance Improvement Through Airfoil Shape Correction from Soaring, 3/80 and Performance Improvement Through Airfoil Shape Correction, Part II from Soaring, 8/80 for examples of how to accurately fair a surface using templates.

    A steel straightedge can be rocked along the surface to feel for waviness. Of course, that is a qualitative evaluation, and it doesn't apply if there is a hollow in the contour.
     
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  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Kim Klaka, the author of the article I referred to, emailed me the following which might clarify a few issues...

    I have also attached the excel sheet he mentions.

    Hi Leo,
    I just came across the discussions in the Boatdesign.net forum sparked of by your posting about my ANA wavy keel article. I'm not a registered member of the forum so I thought I'd contact you direct, to add to the debate. Feel free to post any of this stuff.

    I agree with almost all the comments made about the article - it really is more of a teaser than a solid piece of research. I believe the conclusions are roughly right, the challenge is to find a scientific technique good enough to test them. And you are probably very well placed to progress this! The challenge as I see it is to find a valid theory to model the detail of the flow over an undulated surface. I don't know much about this aspect of fluid dynamics beyond 2nd/3rd year fluid mechanics, and I am very happy for you and others to question or refute my views…..

    I doubt whether traditional boundary layer theory captures small geometric variations and I am also doubtful about surface roughness theories' ability to capture the larger scale effects of waviness. Consequently I would have little faith in any results generated by commercial CFD codes. So my first question to throw to the hydrodynamic lions is: what theory is applicable to the flow over surfaces which can reveal flow changes due to random variations of surface geometry of the order a few mm over a distance of a metre?

    One of the difficulties already posted in the forum thread is how to define waviness non-dimensionally. I recall this is already a poser for surface roughness definition; waviness is more complicated because of the greater range of scale involved. So my second question is: how do you define waviness non-dimensionally in a physically meaningful way?

    Then we ought to ask whether there is an experimental technique that could be used. I suppose the wind tunnel would do the job, so the issue boils down to one of geometric precision of making the model and measuring that geometry. Probably expensive to do properly.

    In reply to the various questions about how and what I measured on my keel and rudder, I attach the rather clumsy excel file containing the as-measured keel and rudder offsets. I measured 8 sets of half-offsets: keel and rudder each at two spanwise locations, each port and starboard.( 2*2*2=8). The raw data is in the "data" worksheet, in columns S through to AH, highlighted in buff and pale blue. The measurement accuracy of the offsets is around 1 to 1.5mm.

    The way I assessed the performance change over the race course was the same way many old-fashioned sailors do - compare how you are going with a benchmark competitor on the race course , and also look at their elapsed time differences for half a dozen races before and after any change made to the boat. My estimate could easily be 50% out, but it is in the same ballpark as the change forecast by my developed rule of thumb ( it is possible to get from lift/drag change to VMG change in about 3 lines of algebra if you don't mind 30% error bars). In my experience it is misleading to quantify race course performance changes from GPS and compass because leeway changes are not properly identified and wind and wave conditions mask other factors unless you can take very long averages in very stable conditions ( it is a different matter if you have fully linked sensors and continuous data logging).

    Some of the forum posts say that I am missing the point and that we should be concentrating on minimising waviness rather than finding out how bad it is. I beg to differ on two counts. Firstly, we need to be fairly sure that waviness is indeed bad before pouring time and money into fixing it (compare with the futile attempts to retain laminar flow over yacht hulls when the incoming flow is already naturally turbulated in ocean waves). Secondly, we need to understand the mechanism of how waviness affects performance in order to know how to minimise it - is it the slope of the wave, or its amplitude, or its effect on the sub layer, is it generating micro-separation, vorticity, or what? No point reducing the waviness at the rear of the foil if it is waviness near the front that is the problem, or vice versa. ( again, compare with the futility of sanding the rear of the hull if the front is already lumpy; or sanding beyond hydraulically smooth)

    If you are not obsessive-compulsive or an insomniac, skip this para…. here is more explanation of the offsets spreadsheet. The profiles are plotted in the four appropriately labelled worksheets, port profiles flipped over to overlap stbd profile. Each plot also has a third profile option, which is a user-controlled best-fit NACA profile. For example, if you go to "lower rudder plots" you will see I have found that a 00012 profile of c 640mm, semi-t 74mm, fits nicely. Note this is not a best fit, it is a fit that allows for a minimum amount of grinding back, most of the fairing being achieved by filling the hollows ( I didn't want to grind away too much of the glass sheathing). If you go to any of the other 3 plots you will see the they display that lower rudder optimum profile, which is clearly not relevant. To get the optimum profile for any other plot, type the chord value on the plot header into cell L2 and thickness in to N1 of the "data" worksheet, you then get an appropriate NACA foil for that plot. Incidentally, please ignore the "Cole 32 Minka" worksheet; that has nothing to do with waviness

    There is a further complication with the measured offsets, which is probably an indicator of the weakest part of the entire article. You will observe in the plots of the raw data that, besides the waviness, there is also port-starboard asymmetry for most of the pairs of profile pairs - one side is thicker than the other. This means that there are two separable components to the keel profile imperfection problem - waviness and asymmetry. I have merged (muddied) these two factors in my analysis and conclusions, but it is quite possible that one or the other is the dominant factor in performance loss. It would be quite a nice student project to use something like X-foils to get a first cut at the effect of asymmetry.

    Whoops! I seem to have done what most research seems to do - raise more questions than I have found answers.
    Best wishes,

    Kim
    Dr Kim Klaka
    Director
    Centre for Marine Science & Technology
    Curtin University
     

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  14. Joakim
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    Joakim Senior Member

    This subject has been thoroughly experimented already in the 30's! Here is one example:

    http://naca.central.cranfield.ac.uk/report.php?NID=1577

    Note that waviness close to leading edge is fatal. But then again adding 10% to the keel Cd is not going to be (easily) seen on the water performance, since keel drag is such a small part of the total resistance.
     

  15. Joakim
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    Joakim Senior Member

    I think the two most important flaws in typical keels are surface roughness and too thick trailing edge. Also there are problems with the profile in the leading edge and the profile may also be concave near the trailing edge, probably in order to make the trailing edge thicker and thus less prone to mechanical problems.

    The trailing edge effect is shown here:
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082748_1993082748.pdf

    It is easy to find 10 or even 20 mm trailing edges from 0.7-2 m chord keels. These will add keel drag 50-100%! The NACA standard is 0.25% of the cord thus only a few mm.

    Antifouling without sanding after painting will be about as bad.
     
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