keel foil design x-y graph

Discussion in 'Boat Design' started by viking north, Jun 30, 2011.

  1. viking north
    Joined: Dec 2010
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    viking north VINLAND

    I'm now into building the male mold to obtain the keel shell for my lifeboat to motorsailer build . ( thread under Boatbuilding The Nancy G) I have successfully blended in the new keels root to the existing keel on the hull. The new keel (long keel/skeg combo)(is approx. 9 feet long with a max. width of 6 in.) I realize i am a long way from the 10 to 15% width to length ratio necessary in developing a proper foil. Using the straight bulkheads of the mold above the blending in tapered sections, I am attempting to create a leading edge foil shape as if I had say 12 inches of max. width 35 to 40 % back from the leading edge. In other words what would be the shape of the foil from the leading edge until it reached 6 in. of width. My idea is to build the keel with a foil shaped leading edge that is followed with a parallel section ending with a foil shaped trailing edge. I am attempting to trick the flow into thinking it is initally encountrring a complete foil thus reducing the breakup of the flow along the inital front section of the keel. Likewise in the flow pattern as it exits the trailing edge of the keel. basically I guess what I need is to obtain a full or scaled plot of a low aspect ratio foil for a keel 9 feet long having 12in. of width some 35 to 40% along it's length. From this I can obtain the pattern for the leading and trailing edge sections and fit the parallel body section between the two. Is there anyone on the forum that could provide me with this pattern either in full or scaled in an x/y axis graph. !00% accuracy is not important I just need a rough guide to go by.--Tnx. Geo.

    P.S. the male mold bulkhead pattern shown is not the the final pattern used. It has been modified to taper in to the 6in. width at half the height shown(5 in. instead of 10) thus giving me a longer straight section which is more desireable in long flat sided keels. The pairs of keel bolts are welded to that SS pipe which runs the entire length of the keel. This creates what I call the keel cage to better hold the 3000lbs. of lead auto wheel weights and resin mixture.
     

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  2. Tim B
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    Tim B Senior Member

    There is nothing wrong with your idea. You are still creating a "Foil" shape, and that will still produce lift; the flow will require no "tricking". what you'll lose is the ultimate efficiency, which I wouldn't worry about too much here.

    I would suggest that you obtain a large piece of paper, a drawing board (or at least something flat) and a T square, then draw the leading edge by hand with a french curve and measure the offsets when you have something that looks right.

    Tim B.
     
  3. viking north
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    viking north VINLAND

    Thanks Tim--I will give it a go--I was a little worried about getting the parabolic shape on the leading edge within tolerance so as not to create an inital turbulance which of course would only multiply as the flow moved aft. I could scale up those small foil shapes in the nautical books i have but they are of such small scale i might create more harm than good. Was hoping for a fee one might beable to go to a design website plug in the numbers I.E. (length & width) and have a 1/4 to full scale printout produced or sent by mail. Thanks again for the reply---Geo.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Hi Viking,
    You can use this little program to generate NACA airfoil coordinates: http://tracfoil.free.fr/airfoils/downloads/nacalte.exe . I have tested it on my PC, no viruses found.
    It will generate a text file with foil coordinates, where x-coordinates range from 0 to 1 (0% to 100% of the chord length) and the y-coords are be scaled accordingly. The so generated data points can then be read by (for example) Excel, where you can scale them up to your actual keel dimensions. If you need a help with this process, just PM me.
    Cheers!

    P.S.
    By the way, see this note:
    100_3800 (2).JPG
    You will probably have a separation of the flow in that point and the consequent increase of keel drag.
     
  5. Tim B
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    Tim B Senior Member

    For a fee I can do the full CFD-based optimisation on the keel, but you don't need it. You just need a rounded bit at the front and a pointy bit at the back, with a smooth transition between them.

    Tim B.
     
  6. viking north
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    viking north VINLAND

    Daiquiri-- thats the 3/4 in. set back on the male mold bulkhead frames. Once it is strip planked it as shown on the third photo it sets even with the side of the origional keel. I have made a big note on my design drawing of the the keel from the the much appreciated techinal info you posted to me a month or so ago, regarding building in a good radius at the keel root of the origional keel. I will check out the foil program. Man 6 months ago if someone had told me this old boat builder would be playing around with such technical info, I would have told them they were dreaming. Thanks to the forum and people like yourself so giving of their time and info I have a much better understanding of the basics. I'm really having fun with this build--Geo.
     
  7. viking north
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    viking north VINLAND

    Thanks Tim, It's good to know I don't have to be too critical on this, especially when I tackle that program that Daiquiri has posted above -- I know-I-know-- whats this crazy guy doing playing around with foils on a lifeboat to motorsailer conversion but this most likely will be the last build for this old boat carpenter, hell why not :) Once again thanks for the reply and say hi to all my south west county distant relatives if you drift over in that direction--Tnx-Geo.
     
  8. Tad
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    Tad Boat Designer

    This may help, NACA section ordinates as a percentage of (C) cord length......

    NACAsections.jpg
     
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  9. viking north
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    viking north VINLAND

    WOW !! TAD thats exactly what I was looking for but wasn't sure it existed outside the computer world, being lost in early aviation and marine design history--I.E. manually calculated Bi-plane wing engineering in X-Y chart form--:)
    One small question just for curiosity--working with foil design NACA 63A, diagram "B" there seems to be an ever so slight hollow between 20 and 35 % of the "C" length (4.900--4.714--4.913--4.995. Also once the "Y" number is calculated do I double that being the "X" axis runs thru the center of the foil thus the pos. and neg. relationship.
    OK- I am going to make a few more accurate measurements on my keel -- work up two possibilities--( a choice has just popped into my head) post them and await the outcome-- This should be a hoot---Thanks once again-- Geo.
     
  10. Tad
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    Tad Boat Designer

    Viking.....

    Yes, there's a typo in the "B" column at 20%, which is why it's circled, it should read 4.400.......25% and beyond are correct. Correction from Theory of Wing Sections.

    It's been so long since I used this particular table I'd forgotten the mistake, but would always rationalize these points anyway. For instance you will want a thicker (for strength) trailing edge than is shown here.

    Many years ago I used this table (and Theory of Wing Sections)to draw full size sections for production of all the Ericson keels (including the wing keels). The wings were twisted asymmetric foils.......
     
  11. viking north
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    viking north VINLAND

    Well that helps my head--I had all these theorys going about the slight hollow creating suction to draw in a more compact flow so it adhered to the remaining length of the keel better. Ya I was compacting a liquid by squeezing out any air that might have been created from turbulance due the inital impact of the flow with the leading edge. :) Amazing how one can rationalize and create new theorys. -- Well here goes nothing -- a boat carpenter -- working with hydrodynamic foils--someone should film this for an episode of Monthy Python at sea--:) Thanks again-Geo.
    P.S. Ericson-- a good name in the sailing world-- good for you
     
  12. Arvy
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    Arvy Senior Member

    With the risk of highjacking this thread (I don't intend to, but as the topic is very very similar and due to this thread I start to worry about my design now :) ).

    I designed a 20 foot launchboat (guess that is how it is called in english), but I have a rather straight "bar like keel". I don't use a rounded front, but the end of the keel is a bit tipped to become smaller in with (from 10cm to 5cm over 40 cm).

    Will this work? it is an engine powered vessel.

    I added some attachment, a wireframe of the keel and the first plate, and 2 renderings, 1 seen from the fore and 1 seen from the side. In the 2nd rendering there is a bit of a weird render error, the little bulp is not in de plate itself.

    Please let me know if the front of the keel really needs to be rounded or will this shape do as well? I normally only do sailboats, this is the first time I design a launchboat :)

    Thanks!

    ps. The boat will be build in steel.
     

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    Last edited: Jul 3, 2011
  13. daiquiri
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    daiquiri Engineering and Design

    Hi Arvy,
    I have two objections on your keel.
    First, the keel in those pics appear to be very thick, about twice what I would expect to see in a boat of that type. The pictures show only half-sections of the hull but even so the keel looks as thick as it would normally be seen in a complete boat.
    Second, I have a bad experience with thick keels in front of a prop. Until you intend to run this boat in displacement speed range, no problem. But if the intention is to go beyond the so-called hull speed (and your flat aft sections would indicate that) then you might (or beter said - will) have cavitation and vibration problems due to turbulent inflow to the prop created by the keel. So, if you aim at higher speeds, my suggestion is to either fair (streamline) the keel's trailing edge, like Viking North has done, or do away with the keel altogether.
    As about your question about leading edge rounding, it is not an issue in extremely shallow keels like this one. Nearly all traditional boats I've seen so far have them squared like this.
    Cheers!
     
  14. Arvy
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    Arvy Senior Member

    Thanks for your reply, I attached an image of both sides of the hull from fore and from aft. The keel in the original one was actually the full width of the keel and. I had to draw it like that so that I can make a single expansion of the keelsole instead of mirroring it :)

    The boat is meant for cruising at hullspeed in canals and stuff, and the back isn't really that flat. And the keel is tapered toward the end to be the width of the bearing of the axis (if you know what I mean). So it cannot be any thinner there, unless I let the bearing of the axis stick out, which is an option as well.

    ps. forgot to copy some plate in the aft view, giving a weird rendering again...
     

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    Last edited: Jul 3, 2011

  15. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    It can be done, just requires some more effort. :)
    See this solution:
    http://forums.bateau2.com/viewtopic.php?f=2&t=15565#p125835

    And this photo:

    [​IMG]

    taken from this site: http://features.boats.com/boat-content/2010/07/johns-bay-boat-company-bucks-the-tide/

    But anyways, like I said it is not an issue for low speeds, becomes a problem only if you want to push it at higher displacement speeds. I have tried the thick squared keel trailing edges (like the one you have there) for high displacement speed crafts, had an unsatisfactory result and now consider them unacceptable.

    Cheers
     
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