Shallow fin keel ?

Discussion in 'Sailboats' started by frank smith, Jan 25, 2011.

  1. viking north
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    viking north VINLAND

    Agree, thus the replacement of what would be an overly wide foils cross section with a parallel (straight) body mid section. My drawing and photos above are not the final design, the molds bulkheads are just a mock up to determine location and height. My next step is to remove them from the ends, leaving those in place for the parallel mid section and and use a batten either side to form the curvature of the 1/3 front section and parabolic leading edge, the parallel mid section, and the tapering tailing aft. section. I will then reduce the width of the straight portions of the removed molds bulkheads to fit the curvature form by the battens and reinstall.
     
  2. sharpii2
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    sharpii2 Senior Member

    A couple of things to think about.

    1.) Having an airfoil shape is useful mainly in reducing drag per volume in keels with such low aspect ratios. They don't produce 'lift' of any amount comparable to a higher aspect ratio keel, say AR 1.0 or more. Most of their lift comes from angle of attack and high pressure on the lee side. With a higher aspect ratio keel, most of the lift is caused by low pressure on the windward side. The higher aspect ratio keel, by the way, should have a similar amount of high pressure on the leeward side as a low aspect ratio keel of the same area. This is why the low aspect ratio keel has to be somewhat larger.

    2.) Having too sharp of a leading edge can move the Center of Lateral Resistance (C.L.R) way forward on a long keel, often causing the boat to round up viciously, especially if the keel is nearly the length of the waterline of the boat. Many full length keels I have seen have very blunted leading edges. I've even seen one where the edge is completely squared off. It did have quite a slope to it, however.

    3.) With a full length keel, the ballast slug should be no more than half the length of the waterline, especially if the boat has a low Prismatic coefficient (cP). This is because, as the boat pitches, the slug has two CGs, one forward of the pitch axis, and one aft it.

    Just for @#$%@ and giggles I will present a formula I came up with for adjusting the area of a long keel to its lower efficiency. I have tried this formula against designs that are known to be successful, including some from the Herrshoffs. It has typically predicted the area of the keel to within 10% of its actual area.

    KA= ((KL/((5*(sqrt A))*A) + A

    Where:

    KA= Keel Area.
    KL= Keel Length.
    A= (Sail Area + Profile Area) * 0.03 for boats that routinely sail less than 3 kts and 0.02 boats that routinely sail faster than that. (I usually use the 0.03 figure myself)
     
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  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Imperial or metric formula? Seems about x1/2 difference in the outcome btw them..
     
  4. sharpii2
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    sharpii2 Senior Member

    Sorry, Teddy.

    I must have gotten my brackets in the wrong places.
    Let's take this process step by step, instead of using the college math professor stuff.

    Here's an example:

    SA+Pa (Sail Area + Profile Area) = 10 sm
    KL (Keel Length) = 3 m
    A ((Sa+Pa) * 0.03) = 0.30 sm

    a.) square root of 0.30 = 0.584
    b.) 5 * 0.584 = 2.92
    c.) 3/2.92 = 1.03
    d.) 1.03 * 0.30 sm = 0.31 sm
    e.) 0.31 + 3.0 = 0.61 sm = Keel Area

    0.61*10.67 (sf/sm) = 6.50 sf

    Now. Lets do this in feet

    SA+PA = 106.7 sf
    KL = 9.81 ft
    A = 3.2

    a.) Square root of 3.2 = 1.79
    b.) 1.79*5 = 8.94
    c.) 9.81/8.94 = 1.10
    d.) 1.10*3.2 sf = 3.51 sf
    e.) 3.51 sf + 3.2 sf = 6.71 sf

    Not exactly the same. But close. Wouldn't your agree?
     
  5. philSweet
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    philSweet Senior Member

    e.) 0.31 + 3.0 = 0.61 sm = Keel Area

    ???
     
  6. daiquiri
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    daiquiri Engineering and Design

    Sharpii... That's pretty unreadable. And the point e):
    0.31 + 3.0 = 0.61 sm ??

    How about rewriting the formula correctly? It's easier to understand than this step-by-step stuff ;)
     
  7. viking north
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    viking north VINLAND

    I can see it all now, filled the aircraft with liters and should have been gallons and we're not going to make it non stop to Vancouver, thank god we're not flying across the pond. :) Ok back to the keel thread- Frank I further tweaked my cross sections and now have a 25% taper combined with a 75% straight section.(thank you again Daiquiri) I am going with the foil LE, a parallel mid section and a foil tapering aft section.(example diagram, your post #23) One important ratio to keep in mind re, shallow keels and draft, as pointed out in Skene's( page 66) anything beyond a 7 to 1 (waterline length to draft) is touchy to dangerious. The point being made is rudder emersion.--- Geo.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Yeah it sounds like a joke, but I've worked in an airline many years ago and had a first-hand opportunity to hear some really incredible stories. Things like that one have happened many times on commercial flights. Not liters for gallons, but pounds of fuel have been pumped in tanks when it should have been kilograms... :rolleyes: :D
     
  9. viking north
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    viking north VINLAND

    Yup, air Canada did it, also NASA, so we mere mortal folk shouldn't feel bad with a few misplaced decimal points --- as long as it isn't money ---
     
  10. CutOnce

    CutOnce Previous Member

    Read "the Gimli Glider" about an Air Canada flight that made this exact mistake. This one was very real.

    --
    CutOnce
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ================
    Great movie loosely based on the Gimli incident-well worth a watch:
    http://en.wikipedia.org/wiki/Falling_from_the_Sky:_Flight_174
     
  12. sharpii2
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    sharpii2 Senior Member

    Exactly.

    As you see, it's more than double the area of a deeper, shorter fin.
    This is generally seen as a disadvantage because:

    1.) There' more whetted surface area to drag through the water, and
    2.) It still needs a larger angle of attack.

    Both of these factors will make the boat point lower.

    But there are advantages as well.

    1.) the shallow keel is less likely to 'stall', or suddenly lose a major portion of its lift,
    2.) because of this, it is more likely to act as a roll dampener than a roll creator (as when the shorter, deeper fin successively stalls, regains lift, then stalls again), and
    3.) it provides directional stability even better than a fin, skeg arrangement, as the short fin provides the pivot point and the skeg gets pushed forward by the currents inside the wave. With the long, shallow keel, there is no direct pivot point and there is plenty of surface area not effected by the inner currents of the wave to resist turning.

    Now that sailboats earn their living by getting around the cans fastest, by covering vast distances with incredible speeds, and by being handy and maneuverable in tight quarters, the long, shallow keel is really quite obsolete.

    But a time may come when better inherent directional stability, better sea kindliness, and reasonable sailing draft will be better appreciated due to the cold hand of necessity (insanely expensive energy) Then the long shallow keel may well make a comeback.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Ah ok, now I get it. It is not 3.0 but 0.30, so the item "e" should be:
    e) 0.31 + 0.30 = 0.61 sm

    So your formula, after some rearranging and simplyfying to 2 significant decimals (it's all very approximate anyways), becomes:

    KA = [ KL sqrt(SA+PA) ]/29 + (SA+PA)/33
     
  14. viking north
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    viking north VINLAND

    Sharpii2--Don't think it ever totally went out of favour especially amoung serious blue water sailers.(cruisers). Like many things in life it's all a trade off and as you state, with the price of fuel never again being within the average Joe's budget, motorsailers and long keels will become the vogue both with the older sailer and the power boaters. In my entire 40 odd years of sailing I've never owned a fin keeler simply because, when compared with a long keel they fail where it counts most to me.--
     

  15. sharpii2
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    sharpii2 Senior Member

    Sorry about the typo. I must have looked at it a half a dozen times and not noticed it. No wonder I always did poorly in math.

    As for your simplified formula, I can't say. I have to first figure out where you got the '29' from.

    As you can see, due to my many clerical errors, abstract math is not going to work well for me.
     
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