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Thread: Keel Fin Planforms

  1. #1
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    Default Keel Fin Planforms

    I’m at the stage of planning to put fins into my two new IACC 120 hulls (ClaudioD NZL 92 plan) & I’ve noted that other hull plans use different rudder & keel planforms & I would like to have a better understanding of the differences between the three most common fin plan forms you can see in IACC 120 yachts design:

    A) Straight “trailing edge” keel fin (which I have on my original hulls)
    B) Straight “Leading edge” keel fin.
    C) Straight “Trailing & Leading edge” keel fin.

    Playing around with the illustration prepared you can see that the Hull Centre of Balance (CB) would move forward.

    Then “theoretically” (which I can never trust my own) I assume would need to move the position of the bulb C.G “forward” as well.

    Cause & effect theory suggests that elsewhere some changes and that is Center of Lateral Resistance (CLR) to Centre of Effort (COE) would need to move forward as well (assuming all 3 fin surface areas were the same)

    ok so far so good...but but the distance between “fins leading edge and the COE position” increases on B & C planforms

    Here’s my questions:

    1) What affect does increasing distance of the "fins front edge to COE" have on boats helm balance?
    2) If helm balance is affected, how do you correct it?
    3) What are the pro's & con's between these 3 different keel fin planforms?

    Cheers Alan

    P.S If I'm not wrong (Matthias?) all of the top boats have straight Leading & Tail edge keel fin planforms.
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    Last edited by K1W120; 10-27-2010 at 09:47 AM. Reason: add comment
    You can only create, what you can imagine

  2. #2
    Join Date
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    Default Re: Keel Fin Planforms

    Hi Allen,

    Assuming that there is an original location for the keel fin where the centers are all working well together and you just want to change the profile then whichever shape you choose the center of the area of the new profile needs to be at the same location as the old keel's center. The bulb location should stay in the same place as well if the boat is floating on its lines. The bulb end of keel fin might intersect the bulb at an unusual location with a new profile, and I suspect most keel profiles are driven by connecting the center of lateral resistance portion of the keel to the midsection of the bulb and filling in the desired area, not for hydrodynamic ideals.

    In a conversation I had many years ago with a very successful M class designer the ideal leading edge should have a 9 degree rake and a vertical trailing edge. In practice this often results in the keel/bulb connection to intersect aft of the midsection of the bulb. Now, if the keel fin has any lateral flex in it this arrangement will deflect the fin to leeward when the boat heels, not really desirable for pointing or balance. Conversely, having the keel fin intersect the bulb too far ahead of the midsection of the bulb will twist the fin to windward, in small amounts this is good but undesirable if the twist is too great. Which brings me back to my prior assertion of how a lot of designers arrive at their keel profiles, although my bet is that none would admit to it.

    Focussing on the boat end of the keel fin changing the profile without changing the fin's center of lateral resistance may require substantially relocating the keel trunk or the keel root. In most boats there isn't much room for restructuring forward because of the mast and its supports, wether a truss system or a mast trunk, are usually pretty close to the original keel location. Moving the root or keel trunk aft will require extra bracing to reenforce the hull to accommodate the new loads on the hull, and moving the keel too far aft will impart a twisting force to the hull which it may not be able to stand for too long.

    So, as you can see designing a boat involves not only an understanding of how the various balance points work together on several angles of heel, and all the other considerations that go into the hull, but the physical considerations to contain the various forces acting on the boat and appendages. When ever you change one thing it affects a lot of other things, not just in terms of overall balance but in engineering as well.

  3. #3

    Default Re: Keel Fin Planforms

    Alan, on a RC model, the fin shape is not important. No one I think can establish exactly what is the better one. Probably because the difference is not measurable. You have to see the keel rigidity, the keel thikness, the keel section shape, keel weight; these are the important factors. Our boats have principally leading & treading keels, and they works very very well!!!

    Cheers
    Matth
    --------------------------------
    The life is short: sail hard!
    --------------------------------
    IACC120 Alinghi Challenge Roma
    Defender of IACC120 CUP 2008-2009-2010-2011-2012-2013-2014
    SUI-100 & SUI-91
    www.iacc120cup.altervista.org

  4. #4
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    Default Re: Keel Fin Planforms

    Hi Neil,

    Maybe I did not make it clear before, but I’m now working with “new” hulls which are the same design as my old hull (actually coming off the same mould) so there are no issues of internal hull surgery to reposition fin boxes and reinforcements etc ... so we are working with a clean bare internal hull at this time.

    Further, I did not clearly explain that I have had professional carbon fins made, not homemade (I can’t make them light enough) therefore I can say they are as rigid as they can possibly with minimal flex. Profile @ 6% tapering chord.

    What you’re say about maintaining the original boat balance centre points designed into the hull, when changing to another keel fin planform, makes sense.

    The straight trailing edge keel fin is what I have on my old hulls, which have good “helm” balance (sail plan positioning) which I understand to be different from “boat” balance (mass distribution) I would like both to be the same or close to as my older boats.

    Following your explanation, I have drawn up your description to see how a straight “leading” edge compares my original “trailing” edge fin while maintaining the hulls designed balance points:

    1. Keel Fin centres same position ](grey dotted line) The “leading edge” fin requires to positioned further aft in the hull to maintain the same fin centres.

    2. Bulb position same place ](grey dotted lines) Keel fin intersection point on the “leading edge” fin moves forward on the bulb.

    3. COE same position for both fins. (grey dotted lines) We still have an increased distance from the leading edge fin to COE when compared to trailing edge fin ...

    Which brings us back to my original questions which are still not answered with our exercise here, but you have introduced some points that maybe lead to the answer questions 1 & 2.

    “I suspect most keel profiles are driven by connecting the center of lateral resistance portion of the keel to the midsection of the bulb and filling in the desired area, not for hydrodynamic ideals.”

    I follow your suspicions, excepting the point of not following hydrodynamic ideals, I learnt from Claudio while building my first hulls that when it came to the Fin & Rudder that these appendage surfaces should correspond to 5.5 max 6% of the sail surface area this has been the rule I work with. If I have a larger surface area fin to have the same leading edge distance to COE, the knock-on effect as a result of this change will be more keel fin surface area which = more drag (slower hull)

    Your valuable next point

    “a boat involves not only an understanding of how the various balance points work together on several angles of heel, and all the other considerations that go into the hull, but the physical considerations to contain the various forces acting on the boat and appendages.”

    This turned the light on into thinking about what Claudio did on his recent AC 100 build by having a adjustable keel fin (fore & aft) by 1% to “fine tune” this unknown variable.

    Neil you helped answer questions 1 & 2 ...THANK YOU !!

    1) What affect does increasing distance of the "fins front edge to COE" have on boats helm balance? YES it will affect helm blance, but it is unknown

    2) If helm balance is affected, how do you correct it? Fine tune keel fin fore & aft (Thanks Claudio)

    So that just leaves just one unanswered question:

    3) What are the pro's & con's between these 3 different keel fin planforms ?

    Hi Matt, You maybe right or wrong about fin shape ...I don't know, but let me ask a you question ...Why have you built so many hulls to get to the rocket you have now my friend ? ... I'm just what 4-5 new boat re-builds behind you ? maybe more I'm learning as fast as I can to catch-up to you
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    You can only create, what you can imagine

  5. #5
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    Default Re: Keel Fin Planforms

    Hi Alan,
    the form of the fin blades do not modify very much the dynamic behaviour of the model boat except for the following aspects :

    1 - a trapezoidal fin has, within certain limits, a better efficiency therefore producing an increased lift at the extreme end. Once the boat is sailing close hauled, the fin blade, like the wing of an airplane, produce a "vertical push" effect as such to increase the heeling effect. Thanks to the bulb weight the vertical stability is controlled.
    A trapezoidal fin has shorter width at the bottom and therefore is presenting some mechanical weakness to attach the bulb.

    2 - the straight fin blade produce, in spite of lower efficency, still an important "push up" once heeled, but exibit stronger mechanical attachment for the bulb. It can be shorter equalizing the same surface of the trapezoidal one.

    A tilted leading edge may presents lower resistance to the incoming fluids. Remember the water is not compressible.

    As an example, assume the water fluid coming in and flushing around the hull, when suddently the fluids met a vertical surface , the leading edge of the fin.
    The fluid therefore shall deviate to both sides of the fin spending part of the boat energy. If the leading edge is tilted, the shock against incoming fluid is progressive compared to a straight edge. Some years ago if I rebember, a Volvo ABM-Amro boat , see pics, was having an hollow area around the fin attachment to the hull . This was ment to create an escape zone to the fluid that suddently were meeting the front edge of the fin.

    My conclusions is that probably a straight fin is better for various reasons: strongher bulb attachement , stronger against bulb oscillations, can be shorter for the same surface, has less "pushing up" effect once tilted because less efficient compared to a trapezoidal one.

    On Class M the trailing edge is often curved in order to reduce the surface close to the hull to reduce turbolences in that area when the boat is heeled. A reverse trapezoidal fin is the one with better efficiency therefore one could decide to reduce the dimensions and accepting a lower ratio with the sail area in the order of 4-4.5%. This was the case for Australia before the evnt of foils.
    Cheers
    ClaudioD
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    Last edited by claudio; 10-28-2010 at 08:08 AM.

  6. #6
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    Default Re: Keel Fin Planforms

    Thank you Claudio ...that explains the differences very clearly.

    Matt, yesterday you published some very informative views on the 3 generations of SUI builds and it is clear to see that lastest SUI 102 has considerably differend keel position (More aft fin attachemnt on the bulb much larger CLR ?) with longer bulb compared to prevoius generation hulls ... does this improve boat balance (mass distribution) compared to the other hulls with shorter bulbs ?

    I picked up that Renato has been tinkering with this concept as well ...

    Cheers Alan
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    Last edited by K1W120; 10-28-2010 at 08:45 AM.
    You can only create, what you can imagine

  7. #7
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    Default Re: Keel Fin Planforms

    Alan ,
    I forgot to tell that if one make a shorter and wider fin is loosing righting moment therefore the sail plan shall be modified too in order to lower the CE.
    Long bulbs exibit less front surface but produce longer gyration radius and induce higher torsional oscillations to the fin.
    On www.nonsolovele.com I wrote an article about long and short bulbs.
    Cheers
    Claudio
    Last edited by claudio; 10-29-2010 at 10:01 AM.

  8. #8

    Default Re: Keel Fin Planforms

    Poor meee!!! Alan I can no longer publish images if you scan them every time with x-ray!!!
    --------------------------------
    The life is short: sail hard!
    --------------------------------
    IACC120 Alinghi Challenge Roma
    Defender of IACC120 CUP 2008-2009-2010-2011-2012-2013-2014
    SUI-100 & SUI-91
    www.iacc120cup.altervista.org

  9. #9

    Default Re: Keel Fin Planforms

    Nice observations Alan!..

    I have also snuck the keel aft a bit on the new one I am building... Not quite as radical as SUI 102 as the balance of my hull with a huge main and small jib (similar to 102) was almost perfect. I am running a small rudder (IOM sized) which helps to keep the wetted surface down.

    But now - I am thinking now that I should have moved it even further aft!
    Fortunately it isn't that hard to change if I need too, and that way I don't need to build a Mk3. Phew!
    I am also lucky enough to have a 40mm x 40mm x 400mm ingot of the perfect density lead (Exactly to the decimal point certified by the IACC measurers) from SWE 96's bulb... I will be fashioning it into my new bulb when I get the time.

  10. #10
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    Default Re: Keel Fin Planforms

    Hi Jim,
    would be interesting to calculate the volume of your lead block and than check the weigth. It should be an alloy I suppose !
    Cheers
    ClaudioD

    PS: my foil lead is 10.5 kg/dm3

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