Reshaping keel of microcup sailboat

Discussion in 'Boat Design' started by Thomas Strandenaes, Apr 26, 2023.

  1. Thomas Strandenaes
    Joined: Apr 2023
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    Location: Norway

    Thomas Strandenaes New Member

    I'm rebuilding a microcup class sailboat from a total wreck. The rebuild of the hull and rig is complete, but the keel leaves something to be desired :) A previous owner has added lead to the keel to improve the stability of the boat. Microcup boats tend to experience knock downs, so I considering to keep the extra weight but reshape it. Does anyone have practical tips on how I should go about to reshape this keel? I'm cosndiering fibreglassing around the lead blocks to get a bulb shape, but I'm unsure on how to determine the actual shape. I'm aiming to reduce drag of course.

    Sorry if this is an experts only forum (if it is then just let me know and I'll have the post removed).
     

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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Welcome to the Forum Thomas.
    This forum is for questions just like yours - it certainly is not for experts only!
    I am wondering about the feasibility of unbolting the lead blocks and cutting a piece off the top off each to then bolt on forward of the existing blocks?
    You would then have a full length section of lead bulb, with a slightly lower centre of gravity, which is always good.
    And that would make it easier to fair.
     
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  3. Blueknarr
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    Blueknarr Senior Member

    It looks like the block is aligned with the bottom of the keel.

    Are there racing class rules governing the keel shape/weight? Or are you free to explore options?

    Gluing foam blocks around the lead would be my choice of shaping options. The foam is easily shaped. If you are near a boat repair facility then foam scraps should be easily and cheaply obtained. Beware, home improvement foams could work BUT will often be dissolved by marine resins.

    Good luck
     
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  4. Thomas Strandenaes
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    Location: Norway

    Thomas Strandenaes New Member

    Thanks! And good to know I'm not crashing into an experts only level party :).

    I agree, that would probably be beneficial in the ways you describe. And also move some weight forward which might also improve the balance of the boat. Micro cup boats (18 feet length over all, 8 feet beam) tend to drag in the water when they are back heavy, which might actually turn out to be a problem with this keel. Must admit I'm a bit afraid of drilling more holes in the cast iron keel, but I think that would be an over all better design.
     
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  5. Thomas Strandenaes
    Joined: Apr 2023
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    Location: Norway

    Thomas Strandenaes New Member

    Thanks!
    I have plenty of pvc foam (Divinycell) sitting around so material is no issue. Do you mind sharing what kind of shape you would be aiming for? This is a classic 80s style IOR keel. Would it be beneficial for a bulb style shape to extend a lot out from the back of the keel like in this pic for instance?

    [​IMG]

    EDIT: as for class rules, there's no active microcup series where I live in North Norway (or even in the south of Norway). So I'm free to experiment, boat will only be used in an open class club regatta for now.
     
  6. jehardiman
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    jehardiman Senior Member

    If the design is open, then the best choice is to make the keel as deep as possible. This will move weight even lower , reduce the interference drag, and improve lift.
     
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  7. Rumars
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    Rumars Senior Member

    As jehardiman said, best way is to move the lead lower. For this I would unbolt the lead bricks, melt them and pour into a mold that extends the tip of the keel.
    To fasten the new tip to the existing keel, you can drill and tap two holes in the sole of the cast iron, or you can reuse the existing holes with a pair of steel straps connecting the pieces. The straps can be inleted into the cast iron (angle grinder is ok, you don't need a milling machine), or can sit on top, in wich case a heavier fairing will be needed.

    While melting the lead can seem like a big thing, it actually isn't, this is a fairly small amount, it can be done with an old pot on a grill. The mold can be done a hundred different ways, but for this I would cut a steel sheet to the profile of the existing keel tip, then bend some more around the keel with a ratchet strap. Weld everything together in an open box, and stick in a hole in the ground while pouring.
     
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  8. comfisherman
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    comfisherman Senior Member

    I'd do the rumars method, make a box a few inches deep with thin sheet metal matching the contour of the base of the keel and pout away, then bolt and fair that ingot to the bottom of the keel.
     
  9. Will Gilmore
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    Will Gilmore Senior Member

    There are a number of videos available to show how to build molds or melt and pour the lead. Mold s can be built from anything from pine, to clay. Perhapse using something like a lost wax technique for molded clay forms. A local potter could probably fire it for you.

    I'd love to see pictures of your minicup. I own a Mariner 19 that I might want to add a little lead to the CB, just to help stop trunk rattle and add a bit more righting moment.

    -Will
     
    Last edited: Apr 29, 2023
  10. messabout
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    messabout Senior Member

    Thomas; as Rumars said, pouring lead is really easy. It can be melted with a a simple electric hot plate. Messing with molten lead can be dangerous but with some careful handling it is not a big hazard.

    One of the things that you need to keep in mind is that you must not, I repeat, must not, pour lead into a cavity that contains any moisture. You can make your own mold with plaster of paris. It makes a very smooth mold and is easy to work with and is cheap.. If you take that easy route then allow the plaster to become bone dry before pouring lead into it. Complete drying of the plaster usually takes several days depending on thickness.. After that, the rest is easy, but be careful with that molten lead. One other thing......try to make the pour in one go. If you have to make several pours the lead can become stratified if you allow the first pour to become too cool. If you mean to add a substantial weight of lead then you may need to use successive pours. Just have it all melted and ready to pour the next batch very soon after the previous one.
     
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  11. Blueknarr
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    Blueknarr Senior Member


    Adding foam adds buoyancy.
    Adding enough foam to make that bulb's shape might add enough buoyancy to negate the lead.

    My intention was to off set the cost and time of fairing compounds without a small amount of foam.

    As others have stated melting lead is easy. You probably don't have enough lead to make your pictured bulb.

    IF you are interested in melting and molding the lead. Measure its existing dimensions. Cut a block of clay to the same dimensions. Smush the clay into your desired shape. Make a plaster mold of the clay.
     
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  12. mudsailor
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    mudsailor Junior Member

    Don’t add foam, add lead…..bulbs work as adding endplate helps reduce drag a little….but the added stability is the big gain.
    Extra lead is normally pretty cheap but don’t make a big bulb, do a few calculations first (basic geometry) to see how much volume a small bulb is.
     

  13. jehardiman
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    jehardiman Senior Member

    Absolutely not; bulbs or end plates never reduce drag, See Hoerner Fluid Dynamic Drag. What bulbs and end plates do is to increase the lift per span (this time see Hoerner Fluid Dynamic Lift). If span is unlimited, you never add a bulb, winglet, or end plate (up to reasonableness...very long, narrow foils have their own issues in the real world). Only when span is limited do you add a bulb, winglet, or end plate...and then very carefully to ensure that the added lift per span is greater than the added drag.

    For the photo given in post #1, and no span limit, the best solution is to move all that lead into extra foil span with no bulb or end plate.
     
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