Casting a keel.

Discussion in 'Sailboats' started by LP, Jan 16, 2008.

  1. LP
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    LP Flying Boatman

    I've read older publications about concrete keels. In the casting process boiler punchings are added to the concrete to increase it's specific gravity along with the rebar etc, etc....

    Would it make any sense at all to consider adding lead shot as a means to increasing the density of the ballast?

    As an alternative, would it make sense at all to forego the concrete altogether and use epoxy with some coloidal silica as a binding agent?

    I'm sure that the most effective use of lead would be to cast a solid lead ballast, but I'm hessitant to go down that learning curve.

    All input is welcome.
     
  2. Omeron
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    Omeron Senior Member

    Concrete has no strength,except in compression which is useless under this application. I would imagine that any keel to utilise concrete would need very substantial steel bar reinforcement and optimal ratio of steel to concrete ratio to keep it in one piece.
    Therefore probably not much volume would be left to fill by lead or scrap material after the structural parts are done properly.
    Mind you i am thinking about a conventional fin keel, which is highly loaded at its root. If you have a long keel, you may have a lot of space to fill with whatever you want.
    Perhaps you can be more specific about the boat and keel you have in mind.
     
  3. LP
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    LP Flying Boatman

    Omeron,
    Thanks for the reply. I suppose "keel" is a misnomer for the application here. It's more "external ballast" than keel.

    My design is a shallow draft, trailerable craft. Its design is specific to cruising the gulf coast and the keys of Florida. The design has taken Chapelle's Southwind as a basis and thrown in some Herreshoff by means of exceptionally thick bottom planking. The plan is to cold-mold 2" yellowpine for the bottom that should come to about 500 lbs and be exceptionally rigid when combined with the side planking and internal structure.

    The external ballast serves multiple purposes. Righting moment, increased lateral area and legs should I choose to let here come to rest on a tidal flat. I don't see a structural role for the keels.

    Originally, I was looking for an easy way to form the ballast without going to a foundry or trying to cook up the brew myself. I had thought that I might be able to use lead shot in a concrete mixture in lieu of sand to increase the density. Further thought was to use filled epoxy as the binder for the lead shot. The characteristics of spherical packing leads to roughly 30% void in the packing matrix that would lead to quite a bit epoxy binder. Lead shot comes in at around $2 a pound and so I've decide the the economics just aren't there.

    The keels would certainly need to be reinforced with rebar and filled with old tire weights to increase the density.

    I'm still curious though about the feasibility of my original concept of using the lead shot in either a concrete or epoxy matrix. Disregarding the economics of course. 800 lbs. of keel would cost roughly $1500-1600. Does anyone know how much a foundry would charge (ballpark figure of course)for a simpley cast lead keel of 800 lbs?
     

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  4. Omeron
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    Omeron Senior Member

    Sorry all; I had some time in my hands, and happened to post this in a haste.
    Sometimes i forget my membership status, and jump forward, without waiting for my turn and due consideration of the status of the topic starter.It shall not happen again.
     
  5. Omeron
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    Omeron Senior Member

    Hi LP,
    My above reply has nothing to do with your post. Pls disregard it. I actually apologise for hi jacking the thread. I had not seen your last post when i wrote it. I love my junior member status. It makes me feel younger!!!

    I think epoxy is still relatively expensive, less dense, and harder to work with compared with the good old concrete, if you do not need its load bearing capability.
    I once manufactured a fin keel carrying a lead bulb for a 21 footer.
    Not a big deal. But epoxy is surprisingly problematic if you intend to use
    it as the main material to manufacture a structure. Even relatively small quantities when poured into a contained volume as a bulk, produce surprising amount of heat, when the curing process begins.
    I remember hosing down the keel to prevent fire, even though only a kg or so of epoxy was mixed at a time.
    Therefore, if you can get away with concrete then i would not bother with epoxy.
    Not my subject, but i believe there are a number of chemical additives to improve the characteristics of concrete vastly.
    Good luck.
     
  6. JohnMc
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    JohnMc Junior Member

    LP, I've posted quite a few posts about the bulb I am casting. (400lbs) I would not use anything but lead! Maybe by the time your ready I can help with the casting. The lead price should be better when we get it in quantity. Is it a bulb(torpedo) you need or just a solid keel?
    JohnMc
    SB18
     
  7. LP
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    LP Flying Boatman

    Hi, John.

    Thanks for the interest. If you'll look at the picture I posted you'll see a couple of "bilge keel" type appendages. These are what I 'am looking at casting. I've just started looking for lead sources, so as yet, I haven't found my source yet. The stuff seems to be going for $2 a lb. on Ebay. I bit pricey if you throw in shipping too. I haven't tried scrounging at the tires stores yet.

    I'm only looking at 800 lbs ballast and a 1/4 of that I may save for internal trim ballast. That leaves about .4 cubic feet per side if I were to make lead ballast keels. On the other hand, The density of concrete is about right to cast each keel as a single piece that is bedded and bolted on.

    The virtures of dealing with a 3' piece of lead vs. a 22' piece of concrete are readily apparent though. Unfortunately, or maybe its fortunately, the build stage is still a ways down the road.

    ================================================================================

    More thoughts:

    I like the impact resistance of the concrete keels as opposed to glass incased wood that would be used to fair in the lead ballast portion. Another thought would be 1" of lead on the lower portion of the keels' full lenght? It would also be flexible enough to conform to the keel contour making the mold very simple to build.
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    I'm having same issue later this year.
    Cast iron is quite practical material. Costs about 1e/kg (75c/lbs?) and you don't have to deal with the casting bcs it's allready included in this price.
    Lead (around here we get it free in certain places) can be done DIY, but a bit tricky.
     
  9. LP
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    LP Flying Boatman

    Teddy,

    Great idea! I really hadn't considered iron as . . . . well, no good reason. If I can really get it for under a dollar a pound, I'll have to put in the running.
     

  10. PAR
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    PAR Yacht Designer/Builder

    Iron wouldn't be my choice for a shallow draft ballast casting. In shoal craft you can rely on firm bilges and beam, but if you want some heft to haul your butt upright, then it's center needs to be as low as you can get it, considering the limitations presented by the shoal requirement.

    Lead is easy to cast, easy to work, tolerates stands abuse, doesn't have corrosion issues, will not create issues when attached to wooden elements and can absorb shock in strikes or groundings, which might be transmitted to the hull structure using iron (braking keels and other elements).

    I've cast lead shot with concrete, using the lead as the aggregate, which was an easy way to solve the issue I had at the time. You're going to dramatically increase the volume of the casting this way though. The same would be true of epoxy as a binder with lead shot. It seems little balls don't "nest" as well as you might think and lots of air pockets account for a substantial amount of volume. Since lead is easy worked you could mash the little buggers to make mini plates, which will lay flat and decrease the amount of volume necessary for the same weight. Using epoxy and flattened shot, you could get 600 pounds a cubic foot, with round shot, you'll be lucky to rival iron, but if you melt it down (real easy) you'll get 700 pounds per cubic foot, which is a significant difference from iron. And yes, you have to do it in a single pour, but it not that bad. Drop me an email and I can nurse you through it.
     
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