what wood for daggerboard?

Discussion in 'Materials' started by sigurd, Jan 20, 2009.

  1. sigurd
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    sigurd Pompuous Pangolin

    It is high aspect and I need it to be stiff.

    What do you think would be best?
     
  2. alan white
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    alan white Senior Member

    Quartersawn mahogany or spanish cedar or luan. Well sealed in epoxy and paint or varnish. A view of the end grain should show all growth rings to be nearly vertical with the board flat.
    Many will recommend plywood, but it isn't as stiff as solid wood.
    The side pieces at the top of the dagger should be oak or similar dense wood to contribute to the board's flatness.
    You can also take a single wide board and rip it into several pieces (say, 3" wide) reverse every other one, and laminate them front to back into a solid piece again. This tends to ensure a board whose movement is self-compensating.
     
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  3. PAR
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    PAR Yacht Designer/Builder

    The most reliable wooden board construction is strip planked, with opposing grains, every other strip, then of course sheathed. If the sheathing is fairly heavy, then a light weight strip can be used, like white spruce. If the sheathing is modest then a stiffer and more dense species like Douglas fur may be used. Hardwoods are an option with the weight penalty and swelling potential they bring to the table. On a daggerboard you'd be well advised to reinforce the trailing edge. A technique I've used for years, is to saturate single braid nylon rope and tack it to a groove along the leading and trailing edges. On the trailing edge it needs to be a fairly small diameter line, but the leading edge could be thicker. Soaked with epoxy, bonded in place and faired, it makes a very durable edge that can take lots of abuse.

    Plywood is an option, but I'd only recommend this on a very small boat with a lightly loaded board. Your high aspect board will be heavily loaded, so do your best to make a light, stiff board.
     
  4. sigurd
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    sigurd Pompuous Pangolin

    thanks alot...
     
  5. mikereed100
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    mikereed100 Junior Member

  6. sigurd
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    sigurd Pompuous Pangolin

    thanks mike, I'll read it.

    Par, Hehe, I already broke two plywood boards, one in the Tornado and one in the Europe Dinghy... kiting.

    I'm getting some carbon tow now to put in grooves in the board I think. And I have some thin glass fabric to sheath it, so it will be super strong.
    Since I am so nerdy I am trying to find strength charts for wood.
     
  7. PAR
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    PAR Yacht Designer/Builder

    You don't need carbon, what you need is to not use plywood. Plywood isn't as strong as solid lumber in this application, because half of it's fibers are running in the wrong direction, to resist the loads imposed on it underway.
     
  8. mikereed100
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    mikereed100 Junior Member

  9. Petros
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    Petros Senior Member

    The two stiffest woods are doug fir and even better is red wood (I do not know how hard that is to find in your location). Doug fir is more stiff than most hardwoods.

    I have made them from pine by laminating several large pieces about 3" widex 3/4 thick, edge to edge, alternating the grain direction as Alan suggests. Fast and easy to build. If you are going to cover it with carbon fiber, just use the lightest cheap wood you can find, all the strength and stiffness will come from the carbon outer skin.
     
  10. sabahcat
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    sabahcat Senior Member


    What size and sort of boat?

    When we know that, and have an idea as to the loads, then we can give you a better idea as to what timber is best suited
     
  11. sigurd
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    sigurd Pompuous Pangolin

    A kiteboat. Since the power is applied so low, the force on the board can get much higher with a mast and its overturning moment. Also the kites can generate huge force from apparent wind.
    Narrow hull, total crewed weight from 200kg but it has many hundred kg cargo capacity.
     
  12. Howaya
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    Howaya Junior Member

    Just because no one likes plywood for boards, I have to ask if you'll accept the following construction method. All of the following wood parts are epoxy coated before assembly.

    Essentially, I used a 2" x 3" x 8' length of douglas fir as a spine. On either side, positioned so the spine is about 40% from the leading edge, I epoxied two thin plywood skins. The 8' edges were pinched together and temporarily stapled to a 1" x 1" x 8' long leading edge of douglas fir and clampled to a 8' trailing edge of flat aluminum while the epoxy set up. Once the epoxy cured I removed the staples and clamps. At this point the plywod skins take on a natural foil shape with open ends at the top and bottom. I fit a thick spanish cedar end cap on one end, filled the forward cavity with pour foam, and then fit an end cap on the other end. Then, I encased the whole shebang in a layer of light biax cloth followed with fairing compound. The boards are 8' long with a chord of just under 2' and weigh around 30 pounds each; they are actually bouyant.

    After 14 months of use aboard a 30' catamaran they seem to be holding up well. One of them took a hit in soft mud and debris at the bottom of the leading edge; apparently the foam not only adds extra stiffness but also acts as a waterproof barrier/cushion during events of abrasion/impact.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Howaya, it's not a matter of disliking a material. Apparently you didn't read the full thread or examine the attached links. Plywood is inherently weaker for the task then solid wood. The veneers and more importantly the wood fibers themselves fail (sheer) with cyclic loading, which is the repeated type of loading a rudder blade, centerboard or daggerboard must tolerate. Personally, I prefer inert materials for these types of appendages, which can eliminate many of the other issued associated with wooden materials.
     
  14. sigurd
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    sigurd Pompuous Pangolin

    par - inert - what do you mean?

    probably it should be reasonable to calculate minimum strength from max CL of the sections, max speed, and the vertical location of the pressure on the board - then give it a safety factor?

    edit - there will only be glass cloth available but the cabon tow can go both crisscross around the board, and along? in little grooves in the core?
     

  15. PAR
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    PAR Yacht Designer/Builder

    Carbon and plywood don't really mix well. Not that they can't be bonded, but if you're going to pay the weight to strength penalty of plywood, then the carbon isn't being used effectively or at least is just along for the ride. The same will be true of a strip planked wooden core board, unless you used a very light weight core stock and a fairly thick (relatively) sheathing.

    Unless you're going to employ balsa, foam or honeycomb cores, skip the fancy sheathings.

    HDPE is a great board material. It's neutrally buoyant for the most part, isn't effected by the marine environment, is self lubricating, so it's less likely to jam, it's doesn't swell with moisture content, easy to machine, durable, etc. It has it's draw backs to, but everything has trade offs. This is one example of an inert material board. Another would be a foam core, 'glassed board (there are others as well).
     
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