DaggerBoard Design Simplified?

Discussion in 'Boat Design' started by jorgepease, Sep 14, 2016.

  1. jorgepease
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    jorgepease Senior Member

    Why aren't daggerboard shafts and their housings machined?

    It would be so much easier, cheaper and faster and you could hold much closer tolerances.

    Here is a drawing of the concept, the daggerboard starts at end of shaft and would be designed to break there before taking out the hull.

    Has this been done before?

    [​IMG]
     
  2. portacruise
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    portacruise Senior Member

    Why aren't daggerboard shafts and their housings machined?

    My guess is the cost for machining/mounting is too high- from materials, labor and extra time required.

    But I like your idea, if you could carry a spare - would mean essentially no downtime.

    PC
     
  3. jorgepease
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    jorgepease Senior Member

    I would think it would be much cheaper and faster to machine a simple round shape than to hand fabricate the non-immersed part into a foil and fit it with close tolerance to a cassette. Machined you could get the perfect tolerance and you could even use delrin rings as a wear surface and have them be adjustable. Can't see why this hasn't been done! ... actually I see one issue, you wouldn't be able to run half up if the fairing plate is on bottom of board. You would have to attach it to the bottom of hull to reduce turbulence
     
  4. messabout
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    messabout Senior Member

    Certain boards are, and have for a long time been, machined. Dinghys like Comet, and scows like M20 have metal boards that have the leading and trailing edges machined. That is all well and good because the edges are straight or have a geometric curve so they are simple to set up in a milling machine.

    To mill a section like a NASA xxxx one would need to use a CNC machine programmed to do the profile. A dagger board for a dinghy like a Laser, Windmill, etc. could be done on the CNC with ease after programming. The problem is that a simple dagger board might take two or three hours to make if it was aluminum or other metal. At machine rates of 2,3, 4 hundred dollars per hour.......You get the idea right? A thousand dollar Laser board would be a bit much. A CNC planer/shaper for wood could be much faster but the board would still cost a plenty. There is promise with 3D printers. Before long we will be able to tell the printer to build a board for us and it will obey. Even the printer will need to be programmed to do what is wanted.
     
  5. jorgepease
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    jorgepease Senior Member

    I was talking about the non-immersed part, the shaft and cassette. That would be really easy to machine. ... though the CNC machine I am getting could easily cut the profile of a dagger board, that is not a big deal these days.

    Another issue I could see would be the aspect ratio. I don't think you would want it more than 12" wide. Do boats ever use multiple shorter daggerboards? This is for a 50' cat.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Appendages of all types are machined and even massed produced. Various materials too, like HDPE, which is very kind to milling tools and produces an inert, basicly neutrally buoyant appendage too. As to short boards, well some study into appendage design is in order to see the effectiveness of a multiple, short board approuch might help. In a nutshell, you'll find this approuch gains a lot of drag and less than effective surface area, compaired to a deeper, single appendage.
     
  7. rwatson
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    rwatson Senior Member

    A tight fit is not always desirable or necessary.

    Some boats have a loose fit to make it hard to get sand and other rubbish jammed inside the case.

    They dont even need to have a hold down strap because when sailing, the leeward forces jam the boards tight against the cases and keep them pointing down.

    This is particularly relevant to pivoting centreboards that are supposed to slam shut when they hit something
     
  8. jorgepease
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    jorgepease Senior Member

    Hadn't thought of HDPE material, have to look into that.

    The reason I asked is because one of the designers of multi-hulls says they can add 30K to the cost of the boat. That seems like a lot of money for a board that gets hoisted in and out of a sheath by a rope. Can't find the web page now where I read that but I believe it was in that same page that referred to the need for exact fitting or they could rattle in their cases.

    I could see where a tight fit, coupled with twisting forces on the case could bind the board, not to mention sand etc... That's why I would only use spaced bearings for close tolerance and they should not be in full contact 360° around board.

    The board and the case seem to be the type of component that would benefit greatly from production mfg ... if 30K is truly what it costs.

    If anyone has anymore info on this please post it, I will be researching it.
     
  9. Skyak
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    Skyak Senior Member

    There are many opportunities to cut the cost of a dagger board system and still beat the performance of low aspect fixed keels. Most of the performance gain is from high aspect plan form -you could have flat sides with shaped ends (PAR's site has an example). I don't understand your sketch but you could machine and insert to be the bearing where the DB exits the hull -this is how I interpreted PAR's advice.

    The one thing you can't get away from -fixed keels are less trouble.
     
  10. jorgepease
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    jorgepease Senior Member

    Hi Skyak, the idea is for a mod above the bottom of the hull only. In other words, why have a super long daggerboard when all you need, above the hull line, is a shaft with whatever tolerance to the slot is desired. I would use spaced bearings to avoid binding but if the tolerance is not that tight as mentioned above, it wouldn't even need that. I would have more than one bearing point, not just at hull, makes it much stronger.

    It's becoming clearer in my head. In this sketch, I think you can see the idea better. I haven't gotten into the details such as materials etc but if your board bolts to the shaft and is destroyed, it's an easy matter to bolt another one on.

    [​IMG]
     
  11. PAR
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    PAR Yacht Designer/Builder

    You're making an inappropriate assumption, in that there's not going to be a need to resist side and twisting loads on the portion of the board captured in the case. As the foil develops lift and the boat cathers way, side loads mash the enclosed portion of the board against the case sides. Additionally the foil presents a twisting load, within the case as well. If you concentrate these loads along a narrow, centrally located bushing as you've suggested, they will have to tolerate exponentially higher forces, than if these were spread out along its chord. This approuch also complicates the board's construction considerably, whereas a board's enclosed portions are simply flat sided, to bear and spread out this loads along the maximum area within the case, permits a lighter case and an easier built board, CNC cut or not.

    You'll find much of the time, the simplest solutions work out as the best, for several reasons and often justify their existence, by being cheaper, easy or better at force dissipation. Simply put, whittling things down for a racer, can yield some results which can be justified on them, but for most boats, the performance envelop doesn't warrant making these types of changes, for a 1/10 of a knot advantage or a few pounds of weight savings.

    What is your justification for making a board more highly loaded, within its case?
     
  12. jorgepease
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    jorgepease Senior Member

  13. groper
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    groper Senior Member

    Par - the upper, non foil shaped part of the assembly could be an elongated rectangle shaft as opposed to a small centeral rod or square section. This would keep the loads relatively similar although its sounding rather expensive now....

    the main issue with boards and cases seems to be about teh ease of which they slide and marine growth and what not poses a problem below the waterline which compromises any attempt at acheiving an ideal tolerance. The other reason they dont move easily is because theyre still under load and pressed againt the case....

    If you have to build a foil section for the lower part, seems not much a big deal to simply make it a big longer section as opposed to complicating the whole thing with multiple components? im curious, what will having a beautiful machined fit actually achieve in practice? - Will it mean they slide easily whilst under load? The lower part which is under the waterline, will always be subject to marine growth and other debris foulng things up, so even if the upper section is perfect, you still have to deal with the bottom bearing being fouled up?

    The last americas cup boats seems to have very good free running boards despite complex geometry, could we not borrow some of their ideas? although they dont have to deal with marine growth complicating the matter...
     
  14. jorgepease
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    jorgepease Senior Member

    Im building it in my head and I read that article that it cost 30K for daggerboards and that builders have trouble making them fit correct etc.. so I was trying to make it easier. But if the upper part is a straight rectangle, and the tolerances aren't tight, then I don't see why the cost or is that article wrong.

    One reason for separate parts though, might be easy to carry extras and cheaper to replace shorter boards if they do break.

    As for fouling, I'm looking at this stuff - http://www.micanti.com/ ... Will cost me about 4K but supposed to last 5-10 years.
     

  15. groper
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    groper Senior Member

    and you think it will stay put on the running surfaces?

    Boards are expensive because people are making profits... has little to do with actual cost... people have been building theyre own boards and cases for years... build your boards first, then take mold off your finished boards with a thin cardboard layer and a plastic barrier for clearance... its not that much work really...

    If you have a good CNC milling machine, machine out female half molds from billet aluminium... infuse your carbon in them, glue the 2 halves together, and tidy up the edges to finish... you should end up with perfect geometry over the entire length of the board and so should run free with tight tolerances if you so desire... could even machine up foil shaped bearing sections from delrin or whatever to fit the actual board itself, rather than a shaft of sorts... break it, make another...
     
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