Best shape for surface piercing foil

Discussion in 'Sailboats' started by grob, Jun 15, 2004.

  1. Paul Scott
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    Paul Scott Senior Member

    Since I am apparently obsessed by leeboards (and windwardboards for that matter), I have remembered this post, and once in a while I have come across a report/thesis etc. that seems to address the problem in some way. I realize that this thread is old, and the mechanism involved may have been understood completely by now, but I haven't seen anything here on BDF, so I've run across a report in the Journal of Fluids and Structures that seems to address the subject, and It is available on the web- Google- "Unsteady free-surface wave-induced boundary-layer separation for a surface piercing NACA 0024 foil: Towing tank experiments" by Metcalf, Longo, Ghosh, Stern-

    Especially interesting is a mention of Zhang and Stern 1996 (in the Reference section), in which contains the following- "Additionally, a necklace vortex can be observed wrapping around the leading edge of the foil for higher Fr. Nodes and saddles of separation and attachment are identified and used to describe the flow patterns." Which seems tantalizing, since it seems to locate a low pressure(?) discontinuity that exists at a leading edge. Could this sort of condition be what is tripped into ventilation by the cup of water that Mark was talking about above?

    I don't know if anyone will take the time to wade through the piece, but if you do, is it germaine? The main reason I ask is there is an observation in a thesis on a page I'll have to look up and post immediately that points out a problem with NACA 0015 at certain re is a microscopic flow detachment that kind of reattaches before the 'officially recognised' flow detachment and subsequent turbulent reattachment that happens very near the leading edge, which is in the general area where ventilation is observed to begin in the water, at any rate, on ventilating foils. It seems there are all sorts of flow discontinuities happening in the laminar flow zone? Or is the idea of laminar flow only a kind of large scale concept with many small scale complexities existing below a certain scale of observation?

    Paul
     
  2. Paul Scott
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    Paul Scott Senior Member

    "System identification for precision control of a wingsailed GPS-guided catamaran" Elkaim
    Stanford University. Look at page 168. Also has a symmetrical airfoil section the author generated for the project that's kind of cool looking.

    Couldn't get the address to paste. Sorry.

    Paul
     
  3. Doug Lord
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    Doug Lord Flight Ready

    =================
    Here it is: (be patient this takes time to load-couldn't post the pdf)
    http://users.soe.ucsc.edu/~elkaim/Documents/GabrielElkaimThesis01.pdf
     
  4. Paul Scott
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    Paul Scott Senior Member

    Thanks Doug
     
  5. brian eiland
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    brian eiland Senior Member

    Bridgedeck Centerboards

    I was looking back at this subject thread for info on ventilation problems that one might incur with vertical foils such as centerboards and dagger boards on level sailing multihulls.

    Gareth, I was also surprised at the direction the subject thread took in discussing 'vessel lifting foils' as opposed to leeway reducing foils or rudder foils as you had asked about.

    Later in the discussion I found this:
    I'll have to see if I can find & wade thru this research, but i have a feeling it may go more deeply into the theory of this ventilation problem than I care to go at this time. I am looking for the best manners to avoid as much ventilation of a vertical foil centerboard(s) as discussed in this recent subject thread,
    http://www.boatdesign.net/forums/multihulls/bridgedeck-centreboard-why-dont-they-work-57051-14.html#post797665
     
  6. brian eiland
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    brian eiland Senior Member

    Surfboard Fins

    Everytime I come back to this subject thread I find another interesting posting.

    So how did these whale tubercle fins work out?
     

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  7. frers33
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    frers33 New Member

    i am trying to figure out the chord and length of a straight daggerboard to reduce the leeway on 30-45ft displacement monohulls. i have already picked an asymmetrical foil based on Cl, Cd, Cm etc...

    my question is how much of a horizontal lift force must a daggerboard generate to work against the leeway when going upwind? typical speeds of 6-8 knots.

    100 lbs ?
    200 lbs ?
    300 lbs ?

    thank you.
     

  8. tspeer
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    tspeer Senior Member

    Probably the simplest way is to assume the boat is sailing in equilibrium. The righting moment from the hull equals the heeling moment from the sail rig and daggerboard. The maximum righting moment places a limit on the maximum heeling moment. The crew will ease the sheets to ensure the heeling moment stays within the capability of the hull. So estimate the height of the center of effort of the sail rig and the depth of the center of effort of the daggerboard, and divide this distance into the maximum righting moment from the hull. That gives you a first estimate of the side force from the rig. The daggerboard has to oppose this side force, so it's also the first estimate of the force on the board.

    A wise man once told me, "You analyze for the loads you know, and you design for the loads you don't know." This first estimate needs to be increased to account for dynamic loads that are greater than equilibrium, inaccuracies in the estimate, and a margin of safety.
     
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