Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    I agree with you Pogo, lets get back to the bridgedeck board idea that started this subject tread.

    And by the way I do NOT feel that hull mounted boards (centerboards or daggerboards) are useful on downwind running boats, particularly bigs seas. Nor do I think those hull mounted skegs or mini-keels are are useful in these conditions,...aft mounted or not. You are dealing with too many CLR verses COE
    variables, particularly when you consider, unlike monohulls, the hull centers can be quite widely spaced apart and not in any alignment with the sail centers.

    When running downwind in heavy seas DON"T utilize the mainsail, but rather a small jib sail well forward, and a couple of good size rudder foils aft is whats call for. Then control your speed with a drogue or trailing warps.

    You don't need ANY centerboard down when running downwind.

    And think about it, if you are tacking downwind in a race, you don't need any leeway resistance either. The leeway you make will get you to that downwind marker quicker.
     
  2. tspeer
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Sharp-edged sections are not representative of subcavitating hydrofoil designs, but I can see how they would be a place to start. Today, custom sections are designed for different stations along a foil, based on the local conditions and design requirements for each station. But something like a NACA 6-series section would be fairly representative since cavitation drives the section design to pressure distributions with flat rooftops. A NACA 6-series section would be susceptible to forming a laminar separation bubble at the leading edge, which would provide your initial separation.

    I've had discussions with the C-Fly designers, as they've paid a lot of attention to, and exploited, ventilation. They thought they'd designed their main foil to be ventilation free, but they discovered that waves breaking over the foil trapped an air bubble and created the initial conditions for cavitation. From there, they said the ventilation progressed very rapidly down the foil - completely ventilating it between video frames. Their lesson learned was ventilation is a 3D phenomenon and 2D section design is not sufficient to avoid it.

    Andrew MacDougal says water temperature plays a role in the susceptibility of Moth foils to ventilation. Moths are more prone to ventilation in cold water than in warm water (I think I have that the right way around). This suggests a boundary layer involvement.

    Although most ventilation studies have focused on the inception and development of ventilation for surface piercing foils, getting rid of ventilation is also a problem that needs work. When a horizontal foil is plunged into the water, the upper surface carries with it a pocket of air to a significant depth. It then takes many seconds for the air pocket to break down and the foil become wetted. Often the air hangs on in several narrow streamers, suggesting a periodic spanwise instability that leads to the breakdown of the pocket. Planform shape definitely plays a role in how quickly the air clears, but I've not seen any research on this topic.

    It would be really cool to be able to tune the planform for the kind of dissipation one wants. The ventilation can actually be useful, because if the foil were lifting strongly it would be harder to push down to the desired immersion. The reduced lift curve slope of the ventilated foil also means the vertical load is less sensitive to the angle of incidence. But if the ventilation hangs on too long, there's a drag penalty. And if it dissipates suddenly, there can be a big transient in the lift that is hard to control. So I'd like to be able to engineer the dissipation characteristics to optimize the handling characteristics during dynamic events.
     
  3. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante

    What i don't understand:

    -- everybody is aware about the disadvantages of bridgeck centerboards concerning ventilation, flow separation, performance , blah, whatever....
    ---everybody is aware about the advantages of bridgedeck centerboards for cruising boats ( no trunks in hulls , kick- up, ....)
    ---when a bridgeck centerboards matches into a SOR, why not ?
    ---the majority , the cruisers, want to have sufficient performance , troublefree and reliable.


    In my opinion we only have to figure out a practcal , a bulletproof solution.
    No, solutions. ALL technical solutions.
    I guess this thread is long enough.
    Someone has to summarize the already presented solutions.
    Everyone should be able to pick out his individual solution then--- more performing, more troublefree, in between, whatever...

    With other words,
    everything has been said, but not by everyone.
    More than 500 posts !


    summarize ( or summarization ?)
    Mmh , not me :eek:


    pogo

    P.S.
    btw., additionally to a kick-up centerboard one needs a retractable rudder, transom hung, bridgedeck hung , telescopic underhung, pivoting underhung , at least a skegged , pretty little ( ineffective ) rudder)
    One needs another, an additional thread ? Shallow draft multihulls .
    Do we wanna talk about retractable appendages ?
    http://whc.unesco.org/en/list/1314
    Do we wanna talk about sailing upwind in excess in about half a meter of water ?
    http://www.kohlus.de/papers/TOPUW8A.gif
    I guess not, not really.
    Sorry.
     
  4. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    vortex rachet? Assuming that vortex pinning is the reason for the persistence of top surface separation and that the vortexes will move spanwise off the foil if the LE is swept back, maybe moderate sweep back and a series of very small LE modifications that encourage the step-wise movement of vortexes towards the tip and off the foil. The foil is never really stationary relative to the water and if the vortexes can be forced to move with foil movement, you might get your progressive wetting.
    For our centerboard the vortexes would move upwards.
     
  5. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    I agree with you Pogo, back to bridgedeck board ideas

    I had to go all the way back to page 25 of these discussions to find this...
    And I just found another good image to go along with those I posted..

    [​IMG]
     

    Attached Files:


  6. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Wood Board Experiments

    If you look back at my dwg for the twin asymmetric bridgedeck CB's you will find that it relatively easy to remove and replace either of the boards, ...and the 'center bearing' of the board itself can be as simple as 'precisely cut hole' that fits around the UHMWP 'bearing rod'.

    That makes it relatively easy to carry out experiments on what particular board shape might best meet our needs.

    We could set up a simple small computer milling machine to cut us a variety of asymmetrical board shapes out of wood blanks (maybe just decent laminate plywood for the testing), and go out testing with a good GPS on board measuring our leeway for different shapes. Then maybe a few waterproof go-pro cameras looking at the flow around the variety of shapes.

    For those concerned about 'identical conditions of testing', we could have a 'pit crew' that can make quick changes during the day of testing. And we might even carry two (2) different asymmetric shapes at one time for comparison on two different tacks.
     
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