gybing center boards

Discussion in 'Multihulls' started by warwick, Jul 4, 2012.

  1. warwick
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    warwick Senior Member

    whats the idea of and how do gybing center boards work?

    Silver Raven sugested an in depth thread on gybing center boards, so I am starting a thread hopefully we all can learn more.
     
  2. brian eiland
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    brian eiland Senior Member

    You might consider these asymmetrical boards in your thoughts....

    http://www.runningtideyachts.com/dynarig/Section_6_Galley_Dinette_Centerboard.php

    "There is a nacelle structure down the centerline of the vessel that acts as a bottom truss member, acts as a wave splitter, and provides a mounting for two asymmetric centerboards, thus eliminating any daggerboard or centerboard penetrations into the main hulls. And everything,…cables, bearings, boards are all above the load waterline…serviceable in remote areas."
    http://www.runningtideyachts.com/dynarig/
     
  3. Doug Lord
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    Doug Lord Flight Ready

    The idea is usually done on daggerboards where the head of the board can be shaped to allow the board to pivot from tack to tack changing the angle of incidence relative to the hull. On any boat it would be good to use a sliding lock to prohibit this when going downwind. I think it can be worked out on a centerboard as well. The point is to try to eliminate the sideways movement of the boat thereby reducing drag by having the lateral resistance developed by the board and rudder and not by the hull since the boards are much more efficient at producing lift(lateral resistance) than is a hull.


    Rough Sketch of gybing daggerboard:

    click-
     

    Attached Files:

  4. Corley
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    Corley epoxy coated

    Brian Thompson noted in a Yachting World magazine on BPV that the trim tab set to leeward on the daggerboard allowed the boat to point 6 degrees higher, quite significant.
     
  5. rapscallion
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    rapscallion Senior Member

    They work veey well on the G32. They make a noticable difference, I can point much higher with them vs. When they are fixed.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    gybing boards

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    Thanks, Corley! Can you post a copy of that article? Or is it in the thread? I'd sure like to see it.....
     
  7. Corley
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    Corley epoxy coated

    It was a magazine article I'm not sure if it's available online I'll see if I can scan it for you when I get home tonight.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Raps, I'm not clear on what you meant-toed in daggerboards or?
     
  9. Doug Lord
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    Doug Lord Flight Ready

    gybing boards

    Tom Speer on gybing boards: (from this thread: http://www.boatdesign.net/forums/boat-design/gybing-daggerboard-5783.html )

    Gybing centerboards/daggerboards have had a mixed experience in dinghies, and I think the reason is people have a misconception of what gybing the board really does.

    First, assume that the hull is well rounded so its contribution to side force is negligible, regardless of leeway angle. Second, let's assume for now that we're operating at board angles of attack below stall, so we're in the linear lift range.

    Now consider a conventional board that doesn't gybe. As you sheet in, the leeway angle increases so that the lift on the board matches the load from the sail. If the boat slows down, the same sail force results in a larger leeway angle because lift is proportional to speed squared if the angle is kept constant. You can estimate the loads on the sail by knowing the stability of the boat because the boat's heel has to equalize the heeling moment from the sails. The same thing happens with the board - the lift on the board equalizes the lateral load from the sails. So if you know the sail force, you know the (steady state) lift on the board - regardless of the shape, size, or orientation of the board.

    Let's say you have a gybing board but hold it temporarily aligned with the centerline of the boat while you get established on a beat. There will be a certain angle of attack required to oppose the side force from the sail and this angle of attack will be equal to the leeway angle measured from the bow.

    Now release the board to allow it to gybe. There will be an initial acceleration to windward as the board increases its angle of attack and the boat continues to sail at the same speed in the same direction. But very quickly that same acceleration will result in a sideways velocity that reduces the leeway angle and brings the lift back to the level it was before you gybed the board. The angle of attack on the board will be the same as it was before it gybed. The boat will be momentarily tracking more to windward, but the steady state course to windward depends on the lift/drag ratio of the hull/board and the topsides/sails. The boat at this point is basically "shooting" to windward in a way that can't be maintained, just like it would if you headed up from your steady course with a conventional board.

    When things settle back down to steady operation again, the boat will have the same speed and Vmg it did before gybing the board. The lift on the board will be the same, as will be the angle of attack. So it's as though the board never moved relative to the water. But the angle of the board relative to the hull has changed.

    So what's really happened with the gybing board is the hull has been rotated to point more off the wind! The gybing board has aligned the hull with the course through the water instead of having the hull experience a leeway angle. As the bow has moved to leeward, the stern has moved to windward, which brings the rudder closer to the wake of the board, as Bethwaite claims.

    The angle of the sails to the true wind is also greater by the gybing angle. If the sails are resheeted to their original angles of attack, the net effect is to basically move the forestay to leeward relative to the mast, much like a ballestron boom would do.

    Now, these changes may improve the performance of the boat in subtle ways, or they may not. I think this is why the experience with gybing boards has been so mixed.

    But the notion that gybing the board increases the lift does not take into account the equilibrium of the sailboat as a system instead of treating it as separate constitutent parts.
    __________________
     
  10. RHough
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    RHough Retro Dude

    This always comes up when talking about gybing boards.

    With a gybing board the hull centreline will be father from the wind and the apparent wind a greater angle to the hull. A compass reads that the boat is pointing lower.

    The track upwind is determined by lift and drag. The foil should be doing 90% of the work and therefore creates 90% of of the induced drag. All you can hope to gain is a reduction of the leeway induced drag of the hull. In anything but flat water the hull is bouncing around in the waves enough to negate any benefit.

    Good boats with good foils have a leeway angle of about 3°. Using that as a target for a gybing foil will probably get you all the benefit (real or imagined) that there is to be gained. When you start looking at gybing the boar more that a few degrees you end up with some interesting challenges id board/slot loading etc.

    I have never seen a study that included hard data to show the benefit ... the idea is cool though.
     
  11. warwick
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    warwick Senior Member

    Thanks Doug for the drawing illustration of how it works.

    Any one else feel free to add any relevant information as this is to help every one, not just my self
     
  12. Doug Lord
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    Doug Lord Flight Ready

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    Randy, years ago I heard the 3 degree limit for leeway for monohulls-don't you think that the leeway angle for a tri is likely to be less? And that brings up a good point: nobody should use a gybing board unless they KNOW the leeway angle of the boat-don't guess....
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    It seems to me that when you consider the benefit claimed by BP5 on their "flapped" daggerboard ,one should be wary of those who say "gybing" boards(the flap does the same thing) are no good. I think we can "assume" that they must have done extensive research on the board+flap. The flap seems good because if you had that research you could set the flap at the optimal angle of deflection for any speed and condition. Boy, it sure would be fun to be paid to fool with that!
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ====================
    You're welcome! PS read Toms comments and maybe check that thread out...
     
  14. Silver Raven
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    Silver Raven Senior Member

    Gooday 'Wazza' The subject was already started ! ! but thanks anyway ! !

    I was hoping to get the 'really smart chaps' to get into the technical side of - not so much what it is in theory only - but much more like Tom has published - how to make it work - in a practical manner - at the 'cold-face' - let us say on a mid sized 40' or so multihull - especially a tri.

    Sure hope to get more from Doug, Corley & especially Tom - Oh & by the way - Where are you Gary ???

    Now I've read what Tom has to say about the subject - but I feer I'll have to re-read all that information many, many more times until I can see how to make it work to advantage in a Kiss manner - so I can do it when sailing - & so I can fully understand - both what 'it' does - how it works - let's say over 10 ks to windward - what & how I might expect as a result of say ??? degrees of forward angle 'tacking' of the board.

    That's to say nothing of the fact I'm not sure whether to 'tack' the trailing edge to windward or to leward - re what Corley had to say.

    I've got so much to learn ! ! ! but at least - I've started thge journey he he !

    Ciao all, james

    Ciao, james
     

  15. Doug Lord
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    Doug Lord Flight Ready

    James, on your boat(40'+?) I'd consider using a flap but you would have to have a top notch foil designer help you with it(unless you're him). Seems on a big boat it would be less commotion to just move a flap(back end to leeward) upwind?
     
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