Gybing Daggerboard?

Discussion in 'Boat Design' started by Konstantin, Dec 3, 2004.

  1. Konstantin

    Konstantin Guest

    Hallo everybody,

    first of all I really enjoy the very interesting postings, but one thing I still can't get right:
    What is a gybing daggerboard?
    Is it a daggerboard wich can be cannted like a the keel ( to the oposite direction) and does dyna yachts have any rights on this technology?

    Cheers for every post
    Regards Konstantin
     
  2. Phil Locker
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    Phil Locker Junior Member

    Gybing boards

    Nope, they don't cant.

    A gybing daggerboard (or centerboard) has a trapezoidal head section rather than the typical rectangular section. The point of maximum thickness should be the same as the max thickness of your daggerboard trunk, and it should be located aft of the midline.

    Pressure on the board will cause it to gybe about its point of max thickness (essentially a pivot point), which increases its angle of attack relative to the centerline of the hull. Greater angle of attack = more lift (or instead, steer the boat a bit lower and get more power out of the sails while still sailing at the same leeway angle as a boat with a non-gybing board).

    Text descriptions aren't all that clear. Take a look at
    http://www.philsfoils.com/designTips.html#gybinghead
    for a couple of pictures.

    Cheers
    Phil
     
  3. tspeer
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    tspeer Senior Member

    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.
     
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  4. tom28571
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    tom28571 Senior Member

    Excellent Tom, excellent.
     
  5. tspeer
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    tspeer Senior Member

    Thanks.
     
  6. B. Hamm
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    B. Hamm Junior Member

    I don't know, I look at gybing boards being similiar in function to a Hobie cat as an example. The Hobie (orriginal 14's and 16's) have asymetrical hulls that are toed in at the front, are not inline with the sailing direction. When sailing upwind a Hobie foots into the wind, the hull lifting to windward, adding distance made good to windward, but not changing the angle of attack into the wind. I think a properly designed gybing board on a round bottom dink would do the same thing. It's very difficult to see or feel by the seat of your pants, but it can be measured.

    Bill H.
     
  7. SailDesign
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    SailDesign Old Phart! Stay upwind..

    If there is only one hull in the water, it is pointing in the direction of travel (barring leeway), no matter what the realtionship of the other hull. Does the water somehow mysteriously "know" that the flying hull is not in line with the one in the water? Actually, all you are doing there is adding wind-drag by not lining the flying hull up with the flow.
    The Hobies worked by having hulls that were asymmetrical in _shape_, thus making them act like an aerofoil. They didn't do very well to windward since the "foil" shape and configuration was pretty nasty, but since they rarely raced except against each other, it didn't matter much :)

    Steve
     
  8. tspeer
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    tspeer Senior Member

    I actually wonder how much lift the Hobie 14 & 16 got from the hulls vs from the rudders. It may be the hull was acting more like a canard and the rudders carrying most of the load.
     
  9. tom28571
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    tom28571 Senior Member

    Good question Tom,

    There are other foil-less cats that make their way to windward as well as the Hobie. The G Force for instance. My guess is that the asymetric hull shape is bound to make for a higher coefficient of lift. The shape of the hull or centerboard is a separate issue from whether it jibes or not though.

    In any case, if the boat is in equlibrium and sailing a straight course, your analysis has to hold. There is the matter of the hull contribution to lateral resistance and drag that clouds the issue. That is, while the CB may be assuming the same angle of attack (leeway) whether jibed or not, the hull leeway will be different and its drag contribution different. In a hard chined boat, I would expect the jibing board to be more beneficial than in a round hull. The total lateral resistance has to remain equal to the sail side force but the individual contribution of the CB and the hull can change. That leads to the conslusion that the jibed board must take a greater angle of leeway to compensate for the lesser lift from the hull. I'm about to confuse myself, so I'll stop here.
     
  10. SeaDrive
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    SeaDrive Senior Member

    Indeed, nowhere is it written that a hull need have minimum resistance when pointed at 0 degress to the direction of motion. Quite possibly, the shapes that have been found best (empirically over a couple hundred years of development) have less resistance at 2 or 3 degrees of leeway. Since the boats are heeled(in general) they are asymetrical in any case.

    I note Tom started his discussion by limiting it to dinghies. Anyone want to extend the discussion to asymmetrical twin daggerboards, and leeboards in heavier boats?
     
  11. tspeer
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    tspeer Senior Member

    I assumed zero effect from the hull to make the effect of gybing the board clear. When you include the hull, the same basic principles apply. The total lift from hull and foil equals the load from the sail rig, and the leeway angle will adjust to make this so. Given that the lift is fixed, the question then becomes, "How to obtain the lift with the least amount of drag?"

    Since the drag due to lift is inversely proportional to the square of the span, when you consider the hull and foil separately, the lift from the hull is "expensive" and the lift from the foil is "cheap" lift, because drag is coin you use to pay for lift. So naturally, you want as much of the lift carried by the foil as possible. The exception occurs when the hull extends the effective span of the foil. Then the drag due to lift of the combination is reduced.

    The optimum combination of foil and hull lift would have a smooth distribution across the hull/foil junction. If there's a sharp discontinuity in the lift distribution, a vortex will be shed that reduces the effective span. How much the lift the hull produces compared to the board, is going to be a complex function of the shape of the hull, the shape of the board, and the leeway angle going upwind. A given board angle may be optimum in some conditions but not in others. But a small amount of hull lift is probably beneficial in most cases. Which one might get by not gybing the board and having a well rounded bottom - sounds like a typical dinghy, doesn't it?

    Then there's the slot needed to give the board freedom to move. The drag of an open cavity is very high. It's a lot easier to seal the slot of a non-gybing board because you know the shape, and it's the same on both tacks. A gybing board has to push the seal aside on one side, and the seal has to fill a larger gap on the other side.

    So I'm not surprised that people have different experiences with gybing boards. I don't see any big fundamental reason for gybing the board to improve performance, like "lifting the boat to windward." Instead there are a whole bunch of secondary effects; some good, some bad. Your mileage may vary.

    Cambering the keel is just like gybing it. Camber changes the zero lift angle of attack. But once again, the total lift has to stay the same. So if you increase the camber, the change in zero lift angle of attack is just like gybing the board. And it would have the same effect of rotating the bow off the wind to align it more with the course through the water.

    The difference with camber compared to gybing is that camber affects the profile drag, while gybing is just another way of changing the angle of attack. With camber, you can place the minimum profile drag at a nonzero lift coefficient, so you can center the drag bucket about your operating point. So aside from acting as a second rudder for big maneuvers, minimizing the profile drag is what I see the AC crews doing with their keel flaps.

    The twin keel is probably going to want to have just enough camber that the drag bucket extends to zero lift and the leeway angle corresponds to the zero lift angle of the windward keel. That way, the windward keel would be completely unloaded and all of the loads from the rig would be resisted by the deeper leeward keel. Once again, going for the cheap lift over the expensive lift. That would also pretty much guarantee that the keels were still operating in their drag buckets when going downwind with little side load. But there would be some drag due to opposing lift on the keels in that condition.
     
  12. Doug Lord

    Doug Lord Guest

    Sails/performance gain

    Tom, if the hull is oriented more along the actual course by gybing the board would you not get a performance gain by slightly easing the sails on the new heading? So comparing two equal boats(one with a "normal" board and one with a well sealed gybing board) the "Gybed" boat would go slightly faster over the course? I didn't get the reference to the ballestron boom moving the forestay; I just layed out a model and it seems that the net effect of the gybing board would allow you to free the sails up a bit?
    -----------------------
    One other question: if the boat had a certain amount of weather helm before the board gybed would the rudder angle relative to the hull centerline of the "gybed" boat have to increase to match the same weather helm ? Could that angle beneficially be reduced by changing the balance of the boat by moving the rig forward slightly on the "gybed" hull? Maybe that would increase the board loading too much..
     
    Last edited by a moderator: Dec 9, 2004
  13. tspeer
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    tspeer Senior Member

    Yes, that's what I meant when I said, "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."

    When you free up the sails, you're really putting them back at the same angle of attack as they had with the coventional boat. So there may be an increase in speed, or maybe not.

    An Aerorig is an example of a ballestron boom rig. The boom is a beam that extends fore and aft of the mast, and the forestay is attached to it. So when you sheet in the boom, the forestay moves to leeward as the mainsail leech moves to windward.

    The same thing is happening with the gybing centerboard. The hull is acting like the beam of the ballestron boom. When you gybe the board, it's like sheeting in the boom. If the sails were properly trimmed to begin with, they have to be eased to get back to their original trimmed angle of attack. But now the forestay is to leeward of where it was in the beginning, and the slot is opened up.

    I don't see why there would be much change in the steering balance of the boat. Of course, you'd have to move the rudder to match its angle relative to the gybing board and the course through the water. So the tiller would be pulled more to windward than in the beginning, but the actual load on the rudder would be about the same. You might experience this as having to pull the tiller more to weather, but not having any more pressure on the tiller than before.

    Possibly a little less weather helm with the gybing board because you'd lose the effect of leeway on the bow.

    All in all, an accumulation of a lot of subtle changes; some positive, some negative. I don't see a way to make any generalizations about the benefit of gybing the board.
     
  14. Phil Locker
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    Phil Locker Junior Member

    rotating keels too...

    Interestingly enough, the current issue of SeaHorse has an article on the Diode 36 with its rotating keel. They're a bit sketchy on what's really going on, but it seems they rotate the keel to align with the "longitudinal shape of the heeled waterline". Upwind performance is claimed to be quite good. And they also say they looked at the gybing centerboards on the 5o5 dinghy as inspiration.

    Phil
     

  15. Doug Lord

    Doug Lord Guest

    CBTF experience

    In trying to understand this better it's interesting to note that the CBTF guys say that their boats exhibit marked improvement upwind against boats that don't have twin foils with a "collective"(both foils turned the same direction upwind) facility.
    In my own experience racing a Windmill and US1 among others it seems like if you could slightly ease the sails upwind and still point comparably to another boat you'd be faster...
     
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