'Self tacking' daggerboard?

Discussion in 'Multihulls' started by kerinin, Jun 19, 2009.

  1. kerinin
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    kerinin Junior Member

    I'm working on a 16' catamaran design and I just had a sort of odd idea. I'm a little concerned about leeway and its implication on extremely fine hulls; as the leeway increases the drag on the hulls does too.

    I've seen some examples of keels/daggerboards with operable trailing edges. I was thinking that if the daggerboard was free to pivot a few degrees to either side and the pivot point was placed behind the center of lift, the daggerboard would pivot itself into a higher angle of attack automatically.

    Has anyone seen/tried anything like this?
     
  2. marshmat
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    marshmat Senior Member

    Hi Kerinin,

    If I read correctly, what you have in mind is a single daggerboard, but on a rudder-like shaft aft of its centre of lift. Thus, if the boat is making leeway to starboard, the daggerboard would turn a few degrees counterclockwise (viewed from above), the larger angle of attack increasing the lift on the board. Tack, and the board should snap around clockwise to work in the opposite direction.

    I've never seen such a setup, but it sounds like it could be feasible.

    Provided the forces balance out OK- the board you have in mind isn't creating more drag than it saves, etc.- you might encounter some of these issues:
    - Bending moment on the pivot mechanism, which is essentially acting like a rudder stock
    - Controlling the motion of the pivoting daggerboard (it'll tend to snap across violently when you tack, unless it has some kind of damping mechanism)
    - Propensity for damage, or for picking up debris

    I was about to ask whether the extra complexity of such a setup would really be better than just having twin daggerboards... but then I realized it's a 16-foot cat. Everything will be happening so fast when you tack that there will be no time to wrestle with daggerboards.
     
  3. BeauVrolyk
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    BeauVrolyk Sailor

    In racing monohull dinghies this has been done for years. It does help by insuring that the hull is moving through the water while crabing sideway less. I used to sail aboard a boat that did this and I'll try to describe how it worked.

    The board is in a trunk that is a fair amount wider than the thickest part of the board. The board has two shims that run vertically near the back edge of the board making the back of the board much wider than the front. The thickness of the shims vs the thickness of the front part of the board and the amount of width in the trunk in which the board rotated will set the amount that the board rotates.

    On the boat I sailed on there were plates that the front of the board rode up and down on that could be moved in and out to make the angle of the board in the trunk larger or smaller. These plates had a set of screws that were operated from inside the boat with lock nuts to hold them in place. If you wanted the board to move back and forth less, you screwed in the plates, to rotate more you let them out. At the back of the board the shims, which were 1/2 rounds glued to the sides of the board, rode in a vertical grove so that the board wouldn't twist aft from the drag. Even with a good smooth fit and a mess of lube, it was hard to pull the board up and down in those groves.

    An important feature was that there were rubber flaps at the bottom of the trunk to insure that water didn't come shooting up through the gap between the board and the trunks. Obviously, there has to be a gap or the board can't move. Also, without the rubber flaps, the extra drag created by having a trunk that is far too large for the board, negated the improvement in drag you made by not driving the hull through the water sideways. The drag of an opening next to the board is substantial!

    In the end, this worked OK. Eventually most people gave up on it because it was simply too much weight and broke down too often.

    In large boats, like the the big maxi Pywacket, there have been rotating dagger boards on the foredeck for years. These are mounted in a rudder post sort of affair and were supposed to "self rotate", it didn't work well at all. The friction on the rudder bearing was more than the energy derived from the balance of the board. So, the board wouldn't rotate unless seriously pressed by water pressure. Then, it popped over and stayed in the new position until a big surge of pressure pushed it some other way. This was slow, as the board was being dragged around by the water and that absorbed energy, slowing the boat. It did work going up wind when the crew would manually position and lock the board with a predetermined amount of offset to the bow. Eventually, they just pinned the board on centerline when off the wind. Generally, everyone believed the thing slowed the boat down a lot and I think it was removed.

    Thus, the results of all this are pretty mixed. Mostly, people have decided that it's way more trouble than it's worth and have given up on it.
     
  4. catsketcher
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    catsketcher Senior Member

    Gybing centreboards

    As BeauV said it has been around for years. 470s. 505s and Flying Dutchman dinghies went through this in the 70s and 80s. Faster in flat water but seemed slower in chop. The Gougeons had them in their 32ft water ballasted cat too. Probably not worth the effort but really easy to do. Just shim the back of the board so it slops in the case less than the front.

    Cheers

    Phil
     
  5. bill broome
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    bill broome Senior Member

    it seems to me it would be easy to put a tab on the board, essentially it's just a very narrow rudder. perhaps the front half of the board cd be fixed in place, the back half turned 5-10 degreeso develop lift to windward.

    hard to tell if it is worth it without experiment. if you are only concerned with making long tacks, on a passage- the way to go is asymmetrical boards, flat on one side. not practical around the buoys.

    you can do more good by cleaning up your rig: lowering air drag is far more fruitful.
     
  6. ThomD
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    ThomD Senior Member

    What about a Canard, if this is on your mind?
     
  7. kerinin
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    kerinin Junior Member

    Actually, I've been thinking about canards a little too! I was considering using large fixed daggerboards aft of the center of lift and then a single control surface mounted centrally (between the pontoons) ahead of the center of lift. That way you can control heading by increasing lift rather than drag.

    I abandoned that idea because it would probably either require moving the fore beam forward (to get the control surface in front of the center of lift) or figuring out some way to integrate two control surfaces midway through the pontoons, which seems like a mess.

    Are there other ways to do canard daggerboards/rudders on a catamaran that I'm not thinking of? Examples of other people trying it?
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    With twinboards the same effect might be achieved using toe-in and perhaps an asymetrical board profile. Whatever method is adopted to reduce leeway, the effect on performance will depend on the hull characteristics. A rounded hull that does not generate much drag when crabbing would show little if any improvement, a hard chine hull might benefit significantly.
     
  9. catsketcher
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    catsketcher Senior Member

    Look at what monos do

    All of the above ideas have been tried in monos

    Canard in AC boats and now in canters. Canard also on a Randy Smyth formula 40.

    The flap at the back end of the board is like the trim tab in 12 metres.

    All doable but the easiest one is the gybing board and it has been dropped in the boats that used it even though it is simple. Seems to me the idea is more trouble than it is worth. The idea of a rattling board in the case is enough reason for me not to use a gybing board anyway.

    Remember the worst Laser sailors come in about 30% slower than the best. So spending more time thinking about and practising how you sail is probably better, more fun and more productive.

    cheers

    Phil
     
  10. catsketcher
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    catsketcher Senior Member

    Forgot - what monos did

    The big thing that monos did was to load the rudder. As keels got smaller (thinner but deeper) rudders got bigger. Now rudders are always acting at positive angles of attack. So I would say make your (normal) rudders bigger, balance them and then put the CE further back. Reduce board size to compensate.

    I did this in my little folder and it sails very nicely. I can't say if it is faster as I didn't build a twin with more normal rudders. What I can say is that is handles like a dream and tacks as well as a mono. It will even bear away from a stall with only a main up. Even with the CE further back.

    Go big loaded rudders!

    Phil
     
  11. kerinin
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    kerinin Junior Member

    Gybing board - that seems to be the existing term for what I was thinking. It's always nice to find out someone already had your idea and tried them out...

    Regarding the large loaded rudders, it seems to me that if the rudder is providing lift in the same direction as the daggerboard, the two together are behaving the same as a canard? Canards are dynamically stable because the larger rear airfoil provides greater lift than the fore airfoil as the angle of attack increases (and vice versa); does this mean that in boats using large loaded rudders the daggerboard is generally placed ahead of the sail's CE?
     
  12. BeauVrolyk
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    BeauVrolyk Sailor

    Two rudders, no keel

    There was a VERY interesting America's Cup boat built by the Kiwis. It had what were really two rudders of equal length spaced about 20' apart (fore and aft). These two rudders could be turned independently or linked together. (more on that later.) Suspended below the two rudders was the lead keel.

    The two rudders basically provided the structure to support the lead as well as providing all the side force (like a keel) and providing the turning force (like a rudder). This was an abject failure.

    Why? Well, to make the rudders strong enough to turn easily (remember it's human powered) and quickly they needed massive bearing structures that weighed a tremendous amount, and never worked properly. Second, the boat was extremely hard to get stable and sailing fast, even though it showed moments of amazing speed.

    The two rudders or blades or keel supports or whatever you want to call them could be turned to both provide lift, meaning they were set up to both push the boat to windward, with the front one basically fixed at about 4 to 6 degrees of angle and the aft one set (by balancing the CE of the rig) to provide a similar angle of attack while sailing along steering. This actually started to make the boat crab sideways to windward - a VERY weird feeling!

    During pre-start maneuvers and mark roundings (remember this is ONLY a match race boat) the two blades were set up to turn in opposite directions, allowing the skipper to use the forward blade to push the bow one direction while the aft blade pushed the stern the other way. It was stunning! You could see crewmen falling down when the skipper didn't warn them, being knocked off their feet the boat turned so fast. But, it was also like applying a giant brake to the boat - it stopped. This stopping/slowing technique did turn out to be useful, but the helmsman had to be very careful not to do it by mistake.

    I think if the Kiwi's had managed to have a year or so to get used to the boat, they would have been really REALLY dangerous with it. But, it was so new and odd that even the best sailors had trouble dealing with it. The AC boat that Blackhauller sailed with the foreward rudder and a small fin keel in the center holding up the bulb was a less radical version of the same idea that was probably a better version. No need for massive bearings to hold the lead up and still turn. So, the boat could put more weight into the bulb and get it out of the bearing system. Again, the difficulty of sailing the beast was significant. Tom told us that he figured you'd need at least a year of testing to really figure out how to go fast all the time. He also said that the boat was astoundingly fast up wind when it was dialed in, it just wasn't dialed in all that often.

    Now, the Kiwi boat is gone, as is Tom's. Tom is gone, and the AC boats aren't doing "interesting" things like this anymore. For some other ideas, have a look at the daggerboards on the foredeck of a Volvo 70, along with the dual rudders. Daggerboards are fixed at about a 4 degree toe-in with asymmetrical shape to provide lift. I don't know if the rudders are asymmetrical also (I doubt it because it's slow down wind with both rudders in the water), but you could do that too if you lifted the windward rudder out of the water. Finally, have a look at the daggerboards on the big cats and tris, like Groupama, which are asymmetrical and also curved to provide lift vertically as well as horizontally - now there's a great idea and the topic of an entirely different thread.

    B
     

  13. CTMD
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    CTMD Naval Architect

    Kerinin,

    it seems I'm stalking you today. Gybing boards are standard on both the AHPC Capricorn and Nacra Infusion F18s. A lot of the top Nacra teams are now bogging up their trunks to lock the boards in place. The designer of the capricorn went on to design the Hobie Wildcat, which I believe doesn't have gybing boards, suggesting he no longer thinks they're a good idea.

    In you modeling you could try toeing the boards in a few degrees. If you consider the boat will be flying a hull most of the time this means that the leeward board will be generating more lift than if it had been parallel with the cl (basically the board will see leeway but the hull will see less). while the windward board will be approx parallel with the leeway, so it won't be trying to lift. This will cut down on the drag associated with having a lifting foil passing through the water's surface. The downside is you may have less lift available at low speeds.

    Also check the f16 rules from memory they restrict you to one set of rudders and one set of dagger boards.
     
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