External Daggerboards

Discussion in 'Multihulls' started by Fanie, Mar 11, 2012.

  1. Fanie
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    Fanie Fanie

    Is there any reason a cat's daggerboards cannot be external (outside) the hull ?

    Instead of the daggerboards running through the hull, they can sit on the side outside of the hull.
     
  2. CatBuilder

    CatBuilder Previous Member

    Ventilation is the usual reason. Surface piercing foils tend to ventilate and lose lift if not specially designed for the purpose.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Numerous cats, including at least one high performance boat, use a single daggerboard-is that what you mean? Or do you mean something like a daggerboard version of a leeboard?
     
  4. Richard Woods
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    Richard Woods Woods Designs

    I guess you mean leeboards

    What Catbuilder says is right

    Anything that cuts the water surface makes waves. Whether it is a hull or a leeboard. As you know, wavemaking is a huge drag, especially at speeds above the "hull speed"

    If you have a 30ft hull your "hull speed" is about 6 knots. But your leeboard will be, say, 18in fore/aft and it also has to do that 6 knot speed. Which means it is operating at a speed/length ratio of nearly 5. Or put it another way, it is "suffering" in the same way as your 30ft displacement hull would be when doing 25 knots.

    No wonder it aerates and sucks water.

    Also I have found they bang in a seaway and also in a sloppy anchorage

    Try to avoid leeboards if possible, they have never worked on multihulls. Look up the Prout Shearwater Mk1 or the Stiletto 27

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  5. Fanie
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    Fanie Fanie

    Thanks, I'm just playing with some ideas.

    I asked a friend to cut me the rudders and daggerboards from polistirene with his foam cutter. When I get it the polistirene gets painted with a water based paint, then it gets glassed.

    I played with smaller daggerboards I had cut previously in the pool and they seemed to do ok, I haven't paid much attention to surface disturbamce though. Doubt I'll reach 25kn running beside the pool with the big boards... :rolleyes:

    Will the boards really make that big a difference ?
     
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  6. CatBuilder

    CatBuilder Previous Member

    Definitely. Once the boards ventilate, they will lose lift and do nothing to keep the boat going in any particular direction.
     
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  7. Fanie
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    Fanie Fanie

    Ok now I don't get it. The rudders for one works protruding out of the water ?

    The small daggerboards I played with sure as hell kicked the slightest I turned them, they just wanted to work forward. I can understand the increase in drag when you exceed the speed ratio, but not that they would stop working.

    Someone must explain please. The rudders will be 1m - 1m200 in the water, the daggerboards will be around 1m500 in the water, and they are 600mm wide - calculated to roughly 2% of the sail size.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    There are, as I said before, numerous boats that use surface piercing foils to good effect. I think that if the boat is not designed to be high performance you should use the board(s) in a manner that is most convenient. Almost every dinghy and beach cat uses rudders than penetrate the surface. Len Surtees 26 footer uses a central surface piercing daggerboard as does Hydroptere.ch..
     
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  9. Fanie
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    Fanie Fanie

    Lets say the boards are made long enough and the sheeth for it ends above the water, but on the outside of the hull. This means that there is going to be a 200 - 300mm piece of the daggerboard that would not be in the water.

    Usually the boards go through the hull and sticks out the bottom of it.

    Now if a board does break the surface, will it make that big a difference if the same length of board is still in the water ?

    The guys that fly a hull... will they pick up speed if they retract the flying hull's daggerboard because it breaks the surface ?
     
  10. Doug Lord
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    Doug Lord Flight Ready

    There is no question that boards that don't break the surface are the best way to go when possible. But there are so many daggerboard/centerboards/ rudders that are surface piercing and work well that I think it comes down to a matter of convenience on many designs.

    click on image-pix#4 Len Surtees "Bat Cat" with center surface piercing centerboard. Pix #6 Kona Kat, center centerboard, designed by me.
     

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  11. Fanie
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    Fanie Fanie

    Of course one wants the fastest for what you are building.

    Logic tells me the only extra drag on a specific size daggerboard if one is underwater and the other sticks out of the water (same underwater size) would be the surface wave it makes.

    How much can that extra drag be... so much that one would notice the difference ?

    Sorry, I'm not being difficult. It could be a matter of convenience though, It would be much easier to work outside the hull as to inside, and you won't have the through-hull that needs to be sealed.
     
  12. Fanie
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    Fanie Fanie

    Hi Dough, you're showing hydrofoils there. Of course the only drag would be the boards and foils in the water - which should be a lot less than the hull drag.

    There is about zero chance I'll fly a hull with the sails I use, simply because there is very little heel.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    --------------------------
    I showed the foilers because they have surface piercing foils-with the same drawbacks as any surface piercing foil. But given certain design considerations they work, as does Surtees centerboard and the Kona Kat board. Many large trimarans have a center daggerboard that is surface piercing when the main hull flys- I'll try to find a good picture. It's like in every area of design-there are always tradeoffs-convenience can be a tradeoff....


    Voila--click on image:
     

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  14. upchurchmr
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    upchurchmr Senior Member

    Fannie,

    The rudder will also work better if it is under the hull with the hull in the water, compared to behind the hull.

    Couldn't try to tell you the difference. If you are not racing you might not care about a loss of 1 knot at higher speed. If you want a boat that goes fastest and best to weather you won't want boards or rudders outside the hull. Look at the Oracle America's Cup tri, the rudders are on the amas and under the stern.

    The practical test would be to take a daysailing cat and convert one hull from center/ daggerboard to your dagger in a case on the side of the hull. Then sail on each tack with your GPS. Does anything show up?
     

  15. rob denney
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    rob denney Senior Member

    None whatsoever. Unless the boat is built like a race boat, the drag from the surface piercing will be more than offset by the smooth bottom of the boat, especially when the boards are lifted downwind.

    There is also the safety aspect. Boards are called daggers for a reason which you discover when you hit something hard with them down. Plus the maintenance and fouling potential of the case and the added weight and lost space inside the boat.

    The ventilation "problem" is not seen as a problem on the vast majority of really fast boats with surface piercing foils (rudders on all beach cats, all dinghies, C class cats, foilers, some ORMA tris, moths, Open 60 monos, centreboards on hull flying cats, etc), so I doubt you have much to worry about. There are many pictures of these boats exceeding 20 knots with no obvious foil ventilation.

    IMO, daggerboards are up there with deck sweeping genoas and 3 stayed rigs as race boat lunacies which have been carried over to cruisers by designers/owners more concerned with looks and trendiness than sensible design.

    rob
     
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