External Daggerboards

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

  1. Doug Lord
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    Doug Lord Flight Ready

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    Just out of curiosity, where does "1 knot" come from? Keep in mind that almost every dinghy and beach cat rudder is surface piercing. I agree that there would be a slight improvement if the top of the rudder was sealed but convenience outweighs anything else in these rudders. It's simply not the problem it is sometimes cracked up to be.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Rob,

    How many stays do you want on a boat?
     
  3. garydierking
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    garydierking Senior Member

    I have been using leeboards on my outrigger sailing canoe designs for the past 8 years and have yet to see the elusive ventilation. As they can reach speeds of up to 12 knots and my hiking seat is just above and behind the leeboard, I think I would have noticed. What you do see is a higher water level on the pressure side and a lower level on the windward side. A sharp leading edge in the area of the waterline reduces any splashing.
    I suspect ventilation would occur with either a poorly shaped board or a board that is too small.
     
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  4. Fanie
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    Fanie Fanie

    I think it would be easy to test to see if there is as much as 1kn in speed difference. All you have to do is push a spare daggerboard tip in the water and see if the speed reduces as much. Only the tip because that will be the extra drag you can expect.

    Thanks for the nice pictures Doug, I'm afraid those boats are out of my league price wise. Won't mind going for a spin on one though.

    All good points, but to be honest, without them, from experience, I can tell it's going to be very very shitty to aim at any destination. I guess it's a matter of you have to remember to pull them up when you plan to bank the boat :D

    One nice thing about daggers as to a keel is you can adjust them, if you go shallow, drop them just enough. Every bit helps and it sure is better than sailing sideways ;)

    So if I conclude that there is a very small loss due to the surface drag created but not a major loss in speed I think I know what I wanted to. Perhaps the idea of having the daggerboard outside the hull doesn't sound acceptable because no one does it that way... almost as foreign as multihulls are to mono-hullers :D

    At least if there is damage due to a dagger sticking in a rock or in a sand bank the damage would be outside the boat.

    Sails have lots of torque, but no so much speed. If one can convert the full sail power into speed we might end up planing around with the power boats... Just saying :p

    Any one ever figured out what the efficiency of sails are ? 20%... like solar panels perhaps ?
     
  5. Harry Josey
    Joined: Jan 2012
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    Location: South Africa

    Harry Josey Junior Member

    Hi Fanie,
    Watch out for polystyrene cores. The stuff available in RSA is generally steam blown and features fairly large cells only loosely bound together. You will need lots of paint to seal the fissures between the cells, and the resin won't adhere to the paint. You'll end up with a hollow board stuffed with polystyrene bubbles. Unless the FRP is very thick(which is pointless) the board will break when stressed. Use ply, shutterboard is OK if well sealed. You will much less FRP which is expensive. You can shape the shutterboard very easily with a 60 grit disc in a small angle grinder.
    You can contact me on my Email hpjosey@webmail.co.za
    Regards Harry
     
  6. Fanie
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    Fanie Fanie

    Thanks Harry, I paint the polistirene with a thick roof paint, it also works excellent on boat hulls and last just as long and as well as some of the expensive paints.

    I usually coat the polistirene 3 coats and in between allow to dry. I'm almost sure the polistirene will survive. And even if it doesn't, the polistirene does not add any to structural strength. Even if it all evapurates afterwards it's no problem. The glassed item will only weigh 100g less.

    The polistirene I use has a fair density and is not the course cell one.

    No planks, only fiberglass.
     
  7. rob denney
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    rob denney Senior Member

    I don't want any, but my point was more about having rigs on cruising boats that made it hard to lower the main with the wind from behind and dangerous when running deep in an increasing wind.

    Fanie,
    No reason to not have daggers, just mount them so they don't rip the hull apart in a grounding, which is what you intend. Mount the cases on a big bolt and they will pivot in a crash, with no damage.

    rob
     
  8. rayaldridge
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    rayaldridge Senior Member

    I think it's true that surface-piercing foils are inferior to end-stopped foils. Consider the Stiletto 27, which came originally with a centerboard hung from the centerdeck. Eventually owners wanting more speed installed daggerboards, and these boats were consistently faster.

    However, if the boat is not an out and out racing machine, leeboards may make sense anyway. As other have said, they are simpler and lighter and have other advantages, especially in small boats.

    As to the rudders referred to above, my understanding is that outboard rudders work better than surface piercing centerboards and leeboards because the water has been entrained by the hull. Because it is moving in the same direction as the foil, the relative velocity of the rudder through the water is less than with a leeboard or central surface-piercing foil. Thus there is less ventialtion potential.
     
  9. upchurchmr
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    upchurchmr Senior Member

    Doug,

    I did say later that I could not begin to say how much difference there was. 1 Knot came out of ... the air. I'm feeling PC for a minute. There is nothing behind the number just an example of what "might be" compared to a 20kt Tornado, etc.

    I agree with everything you just said. Fannie was commenting that rudders were surface piercing, I just offered an alternative view from the top of the sailing food chain - since we are saying that it probably doesn't matter if you are not racing.

     
  10. upchurchmr
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    upchurchmr Senior Member

    Didn't Sew Sew in the EC break a polystyrene ama? No telling what actually happened, but I think polystyrene has no real place in boat construction except to keep the beer cold.

    I know boats have been done with it, but they probably survived in spite of the foam. It does make good one off tooling if you are making a fiberglass boat. It also provides some minor buckling support for a skin, but anything else would be better, IMHO. That ought to start a war.

    This reminded me of talking to Burt Rutan of wonderful airplane fame. He often used styrofoam in various weights to make a shape and then fiberglassed it. But after that he put it in an oven at around 200degrees. The foam collapsed back to a non foamed shrivelled up mess, which he would then pull out of the part - to save weight.

     
  11. rob denney
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    rob denney Senior Member

    I don't understand. The water "entrained" by the hull will be the boundary layer, which will not be more than an inch or so of turbulent surface water, which is more likely to cause ventilation.
    The rudder works in water "flowing in the same direction as the centreboard" except when the rudder turns, which is often.

    Bethwaite found that using scale models of foils off the back of full size boats, the drag from the boundary layer was huge. This is another reason for offsetting the rudder and leeway preventer away from the hull.

    The Stiletto case is often referred to. Do you have any solid data about what they did, and what if any, other alterations were made? My experience is that the experimenters who make these changes often do other go fasts (bigger, better sails, etc) as well. These get forgotten and the wrong conclusions end up being drawn.

    rob
     
  12. Harry Josey
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    Harry Josey Junior Member

    I believe there is a definite place for leeboards in non racing multis. Both cost and accommodation benefit. Most of the drawbacks can be solved with good design. I also think that all multi centreboards and daggerboards should be offset to avoid intruding into the accommodation.

    Regards Harry
     
  13. dstgean
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    dstgean Senior Member

    leeboards

    Good shape has to mean a lot. I had lots of ventillation on my Hobie rudders toed too far in, but none on my Leeboard at teh same speed of 10 knots. Fariers often have some rudder ventillation issues but added a fence to help the air sucking down the blade.

    I suppose if your boat is going to spend much time over the 12 knots Gary mentions, then you should pay more attention to it. However, most multis, even hot ones like Randy's Sizzzors ends up spending lots of time sub 10 knots anyhow. For me, leeboards or even an external dagger board is likely fine and really wouldn't end up affecting performance much at all.

    Dan
     
  14. rayaldridge
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    rayaldridge Senior Member

    I'd think it might be less likely to cause ventilation, since any disruption of the flow of air downward would be a good thing. But I'm just reporting Thomas Firth Jones' opinion on the matter. He could have been wrong.



    No, I really don't have any hard data; my information is much the same as yours, except that I talked to a Stiletto owner who made the change, and he said it was a definite improvement. As you say, there could have been confounding factors.

    Some info here:

    http://stiletto.wildjibe.com/cgi-bi...in/info/artical-db.txt&function=view&viewme=8
     

  15. Timothy
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    Timothy Senior Member

    A daggerboard or rudder swept forward by as little as three degrees is said to stop or reduce ventilation. Is this true?
     
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