Rudder Blade Shape Question

Discussion in 'Hydrodynamics and Aerodynamics' started by BobBill, Mar 30, 2011.

  1. BobBill
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    BobBill Senior Member

    I put this up in past and need some more advice.

    The pic is of two dinghy blades. The original is non kick-up barn door. I would like kick-up rudder.

    Easy to switch rudder heads to get a kick-up blade.

    But what about using the narrow blade assy, adding some inches to leading edge to move blade forward to offer lighter helm and maybe add length?

    Does the smaller narrow blade have to match the barn door surface area to offer similar control and less slip?

    Class rules do not bear, class more or less extinct.
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Imho, the best way to discover the difference is to try them both. The practice will give better answer than the theory.
    They are barn door rudders, so it's easy to change them on the go. If it was my boat, I would fix temporary gudgeons for the new rudder if the pintles doesn't match with the original ones.
    Cheers.
     
  3. BobBill
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    BobBill Senior Member

    Thanks, Daiq, Geniouses think alike...

    I planned doing exactly as you said, first, then deciding. I was looking for some "theory" which I do not possess.

    I read in a recent issue of Sailing World that moving a rudder forward ahead of pivot decreases helm, and kind of took to that.

    My best boats have been keelers, save two, and all suffered from weather helm, which is normal, but never liked it.

    This wee dinghy, without significant helm, might be quite sprightly, at least in my mind. Also, I surmised adding some leading edge would tenderize the helm a bit and maybe lose some helm...

    The kick-up rudder will allow beaching and so on, that the barn door now does not favor but easily fixed now that I have the other blade and kick-up head.

    That Sailing World piece offered some other advice that was either printed wrong or not well thought out (p76), so I guessed maybe it would be better to ask those whom I know know their stuff. (I do not think much of that periodical, I must say.)

    Thanks.

    We will know soon as I finish the rebuild and I will publish and maybe add video.
     
  4. Calculated Risk
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    Calculated Risk New Member

    Moving the center of area of the rudder forward relative to the hull will not decrease weather helm; it'll actually increase weather helm by shifting the boat's underwater center of effort forwards. In your case, however, the increase in weather helm will likely be imperceptible.

    However, moving the rudder's center of area forward relative to the pivot point will decrease the amount of pull you feel from the tiller extension. Essentially, by moving the rudder's center of area forward relative to the pivot point, you are decreasing the lever arm of the transverse force on the rudder, consequently reducing the moment transferred through the rudder post, which you feel as a force tugging the tiller away.

    Adding area to the rudder blade forward of the pivot point will give the helm a lighter feel, but will not decrease the rudder angle required to maintain a windward course in any given conditions.
     
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  5. BobBill
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    BobBill Senior Member

    Rudder Shape


    Thanks, I kinda figured it was a "caluculated risk," if you do not mind the word play.

    I was just reiterating what the article said, and it was referring to keelboats, and I think it was wrong illustration.

    Yes, I understand the lever issue, and that it makes the helm more finicky as well, so, as noted, both will get the try.

    I did forget to mention that this is a cat boat, very much like a Finn, which still employs the barn door rudder, but they do have lots of strings that the Kite does not have aboard.

    Still, if the blade is longer, would that not reduce weather helm? Seems it would logically, but like I said, I know nada.
     
  6. daiquiri
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    daiquiri Engineering and Design

    It will, because the center of pressure of the straight rudder is closer to the pivot (gudgeon) than the center of pressure of the wider barn-door.

    However, only the sea-test will tell if the maneuverability will improve with the new rudder. I see two possible (probable) problems:

    1) looks like the new rudder has a smaller chord and a much smaller area. It means that:
    - it will stall at angles when the old barn-door was still effective
    - it will stall more easily at low speeds
    - it will require higher tiller angles to achieve the same lateral force​
    2) since the center of pressure of the new rudder will be closer to the hull, the yawing lever arm and moment (the one that makes the boat turn) will be smaller than the old one's, so your boat might easily get less helm-responsive with the new rudder, for the same lateral force.

    And due to the fact that the effects 1 and 2 will act together, the final result might easily be a much lower boat maneuverability with the new rudder.

    That's the theory, now it will be interesting to hear from you how well or bad did the new rudder do in practice. :)

    Cheers!
     
  7. BobBill
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    BobBill Senior Member

    Rudder Shape

    Yes, I agree. Less maneuverability overall.

    The very reasons I thought to make the new rudder with longer fore/aft chord, if the correct terms, by adding to leading edge to forward of pivot points and also adding some length at bottom or a bit of a foil.

    I will likely refinish barn door rudder and sail with it for a bit, then try the Laser II rudder, and maybe reshape it as above and see.

    Snow is near gone; time to get cracking here.
     
  8. Phil Locker
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    Phil Locker Junior Member

    Another quick question: does the boat in question have a plumb transom? If not, then that Laser>> rudder will have forward rake, which brings its own issues.
     
  9. BobBill
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    BobBill Senior Member

    Rudder Shape

    Yep, transom is typical dinghy transom...here is pic, Phil. Thanks.
     

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  10. daiquiri
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    daiquiri Engineering and Design

    Hey Phil, just wanted to say that I've seen some very neatly engineered rudders at your web page. Good job! :)
     
  11. Phil Locker
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    Phil Locker Junior Member

    BobBill ... the biggest difference you'll notice is at low speed where the boat will feel less manouverable. That's when that big beavertail shape has an advantage in forcing a low speed turn. But at higher speeds you'll like the decreased steering effort from the smaller Laser>> shape.

    (Daiquiri: thanks! :)

    Phil
     
  12. Petros
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    Petros Senior Member

    change the location of the center of pressure on the rudder will have very little effect on weather helm, only on the amount of tiller force you have to hold. To correct weather helm you have to change the relationship between the center of pressure on the total sailing rig and the equivalent center of lateral resistance of the total below waterline shape. The effect on this relationship of the location of the center pressure on the rudder is very small.

    Reducing the size of the rudder will move the center of the lateral resistance forward somewhat, but at the cost of less effective rudder. This may increase the weather helm.

    The higher aspect ratio rudder will have less induced drag, and less rudder area will also reduce total rudder drag, but again at the expense of less a responsive rudder. A high aspect ratio rudder also has much less stall resistance as pointed out above, and it will stall at a much smaller angle of attack. You would be better off making a higher aspect ratio center board, and keep a larger rudder with much more modest aspect ratios.

    If you want to reduce the weather helm rake your mast forward, or get a larger jib, or reduce the size of the main sail.

    There are good reasons recreational sailboats have large low aspect ratio rudders.
     
  13. BobBill
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    BobBill Senior Member

    Rudder Shape

    Petros, Thanks. It is a cat boat, unstayed Finn type mast, with dagger board, instead of Finn's more traditional centerboard. I could lengthen the board, but wanted to have kick up rudder. I thought as long as fiddling with rudder, might use the higher aspect type and widen to ease effort and gain more lateral area. Being a cat boat, with mast nearly at bow, thought maybe to reduce weather helm some, is all, as added benefit.

    As noted, will likely try both. If I refinish the older barn door, will likely keep it on boat, but use the other head. It (wood) would be too pretty to not use.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Why don't you just make a kickup barn door?
     

  15. BobBill
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    BobBill Senior Member

    Rudder Shape

    Hi Par. Yes, the logical move. Oddly, with some 1300 plus boats made, very few had kick-ups.

    Well, I have concluded that will likely be the end result, but I cannot help but experiment.

    I bought the Laser II rudder to use that head on the old barn door, but got to thinking, why not fool around a bit? No harm done, not big expense.

    Moreover, seeing as the class is kaput and likely will stay that way, those who have the same boat, whereon the rudder is shot, will have an alternative.

    I also acquired a Force 5 spar set in case the Sitka Spruce mast fails...though I am refinishing and carefully reinforcing with epoxy. Laser practice sails are low cost, if need be. Or, I can add lugs or bolt-rope as well.

    Thanks. Will be finishing the little boat up shortly and post the results.
     
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