Asymmetric Leeboard Design?

Discussion in 'Hydrodynamics and Aerodynamics' started by stepcut, Dec 23, 2020.

  1. stepcut
    Joined: Jul 2015
    Posts: 3
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Chicago, IL

    stepcut New Member

    I am building an 8' dinghy with leeboards. (If you hate leeboards, don't worry, I am also building a different dinghy with a daggerboard). I have seen it suggested that for optimal performance the leeboards should be asymmetrical foils with the 'flat side' on the outside. (other sources specify convex on the hull side and concave on the outside). That makes sense -- but I am unclear how to get from 'asymmetrical foil' to a specific shape.

    Let's assume I have access to a CNC router -- so carving complex, accurate shapes is not a major hardship. Do I just pick one of the NACA profiles which has some camber? And, if so, how much camber? Or is there some other shape which is better suited to a leeboard?

    I realize the performance gains compared to a symmetrical leeboard are likely to be difficult to notice on an 8' dinghy. But I am still curious about understanding the theory.

    Related Questions:

    A design guideline I have read is that the planform area of the leeboard (surface piercing foil) below the waterline should be 4% of the sail area.

    So, to design a leeboard, I think I would first calculate my sail area, and also the longest leeboard I can tolerate. Once I know the maximum length, I can calculate the chord (width) to make sure my planform area hits the 4% mark. Then I pick the maximum thickness (specified as a fraction of the chord).

    For a dinghy with a top speed of 10 knots, (and usually much less), I've heard that a thickness of 6-9% should work? For 6% thickness, a Pollock foil should perform well. For a 9% a NACA 0009 should be good? Those are both symmetrical foils.

    The 'class rules' for this dinghy limit the leeboard to being no more than 1.5" thick. Not sure if I should aim for the maximum allowed thickness. Or if I should pick the minimum thickness that I think won't snap or flex too much. Or just stick with something in that 6-9% range.

    I am also not sure what adjustments would be needed for picking an asymmetrical foil.

    Thanks!
     
    Steve Clark likes this.
  2. messabout
    Joined: Jan 2006
    Posts: 3,131
    Likes: 296, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    I suspect that an assymetric lee board is not likely to perform much, if any, better than a slab sided piece of 3/4 plywood. It should, by virtue of technical specifications, be a better choice but it will be operating in a very low velocity realm in which aerofoils (hydrofoils) do not generate significantly more lift than a rusty beer sign.

    You need lee boards therefore the boat is presumed to be a sailboat. In manageable winds, the little sailboat boat might get to 4 knots or so.... Six feet per second more or less.... An eight foot dink is not a speedy boat. Keep in mind that lift of a foil is a function of the square of the velocity and a bunch of other factors but velocity is the dominant one. . Thus the difference between four knots and eight knots is enormous. If the board is to be a lee board then you have the surface piercing problem that will probably compromise the lateral resistance of a board of a given size.
     
  3. stepcut
    Joined: Jul 2015
    Posts: 3
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Chicago, IL

    stepcut New Member

    I have no doubt that is true; in fact, I said as much in my original post. In practice, I will probably pick an appropriate symmetrical foil, like the NACA 0009 or 0012. And, even then, the time spent in trying to build them precisely will mostly be an exercise in improving my craftsmanship in a situation where screwing up has little penalty.

    But since I have to pick some shape for the leeboards, it has peaked my interest in learning the theory.
     
  4. Doug Halsey
    Joined: Feb 2007
    Posts: 503
    Likes: 150, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    Do you really need to have two of them?

    Sabots seem to do just fine with only one. (It's symmetrical, of course.)
     
  5. stepcut
    Joined: Jul 2015
    Posts: 3
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Chicago, IL

    stepcut New Member

    For asymmetrical -- yes. If it is symmetrical, then you can get away with one, if you design for it. When it is leeward it is being pressed against the side of the boat, but when it is windward, it is being pulled away. That often means needing more reinforcement near the attachment point.

    If you have two, and one breaks, you can probably get home with the one remaining one. If you only have one, then you better hope you live downwind.

    I've also seen it suggested that if you have asymmetrical leeboards, but deploy them on the windward side, you can actually produce 'propulsion' -- but I do not yet have a deep enough understanding to know if that is true:

    Duckworks - Propulsion - Part One https://duckworksmagazine.com/13/howto/propulsion/#.X-SzRuBOkhc

    Once again, I do not expect to realize those gains on a little dinghy, but perhaps on a larger future boat.

    But, at the moment, my question is not *should I?* but *how would I?* I am more interested in the theory, than the practicality of applying it to my current boat project. For the sake of discussion, let's pretend I am building a 40-60' boat where these things could actually make a measurable difference.
     
  6. Steve Clark
    Joined: Jul 2004
    Posts: 203
    Likes: 14, Points: 18, Legacy Rep: 163
    Location: Narragansett Bay RI

    Steve Clark Charged Particle

    All center boards and leeboards have to produce enough lift to counter the maximum side force of the sails. This is in turn a function of righting moment. An asymmetrical board will be able to produce this force with less surface area than a symmetrical foil. Potentially with less leeway. Making “negative leeway” that is to say driving the hull to Windward, is slow and should be avoided. So you should be careful not to oversized the leeboard.

    The advantages of asymmetry are countered by having the weight, complexity and incidental drag of having a second leeboards. These can be significant enough to cancel the benefit. An alternative is to gybe the leeward so it has several degrees more angle of attack relative to centerline of the boat. This will increase the lift coefficient. There are several ways to do this.
    NACA 63-412 is a nice section we used on the asymmetrical daggerboards go C Class catamarans.
    SHC
     
  7. tlouth7
    Joined: Jun 2013
    Posts: 177
    Likes: 64, Points: 28, Legacy Rep: 10
    Location: Cambridge, UK

    tlouth7 Senior Member

    The easiest solution in terms of design would be to use a symmetric profile but toe in the boards, so the leeward board has an angle of attack when the hull itself is making no leeway.

    In terms of manufacture you could leave one side of the board flat, and cut a profile into the other side. You would want to round off the leeward side of the leading edge also. I'm afraid I can't advise on the best profile for the job. Don't forget that you can vary the chord, thickness and profile along the span of the board. The classic aim is to achieve an elliptical or trapezoidal lift distribution to minimise drag.

    In reply to messabout, profiled boards absolutely make a difference in small dinghies. If you have ever sailed a dinghy with a flat ply or aluminium board (such as a Lark) then you will discover that it stalls much more easily in low wind conditions, or when the dinghy is stopped by waves.
     
  8. messabout
    Joined: Jan 2006
    Posts: 3,131
    Likes: 296, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Tlouth7 i agree that there might be a difference between performance of flat sided boards and profiled boards. I do not agree that it would make a measurable difference on an 8 foot dinghy using lee boards. Lee boards notoriously ventilate.

    I cite my experience with the Windmill class that uses a flat sided board as demanded by rules. The Windmill can and does go to windward with the best of boats in its size and category.. My M-20 scows had flat aluminum plate dual boards. Those boards allowed the boat to go to windward with the likes of FDs and other very good boats.

    I agree that a profiled board is superior in some circumstances. Say on an international 14 or A cat or other very fast boat. On a tiny lee board dink it does not matter enough to quarrel about. The VMG of an 8 foot dink with or without elegant boards will not change much if at all. Matters of sail trim, skipper skill, and sea state will be far ,more influential than the style of whatever lateral resistance is at play. ......................yes I know that my stubborn refute of some of the most revered principals of aero and hydro dynamics are at risk of academic scorn.
     
  9. sharpii2
    Joined: May 2004
    Posts: 2,025
    Likes: 195, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    This is probably true because the actual speed range is likely to be around just 1.5 knots. So, in ideal conditions, your VMG
    may be 2.8 kts. Maybe with a flat board, it will be 2.5 kts.
     
  10. Doug Halsey
    Joined: Feb 2007
    Posts: 503
    Likes: 150, Points: 53, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    If you say that it's your opinion that having a better section shape wouldn't make much difference, then I wouldn't argue with you.

    But you can't say that you've refuted any aero or hydro theories, because refuting something means disproving it, and you certainly haven't done that.
     
    Last edited: Jan 13, 2021
  11. tlouth7
    Joined: Jun 2013
    Posts: 177
    Likes: 64, Points: 28, Legacy Rep: 10
    Location: Cambridge, UK

    tlouth7 Senior Member

    I doubt it would make a difference in terms of speed or even VMG to windward in most conditions, but in very marginal low winds a flat board will stall earlier which can make the difference between making progress against an adverse tide and going backwards.
     
  12. The Q
    Joined: Feb 2014
    Posts: 173
    Likes: 19, Points: 18, Legacy Rep: 21
    Location: Norfolk, UK

    The Q Senior Member

    With such a slow boat, fancy NACA or other shapes are not going to work, you'll never get laminar flow to any extent..
    You will however get some benefit from assymetrical boards.
    Flat one side, just the nose shaped a little, then something like a Naca 12 or 15% for the nose and tail on the inside, with probably a flat between for strength and ease of use..
    The larger the NACA number the more it can take slow near to stall speeds..
     
  13. DCockey
    Joined: Oct 2009
    Posts: 4,914
    Likes: 449, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Laminar flow type airfoil sections continue to work if the flow is not laminar but the drag wll be a bit higher.

    Thicker sections are not less prone to stall at slow speeds. Thicker sections can usually work without stalling at a higher angle of attack than a thinner section.
     
  14. messabout
    Joined: Jan 2006
    Posts: 3,131
    Likes: 296, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Doug; in no case would I dare to refute the rules of physics. I just looked across the room at my bookcase. My eye fell upon a book titled; Shape and Flow, another is Principles of Aeronautics and several others of that sort. I am a true believer and have done some semi serious work in those fields.

    My curmudgeonly critique is only based on years of messing about in sail boats. I am only arguing that a shaped Lee board will not make a lot of difference on a tiny dink.

    I hasten to confess that I have spent many hours in my workshop carving exotic shapes and profiles into boards and rudders. If the OP wishes to do that sort of work, I encourage him to do so. Actually we sometimes do some exotic work just because we can.......and because we have high hopes.
     
    Doug Halsey likes this.

  15. tspeer
    Joined: Feb 2002
    Posts: 2,314
    Likes: 289, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    The main benefit of an asymmetrical section is drag reduction. The lift on the board has to equal the side force applied by the sail rig, so the lift will be the same for different sections. A symmetrical section typically has a U shaped drag polar, with minimum drag at zero lift. An asymmetrical section shifts the U so the minimum profile drag occurs at positive lift. But bear in mind that board profile drag is not a large proportion of the total drag, and differences in profile drag between section shapes are likely to be fairly small.

    A symmetrical section has zero lift at zero leeway. This requires the whole boat to experience leeway in order to generate the required lift. An asymmetrical section produces positive lift at zero angle of attack, and there will be a negative angle of attack at which is produces zero lift. If you measure the angle of attack from the zero lift line instead of the chord, you'll find the zero-lift-based angle of attack is essentially the same for both the symmetrical and asymmetrical sections. So adding camber is equivalent to toeing in a symmetrical board, but the rotation axis can still be at 90 degrees to the centerline. It also has the effect of reducing leeway by rotating the bow off the wind, pointing it more in the direction of travel. This may reduce the drag, and it may improve the effectiveness of the sails.

    The NACA 4-digit sections would be good choices. If you want something that is 9% thick, a NACA 3309 section might be a reasonable choice. It has a zero lift angle of -3 deg, so it will reduce leeway by about that amount. The leeward side is close enough to being flat that you could actually make it flat to ease construction without affecting the section characteristics.

    I think you have the right idea as to how to size the board planform.

    My first boat was a 16 ft canoe to which I fitted leeboards and a lateen rig. I extended the boards above the pivot to form a handle that was angled somewhat aft. I ran a shock cord forward from the top of the handle, and a line with a hook on the end aft. The shock cord would hold the board in the retracted position. To tack, I pulled on the line to the windward board and hooked it on the center thwart to lower the board. Then I tacked with both boards down and released the line on the new windward board when sheeted in on the new tack. The board then retracted by itself, thanks to the shock cord. I really liked the arrangement. The leeboards left the interior unencumbered, and the boat had twice the board area when down speed in the tack.
     
    Tom.151 likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.