sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. CT249
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    CT249 Senior Member

    Yep, it is a multi thing. Shortboard windsurfers also have loose leaches, but significantly those rigs are slow on a longboard. But on types with limited top end speed, tighter leaches and higher pointing certainly works since even if you foot off, you won't really "go" like a multi does.
     
  2. brian eiland
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    brian eiland Senior Member

    Tight Leeches UNPRODUCTIVE

    Perhaps its not just a multihull thing?

    I was looking up some material to construct some model boat sails from and found this interesting quote:
    http://www.magicksails.com/KiwiSails10.html

    He goes on to say:
    http://www.magicksails.com/KiwiSails13.html

    Alec Newald
     
  3. CT249
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    CT249 Senior Member

    By definition "over tight" is too tight, but what is "over tight" in one class is too loose in another and vice versa. In the classes I race the angle of the upper leach when sailing upwind and winning championship races would range from maybe 15 degrees to windward of the centreline, all the way out to perhaps 20 degrees to leeward of the centreline. There's no mystery about it, it's just matching sail shape to the class in terms of lift/drag and other factors.

    Model yachts could be different because of Rn and the amount of wind shear they sail in, but that doesn't change the fact that many classes perform very well with tight leaches, just as others perform better with loose leaches.
     
  4. sigurd
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    sigurd Pompuous Pangolin

    Teardropz

    A few years back I was creating and comparing a few teardrop sections in Xfoil and Javafoil.
    The mirrored clark Y I tried didn't look quite right, in the transition to the sail.
    The best I found was UI1720 mirrored. IIRC I was using around 30% thickness.
    EDIT: And I think mast chord was 20 to 25 % of total.

    The original UI1720 is a low Re, high lift section with most of the foil above the chordline, max thickness way forward and a loong straight trailing edge. When the top side is mirrored making a teardrop, the resulting fine trailing edge makes a slightly lesser kink meeting the sail.

    Try it out in a flow simulator and let me know how you like it.

    Any other thoughts about what makes a good wingmast section?
     
  5. tspeer
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    tspeer Senior Member

    The mirroring technique is one that I devised to allow a designer to create a reasonable mast section without using any CFD tools. The best way to design a mast section is to use an inverse design method, such as XFOIL's MDES mode.

    I don't know of any inverse method that can enforce the symmetry constraint of the mast, so even an inverse-designed section needs to be touched up using the mirror technique to restore the symmetry.

    When using an inverse method, there is something like a singularity at the windward mast/sail junction. With the old Eppler code, the alpha-star values were singular there, with a change in sign across the junction. A method that is based on specifying a pressure distribution, like XFOIL, requires stagnation pressure at the windward junction, creating three stagnation points in the inviscid pressure distribution (leading edge, junction, and leech).

    Jibs don't seem to have a big influence on the shape of the pressure distribution of the mast and mainsail, so single element codes like XFOIL and the Eppler code can design mast sections for sloop rigs. Javafoil is capable of including the effect of the jib.

    Besides the performance of the mast and mainsail, another design point for the mast section is how it works under bare poles. This can be important for minimizing the drag of the mast above a reefed mainsail, especially for a rotating mast. If the separation can be minimized on the mast without a mainsail, it will probably work well when paired with the mainsail. So much of the mast design work can be done by considering just the mast section by itself.
     
  6. brian eiland
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    brian eiland Senior Member

    Drag of the Bare Mast

    As I am sure you recall, I've been concerned about reducing the drag of my bare mast on my aft mast rig design. I've played around with some ideas of twin faring 'plates/strips' added to either side of the mast section, and their trailing edges capable of sliding against one another to form a overall smooth trailing edge.
    http://www.boatdesign.net/forums/sailboats/i-call-your-bluff-naked-mast-drag-myth-48927-6.html

    But I keep coming back the more simple idea:
    So my question Tom, if I were to just adopt this simple 'single splitter plate' configuration, what bare mast section (non-rotating) might be the best one for me to consider,....and how long of a splitter plate should I look at??

    For reference:
     
  7. tspeer
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    tspeer Senior Member

    The problem with a fixed mast is the apparent wind angle.

    When I designed the sections for the BOR90 trimaran mast, I was looking at a +10 deg angle of attack range, even though the apparent wind angles were more like 20 - 25 deg. I also had the benefit of a very high Reynolds number, due to the mast chord being around 2 m. Those two factors made it possible to have attached flow on the mast, even though the thickness ratio was 40%.

    There's no way to have attached flow on a fixed mast when operating at angles of attack over 20 deg. If you add a splitter plate (on centerline for symmetry), you are only making the separated wake larger.

    You could consider a rotating fairing, similar to Chris White's mastfoil, and that would reduce the drag of the mast. How big the chord would need to be would depend on the shape of the fairing, but it would be hard to get away with a chord that was less than 250% of the thickness.
     
  8. brian eiland
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    brian eiland Senior Member

    Interesting, I'll have to reread your post several times.

    Are you saying the splitter plate on centerline would actually increase the drag?
    ( I was always thinking of the example where they placed some streamers on a round piece of rigging wire/rod and reduced its drag substantially?)
     
  9. tspeer
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    tspeer Senior Member

    An increase in drag is what I would expect. All the splitter plate studies I've seen were conducted at zero angle of attack. The operating conditions for a fixed mast are quite different.
     
  10. sigurd
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    sigurd Pompuous Pangolin

    I have a small question regarding the gap between the boom and the deck.
    Using a una rig as example.

    If we swept the deck with the boom, e (oswald's span efficiency number) could be higher than 1 and approaching 2 if everything was on the ball.

    If we raised the boom 1m, and added a 1m tall square mizzen, with the same chord and 2d lift coefficient as the main near the boom, would this give the same e as with the gap closed?

    Something about the very large effect of very small gaps has me thinking this isn't so, but I'm not sure why yet.
     
  11. tspeer
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    tspeer Senior Member

    What matters most is the spanwise distribution of how the wind is deflected (wake wash), and this is determined by the spanwise lift distribution. Ultimately, what you want to achieve is a linear wake wash distribution.

    Simple theory says it doesn't matter how the lift is distributed in the streamwise direction, so a mizzen could, in principle, fill in the missing segment and effectively extend the span of the mainsail. Simple theory says the drag would be the same. Basically, the trailing vortex from the bottom of the mainsail would be cancelled by the trailing vortex from the mizzen, which would be rotating in the opposite direction.

    In practice, it's not possible to maintain the same lift distribution as a lifting surface is moved in the streamwise direction.
     
  12. sigurd
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    sigurd Pompuous Pangolin

    Aha, thanks for the explanation.
     
  13. brian eiland
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    brian eiland Senior Member

    Apparent Wind(s)

    I have another question about 'apparent wind' that has been nagging at me lately.

    It seems that in many cases of discussion (text books et al) we just accept the proposition that a sailing vessel as a whole experiences a certain apparent wind that is a result of the combination of the wind speed and the vessel's speed. Thus the rig on our sailing vessel experiences this apparent wind.

    But my question, are there some portions of the rig that that experience different apparent wind(s) as a result of other influences? If I stand on deck at various locations around the vessel I may experience differert apparent winds than that singular one we often associate with the vessel as a whole,...other obstacles on deck might redirect some of the apparent wind that strikes the vessel.

    We have seen how the 'slot' can redirect some flow over the foremost headsail, such that it receives a more favorable apparent wind.

    Is it possible that the fixed mast on either the std sloop, or my aft mast, or other rigs, might also receive a smaller angled apparent wind as a result of being at the trailing edge of the headsail's redirected flow in a close beating situation?
     
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I think this will just be a reminder for you, Brian, of something you already understand full well.
    Wouldn't the primary cause of differential angle of incidence be wind gradient: the greater velocity of true wind at the top of the mast vs the bottom?
    Just in case either you, or someone else reading this, doesn't follow what I'm saying:

    • True wind velocity varies by elevation, being stronger aloft.
    • Forward velocity of boat at any moment in time is constant (if we discount pitching).
    • The forward velocity of the boat will therefor have a greater influence on the apparent wind direction closer to the deck - one reason twist is sometimes desirable.
     

  15. brian eiland
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    brian eiland Senior Member

    Yes Stephen, I'm well aware of that change in velocity and direction with increasing height of the mast. I was ignoring that variable with that recent question I posted.
     
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