sail aerodynamics

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

  1. Guillermo
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    Guillermo Ingeniero Naval

    Thanks for reporting, Brian.
    RIP Marchaj, one of my forever masters.
     
  2. sharpii2
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    sharpii2 Senior Member

    It was from him that I learned that there is significant difference between a good racing yacht and a good cruising one. I still have his SEAWORTHINESS: THE FORGOTTEN FACTOR on my bookshelf.
     
  3. brian eiland
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    brian eiland Senior Member

    I don't think I ever read that one,...but probably should have.
     
  4. PI Design
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    PI Design Senior Member

    With so much of the drive of a sail coming from the aft 1/6 of the sail, does this go some way to explaining the performance of square top mains? With the sail more able to twist open from the head down, you would be losing surprising amounts of lift right where it produces the most moment - so excellent for gust response. But only if tuned properly. A square top that opens too early would destroy drive when it's wanted and be slower than a pin head.
     
  5. brian eiland
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    brian eiland Senior Member

    I've seen some controversy on exactly what the total benefits are with square top mains?

    I don't think this square top main on the Gunboat G4 was too good at gust response :eek:
    http://www.boatdesign.net/forums/multihulls/gunboat-g4-uptip-foils-50397-7.html#post733502

    PS: And I'm sorry, but after looking back thru Marchaj's book, I am just NOT convinced that so much of the DRIVE is coming from the aft portions of a sail.
     
  6. philSweet
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    philSweet Senior Member

    Brian and PI. There is a distinction here. The drive isn't coming from the aft part of the sail. But the drive is being determined by the aft part of the sail. The drive comes from the pressure difference across the membrane times the sine of the angle the membrane makes with the vessel's course. You integrate this product over the entire sail (and subtract the amount of the drag vector's course component) to get drive. Both the pressure difference and the angle effect is greater near the front. The drive is biased towards the front. But it is the departure angle of the rear of the sail that has largest impact as far as generating the pressure difference over the entire sail.

    Perhaps an unnecessary clarification, but I thought I'd give it a shot.
     
  7. brian eiland
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    brian eiland Senior Member

    Thanks Phil, that is an interesting manner to phrase it,....something more to my understanding, but I still have to give it more thought to 'visualize it',....just too old school I guess :eek:
     
  8. brian eiland
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    brian eiland Senior Member

  9. brian eiland
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    brian eiland Senior Member

    Sail Rig 'SLOT' considerations, vertically speaking

    Slot considerations with increasing mast height.

    By now I believe we all agree that the slot phenomena (between headsail-mainsail, or between 2 headsails) does exist,....And that generally speaking it states that the mass of air encountering such a 'restriction' between the two sails gets split up into portions that choose to flow around the edges of the 2 sails, rather than necking down to flow thru the slot.

    The slot that most of our sailboats have (fractional or masthead sloops) is a triangular area between the headsail and the mast. This slot significantly decreases in 'size' as we move up from the deck to the masthead. Meantime the velocities and mass of air increases in size as we move up from the water's surface (nature's wind gradient).


    I have in the past suggested that 2 parallel headstays/headsails, as designed into the front portion of my aftmast rig, that might be a more favorable configuration,...one such posting
    Double Headsails vs Fractional & Masthead Sloops
    double head sails, parallel slot.jpg

    Recently I was made aware of another double headsail arrangement (no mainsail), that tested out quite promising.
    Wind tunnel and CFD investigation of unconventional aftmast rigs
    But in this arrangement the 2 headstays both terminated at the masthead, thus forming another triangular slot area.
    Double Jib model with overlap, ps.jpg

    So my primary question with this posting:
    1) Is this decreasing slot size (with height) compatible with the increasing wind velocity/mass?
    2) And/or are there better configurations to process this airflow,....more effectively (more driving force, better pointing, less drag, etc) ??

    Which of these 2 slot configurations should prove superior by aerodynamic theory?

    (note: For the moment forget the rest of the rig's construction holding things up, and just consider the 2 leading headsails)
     
    Last edited: Sep 16, 2015
  10. Petros
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    Petros Senior Member

    slots in aircraft leading edges increase drag and reduce best L/D, a head sail works the same way. What they do is allow for a much higher CL before stall, in a sailboat that means pointing higher, but it does not mean you will make best headway.

    though sometimes being able to point higher than your competition in a race could be useful, even if you are not sailing at best L/D. the slot has the effect of releiving the adverse pressure gradient on the "pressure recovery" area of the low pressure side of the sail. The air pressure has to go from low near the lee side LE, up to free stream pressure at the TE, and is likely to trip and separate, deducing total lift. This is the "pressure recovery area", the slot on a wing releives the adverse pressure gradient so it is less likely to stall, and allows for high lift, even if at higher drag.

    A decreasing slot size means the flow between the surfaces is reduced, and the effect of relieving the adverse pressure gradient on the lee (low pressure) side is also reduced.

    A large single foil or sail would be the most efficient in terms of producing the most amount of drive for any given wind, and best L/D is useful in most points of sail to acheive best thrust. Best L/D should also result in less healing moment as well. But a slotted LE (presuming it is designed well) will give you best Cl, and hence, best ablity to point higher into the wind. but that may not be your best time made good.

    hopefully this does not confuse you too much.
     
  11. Doug Halsey
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    Doug Halsey Senior Member

    I think you have it the wrong way around. Higher CL maximizes the sail's thrust at apparent wind angle of 90 degrees. For best pointing ability, trimming closer to best L/D is the thing to do.
     
  12. BobBill
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    BobBill Senior Member

    Screw it, am adding sprit to bow...and running large slot on port...narrow on starboard...and go from there...

    May sound goofy, but most of you guys are mentioning stuff I know little about and do by seat of pants...and I love it...thanks.
     
  13. brian eiland
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    brian eiland Senior Member

    I need to digest your posting a bit more, but I am presently on a road trip visiting a few friends up north, and lack the time to fully explore it.

    One thing I did notice is your comment about the most efficient form being a single foil...? Did you read that test I posted?
    http://www.researchgate.net/publication/262797205_Wind_tunnel_and_CFD_investigation_of_unconventional_rigs
    They evaluated both a single foil configuration, and a conventional fractional rigged sloop,....and found them both less performance than the double foil (double-headsails with NO mainsail) configurations. :?:

    Also I do not think you addressed that primary question I had with that posting...

     
  14. sharpii2
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    sharpii2 Senior Member

    This is all very interesting, but I think there are a few caveats.

    1.) is this single lifting foil rule true even when you have to settle for a much lower aspect ratio foil to get enough SA without increasing the CG height of the rig and its Heeling Arm?

    2.) Is best LD always what you want on a sailboat which sails at less than say 15 kts? I would think that best Lift would be what one wants, especially for a displacement hull boat, which has its hull drag increase more geometrically with speed, rather than more linearally, like an airplane?

    I have seen examples of wing sails on multihulls, but have not seen any on mono-hulls.
     

  15. Doug Halsey
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    Doug Halsey Senior Member

    For a heavy displacement boat, maximum thrust is what you want. Thrust is related to lift, drag & apparent wind angle by

    Thrust = Lift * Sin(Angle) - Drag * Cos(Angle)

    When the angle is 90 degrees, then maximum lift is what you want. When the angle is less than 90 degrees, it pays to reduce the drag, but when the angle is greater than 90 degrees, it pays to add drag.

    The only time it pays to maximize L/D is in the limit when the hydrodynamic drag is zero, which never happens when sailing in water & is only approximately true for a very efficient iceboat.
     
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