sail aerodynamics

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

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes... I have done an analysis on the A-Cat with a wingmast. Maybe a bit to my surprise, the wing mast does not contribute here more than 5% of the total drive. It was downwind, and there is a huge wind shear then, 25 degrees from foot to top. While the sail part can be twisted, the mast cannot, so the mast is over rotated in the lower part when the upper part is OK.

    Rule makers have spotted this and penalize you by adding double the foil width to your sail area.
     

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  2. Mikko Brummer
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    Mikko Brummer Senior Member

    Well, like a said earlier, the IMS-rule does include the mast, but in the wrong way: they give you a better rating for a big mast, based on the old drag device thnking.
     
  3. Paul Scott
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    Paul Scott Senior Member

    Here is an interesting piece that might have some contribution to make to Mikko's point, esp. as the re numbers are interesting:

    " Effect of Regular Surface Perturbations on Flow Over an Airfoil"

    Santhanakrishnan and Jacob

    Dept. of Mechanical Engineering, U of Kentucky

    I think if you Google it, you will get it. I got it off the net, but my printout doesn't have the address.

    I know there are those here who don't find trips elegant, but if the mast was leading edge and trip of the rig et al, would it be more efficient? How close could it come to a wingmast in performance, given that twist and bend viz wingmasts are somewhat problematic? The Tasar and Finn masts come to mind.

    Paul
     
  4. Design_1
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    Design_1 Water Logged Hack

    I am not sure if this is on topic, but I need a little input. I am designing a
    6.2m sailcat for myself and need help with mast location. Can anyone point me in the direction of the formulas used to locate the mast origin of placement?

    Thanks,
    Chris
     
  5. GEDaggett
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    GEDaggett Junior Member

    So after reading most of this thread my brain hurts....badly. I know that I trim my sails try and pay attention to the wind and let God do the rest. I guess I should probably learn a bit more about it but this works fine for now.
     
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  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Interesting report, yes. Sails certainly are often "bumpy". However, do I sence here a little bit of the "old school" thinking? My point was that the mast is not a trip wire, but rather part of the sail area. In case of the Finn, the mast contribution to drive is up to 8,5%.
     
  7. Joakim
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    Joakim Senior Member

    Could you be more specific on what you have actually calculated? I assume you have calculated the whole sail plan with mast using CFD (OpenFoam?, with full N-S? or a panel method?) and the calculated the drive of each component using pressure (and friction) integrals on surfaces. This tells the forces on the objects, but it does not tell the reason for these forces and you can not say from these if the mast is beneficial or not or would a bigger profile even be better. You have to compare to identical calculations with a bigger mast and no mast at all. Then you can calculate the contribution of the mast to total drive as a difference of total drive between these calculation.

    The problem with rating rules is that they have to try to be able to deal with all the possible loop holes, sometimes at the price of accuracy. If the current model would be clearly wrong, I think there would have already been huge mast profiles. But I think there is no real difference between IMS/ORCi profiles, IRC and modern OD profiles. Instead there is a clear difference to the old IOR profiles which were very thin, which is probably due to not taking into account the mast profile in the rule.

    Joakim
     
  8. Erwan
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    Erwan Senior Member

    Mast's Twist? What's for?

    Hi Mikko,

    Congratulations for CFD modelization an A-Cat sailing the "wild thing"

    I would like to provide a more "rock-bottom" perspective about mast's twist issue.

    A lond tile ago I used a carbon Italian mast, with a Greg Goodall sail on my A-Cat.

    I made my batten myself with 5.5 mm pultruded carbon-rod.
    In order to achieve a consistent sail section including the mast and a theorical 17° twist all along the mast, I took a pencil to design the mast section on the floor, and each batten with its twist.

    Doing that I discover how well each of the batten fits perfectly the mast, providing a perfectly smooth "wing section" (leeward side) at any level, without mast's twist of course.

    And the reason is straigtforward: As you go up, you have less chord (shorter battens) and twist.

    At the lower levels, mast account for 6.5% of total chord, at the square top, it accounts for nearly 20% for moderate Square top.

    At the same time the radius of the batten gets flatter, and fits perfectly the mast as the twist increase at the same time.

    In the A-Cat case it seems to me difficult to consider separetly mast and sail. Instead I think Tom Speer XFOIL workpaper about wing mast support this view for a 2D analysis.

    Regards

    EK
     
  9. schakel
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    schakel environmental project Msc

    Hoyt booms for genua

    Hi,

    To obtain the most effective sloteffect I had always wanted to apply a kind of Hoyt boom to a genua. When I sail Half wind the genua is bended towards the ship in such a way that in my intuition the airflow is bended to much and a lot of turbulences is leeward of the genua. Esspecially in the sheetcorner.

    The hoyt boom is a self tacking system and is to short for a genua nor can it be swiveled up or down. It might be appicable on the long run for ocean racers. I have a feeling this might work.

    Does anyone have experience with this?
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes, the whole sail plan including the hull and mast with spreaders, and, in case of the Star for instance, also the crew. Full N-S, and extracting forces on separate objects (like sails, hull, mast etc.). Yes, what I am suggesting here more work should be done with different mast sizes and shapes. For the Star, in a 2D-model, the sails completely without a mast were slightly better than those with the mast, but I have not tried a 3D mastless model, since that would not be very realistic.

    Hmm.. I believe that masts have grown in size under IMS, because the rule interprets the mast as a drag device and gives you a beneficial rating for a bigger mast. To me the current mast profiles are huge ;-).

    The AC-masts have grown into the max. permitted size within the rule, so that would suggest a bigger mast is beneficial. The same happened in the Finn, when they permitted a (small) wing mast in mid 90's.

    However, I still think that the small IOR profiles were best for all round performance. The mast also has a supportive job to do, and bending is beneficial for sail trim. Under IOR we learned that the ability to bend the mast a lot is a real bonus, and small profiles were needed for that.

    The IMS has in practice forbidden running backstays. So the permanent backstay has taken the role of the runners, for keeping the forestay tight. This (and the IMS aeromodel) has led to the full-width, back-swept spreaders, the almost masthead-, large profile jib-rigs that we see today. This could be another reason for the big masts. The one-design classes have copied the IMS, because that's trendy and also because they have to buy the same profiles from the mastmakers.
     

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  11. Paul Scott
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    Paul Scott Senior Member

    Mikko, sorry about the cryptic nature of the following post, but my anti virus program decided to restart my computer just as I was finishing my more stylish initial missive and I lost it, I lost it, and had to re do it-

    1-If twist and side bend in wing masts is generally problematic, and therefore upwash is discouraged in a wingmast rig through lack of rake and little, if any taper in planform (I'm not talking about a Finn mast here), and additional upwash through rake and taper is beneficial for an Una rig, then a more conventional mast may be more useful that a wingmast, esp. at re numbers under say 200,000.

    2-Windsurfing rigs with smaller diameter masts are finding some problems with detached flow, vs bigger windsurfing masts, which are not (as much).

    3- The mast is part of the sail area (you agree with Herreschoff, who reasoned this out in the 1920's, trying to explain why square masts worked well).

    4- Most of the forward drive of a sail is generated around the leading edge

    5- So what shape could a more conventional mast take, and still have fore and aft bend, easily allow twist, generate a lot of forward lift, somehow reattach turbulent flow back to the sail.

    6- I found this report that looked at bumps and flow (and in the re of smaller sailboats, at least). In one case, one bump.

    7- A mast can be thought of as a bump (?). But the report, if I understand it correctly, also seems to imply that a bump is, in some sense, a trip.

    Seems to beg the question- what shape and size can the bump be (viz the cross section of a mast) and still allow easy twist of the sail, fore and aft mast bend, forward drive, and flow getting back to stay on the sail?

    This is very very very very very old school. Back to the future, as it were. :p
     
  12. Joakim
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    Joakim Senior Member

    I don't think you can blame IMS for that anymore, although it might have been the initial reason 15 years ago. There are other rules and lot of new OD classes, but still all new designs use bigger profiles and no running backstays. I guess that is what sailors want. Large jibs are more due to IRC than IMS.

    Joakim
     
  13. schakel
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    schakel environmental project Msc

    Up wash as in where the wind leaves the sail not backwards but upwards is a loss in forwarding power. If you apply canting masts that tilt windward you gain forwarding power and up warding momentum although I have never seen them and I think they are forbidden in most classes except C-cat of course but there it is not beneficial.
     
  14. Mikko Brummer
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    Mikko Brummer Senior Member

    I must be getting old and cynical... but I also "blame IMS" for:

    - the plumb stem

    - vertical topsides with no flare

    - the flat bottom canoe body, excessive mid section coefficient

    - favoring too much rocker in the aft run

    - a rig completely lacking automatic gust response

    - a sailplan with insufficient power control possibilities

    None of these features, I feel, have really taken yacht design forward... but are rather, in my opinion, the reason for the demise of offshore racing. With today's breed of boats, people only want to sail round the buoys, windward-leeward, never overnight in open waters.

    Motion comfort & ability to cope with louzy weather is so bad in modern boats that people don't want to race them offshore - or how else do you explain the nearly 50% rate of retirement during the first 24 hours of the 2007 Fastnet race, for instance. The weather report was bad, yes, and so was the weather itself, but in the IOR times boats (people?) coped much better with this kind of weather. Fastnet is raced under IRC, but today's boats carry their inheritage to the IMS. IRC is more permissive towards seaworthiness, but things change slowly.

    I'm sorry for the negative attitude in my reply, it is not meant to be, pls. prove me I'm wrong. I also apologise for wandering off of the subject, sail aerodynamics.
     

  15. yipster
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    yipster designer

    i too am wondering about these flows, as a rig heels how much loss it makes, mast rake, and what if when battens or a top boom would twist more windward
     
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