Sail Angle to the Wind

Discussion in 'Multihulls' started by Fanie, Oct 17, 2018.

  1. Fanie
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    Fanie Fanie

    Hi Guys and girls,
    According to this page
    Best Angle to the Wind http://www.nauticed.org/freesailingcourse-m1-34
    the best angle to wind is 30 degrees
    upload_2018-10-17_19-39-27.jpeg

    It says the best angle achieved is 30 degrees to wind with a wing ?

    I have made a model for the aft mast sail I made for the little tri, but I fitted it on a frame to simulate a catamaran to test the sail with, and one sail on each hull.

    Remember that Manie said he has never seen anything sail to wind the one day we went out ? Unless there is something wrong with my protractor, my sail is already working at about 20 degrees off wind where it begins to make power. The boat will swim when the wind drag on the boat is overcome by the power off the sail.

    The weekend the wife and I took it out to the Vaal, we had a stiff wind blowing off the sluices, and sailing towards it I got the impression that I could sail directly into the wind.

    The experience I had when sailing more and more to wind is that it just loses power as the angle to wind becomes smaller, but even close to wind I could still sail ahead.
     
  2. CT249
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    CT249 Senior Member

    It's generally accepted that about 34-40 degrees to the true wind is as close as anything can sail with efficiency, as far as I can recall. With respect, people have been making sails for centuries and none of them could sail directly into the wind. The chances that your sail achieved it are similar to the chances of me picking up some spare parts and building something that could win the F1 world championships. Your estimate of wind direction must have been incorrect, or you were pinching, or your terminology is hard to understand - for instance I've never heard of a boat "swimming" in the way you appear to be using the term.

    I'd be cautious about that website and what it says about sailing. It was apparently written by someone who hadn't sailed a wide range of craft, and also apparently had never even looked at a photo of a popular cat like a Hobie or a 1950s Shearwater, a modern windsurfer, a skiff, a Moth and the many other boats that use full battens. This is apparent from the claim "Sails, unlike wings, get their shape from the wind pressure against the fabric. After the shape is filled, then and only then does the lifting effect come to bear. Thus some efficiency of the shape is lost in creating the shape itself and so sails can't fly as close to the wind as a wing. About the best angle off the wind that most sailboats can achieve is 30 degrees. Any less than this and the wing shape of the sail begins to deflate."

    It's complete rubbish to say that sails only get their shape from wind pressure against the fabric and that they "deflate" at higher angles when some of the world's most popular craft prove that it's not so. If the writer cannot get such a basic point right it's a bit like someone writing about driving and saying all cars use rear wheel drive - it indicates they don't know their subject.
     
    Last edited: Oct 17, 2018
  3. rwatson
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    rwatson Senior Member

    Oh really ? Perhaps you could post a few pictures of self inflating sails ?
     
  4. Fanie
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    Fanie Fanie

    It's called a rubber duck :D

    Let me show you. This is the wind tunnel, a big fan I have.
    upload_2018-10-18_13-20-35.jpeg

    The wind is not the same as on the water because it tabulates, never the less creates some wind to see how the sails behave. The sails I made are relative heavy at the scale and reacts a bit like a heavy tarp, so they are not as sensitive as in real life.
     
    Last edited: Oct 18, 2018
  5. Fanie
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    Fanie Fanie

    This is the sails at 20 deg off the wind
    upload_2018-10-18_13-25-57.jpeg

    Both the sails make power, but the one on the left is trimmed in. The luff is canted directly towards the wind. The stronger the wind, the more power it makes (of course) but at this angle it wants some wind to sail. If there is only a breeze the angle to wind has to be increased.

    upload_2018-10-18_13-30-19.jpeg

    Picture taken directly into the wind at 20 deg.
     
  6. CT249
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    CT249 Senior Member

    The vast majority of fully battened sails are "inflated" in the sense used in that piece - that is, they have draft/camber even without wind pressure. Surely that's obvious?

    Here's some examples; in the top case the area around boom level shows it most strongly;

    [​IMG]

    [​IMG]

    As is perfectly normal with fully battened sails, they do NOT always "get their shape from the wind pressure against the fabric" and they they don't "deflate" (certainly not in light winds) as claimed in the piece that was quoted. These sails have shape even when there is no wind, because of the tension of the battens, luff curve and sometimes seam taper.

    You can particularly feel this when you use a windsurfer with a camber induced or RAF sail, which use batten tension to maintain sail draft in light winds even when completely feathered. Despite this rigid shape, such sails are not noticeably closer winded as a whole than "soft" sails.

    The idea that all sails get their shape from wind pressure is very obviously and very simply very wrong, as many good fully battened sails show. To have ignored the sails used on such common and significant boats as Hobies and Moths shows that the author of the piece was simply very wrong.
     
  7. Fanie
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    Fanie Fanie

    There are two sails, making double the power.
    The model is scaled roughly for an 8m catamaran at 6m wide and mast height is 10m of which thumb suck 1m will sit in the hull.
    Each sail area will be 34 sq meter making a total of 68m. If the mast is taller then the sail area will increase, of course.
    And yes, a "spinnaker" is also provided for. It adds about 54 sq meter to the model's sail dimensions.

    upload_2018-10-18_14-6-6.jpeg

    Sails at about 35 degrees off wind, and this should be where the two sails will perform best

    upload_2018-10-18_14-8-2.jpeg

    The view to wind
     
    Last edited: Oct 18, 2018
  8. CT249
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    CT249 Senior Member

    Sails can make some power at higher angles, but the point is whether they make efficient power, and in particular enough power to overcome sail and rig drag. In one class I know, for example, you can point very high (maybe 30 from true wind?) when creeping into a start line, but only by going very slowly. The sail's true effective angle of attack to the true wind is more like 45 degrees.

    It's very hard to see how such low aspect sails will work well as close as 35 degrees. Whatever their qualities, high pointing is not among them.
     
  9. Fanie
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    Fanie Fanie

    One at about 45 deg to wind
    upload_2018-10-18_14-10-27.jpeg

    As you can see bot sails still give full performance
     
  10. CT249
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    CT249 Senior Member

    [/ATTACH]

    As you can see bot sails still give full performance[/QUOTE]\

    No, I can't. How can you claim that it's apparent? Where are your lift/drag measurements? What do you define as "full"?

    Are the two sails going to make double the power? Compared to what? What about interference between the two sails, upwash etc?
     
  11. Fanie
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    Fanie Fanie

    Over 90 deg it becomes interesting, This is roughly at 135 degrees
    upload_2018-10-18_14-18-39.jpeg

    or using the spinnaker with the windward sail furled
    upload_2018-10-18_14-19-38.jpeg
     

    Attached Files:

  12. Fanie
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    Fanie Fanie

    Running all three sails used for optimal speed

    upload_2018-10-18_14-21-39.jpeg
     
  13. Fanie
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    Fanie Fanie

    You won't see anything because you see a flat earth. I'm trying to indicate a principle. You're asking how long is a string, like what is the wind speed, and the boat speed and I cannot tell you that. All I'm saying is there is something to gain from my little experience and it was confirmed by someone else. Besides, the smaler the to wind angle and the faster you sail the more power the sails will make and the less effect apparent wind will have. At 0 degrees there is no apparent wind.

    Besides, I think you're missing the whole point. The normal sails o boats don't make lift, that is why they pitch pole.

    The example I show hardly has any heel, and the sails do lift, that is the force is up, not down into the water.
     
    Last edited: Oct 18, 2018
  14. Fanie
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    Fanie Fanie

    Running, if the spinnaker head is let out some the sail makes less upward lift when it becomes more vertical, making more forward force and less lift.
    upload_2018-10-18_15-24-35.jpeg
     

    Attached Files:


  15. Fanie
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    Fanie Fanie

    Basic advantages -
    Relative short masts (if desired) with a good center of wind force on full sails.
    Stand alone, no rigging.
    Furling in and out is quick.
    Each sail use only two lines.
    Easily adjusted smaller as storm sails or for reduced speed.
    Sails must be flat. Especially tight luff and foot.
    The spinnaker can as fast be furled in or out.
    The spinnaker use three lines, but can be used as two.
    If made like a normal sail it can be used in higher winds.
    The main sails force components are up and forward, attempting to lift the boat.
    So does the spinnaker if trimmed against the luff angle of the main sails.
    Close to wind (or apparent wind :rolleyes:) sailing
    Very little heeling, even on a beam reach
    Tacking is almost effortless (especially when running CT249 ;))

    Of course, all of this work only if there is wind.
     
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