Draft in the Junk Sail

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  • 01 Jun 2024 18:59
    Reply # 13364557 on 13317633

    Based on the simulation picture it also appears that maximum draft (or what the depth of the sail is called) is moved further aft. Might be a factor?

  • 31 May 2024 14:04
    Reply # 13364079 on 13317633

    Today I crawled to the foredeck to take some fotos of the mast on the leeside, being burried into the cambered sail. May camber is rather high, atv9%, but one can see the forward part of the sail also forming a bit of camber. From those views, I can imagine that the mast drag is negligible, compared to the fully exposed mast on the other tack.

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  • 05 Mar 2024 16:12
    Reply # 13324957 on 13317633

    Ti Gitu also sails best when the sail is pressed against the mast. We first noticed this in the Med and had a chance to test this when behind an island in the Caribbean in perfectly flat water and a nice steady f 3 to 4 wind. The only explanation I could think of (as already mentioned) is that when the mast is buried in the sail the drag of the mast is lessened.

    We are currently fitting new cambered panel sails made as individual panels between each batten. Not sailed yet but just hoisting them in the marina looks good. Will write it all up later in the year.


  • 03 Mar 2024 16:17
    Reply # 13323941 on 13317633

    By the way, it is interesting that AD on his SJR also notices a similar phenomenon. This would probably lead to the conclusion that e.g. D-formers in Origami (and possibly SJR) rigs should not have a wide transverse gap allowing the battens to be moved to the leeward (i.e. unfavorable) side, e.t.c.

    Last modified: 03 Mar 2024 21:28 | Anonymous member
  • 03 Mar 2024 02:24
    Reply # 13323793 on 13322983
    John wrote:

    I have a cambered junkrig sail on my boat. My expectation was that the sail should sail better with wind from the mast side, i.e. with the sail in good foil shape instead of pressing against the mast. In actual performance, I was suprised to find the reverse result. The boat sail better with the sail pressing against the mast! I played with simulation software trying to find out the effect of the distorted sail when pressed against the mast. It showed that there isn't much damage on the laminate flow. (see image). My guess was that the transmission of propulsion force is more "solid" with the battens pressing against the mast, instead of transmission through the parrels on the reverse side.

    It's a well known phenomena and it occurs with all sails that lean against a mast on one tack and away from the mast on the other. So junks, balanced and standing lug rigs all experience it but dipping lugs don't (they  "dip" the sail for each tack).

    The best explanation for it that I've come across is that on the tack where the mast is exposed, the high parasitic drag of the mast is in full effect and you pay the price. On the other tack, the mast is buried in the sail, inside the lammar flow and the "air" does not "see" it, so it's drag is much reduced.

    If one wants to reduce the effect the answer is to go for a skinny carbon fiber mast. There is still an effect but the smaller you can make the mast section, the less effect the mast will have. I have seen this when I have replaced wooden masts with steel ones or carbon ones. Both tacks always improve but for the one where the mast is clear of the sail, the improvement is most marked.

    Last modified: 03 Mar 2024 02:25 | Anonymous member
  • 02 Mar 2024 23:54
    Reply # 13323772 on 13322983
    Anonymous member (Administrator)
    John wrote:

    I have a cambered junkrig sail on my boat. My expectation was that the sail should sail better with wind from the mast side, i.e. with the sail in good foil shape instead of pressing against the mast. In actual performance, I was suprised to find the reverse result. The boat sail better with the sail pressing against the mast! I played with simulation software trying to find out the effect of the distorted sail when pressed against the mast. It showed that there isn't much damage on the laminate flow. (see image). My guess was that the transmission of propulsion force is more "solid" with the battens pressing against the mast, instead of transmission through the parrels on the reverse side.


    John,

    thanks for presenting those diagrams. They confirm my conclusions after studying the phenomenon on my sails.
    I then wonder if you could simulate the same sail with the angle of attack at 10, 15 and 20°.
    It would also be useful to know what happens if you set the mast 15, 20, 25 and 30% of the batten length (B) aft from the luff.

    In the early days I clung to 10-15% mast balance rule, but later, inspired by Paul Thompson, I have moved to over 20% balance. I would therefore be useful to see what happens if this mast balance is adjusted up to 30.

    Here is what I found about the good and bad tack, some years ago.
    http://goo.gl/khii3l

    Cheers,
    Arne

  • 01 Mar 2024 21:44
    Reply # 13323423 on 13317633
    Anonymous member (Administrator)

    Bonjour

    On the John's images we can see that the air flow remains stucked to the sail longer on the "bad tack". It is sometihing welle know in aérodinamic. The turbulent flow has less a tendency to leave the extrado surface.

    It seems that the images are representative of "solid" sail with a very small incidence angle. If it's true it is not representative of a junk sail headwind. the incidence would be rather about 10°.

    On some aircrafts, spoilers (small asperitiies on the surface) are added to the surface to prevent the flow to leave the surface.

    So, in the exemple presented by John, the turbulences due to the mast in the airflow maintains the flow attached to the surface.

    An other explanation, especialy whith flats sails, may be the turbulences around the mast create a virtual camber of the airflow. So with flat sails, the bad tack becomes the better one but with cambered sails the effect should be less evident.

    Eric

    Last modified: 01 Mar 2024 21:48 | Anonymous member (Administrator)
  • 01 Mar 2024 17:51
    Reply # 13323270 on 13323239
    Anonymous wrote:

    Very informative experiments John! Keep it up !

    So this is also confirmed (by AD) with the SJR rig?!

    Affirmative.

    AD

  • 01 Mar 2024 17:03
    Reply # 13323239 on 13317633

    Very informative experiments John! Keep it up !

    So this is also confirmed (by AD) with the SJR rig?!

    Last modified: 01 Mar 2024 17:27 | Anonymous member
  • 01 Mar 2024 10:03
    Reply # 13323002 on 13322983
    Anonymous wrote:

    I have a cambered junkrig sail on my boat. My expectation was that the sail should sail better with wind from the mast side, i.e. with the sail in good foil shape instead of pressing against the mast. In actual performance, I was suprised to find the reverse result. The boat sail better with the sail pressing against the mast! I played with simulation software trying to find out the effect of the distorted sail when pressed against the mast. It showed that there isn't much damage on the laminate flow. (see image). My guess was that the transmission of propulsion force is more "solid" with the battens pressing against the mast, instead of transmission through the parrels on the reverse side.

    This paradox is well known and my own experience confirms that junk sails perform better with the mast to leeward. I believe the reason to be that the windage of the mast itself is significant when it stands fully exposed to windward of the sail.
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