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Junk Rig Glossary -updated and expanded, and an invitation

  • 12 Mar 2015 09:51
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    Reply # 3249129 on 3171528
    Anonymous member (Administrator)

    Slot effect or whatever...

    I have sailed quite a lot in Bermuda-rigged sloops with partial rigs, where the jibs were small and not overlapping. The effect of setting the jibs (Folkboat, Knarr, Oslo Dinghy) was so positive that my hunch is that the added performance stemmed from more than just adding sail area or improving balance. There appeared to be some sort of positive interaction between the sails.

    Just a hunch.

    Arne

     
  • 12 Mar 2015 04:29
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    Reply # 3248099 on 3171528
    Deleted user

    Unfortunately, without overlap of the sails a Junk Rig won't see benefit from the slot effect.  I don't know of a reference that talks about the degree of overlap that must be necessary, but even a genoa and main must be sheeted properly to see the benefit of the slot effect.  By the time you get to a Junk Schooner or Ketch, the sails are sufficiently far apart that they shouldn't benefit from the slot effect.  In fact, the aft sail will be in the "downwash" of the foresail (thus giving the aft sail a header) and it will also have more turbulent air spilled onto it by the foresail.  Both effects are going to make the aft sail less efficient.  I suspect Junk Rigs would benefit most by getting the masts as far apart as possible or the often impractical solution of a single large sail.  Of course, all this only applies going to windward close-hauled.  
  • 11 Mar 2015 23:01
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    Reply # 3247899 on 3171528
    Anonymous member

    Hi all,

    Thanks so much for all the explanation – and to Annie, who gets credit both for this round of glossary project activity AND for proofreading, when the rest of us are all bleary-eyed when it comes to looking at glossary definitions!

    Fascinating about the two-sails question, as far as aerodynamic benefits. If the headsail has to be overlapping, does this mean that Bermudan rigs using a 100% jib do not have the same slot-effect advantage? Not that we are here to talk about Bermudan rigs, but I'm still stuck on this question of possible advantages of schooner or ketch junk rigs… I get it about the overlap issue, but still wonder if there's something useful going on there, even when there's no actual overlap, which might be demonstrated (or not) by Bermudan sloops with 100% jibs…

    This is beyond the scope of the glossary question, rather along the lines of "inquiring minds want to know" – but if anybody would like to chime in it would be a treat!

    Many thanks,
    Shemaya
  • 06 Mar 2015 01:53
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    Reply # 3243368 on 3171528
    Anonymous member
    Well, guys, you all know a hell of a lot more than I do about all these things.  My only claim to credit is that it was my proofreading that got the issue going again.  Thanks for responding to help out the Glossary team who have done (and are still doing) a fantastic job of this time-consuming and, necessarily, much-researched project.
  • 04 Mar 2015 04:28
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    Reply # 3241470 on 3171528
    Deleted user

    I agree with Arne, for sails.  I suppose there might one day be a souped-up Junk racing rig with a polished hull that manages laminar flow over much of the hull, but as far as sails are concerned attached and detached are more useful terms.

    Darren

    (Also not an aerodynamicist)
  • 03 Mar 2015 10:46
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    Reply # 3240626 on 3171528
    Anonymous member (Administrator)

    Laminar flow or attached flow?

    It appears to me that the term laminar flow  often is used when the airflow is turbulent but attached, that is, anything but laminar. Laminar flow is a rare condition, even on aeroplanes. To get laminar flow, the wing must be super smooth (polished, without raindrops or dust), and even then, the airflow will soon gradually turn turbulent and the boundary layer of slowly-moving air will build up thicker and thicker as the airstream passes from the leading to the trailing edge.

    In un-even aerofoils like our junk sails, we can just as well forget about laminar flow, and rather use the term attached flow (meaning attached and turbulent).

    Arne

    PS: I surely am not an aero-dynamist, but this is how I understand what I have read about this matter. I could be all wrong...

     

    Last modified: 12 Mar 2015 10:17 | Anonymous member (Administrator)
  • 03 Mar 2015 04:22
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    Reply # 3240070 on 3171528
    Anonymous member

    Hi All,

    Thanks very much for setting me straight (or at least giving me the opportunity to wrap my head around all this!).

    I suppose, to be honest, I've kind of dozed through that little aerodynamic theory to which I've been exposed. Skimming text and superimposing my misbegotten mental picture over those much better informed. Bad habits!

    As Shemaya says, I very much appreciate the amount I've learned in helping to put the JRG together. It truly is a great perk of the job.

    Meanwhile, the entire 'aerodynamic' cluster of definitions is in need of overhaul... please continue to point out flaws in my understanding. I'm not at present able to give it the attention it deserves, so it will be a while before they begin to reflect your inputs. But at least I'm now aware that there ARE issues, and have your words and references before me.

    Off the top of my head, I'm thinking the following terms should be corrected or added:

    Aerofoil, Foil, Upwash, Downwash, Slot Effect, Bootstrap Effect, Stall, Laminar Flow, Turbulence, Airflow, Lift, Drag, Angle of Incidence/Attack. Others?

    If you see problems with any of these, or would like to propose definitions for the meantime, please post them here.

    Thanks again!

    Dave Z

    Last modified: 03 Mar 2015 04:43 | Anonymous member
  • 03 Mar 2015 01:47
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    Reply # 3239645 on 3171528
    Deleted user

    I agree with what Chris has said.  To answer directly Dave, I think I have things correct (other then I've spelled upwash upawash).  The mainsail causes more air to flow on the windward side of the foresail which increases lift, but the mainsail also causes less air to flow between the main and foresail than would flow under the foresail alone.  This causes lower pressure, but more importantly higher velocity.  The Kutta condition states that the airflows above and the below the wing match velocity at the trailing edge.  Thus the mainsail allowing higher velocity flow on the windward side of the jib means that the flow on the upper side (leeward) of the jib does not decelerate to match the flow under the jib and that more airflow (higher velocity) over the top of the jib makes more lift.

    Marchaj and Gentry are a much better place to learn about these things compared to many other sources which rely on older theories which are incorrect in whole or in part.  It is ironic that we are putting a bunch of thought into this, since as Chris points out we can't really benefit from the effect with most Junk platforms (with the exception of the Split Junk Rig, which probably sees some benefit).  However, it is cool and part of an understanding of aerodynamics which when applied to Junk rigs can make them better.
  • 03 Mar 2015 00:23
    Quote
    Reply # 3239567 on 3171528
    Deleted user

    According to Marchaj the overlap is essential, and optimal - when close-hauled - at around 0.65 of J (the base of the fore triangle).

    The downwash from the jib reduces the large suction peak which appears just behind the luff on the leeward side of the main when the jib is not present. Although this peak is responsible for a lot of the mainsail's drive, the rapid change in suction and lift along the leeward side of the main sail makes the flow prone to separation with associated turbulence and drag, and therefore makes the sail prone to stalling. While reducing the total lift due to the main by reducing this peak, the presence of the jib  also reduces the sensitivity of the main to stalling by improving overall flow around the leeward side of the main sail.

    None of this is directly relevant to our junk rigs – we don’t have the option of an overlapping foresail. But it does explain what we observe – that the Bermudan rig is a ‘going-to-windward’ machine. Once the wind moves aft of the beam and the incidence angles of the jib and main move away from the optimal, all this advantage is lost. The pointy rig rapidly loses out to the flat-tops, and the donkey comes into play.

    It also explains why Tom Colvin didn’t get the advantage he hoped for by putting a jib on his junk-rigged Gazelle. No overlap and no mutual advantage for the sails. He dispensed with them on later junk designs such as Kung Fu Tse.
  • 02 Mar 2015 21:15
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    Reply # 3239460 on 3239260
    Deleted user
    Shemaya Laurel wrote:

    My biggest question is: does the foresail have to be a jib? Or will a schooner or ketch-rigged junk have the same kind of dynamic going on, if the sails are close enough?

    I think the important thing about the jib and this effect is the overlap - therefore it doesn't apply to main and foresail or main and mizzen. Paul Bogataj explains it as follows:

    (See diagram in Technical illustrations)

    "The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The observed affect commonly referred to as "backwinding" is partially a result of downwash from the foresail, but is also due to the higher pressure on the windward side of the genoa being very close to the forward, leeward side of the mainsail, causing the flexible material of the mainsail to move away from that higher pressure.

    "The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of  force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses."

    This is also why the main must be sheeted more closely than the jib.

    Last modified: 02 Mar 2015 21:36 | Deleted user
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