Aerodynamics of 'overlapped' staysails

Discussion in 'Hydrodynamics and Aerodynamics' started by Revboat, Jun 23, 2021.

  1. philSweet
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    philSweet Senior Member

    Another effect is to make the quarter chord lines (or whatever cord line is applicable here) of the staysail/mainsail combo and the A-sail more parallel. This means that the circulation boost of the staysail/mainsail will be in the same plane as the self-induced circulation of the A-sail. If I'm not mistaken, that maximizes the effective aspect ratio of the overall combo and minimizes induced drag.
     
  2. Rumars
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    Rumars Senior Member

    Yes, something like this. The key is the separation between sails, and the specific sheeting angle. Seperation has to do with boat length, the longer the boat the more space there is between the individual sails. I believe a class 40 is to short to fly 3 sails without interference (it would need a truely long bowsprit), while over 60' there might be enough space at certain apparent wind angles. On every photo of three foresails flying the separation between the sail on the bowsprit end and the jib is bigger then the one between jib and staysail, and that is not by accident, it translates to horizontal separation at the clews, wich means the inner sails are not "burried". Important is not what we see, but what the apparent wind sees.

    There are also practical reasons for using this approach, mainly overall sail area and sheeting geometry. If you use a big overlapping first foresail, like a 150% genoa, you effectively kill much of the mainsail drive, as observed in the IOR era. That was no problem then with their tiny mains and non planing boats, but it would be a problem now. Any sail you put forward of this 150% genoa must be smaller, you end up with less overall area, and a not really working main. Given how fast this boats are, in order to sheet the more forward sail correctly for the apparent wind, the sheet must pass through the previous sail.
    Draw the sheeting angles of such a configuration when the boat is traveling at over 20kn and you will see what I mean. It worked for the ancient cutters and square riggers, but those were never that fast.

    What is important is the end result expressed in knots, not the ideal working efficiency of the sail. The ideal is the rigid wingsail that can change shape and reef, but we are not there. What we do is pile on more sail area, even if we loose some efficiency on some sail combinations, the net result is positive, giving us more speed. This is just like the tradeoff on pointing, or downwind gybing, you sail a longer distance, but you are faster, and arrive sooner overall.
     
  3. Revboat
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    Revboat Junior Member

    Sorry, not quite following...

    Do you mean all the sails should run in parallel... which I don't think makes sense because the point of multiple elements foils is to allow the aft edge to achieve a narrower angle to the wind than otherwise.

    Or, as I suspect, do you mean to make the quarter chord of all the sails parallel but to tune the aft edge of each to progressively closer angles?

    Or do you just mean that the staysail/mainsail combo should have parallel quarter chords because they are acting as one element?
     
  4. Revboat
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    Revboat Junior Member

    Yes, I see the point. If the clew of the A-sail is far enough apart from the staysails, then one does not need to worry about interference. In that instance, it's just about adding sail area for drive. Or, maybe the inner sails work with the main as a multi-element foil, but the big headsail sees them as one unit and does not interact with any of them separately.
     
  5. Revboat
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    Revboat Junior Member

    I accept this explanation completely. Very helpful. Your sketch passes my 'intuition' test for what that's worth (probably not much!).

    But imagine your sketch with the staysail line drawn only 3/5 as long. Does that make a difference? Or, does it make a difference if the staysail line is 1/5 longer or positioned with its tack a bit closer to the main?
     
  6. Remmlinger
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    Remmlinger engineer

    If the gap between foresail and main is small, the genoa will create a positive pressure at the rear side of the main. If the gap is large, then the distance between the two sails is too large for a strong interaction. In this case, an additional staysail can create the support for the rear side of the main. The further the staysail extends towards the leech of the main, the more positive pressure it creates on the main. However, at the same time the staysail will create additional drag, so there is a trade off. To find the optimum overlap is only possible by trial and error, either in the wind-tunnel or with CFD.

    Arvel Gentry shows in fig.14 the pressure distribution on the main with and without the jib. With the jib, the negative pressure spike at the main is dramatically reduced and the pressure gradient towards the trailing edge is much milder. But keep in mind, that Gentry shows the flow pattern for potential, inviscid flow. In reality, there is always some kind of separation. http://www.remmlinger.com/2D aerodynamics.pdf
     

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  7. philSweet
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    philSweet Senior Member

    Revboat - I meant the second one. The idea is to get the interaction of the A-sail and the Staysail/mainsail to be as constructive as possible. That is helped by having the spanwise line connecting the quarter-chord points of the two foils systems be parallel.
     
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  8. Revboat
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    Revboat Junior Member

    Yes, that's what I figured. Thank you. This makes intuitive sense to me. Do you happen to know of any articles or resources that discuss this idea of getting the quarter-chord of the sails interacting with each other in parallel (in conjunction with the trailing edges nonparallel)? It makes me think of the potential value of vertically parallel luff lines (as opposed to converging luff lines, as on boats with multiple masthead sails). Also a number of things to do with leech tuning.
     
  9. Revboat
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    Revboat Junior Member

    Thank you, Remmlinger. And thanks for posting all the material on your website. Took me a few days to get through it!
     
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  10. Remmlinger
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    Remmlinger engineer

    And thank YOU for posting this interesting and challenging question about staysails. Such questions keep the forum alive and interesting.
     
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