Aspect ratio on headsail vs shroud angle?

Discussion in 'Hydrodynamics and Aerodynamics' started by sailingdaniel, Aug 18, 2011.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    Missed the attachment,
     

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  2. farjoe
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    farjoe Senior Member

    I have often wondered why a mail sail with that shape is faster than a reefed mail which can then be properly set. I just don't understand it.

    Yet most racers seem to prefer racing with a distorted sail so it must be faster.
     
  3. viking north
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    viking north VINLAND

    The art -magic-engineering tradeoffs of building a good hull and rigging is all lost if you don't have that crewperson on board to do magic with those sails. I consider myself not a bad sailer but I have several friends who put me to shame on sail control. Then again i'm a lazy non competitive sailer when compared to these constant adjust this , tweak that, masters of canvas control. Don't know if I have the personality for it but I promised myself to improve and as such have been studying an excellent book--Sail Power by Wallace Ross-- Be a great upset to win the Cup in a Motorsailer :)

    A yacht is not defined by the vessel but by the care and love of her owner--
     
  4. brian eiland
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    brian eiland Senior Member

    Case Made for Fractional Rigs

    Dear Mikko,
    That presentation you made in postings #74, #75 was one of the better I've seen for the positive case for fractional rigs.
     
  5. tspeer
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    tspeer Senior Member

    Articulating Bowsprit

    Here is the design for the bowsprit fitting on my 34' trimaran. The stemhead fitting had existing holes for a horizontal pin. I designed a tetrahedral shaped piece that was pinned horizontally to the stemhead and vertically to the sprit. The sprit is a piece cut from a broken carbon mast that was lying in the boatyard. The tetrahedron is composed of four triangular carbon plates bonded to two bronze tubes and wrapped with carbon cloth. See attached drawings.

    This toggle eliminated any vertical or horizontal moment being transmitted to the stemhead fitting. The sprit is restrained by a bob stay that go from sprit to shackle at the cutwater, back to the sprit, and to a clutch on the steamhead. Whisker stays go from the end of the sprit to padeyes on the ama bows and back to padeyes on the inboard portions of the forward beams. An uphaul goes from the intersection of pulpit stanchion and rail, to a shackle on the end of the sprit, and back to the intersection of stanchion and rail on the opposite side of the pulpit. This is a stronger arrangement than a single line from the end of the pulpit, and won't bend the pulpit down if there's a sizable download on the sprit. The uphaul ends in a block with a 2:1 purchase to a cam cleat.

    Instead of the ridge shown on my original drawings, there is now an eye laminated to the top and bottom of the sprit. Each line to the sprit has a loop spliced into it that goes through an eye and around the sprit. The only function of the eyes are to prevent the loops from sliding down the sprit. All the loads are transmitted to to the sprit by the loops wrapping around it.

    The design has worked out well. It is easy to retract the sprit by releasing the clutch on the bob stay and pulling in on the uphaul to pull it up to the pulpit. Although I haven't implemented the articulation, yet, the sprit will go roughly 30 deg to either side. Right now I just have the whisker stays lashed to the padeyes on the beams. But all I need to do to have a fully articulating sprit would be to replace the lashings with tackles and cleats.

    Originally, I had shock cord incorporated into the uphaul so that the sprit would give in the event of, say, a port-starboard collision. However, this proved to be a bad idea when we shrimped the spinnaker and the shock cord allowed the sprit to drop into the water and fold backward in a hammer lock. This exceeded the limits of articulation of the toggle and ripped the carbon cloth holding it together. So now the uphaul is much more robust and able to handle large downloads.

    The length of the sprit was based on the sailplan for the boat, but it's on the short side. I sometimes wish I'd gone with a longer sprit. But there's still plenty more broken mast in the boatyard!
     

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  6. tspeer
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    tspeer Senior Member

    Don't forget RRS 54, "Forestays and headsail tacks, except those of spinnaker staysails when the boat is not closehauled, shall be attached approximately on a boat's centerline."

    A movable jib tack is OK on a cruiser, but you can't move the tack in a race.

    The same restriction would apply to a screecher used to go upwind in light air. Even if tacked to an articulating bowsprit, the articulation couldn't be used when going to windward.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    The RRS can be modified by class rules ,right? Thanks for your info, Tom.
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    Tom,

    What has been your experience in boat performance with your articulating bowsprit? Have you seen improved pointing abitility and/or boat speed as a result?

    Eric
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    I can give you 3 reasons for that:

    1) The drag of a bare mast is much worse that a mast with a sail behind it. This is the main reason why some Open 60 boats choose a wingmast, although the verdict is still out whether a wingmast is better than a conventional one for them.

    2) With that in mind, it is worth while to try to keep the main up as long as possible. Even if the sail is inverted 50% of the time, it may be useful during the other 50%. Works much better with a fractional rig than a masthead/modern IMS rig.

    3) Downwind you want all sail area , so whether to reef or not also depends on the length of the upwind leg: In an offshore race it usually pays to reef early, while in a round the buoys race you want to try to go on with full hoist. You always loose something while reefing/unreefing, and the increase in wind may be only a passing gust.
     

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  10. Mikko Brummer
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    Mikko Brummer Senior Member

    In this video clip of the X-35 main you are on the limit of reefing or not. You can see how the main fills in/fills out in every wave, providing substancial righting moment with the head filling out. In this respect the fractional rig works nicer, as the lower part of the main always remains effective even if the head is backwinding. With X+35 rig the main tends to "flag" more globally when overpowered. (7,1 MB - a big upload)
     

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  11. farjoe
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    farjoe Senior Member

    I was glad to read that there is actually some explanation to an over powered main although looking at the video i am not sure that this is less draggy than the bare part of the mast of a reefed sail. Too my eyes only the lower third of the main in the video is providing forward drive.
     
  12. DennisRB
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    DennisRB Senior Member

    My guess is it has something to do with the main preventing this from happening

    [​IMG]
     
  13. brian eiland
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    brian eiland Senior Member

    That is a round mast section which is the worst...but your point well taken.
     
  14. brian eiland
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    brian eiland Senior Member

    Masthead vs Fractional for Cruising Vessels

    Just happened across this older posting I had made:

    Aspect Ratio (AR) of Rigs on Cruising Vessels
    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-12.html#post93500


    Shorter Rigs, Lower Aspect Ratios, posting #367
    http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-25.html#post420889


    ....and this
    Fractional Rigs on Class Boats
    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-12.html#post94004
    "Or in other words, on many boats the capability to utilize the bendy characteristics of the mast to reshape the mainsail for a variety of conditions does not lend itself to the masthead configuration. Many racing classes depend upon this mast shaping feature and thus utilize fractional rigs. Or in the case of multihulls with rotating mast, the fractional jib is practically a necessity.

    But this should not be taken as an across the spectrum endorsement of the fractional rig, nor a condemnation of the masthead rig from a purely aerodynamic standpoint."
     
    Last edited: Sep 8, 2011

  15. brian eiland
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    brian eiland Senior Member

    Movable Jib/Genoa Tack

    There have been several references to movable jib/genoa tacks on this subject thread, and in several instances an indication that a vessel might be able to point up a little higher if that tack position could be moved a little bit to leeward. I believe this is a very real possibility as well, even though I can not specifically remember experimenting with it.

    In contrast to a leeward move of the tack position, how about the windward moves of the tacks on this A-frame masted 63' catamaran 'cutter' I just became aware of, and posted the info over HERE:
    http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-12.html#post485560

    Real interesting full size experiment!
     
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