Aspect ratio on headsail vs shroud angle?

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

  1. philSweet
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    philSweet Senior Member

    a bit off topic but regarding Eric's comment about a jib with a sliding tack, this is a wonderful thing indeed. If a boat carries her dink on the bow, rigging a temporary babystay probably won't work in the usual manner. I run a bridal from toe rail to toerail over the dink and set a small sail flying. It is a boom sail with the sheet tackle run to the base of the mast. I fly it quite high and found I couldn't sheet it in enough or control the twist. Then I redid it so the tack could travel a bit on the bridal and viola, I can set it and forget it in pretty much anything, and she pulls like a mule. A key factor in this setup is being able set the jib flatter than the main which is a necessary heavy weather tactic on fin keel boats, it lowers the helm effort and saves battery power. This is how I get a three hour nap before the storm really clobbers me. With the tack fixed to the center, I would have to fall off another 15-20 degrees to get the main out as far as the jib. Say an 80 degree course instead of a 65 degree course to the wind. On a proper cruiser, none of this nonsense is necessary. At least not in a 40 knot wind.
     
  2. Stumble
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    Stumble Senior Member

    I keep thinking about converting a mainsheet track and mounting it perpendicular to the boat. That way you could control the exact placement of the headstay on each tack relatively easily. I don't think that most traveler tracks could handle the pulling load that a sizable forester would exert however.

    If you wanted to get really fancy you could install a 2d track that would also allow you to control headstay sag at the same time..... God I hate it when I get these ideas. Now I need to go buy a laser to butcher and try it out on.
     
  3. viking north
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    viking north VINLAND

    Thats why I love motorsailers--getting to dam old to be fussin around with more rigging--:)
     
  4. Doug Lord
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    Doug Lord Flight Ready

    moving jib tack to leeward

    I thought about moving the jib tack but never did any experiments at all.
    However, I did make a bit of a "breakthru" in the East Coast 12 RC model when I devised a way to slide the jib pivot to leeward allowing the forestay to intersect the boat cl. Made a significant improvement in upwind sailing compared to other EC12 models that were sailed with the jib pivot on the centerline.
    The pivot was located on a jib boom about 30%(of the jib foot) aft of the luff and most models consequently have the jib luff to windward of the centerline.
    Came up with this(below) system to be used with a jib boom and a molded in track
    across the front deck to give some of the advantages of the model arrangement to a tall narrow square top jib-rough sketches:

    click on image for detail--
     

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  5. philSweet
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    philSweet Senior Member

    Stumble, please tell us you'll revisit this issue before trying it on anything much bigger than a laser. Moving the headstay downwind will do horrible things to the span loadings on the mast of a conventionally stayed boat. However, if designed in from the start, no reason that its effect on the mast and main couldn't be accommodated. Or get an old Cal 28. Those things had a mast like a cannon barrel.
     
  6. Paul B

    Paul B Previous Member

    Considering Lasers don't have forestays or headsails your buying a Laser probably won't help you much.

    Tacking headstays have been tried. If anyone believes you will gain a degree in point for each degree you drop the headstay off CL, well I have a bridge to sell you.

    My boat was originally built with a tacking headstay. It doesn't have one now. When it had one it did not gain in pointing. That wasn't the purpose.
     
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  7. Paul B

    Paul B Previous Member

    Wow, that is some post. I assume some of it is in response to my post above, so let me reply.

    I was pointing out that Eric's comment about needing more than 20 degrees between the mast and headstay was a bit off. Probably more than 90% of all designs built in the last quarter century are below this.

    This has nothing to do with shroud angles, sheeting angles, clew heights, or sail areas. So I don't know why you were bringing that stuff up.


    However, I find your comment about Aspect Ratio strange. Quite an odd definition you have. I wonder where you learned such a thing. By your reckoning a full hoist blade jib is lower aspect than an old style "mule" of 3/4 hoist and 140% overlap. I doubt anyone agrees with this.


    I suggest anyone who has "learned" this about aspect ratio needs to somehow turbo wash thier brain to unlearn it.
     
  8. philSweet
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    philSweet Senior Member

    Fixed the formula in the original post. :rolleyes:
     
  9. philSweet
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    philSweet Senior Member

    Paul-
    My post wasn't addressed to your post at all. I was simply trying to point out that the choice of stay angle and jib aspect ratio were made largely independent of one another. The idea that the two were not wed to one another had not been mentioned. Also that just because the aspect ratio of the measured foretriangle tends to be higher in racers, that doesn't mean that the actual sails flown will higher AR. Reading my previous post now, It could have been clearer and briefer.
     
  10. Stumble
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    Stumble Senior Member

    Paul,

    Good point... Time to breK out the junk Flying Scott
     
  11. viking north
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    viking north VINLAND

    I reiteriate ; Thats why I love motorsailers getting too dam old to be fussin around with more rigging-:) :)
     
  12. Paul B

    Paul B Previous Member

    Well, you are sort of limited by the rigging geometry if you build sails to fit the rig. I tend to compare apples to apples, but what the heck. Clew height really doesn't make any difference if you use a true LP.

    Your comparison of apples vs plums does suggest it is possible for a boat with greater headstay angle to fly a higher aspect sail. I think this surprises no one.

    Maybe you should have gone all in and used 18 degrees and a 155% genoa vs your 22 degreees with a 90% jib. That works.

    The choice you did make, 20 degrees w/135% vs 22 degrees w/ 125% doesn't quite work out the way you thought it would. I wasn't sure how you wanted to do the comparison, so I sketched it two ways.

    I used a "J" dimension of 10.00 units as a baseline, just to make things easy.


    Option One: Using the baseline "J" and a 20 degree angle we get an "I" of 27.47. A 135% sail would have an LP of 13.5.

    Keeping the same "I" and a 22 degree angle we get a new "J" of 11.1, and 125% of 11.1 is 13.88.

    In this case the 20 degree, low clew 135% example is a slightly higher aspect ratio.


    Option Two: Keeping the baseline "J" for both sailplans. The 20 degree "I" is still 27.47. The LP is 13.5.

    For the 22 degree headstay the "I" drops by 2.72 units. The LP is 12.5.

    The result is once again the 20 degree example is slightly higher aspect.



    In both examples we are using your skew of 135% vs 125% and still the steeper headstay is higher aspect. Not what you expected...
     

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  13. philSweet
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    philSweet Senior Member

    Paul-

    From my post #13-
    I looks to me like you transposed these values, drawing a 135 on a 22 degree stay and a 125 on a 20 degree stay. What I was trying to do was keep the chord to the clew similar in both cases and isolate the effect of cut. Like I said above, I kind of made a hash of it.
     
  14. Paul B

    Paul B Previous Member

    Nope. You need to take another look.
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    I'll stand corrected on current design practices as to headstay angle. I checked a number of custom and production boat designs from US builders and found practically all of them had headstay angles less than 20 deg, the lowest being about 16 degrees. Most of these boats had overlapping headsails, so the headsails themselves probably had angles between leach and luff of 20 degrees or more. So be it if that is what the industry seems to be doing.

    Eric
     
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