Aspect ratio on headsail vs shroud angle?

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

  1. sailingdaniel
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    sailingdaniel Junior Member

    A Newbies question..

    In every thing i read aspect ratio seam to be on of the most important thing for booth sails and fins.. I found plenty of info about sweep angle on keels.. But nothing about shroud angle on sails (im thinking headsails)

    Upwind a high aspect ratio sail is good. But is that mostly due to a low shroud angle ore a has it more to do whit length/hight of the sail..

    I understand that if u make the shroud angle close to horisontal it would not be good.. But for ex between 15 and 30 deg. to vertical that is not un-normal..

    Me and some fellow cruisers got in to this discussion, but no one really had an answer , just diffrent ides..

    Just cas im curios..

    Cheers

    Daniel
     
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  2. sailingdaniel
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    sailingdaniel Junior Member

    Ah .. shroud angle . Maybe the right word is stay angle..
     
  3. viking north
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    viking north VINLAND

    Interesting question -- a sail is a "foil" so logic would dictate it should follow the same rules as the leading edge angle of foil keels but and there's always a qualifier--One operates in air the other a liquid and there in lies part of the answer. The never ending shades of grey when engineering a machine to function efficiently in two mediums. My guess the leading edge angle is not as critical for the sail as it is for the keel, however it still plays a role in that the angle will dictate the aspect ratio of the sail for a given mast height.


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

    In general, in sailing yacht design, you want the headstay to be angled at least 20 deg ahead of the mast. This is because if it is angled less, the head of the sail becomes simply too narrow and won't provide any real lift, mostly just drag. Remember, triangular sails are the absolute worst shape and sail can be, and the narrower they are at the head, the worse they get. The headstay angle can be greater, but by 30 deg, the slope is getting to be too much, and the aspect ratio of the rig too low. Headstay angles in the region of 20-25 deg are pretty normal, as this is what has evolved over time as a good design.

    As for shroud angles, they should always be at least 10 deg to the mast, not less. If they are much more, then the spreaders are wider, which is good for staying of the mast, but then any overlapping headsail such as a genoa or Code Zero will get hung up on them more easily and they cannot be sheeted as close to the mast as they could be. If the shroud angles are less, then their sizes go up more and quite dramatically (their geometry to the mast determines the loads that they carry) and start to become too big and heavy = expensive, with more drag. So shroud angles between 10-15 deg are common because that is what works best.

    I hope that helps.

    Eric
     
  5. viking north
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    viking north VINLAND

    The master has spoken--thanks Eric I have learned something new today--Geo.

    A yacht is not defined by the vessel but by the care and love of her owner--
     
  6. sailingdaniel
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    sailingdaniel Junior Member

    Thanks Viking an Eric for your answers..

    Stay angle was the thing i meant, not shroud as i first wrote..

    The question came up when we sat in a marina looking at different boats , some modern whit very small angle and an old schooner whit a long blowsprit and a really big angle..

    My thought was that the smaller angle would be more effective uppwind , as i guess it is. But I did not think about that it also bring the sail closer to the mast.. Interesting..

    I read on your web page Eric , (and i think u uptaded/change it a bit?) , I see that u are a big fan of freestanding wing masts. Your boat Saint Barbera is really nice. And it was interesting to read about moving the clew to leward when going uppwind. This could be done an a boat where the head stay doesent go all the way out to the bow , but how big must the change angle be to get an effect? It could also be done on a cutter stay, I will try that one day..

    I have done some experiments whit a free flying headsail where i took a big bamboo pool and tied it diagonal on both toerails so it became as a pivoting bowspear.. I only tried to move the clew to windward. That worked well, and i could go an much deeper windangles and the sail worked much better.. Until the bamboo broke.. the designer of the bamboo pool had not done his homework.. :)

    It helps.. Thanks

    Daniel
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    I have shown in classroom display models (rigs on gimbals sitting in front of a fan) that for every degree of angle offset to leeward, when sailing upwind, you gain a like number of degrees sailing to windward, just about equal. Admittedly, these are static gimballed models, but the principle works. Just think if you could get your boat, any boat, to point 5 degrees closer to the wind--that's a huge amount--simply by moving the tack of the headsail about that many degrees to leeward! I have proposed designs that do this with an articulating bowsprit. An early version of Project Amazon had one, but the owner decided to not go that far. However, some readers may recall that Hunter's Child, which sailed in the Around Alone Race a few times, had "Bull's Horns" on the bow that did essentially what I describe here. However, by using an articulating bowsprit, you get the headsail out farther away from the hull, plus you can get much more movement away from the hull centerline. Unfortunately, all round-the-buoys racing rules forbid the movement of the tack of the sail at all--it must stay on the hull centerline. The offshore open class rules do not have this restriction. Nor, of course, is there any restriction on cruising yacht designs which do not have to conform to any racing or handicapping rule.

    You found out the beneficial effect yourself with your bamboo pole. As you fall off the wind, the sprit must move more to windward, such that on a run, the tack of the headsail is to windward of the hull centerline. The result is less blanketing of the headsail by the mainsail, and you can sail deeper.

    So just my moving the tack of the headsail from side to side, we can greatly improve the sailing performance of boats by sailing closer to the wind to windward, and deeper off the wind when running.

    Eric
     
  8. sailingdaniel
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    sailingdaniel Junior Member

    When i asked my first question i was thinking about the angle seen from the side , this turned out to be much more interesting..

    Tomorrow Im off to Madagaskar, There I will be rounding the top and going down the westcoast and there meet a lot of headwinds.. Then I will experiment whit moving the foot of the sail to leeward.. If half of what u are saying is true ( not saying u are a lier) , then i should see some affect of that also i hope.. Will be very interesting.. I post a reply about that when i can..

    I read something about Hunters Child in one of Dashews books. Lars Bergstom was the designer i think..? Dashew talks about his pivoting bowspeer on Beowulf and it seams to work well for him. But only to windward from what i remember. Thats where i got my bamboo ide..

    Racing boats seam to be the "drive" in boatdesign.. But there rules sometimes just seam to mess things up.. The *** of my IOR like boat is one exampel.. This sail ides seam to be another..

    In my mind i can see the use of this in cruisingboats (ore any boat) on a lightwind sail on a pivoting bowspeer, or a cuttersail.. The tripwire/lifeline might make it harder to use on a normal headsail.. Any Cat sailor should know a lot of this i would expect?

    Cheers

    Daniel
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    'Twas ever thus. This has been going on since the advent of rating rules than can be traced all the way back to the reign of Queen Elizabeth I in England.

    I will be interested to read the results of your experiments on your trip.

    Eric
     
  10. Doug Lord
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    Doug Lord Flight Ready

    Daniel and Eric: thanks for the inspiration regarding moving the tack of the jib to leeward. I'm going to incorporate it in my trimaran test model and on the full size version.
    Important idea that could offer a lot-thanks!
     
  11. DennisRB
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    DennisRB Senior Member

    Great thread guys.
     
  12. Paul B

    Paul B Previous Member

    I think if you look around you will find that most sloop rigged boats in the past 35+ years have an angle range skewed lower than your estimate.

    Most sloops these days have an aspect ratio of greater than 3:1. At 3:1 the headstay angle is about 18.5 degrees.

    To get to your 20 degrees you would be looking at a 2.75:1 aspect ratio, pretty low for a modern boat.

    To get to your 25 degrees you would have to have an aspect ratio of about 2.2. I can't imagine anyone designing a rig like that unless they were trying to emulate some ancient type of rig.
     
  13. philSweet
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    philSweet Senior Member

    We probably shouldn't associate headstay angle with aspect ratio so strongly. A high cut 125 on a 22 degree stay has a greater aspect ratio than a low cut 135 on a 20 degree stay. Unless, of course, it's a real deck sweeper, then you can double the AR through mirroring. AR is the (edit) hoist2/area. (the correct furmula now- thanks). The point being that cruisers can often be found running higher aspect headsails than racers. In order to make the sheet handling easier, they run higher cuts, and can have more stay angle and less overlap at the same AR. On a single spreader cruising rig with 11.5 degree shroud angles, the ability to sheet the jib in close effectively limits the stay angle. A 20 degree stay sheets to 16.2 degrees. A 22 degree stay would sheet to 14.9 degrees. A double spreader rig would work out to be about 11.5 degrees for 20 degree stay, 12.4 degrees for 18.5 degree stay. One last point. The AR of a particular sail is far less important than either the AR of the entire suite of sails or how well the suite of sails is spreading the load. High AR sails can be complete dogs if the vertical load distribution is wrong, and this is very easy to do with a skinny triangle.
     
    Last edited: Aug 22, 2011
  14. viking north
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    viking north VINLAND

    It always amazes me how what seems like a basic question posted will turn out to be so educational. In my first post responding to the OP i had a rough idea of the effect of forestay angle on the headsail but as this thread progresses, man was I a babe in the woods--excellent posts and very well written for those of us not of the engineering persuasion(local slang)
    PhilSweet -- excellent post --always thought of the high cut jib as more of a trade off between better visibility verses the negative effect of sail power being higher up resulting in more heeling. For some reason aspect ratio never entered my brain. So in effect one could use less canvas (hi cut) gain better visibility and still have the same driving power.(higher aspect and closer sheeting) I realize there are tradeoffs here but properly designed it's a plus. Good Stuff--Geo.


    A yacht is not defined by the vessel but by the care and love of her owner--
     
    Last edited: Aug 20, 2011
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  15. gonzo
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    gonzo Senior Member

    Eric's experiments disprove the "wind tunnel" theories. I never understood how people kept on believing them. All it took was a look at the leward telltales to see them limp and hanging. However, they continued to claim the air was accelerating in that area. A wider slot with clean air flow is the most efficient.
     
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