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

    Paul- I was editing my post but you snuck in ahead of me, so I'll leave it stand. Your illustration is correct.
     
  2. viking north
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    viking north VINLAND

    So gentlemen-- for the sake of us common folk struggling with inferior craft and fighting for fractional increases of windward power and every minute of windward pointing ability could someone summarize, lets say with a fixed headstay angle say of 20deg. I.E. What would be the result (in theory) of the following using a 100% jib(headsail) as a base.
    (A) Changing to a higher cut. Power ? Pointing ability ?
    (B) Not changing the cut but increasing to say a 125% sail. Power? Pointing ability.
    (C) Which of the two above would be more benificial in power and pointing ability.

    A yacht is not defined by the vessel but by the care and love of her owner
     
    Last edited: Aug 22, 2011
  3. Paul B

    Paul B Previous Member

    A) A higher clew usually decreases pointing ability.
    B) Increasing to a 125% would be more powerful. However, on most boats the shroud base works against you with sails of this size. To get a better sheeting angle it is better to go up to about 140%.

    One thing you did not mention is sheet angle. If your boat is of a decent, modern design you might look at where your jib sheets. If it is out on the rail it is too far out (normally).
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    I'll concur with that.

    Eric
     
  5. viking north
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    viking north VINLAND

    Excellent--that gives me a base to work from when I get to my rigging--and of course it will end up being the typical tradeoff :) Tnx. Geo.
     
  6. DCockey
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    DCockey Senior Member

    Depends on whether someone is talking about the direction the boat is pointing relative to the wind, or the direction the boat is moving relative to the wind. Only one really matters when going to windward and it's not how most people measure "pointing". :)
     
  7. Stumble
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    Stumble Senior Member

    Viking,

    Down here on the Gulf Coast almost everyone uses a 155% in large part because of our perenial light winds, particularly during the summer. There is no question in light breeze you will get more power, but it will cost you a bit in pointing ability. Though your vmg to windward will defiantly improve as the boat speed difference is substantial.

    This raises a good point, which is that except in very special and almost always racing circumstances absolute pointing ability is usually less important than vmg to weather. In a lot of boats for instance it is better to bear off a few degrees, get the keel working, and boat speed up, than it is to put the boat in pure pointing mode.
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    One universally talks about the direction the boat is moving--speed made good to windward, speed being a vector. It is understood there is always leeway angle between the boat's centerline and the boat's course. If you improve the boat's pointing ability, you are talking about its course, and understand that the bow of the boat will be a few more degrees into the wind than that.

    Eric
     
  9. DennisRB
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    DennisRB Senior Member

    Eric. I just spent hours on your website. Very informative and entertaining (apes VS triangular sails :p ) A major part of your favor for free standing masts seems to be the ability to get rid of the narrow triangular tip of the mainsail due to not having a backstay. If I had a square top or very large roach main to reduce tip vortex. How would the airflow around the tip of a triangular top jib be affected, specially if it were fractionally rigged?

    To my thinking, a masthead rig with two triangles meeting together would be the worst for the generation these vortices. However if you have an overlapping fractionally rigged jib (particularly with a square top main) the blanket effect of the main would have a tendency to smooth these vortices out. Is this true?

    Also. I have been looking at some freedom yachts for sale and they mostly still have triangular main sails. Why is this, and why are they reported to sail badly to windward compared to a normal rig?
     
  10. viking north
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    viking north VINLAND

    So true Eric and Stumble-- I had one hell of a time convincing a nervious (closer to panicing) fellow sailer(owner) that he was losing ground big time beating to windward in one devil of a storm. The goal was to get into a harbour and shelter some 15 miles distant and he was convinced by close hauling her and pointing toward the harbour we could sail directly in on one leg. I pointed out our speed-- down to 3 knots and tried to explain we were going sideways as much as forward as the keel was not biting. Finally after some mutinous language :) I convinced him that by falling off a few degrees and getting the speed up we would make harbour faster, on two legs, which we did of couse and once docked it earned me a thankyou handshake and a double of rum. The point being it's an ever so common problem convincing, that two legs at greater speed is faster to windward than too close hauling and losing speed. I think it might have to do with, by falling off the bow is no longer pointing at the target --especially in heavy weather and a nervios crew--

    A yacht is not defined by the vessel but by the care and love of her owner--
     
  11. Steve W
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    Steve W Senior Member

    I believe the Jarcat catamarans in Australia have the forestay on a traveller across the bows.
    Steve.
     
  12. philSweet
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    philSweet Senior Member

    Viking-
    It might help at this point if you post a picture of your boat. Your question is best answered if an overall idea of the proposed rig is presented. Before we worry about the details of jib cut, lets get the basic geometry of the mast and rigging figured out and the balance of main area to jib area. If its the boat I'm thinking of, a largish main and a smallish jib are probably best.
     
  13. viking north
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    viking north VINLAND

    Phil-- It's the build "Nancy G" under Boatbuilding on this forum. I am working on the keel/skeg/rudder install and will be forwarding the added appenges info.to a retired/part time/as needed/boat designer with a computer :) so he can add the info to the canoe hull stats he has already processed. Once this is all updated i can then post her lines -- hydrostatics and get into some fine tuning. Thats if I havent made a complete bomb on the keel design and install which was the part of this build that I wanted to do on my own. You see i'm a builder not a designer, this is my retirement boat and I wanted to play around with designing the easiest part :D. Might have bitten off alot more than I can chew but after some 4 months study and bugging the hell out of all you most patient forum members, it seems to be coming together. My rough( and i mean rough) design right now is a 14ft.to 15 ft boom on a 32ft. deck stepped mast approx. 140in. back from the bow. This is on a 28ft 7 in. LOA hull which places it at approx. waterline station #4 based on 8000 to 8500 lbs. displacement. I'm fighting weight so 9000lbs is a possibility which will throw a monkey wrench into the works but the good news is it's a motorsailer, it's my own boat and if I mess up a little i'll live with it, At near 70yrs. old I dont sweat the small stuff as they say. I do thank you for the input and as Mc Carther said "I will return"( for help that is) ---Thx. Geo.

    P.S. often pass your way in route to the Keys, have a friend in Ashville/ Friend in St. Augustine(Erics territory) and a sister in Daytona(PARS territory) and a bunch others on this forum so might get to meet a few in person one day, thats if I don't go down with my latest creation :p
     
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  14. Eric Sponberg
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    Eric Sponberg Senior Member

    For a triangular top jib in front of a square top main, you still have to consider that the top of the jib is not providing very much in the way of lift. Farther down the jib, its airflow gradually increases its effect of the flow over the main. Really, only the lower 2/3rs or so of the jib is working well, and the upper part of the main works a lot better for the fact that it is not triangular and doesn't really need a jib. The area of the jib provides exactly that--area--and more area = more speed because there is more pounds of lift from the increase in area. But the sail interaction at the top of the jib where it is small and closer to the mast just isn't that good--we give it up to gain whatever we can from the rest of the sail down below.

    As for the older Freedoms with triangular sails, that is just they way they did things back then, prior to the advent of full-length battens and highly roached sails. Also, the masts are round in cross-section. Round is a really bad shape for airflow--airflow separates off a round mast much closer to the stagnation point than it does on a non-round mast. This is one reason why conventional mast sections are longer fore and aft than they are wide, and the leading edge is not round--it is parabolic or elliptical. Parabolic noses and elliptical shapes (my favorite) are much kinder to the flow--the flow does not separate and cause a big separation bubble behind the mast. Rather, the flow is more continuous, it stays attached easily, and the lift is more powerful. So with a poor mast shape and triangular sails, some of the Freedom designs, and other early free-standing rigged boats, just did not sail that well. The later Freedoms had more pronounced roaches, and that helped considerably. The mast shapes that I designed for the Herreshoff cat ketches, the Sparhawk cat ketches, and the Sea Pearl cat ketches, all were elliptical, for that better airflow. All those boats sail remarkably well.

    And this is why I like rotating wingmasts which have elliptical cross-sections--the overall shape, and particularly the nose, is great for airflow--it stays attached--and ellipses are easy to engineer structurally. Ellipses are kind of like circles (a circle is a special kind of ellipse) in that you can define the shape of an ellipse with an equation which you use for the engineering--engineering becomes relatively quick to do. You have to calculate the size and wall thickness of every section (1' increments) all the way up the mast. If you use an airfoil shape for a mast, the engineering is many times much more complicated--you cannot calculate cross-section shape and wall thickness with an equation--it takes a whole spreadsheet to do each section--and that takes a lot more time.

    Eric
     
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  15. DennisRB
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    DennisRB Senior Member

    Thanks Eric. I think I have read your whole site now. Took a few days :)

    So the triangular jib with the square top main does indeed reduce the drag caused by tip vortices on the jib, but the top section of the jib is still pretty much useless? So it is still a better idea to run a fractional jib and or square top main for this reason? This is of course when you are bound by a stayed rig.
     
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