Jib question

Discussion in 'Hydrodynamics and Aerodynamics' started by BobBill, Jun 9, 2012.

  1. Paul B

    Paul B Previous Member

    I am also waiting to hear about the proof people have that it works. Why has Eric not given details about the boat, the circumstances, etc of his experiment? He claims he tried it and "it works".

    Why has Petros not provided any information about the race or the results that he mentioned?

    Going back to the original claim by Eric. I have already put forth this challenge on this thread. Take any boat you currently have. Sail under mainsail alone. Pull the traveller up to windward so the boom is over the CL by 10 degrees. Head up 10 degrees, so the mainsail is now in the same position relative to the wind as it was before. The bow is now pointed 10 degrees higher. Has your VMG increased or decreased?
  2. Skyak
    Joined: Jul 2012
    Posts: 1,194
    Likes: 25, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    Since I last posted here I have been exploring the benefit of applying this concept to high performance catamarans. I came up with eleven ways it could improve windward performance and versatility. I am convinced that a cat would benefit most, and I am convinced the benefit would be significant and offer an attractive cost/performance compared to other popular upgrades. I made some inquiries but have not found a plan to demonstrate or benefit. The only cat I have is too small and low performance to prove anything but if anyone has a good small wire luff jib I could borrow, I would give it a go.

    The improvement that I can't quantify or am the least certain of is the aerodynamic benefit of separating the lower part of the jib and main. The main should gain more than the jib loses, effectively giving more power, but what about drag? Is there a penalty for making the slot asymmetric? Does anyone have any resources or references to recommend?

    I second the emotion that we need to move the discussion to numbers and proof (which is why I disproved the 'X for X' claim a few pages ago). I only want to hear what it is good for (numbers) and how could it be done.
  3. TeddyDiver
    Joined: Dec 2007
    Posts: 2,564
    Likes: 113, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    That's not related to the quest..
  4. Paul B

    Paul B Previous Member

    It is precisely related to Eric's claim. Only instead of dropping the jib tack to leeward we are doing the same thing in a Una Rig.
  5. TeddyDiver
    Joined: Dec 2007
    Posts: 2,564
    Likes: 113, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    It's a matter of interaction of the jib and main.. IMHO
  6. Paul B

    Paul B Previous Member

    In Eric's claim the interaction between the sails remains constant. Basically the only change is rotating the entire sailplan relative to the hull. The centerpoint of this rotation woud have to be the mast.

    So as i mentioned earlier, draw the hull on a sheet of paper (or on a layer of a drafting program). Draw the sailplan on a sheet of acetate (or qa seconf layer). Sick a pin through the center of the mast. Now rotate the hull 10 degrees to windward.

    There is no change in the relationships or interactions between the sails. They are seeing the same TWD as they did originally.

    But wait, let's consider the apparent wind angle. If Eric is correct the boat is now sailing 10 degrees higher than before. Think about what that is doing to the AWA. How are you going to deal with that?
  7. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,002
    Likes: 205, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    I wish I had documented evidence for what I claim, but unfortunately I don't. As I had stated originally, however, I have seen anecdotal evidence that it works. I have also heard stories from sailors, the likes of which that Brian Eiland posted a few pages earlier, where multihull sailors, whose gear is ideally suited to tack jibs off centerline to the appropriate hulls, have reported evidence that substantiates my claim.

    I think this discussion is very healthy and am generally pleased with the way its going--really has sparked a lot of discussion. I have also had private correspondence with Skyak about securing a suitable boat on which to do testing, and I have promised my own time for technical advice and analysis should any testing proceed.

    Next, I would like to reiterate that when setting the jib off centerline to leeward when on the wind, you definitely must retrim the sails for best flow. It would be impossible to hold the same jib sail shape in the offset position as for the centerline position because the sheeting angles and cambers would be different. You have to sheet the jib in the new position that gives the best airflow around the total rig.

    Again, what I think is happening is that the airflow and circulation around the rig changes significantly. The two sails, main and jib, generate more lift slightly farther apart than they do as close together as they normally are. I stated before a few pages back that I had not had the opportunity to test this effect in a wind tunnel. Well, I was mistaken (forgive a old man for his senior moments). In fact, in 1986, my client at the time for a new wingmast did travel to the Wolfson Unit wind tunnel in Southampton, England to test the design of the wingmast rig that I had designed for his 45' boat. During those tests, we did test the offset jib idea. Our model jib was not well made, however, so we did not get the best results, but we did see an effect that increased lift-to-drag ratio slightly, about 6%. This suggests modified airflow around the rig that effectively generates more power. The other drawback of this testing, however, wat that it was the rig only, no boat was included. The problem has to be defined by the system of the rig on a boat because it is the boat that is going to experience the benefit of the offset jib.

    But I don't want you all to take my word for it; I am just one point of observation. I encourage you all to test the effect. Your resources--actual boats--are probably more numerous than mine. I want to see what others can measure.

    Finally, with regard to Paul B's suggestion that one could see the effect simply by sailing a boat under main alone but sheeted more to windward is not what I am claiming. That won't work because 1) there is only one sail, and 2) that sail is changing its relative angle with respect to the keel and rudder. The effect of an offset jib is precisely that--an offset jib, not a rotated main, and in the offset jib case, the main stays sheeted pretty much the same way in the same relative position and angle to the keel and rudder.

    If I had a boat myself to test this and record it, I would, but I don't, so I leave it to you all to see for yourself.

  8. Paul B

    Paul B Previous Member

    Nothing, not even any anecdotal evidence? Funny that. Not even a vague recollection of the type of boat, the amount of change to the tack (in degrees), how you qualified the results, etc?

    BE's post has nothing to do with the subject of sailing upwind with the tack moved to leeward.

    You really should draw things out before you post such nonsense. Draw it up and then see if you want to maintain this position.

    Remember, you've rotated the hull and keel and rudder to windward by X degrees, so now you are rotating the mainsail to windward X degrees as well? So now the jib is re-aligned similar to it's original setting, but the main is now under trimmed by X degrees? How does that work?
  9. Grey Ghost
    Joined: Aug 2012
    Posts: 194
    Likes: 9, Points: 0, Legacy Rep: 94
    Location: california

    Grey Ghost Senior Member

    Have you tested it? I don't see any numbers in your posts at all. "Funny that."
  10. Skyak
    Joined: Jul 2012
    Posts: 1,194
    Likes: 25, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    Welcome back Eric. I think you need to concede the 'X for X' is too much to expect. You admitted as much to me but I think a lot of posters here still sense the impossibility.

    A 6% improvement in lift/drag is terrific. If you can find any documentation, I would love to see it.

    Regarding post 100, it only demonstrates that headsails set well off the lee bow of a cat. It was on a beam reach, and there was another headsail on center.

    Of course performance still depends on the whole boat, but if there is improved capability to be had from a rig, or any other single component, we should pursue it. And the best way to develop is to isolate and characterize the change. When it is characterized, you can consider the whole boat in a velocity prediction program.
  11. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 277, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Just to aggravate the thread further, this is how I see the development of this most educational thread -

    No mention of airflow here .. and its patently obvious from subsequent comments that its totally wrong

    Now its aerodynamics that does the trick

    Why professionals like to create 'head wanks'

    I could be wrong. I look forward to further comments

    Personal Note on hackneyed metaphors -

    I am always wary of anyone quoting this hackneyed and meaningless metaphor "thinking outside the box"

    If your argument is sound and well presented, you don't need to add this ridiculous postscript, and you mark yourself as a "herd thinker". When you do use it, it reinforces your weak exposition. If the concept is truly innovative - it should be a readily apparent.

    If any of the readers are management educators who use this phrase regularly, along with "left brain, right brain thinking" - please feel free to shoot yourself now !
  12. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 139, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    The reason I think any benefit comes from improve the flow over the mainsail is because the flow over much of the main sail is separated. The area for several feet aft of the mast on the main sail is fully separated flow and does not reattach until further aft on the sail (that is the only part that generates effective lift). the whole upper third of the sail is typically fully separated too (one of the reason I suspect the now popular flat top sail is more effective is there is just more sail higher up to allow it to reattach aft of the mast). So if moving the jib to lee alters the flow on the LE of the mainsail and allows it to attach sooner, I would expect it to greatly improve the L/D, total lift, reduce drag and delay stall, allowing you to point higher. For a rigid wing that does not have large areas of separation you are correct, it does increase drag a lot, but soft sails usually have large areas of separated flow and would greatly benefit if these areas could be reduced or eliminated.

    I have not tried it, but I have seen both wind tunnel and CFD test of flows around typical main and jib configurations, the flow sucks over much of the main sail. So if there is improvement to be made by moving the jib to lee, I would suspect it has to do with this problem. I have been meaning to do some experiments with changing the shape of the mast/sail LE to see if I can reduced the separation and try it out on my dinghy, or on a new one I have yet to build. Putting a moveable jib would be another thing to try. I would need to find a way to accurately measure the wind angle as well, only a few degrees could give you a significant advantage. Six degrees would be amazing.
  13. Mikko Brummer
    Joined: May 2006
    Posts: 488
    Likes: 35, Points: 28, Legacy Rep: 258
    Location: Finland

    Mikko Brummer Senior Member

    To me, Paul B's words is all the proof I need. He has actually tested it on his boat. He says it did not make it point higher. A clever way to overcome the problem with long spreaders & overlapping headsail, though.

    Rwatson: Your sketch is what the original claim was about. I think the sketch says it all. Wish you had come up with it on page one, to stop this thread right there.

    But no, the superstition still goes on, after 10 pages of speculation against common sense.
  14. TeddyDiver
    Joined: Dec 2007
    Posts: 2,564
    Likes: 113, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Superstition.. It's the same thing that makes it obvious to pole out the chute to weather when running, sprit the gennie out from the bow while reaching, and then to rig the jib to lee when beating to weather...

    Ps. There's some recemblance to the thread about DDWFTTW, some just don't get around to the wind.. ;-)

  15. philSweet
    Joined: May 2008
    Posts: 2,243
    Likes: 142, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    From an engineering standpoint, rotating the jib's tack to leeward is a poor way of going about it.

    I ran the calculations of cap shroud geometry. If the cap shroud carries a preload of about 1/6 the headstay tension then the upper spreader will have to grow by the same amount as the headstay falls to leeward at that height. Ie no sheeting angle gain. (This was a pretty loose calculation. It didn't consider updrafting or the change in updraft due to the different angle to vertical. Nor did I consider spreader sweep. 20 degree forestay and 11 degree capshrouds all intersecting at a point. The number 1/6 isn't going to vary much over the geometries usually seen. To the extent capshroud tension exceeds 1/6 of headstay tension there would be an improvement in sheeting angle.)

    This suggests that a low forestay tension and single spreader rig would be the best candidate rig.

    An alternative would be to use twin backstays and take advantage of the windward pull to shorten the upper spreader- a common practice.
Similar Threads
  1. Roelina
  2. Earl Boebert
  3. massandspace
  4. Southern Cross
  5. laukejas
  6. cristianon
  7. six four
  8. mydauphin
  9. mcarling
  10. Anatol
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.