Jib question

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

  1. Paul B

    Paul B Previous Member

    Mikko, my words should not be required. Anyone who sails, really knows how to sail, would have read Eric's initial post with utter disbelief.

    His latest attempt (drop the jib tack 10 degrees to leeward, turn the boat 10 degrees into the wind, and keep the main trimmed as it was before you turned the boat) is even more laughable.

    He refuses to give any details about his "it works" experience. He will not produce a drawing defining his idea. He knows if he did he would make it too easy for people to see the truth.
     
  2. Paul B

    Paul B Previous Member

    I have read your post 3 times and I still have no idea what you are trying to say.

    Caps have 1/6 the load of the headstay????

    Headstay "falls to leeward at that height"????

    Let's see. I have a boat. It had a fractional rig with single spreaders that were too wide to allow a 150% genoa to sheet at 8 degrees or so. So the tack was attached to a t-track slider and could slide to leeward about one foot. The genoa sheet track was also moved outboard a similar amount (keeping the sail aligned with the hull/keel/rudder).

    You are saying, what? The rig would fall down? It didn't.
     
  3. Skyak
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    Skyak Senior Member


    Mikko,
    Paul B's experience does not disprove the concept. He did mount the tack off center, he did point higher, but it was because it allowed better sheeting of a larger headsail. He simply did not get better performance than he expected from a centered setup he could not acheive.
     
  4. philSweet
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    philSweet Senior Member

    Paul, the post was a lot longer when I first wrote it, then I shortened it- by too much apparently. Your single spreader rig is the best candidate because the ratio of cap shroud tension to the forestay tension is highest on a single spreader rig.

    The calculation was based on keeping the tension on the capshroud the same by extending the spreader as the tack of the jib was swung downwind, thus keeping mast compression and most everything else comparable between the two cases. Realistically, I think you would also shorten the upper panel in the spar design.

    My point was that it seems an awkward thing to do compared to using double backstays.

    if phic is cap shroud angle to vertical (evaluated at 11 degrees)
    phif is forestay angle to vertical (evaluated at 20 degrees)
    theta is the angle the forestay is shifted from the centerline
    and T is tension

    then the force balance becomes

    2*[sin(phic)-sin(phic0)]=(Tf/Tc)*sin(phif)*sin(theta)

    The 2 comes from there being two cap shrouds, one getting tighter and the other getting looser.
     
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  5. Paul B

    Paul B Previous Member

    I still don't know where you think you are going with this. Forestays have toggles at the top and bottom for a reason. You might think about that when you are doing your calculations. You might also want to consider the fact that on most rigs the forestay and the caps are the same size.

    I see no reason to change any panel lengths, and would not. We use the ratio for panel lengths that we do for a reason.

    I don't know how using double backstays (is this split topmast backstays, or topmast and runners?) would be of any help in getting the sail to sheet around the spreader.
     
  6. Paul B

    Paul B Previous Member

    Higher than what? Higher than an existing rig that was not optimal?

    If the rig was modified to a smaller CPW and went to double spreaders the sail could have sheeted correctly and would have pointed as well, or better, compared to the sliding tack modification.

    Moving the sail outboard as it was done did nothing to enhance the aero properties to make the boat point higher, compared to having the same sail sheeted the same way with the tack on the CL.
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    This thread is going so fast it is hard to keep up. I have regular design work that I have to get through, so I have to work these entries in somehow in my spare time.

    First, see the attached sketch of sailboat forces (pdf format, clearer that way) which explains my thinking.

    Second, I found the wind tunnel test report and scanned that and post it here. Look at the first paragraph on page 5, where it says that for the offset jib (on this particular model) the lift/drag ratio increased from 5.8 to 6.1. Do the math and that is an increase of 5.17% (I stand corrected, not 6% which I tried to recall from memory). See also Figure 4 for the lift/drag curves. This is for one particular mast design and one rig configuration. It is not meant to prove the entire science of this problem as it covers only the rig and not the hydrodynamics of the hull.

    See also the photographs attached which show me (yellow sweater), the owner Mr. Robin Lagemann (to my left), Bill Peterson the designer of the boat (to my right), Richard Boehmer a writer and researcher on speed sailing (at far left), and Tony Marchaj (between Richard and Bill) who advised us on our testing. Included in these photos are pictures of the model in the wind tunnel at Southampton, and the one with the 3 gentlemen gathered around a desk show Mr. Lagemann (left), aerodynamicist Andy Claughton of the Wolfson Unit (right), and another technician, back to camera, don't know his name.

    Mr. Lagemann died, unfortunately, before his boat could be completely finished. The very expensive carbon fiber wingmast, my design, has been given to the Landing School who had wanted to use it for a flagpole (not put up yet, the city says it's too tall). But it is probably one of the most expensive flagpoles for its size ever.

    Finally, another source of my findings, again for wind part only, were demonstrations that I carried out for the Landing School with some gimballed balances that I made of sailboat rigs. The only photograph that I know of taken during these demonstrations is published in my PBB article on Project Amazon and the Unstayed Rig, copy of which you can download from here: http://www.sponbergyachtdesign.com/Project Amazon PBB.pdf. See page 46. On these balances, I was able to set the jib to leeward any small arbitrary amount of degrees, say up to 10 degrees or so, and the model would turn to windward a like amount, all measured by scales on the models.

    Sorry, I do not have documentation of the other anecdotal stories that would illustrate my point--I did not take notes. They would have been meetings with other sailors at boat shows, sailing events, and the like.

    I will concede that in any real life, full-scale demonstrations, one may not see and "X degrees jib offset = X degrees higher pointing" correspondence because as I said, I have only anecdotal evidence and not conclusive proof. On the other hand, you may. That is why I have advocated for any one else to verify if this is true.

    Eric
     

    Attached Files:

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  8. Skyak
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    Skyak Senior Member

    Again, it could well be true, but we don't have any evidence. You are comparing to a theoretical case that has not been created or tested.


    All we can say is the configuration was used and no dramatic improvement was noted. No direct comparison was tried.
     
  9. philSweet
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    philSweet Senior Member

    Paul,

    The panel lengths and spreader lengths can be set up to provide a specific load ratio and length ratio between the uppers and intermediates. Using Skein as an example, he recommends a load percentage of 28% for intermediates and 20% for uppers. Relative to one another, the upper is 71% as loaded as the intermediate. The length of the intermediate happens to be about 71% of the length of the uppers. When the load is applied, both shrouds stretch by an identical amount. This tends keeps things in column, is convenient for sizing turnbuckles, and equal turns will keep a mast tuned. There are just enough variables to play with to get this to work and deal with the mast section at hand (or better yet, craft the stick to suit the fancy of the rigging). Are we off topic yet?


    Now when you go and change the sideload a whole bunch by swinging the forestay, you will want to dither with every measurement on the shrouds to recover as much of these conveniences as possible. It will not be possible to keep the mast in column at all angles so the forestay should be swung to a specific point and the rig tuned for that point.

    I look at the engineering first and the aerodynamics second I guess. I seem to be the only one voicing engineering concerns. As far as I can tell, the jib/shroud clearance problem can be better solved by other methods.

    As far as the backstay is concerned- any arrangement that pulls the masthead to windward will permit a narrower upper rig once the rig is redesigned. It may surprise you just how well this can work. For a three spreader rig, moving the backstay a foot to windward might do as much for the sheeting angle as moving the jib tack downwind a foot. This assumes a clean sheet design for each case. Not intuitive, I admit. For a two spreader rig, it looks like it's about two feet to equal a one foot move of the jib tack.

    Surely there are some rig designers here that can help me out some here. The offset backstay can be modelled as a phantom capshroud plus centerline backstay and the remainder of the shrouds worked out from there. I somtimes see the upper shrouds fixed to the mast well below the masthead. I'm thinking they must use split backstays for this to make sense.
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Great info.. thanks to Phil and Eric.
     
  11. upchurchmr
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    upchurchmr Senior Member

    I really hate made up acronyms.

    What the heck is DDWFTTW....... BXCCSSRERWSFGE
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

  13. Paul B

    Paul B Previous Member

    Two things.

    First, your conclusion that sailing with the genoa to leeward resulted in the boat sailing higher that with the jib alone is not correct and is not mentioned in any of my posts. How did you come to that conclusion?

    Next, the "evidence" of the boat's ability to point as well if we made the rig a double spreader, narrow CPW style, with headsail tacked to the CL. You are correct, there is no direct evidence since we didn't do this. We do know that it works on all the similar and even not-so-similar types from the same designer and most others.

    To clarify, the boat, sailing with the headsail tacked to leeward, did not seem to point any higher than the other boats around her in fleet racing. The angle of the dangle was 7 or 8 drgrees. If we could sail that many degrees higher than the fleet (or even half that) it would have been a huge breakthrough and the talk of the industry. It was not.


    The closest thing to a "direct comparison" is sailing with the jib. The boat has been sailed in both configurations, fixed tack and sliding tack, with a jib only. There is no noticeable difference in pointing or tacking angle.

    Outside of the AC most advances in sailing yacht design are made by evaluation against the known fleet. It is rare to find well documented science, as the costs are too high. So if that is the only data that matters to you then I doubt you will find it.
     
  14. Paul B

    Paul B Previous Member

    Not only do you seem off topic, but confused. My single spreader rig has no intermediates. Even if it did, I think you shoud probably go to the nearest marina and take a look at every modern multi-spreader rig there. You will find the Ints are smaller than the caps in every case. There is a reason for this.

    Perhaps you could read the significant chapter in Principles of Yacht Design to see what those authors think about the loads on the cap diagonal vs the int diagonal.


    If the forestay is changed to 8 degrees off CL they loads do change. But if you look at the headstay sag of most non-racing boats, the angle of the forestay would be similar due to sag (note my earlier comment about toggles).


    What methods? It seems like your solution is to add running backstays that are off center so that the spreaders and CPW can be shortened, allowing the sail to be sheeted correctly. So now we have a rig that is dependent on someone not missing the runner, or not getting it "in" during a heavy air tack. That doesn't sound like a better method for this application. More expense, more fiddly, dependent on perfection in the human element.

    If we were going to have the expense of moving the CP, cutting down the spreaders, etc it would have been better to just make it a double spreader rig with the tack on CL and the genoa track at 8 degrees.


    I am surprised you find this interesting or significant. I think you will see the uppers below the forestay on virtually every modern rig. Whether the designer uses 97% if "I", 98%, or some other number depends on the formula they use. Split backstays are not required. I don't know why you would think they were.
     

  15. Skyak
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    Skyak Senior Member

    Understood. I had some email discussion with Eric about why that is. I think it relates directly to a lack of respect builders have for intellectual property protection. My own interest in spending to develop this concept evaporated with no chance of a return. I had an idea that this could be solved at the class association level. Kayak racing rules have a line that says that proprietary designs would be considered if they are offered at a reasonable cost to all builders. If the class went a step further and could disqualify for patent infringement there would be a great incentive to develop and produce the data to sell the class on high performance low cost improvements.

    Speaking of data THANKS ERIC! Still reading...

     
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