Jib question

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

  1. TeddyDiver
    Joined: Dec 2007
    Posts: 2,614
    Likes: 136, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Waiting Paul and Mikko to comment Wolfson unit test data :)
     
  2. Paul B

    Paul B Previous Member

    Patience Grasshopper. I haven't even read the thing yet.

    Eric's post is probably going to have to be addressed in bites. First there is the post itself, then the sketch he attached, and finally the report.

    Since I'll be out and about the rest of today and all of tomorrow and Fri AM you shouldn't expect to see much from me before next week.
     
  3. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    The flaw in your analysis is the idea that beta will be reduced to 45 deg. Although your drawing doesn't show it, I'll accept that the mainsail can be retrimmed so the rig produces the same lift and drag in Figure3 as it does in Figure1, and is not reduced by the 10 deg reduction in angle of attack due to the rotation of the mainsheet location.

    The problem is that although the total aerodynamic force has the same magnitude and direction relative to the apparent wind, it has a different orientation relative to the course through the water. This requires a different hydrodynamic lift and drag to oppose it. The smaller apparent wind angle, beta, requires more hydrodynamic lift and less drag than the original case to react to the same total aerodynamic force. So the leeway angle will not be the same, as you have indicated. How the hydrodynamic lift/drag ratio will change depends on the sizing of the keel and the hull design.

    If the keel was deep and large, it is possible that in the original case the boat was sailing at a leeway angle below that for best hydrodynamic lift/drag ratio. In that event then, yes, pointing higher would improve the L/D and allow the boat to sail at a smaller beta. However, I suspect this would not be the case for most boats, and there would be a disproportionate increase in drag compared to the increase in lift. In that case, the hydrodynamic L/D will suffer and the course will be lower than it was originally.

    If the offset jib results in a reduction in drag, then the boat will be able to point higher. But it's not going to correspond to angular difference between the tack locations, it's going to correspond to the effect moving the tack has on the aerodynamic lift/drag ratio (including the windage of the hull).
     
  4. TeddyDiver
    Joined: Dec 2007
    Posts: 2,614
    Likes: 136, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    IMHO the main point in this quest..
     
  5. Paul B

    Paul B Previous Member

    This is the first bite at the very big meal Eric has set out for us. This post will comment on Eric’s post. I will have comments on his “Sailboat Forces” diagram in a second post, and will have to take some time to analyze the Test Report and will comment on that in a third post.


    I have looked at that document and will comment in another post today.


    Thank you for posting that. I have not had time for this one yet, so I will have to comment in a 3rd post, probably early next week. A quick scan through the text portion does set off some alarms.


    This is no surprise. If you have a balance sailplan that is in a static gimbal it is basically decoupled from hull forces. So the position of the hull is irrelevant. You could have turned the sailplan 90 degrees to the hull and had a similar result.

    I can’t imagine you think this has anything to do with actually sailing a boat in this configuration.


    That doesn’t seem to be what you were saying earlier in the thread.
     
  6. Paul B

    Paul B Previous Member

    This is the second bite at the very big meal. This post will comment on Eric’s “Sailboat Forces” diagram.


    The first observation is this diagram is not related to upwind sailing. When beam reaching you have more freedom than you do when sailing upwind. Regardless, this diagram set is still flawed.

    (Note: At a beam reach wouldn’t the sailplan be more efficient with the tack moved to weather anyway?)

    Going from Fig 1 to Fig 2 the tack has been dropped 10 degrees and “sails retrimmed”. The jib sheet would have had to move outboard about the same amount as the tack moved to leeward, to maintain the sheeting angle of Fig 1. The main seems to have been trimmed in 2 or 3 degrees. This seems odd, since the slot would be more open one would think the main would not need to be trimmed in. The “forward” component of the rig forces has grown quite significantly. How?

    Now we transition to Figure 3. As the jib tack comes back to the original position relative to the wind the jib sheet position would have to change back to more inboard, similar to the sheeting angle in Fig 1. The main as drawn is now undersheeted by 7 or 8 degrees compared to Fig. 1.

    Let’s assume this is an oversight and assume the mainsail would be trimmed to equalize the sailplans of Fig 1. and Fig. 3.

    Now, if the sailplans in Fig 1 and 3 are equalized, the resolved Rig Force of both sailplans would be equal. But Eric’s sketch shows a 10 degree gain by his sketch in Fig 3. This should not be true.

    So if this mistake is fixed we have the resolved sailplan force at a greater angle to the hull and the direction of travel compared to Fig 1. This suggests an increase in leeway and hydro drag that Eric seems to ignore.

    As I mentioned in an earlier post, the whole idea of gybeing boards (as in I14s) allows the hull to rotate to be more in-line with the sail force, and a reduction in hydro drag. Eric’s idea is basically the opposite of this.


    In Eric’s sketch the direction of travel is now to windward of the CL through the tack and the mast. So the boat is making “negative leeway” compared to the sailplan. This sounds dubious. Actually, if the boat could do this the AWA would change and not stay constant as Eric has sketched.


    It is very important to remember the direction the bow points is not indicative of the “pointing” of the boat.
     
  7. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,021
    Likes: 248, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    This comment doesn’t make sense. Of course this diagram is related to upwind sailing, the apparent wind is forward of the beam (definition of upwind sailing) and the whole point of this entire thread is primarily about upwind sailing. And, as I mentioned in my first post, #3, of course as you fall off the wind, the tack of the jib should move more to windward such that on downwind legs, the jib tack is to windward of the mast.

    The purpose of my three figures is to illustrate the principle of how the forces could change, not to be precise instruction on how to trim the sails. Sail trimming will depend on any individual rig, and some rigs may be trimmed differently than others. These diagrams do not illustrate any particular boat, the way its sails are trimmed, or forces derived from actual measurements. What we can say about the change from Fig 1 to Fig 2 is: Imagine, if you will, that instead of moving the jib tack 10 degrees to leeward, you just turned the boat 10 degrees to leeward. The angle of attack of the air to the rig will go up, so the forces will go up. This resolves to more drive, and on most sailboats, when you do this, the boat goes faster—look at any VPP polar diagram for upwind sailing. So in the case of these figures, the jib tack moves to leeward by 10 degrees, and it goes from the windward side of “Course Sailed” to the leeward side of “Course Sailed”. First, the angle of attack to the rig increases. In addition, recall that the amount of force derived from the rig is created by the air circulation around the jib-main pair. The relative position of the jib to the main has changed—the relative positions are different, the slot between the two sails is different, and the flow around the two sails changes. I think that this creates greater circulation, and therefore, slightly larger forces around the rig. This is indicated by my wind tunnels tests wherein the lift increased slightly, and the lift-to-drag ratio improved slightly. That is, greater circulation, more force. Precisely how the sails are trimmed is immaterial at this level of discussion because it would be slightly different for any different rig. The point is, the jib tack changes to leeward, trim the sails as best you can for your boat, and the forces on the rig go up, providing a little more driving force. Boat speed would go up.

    To be perfect for a steady state case, the apparent wind in Fig. 2 would also increase and the Beta angle would reduce. But I am showing Fig. 2 as a transitory state between Fig. 1 and Fig. 3 before that has a chance to happen for the purposes of discussion. Note in Fig. 2 that the angle of the apparent wind to the rig centerline has increased by 5°, so it would be 60°. But let’s go onto Fig. 3.

    Yes, it doesn’t matter what the actual trim angles of the sails are for the purposes of these diagrams, they could be anything. If you are going to nit pick the sail trim, you can also nit pick the whole shape of the boat, the rig, the keel, and the centers of where the forces act. There are not all meant to be that representative—these figures are meant to illustrate the forces in general for the purpose of discussion of the principles. I was asked by the readers here to provide such diagrams to illustrate my point because no one in this entire thread had gotten it quite right yet.



    In Fig. 3, the forces of the rig reduce slightly because as the boat and rig turn to windward, the angle of attack to the wind reduces, so forces go down as a result, but the air circulation is still different by virtue of the fact that the jib is in its new location. The air circulation will be a little less than Figure 2, but still more than Figure 1. For the simplicity of drawing, I drew the force vectors in Fig. 3 the same length and direction as in Figure 1, but they could be slightly greater. The boat still has leeway, the difference between the boat centerline and the course sailed, so the hull (hull, keel AND rudder) are still generating the necessary forces equal and opposite to the rig. The rig, because of its improved circulation, however, in generating enough force to improve its angle of its centerline to the apparent wind, now 50 degrees (the angle to the boat showing as 45°--this is best case, perhaps, it could be a little more, whereas in Fig. 1 is was 60°

    Of course, in that situation, the gybed daggerboard has changed its twist relative to the hull for increased angle of attack for increased lift, which carries with it increased drag, but by virtue of the hull tracking at with a little less crab angle to its direction of travel, maybe the net result is lower total drag. But your direction of travel may still be as it was before. You may have faster boat speed, but will you point higher? Perhaps—everything at that level now depends on the boat at hand. And yes, my diagram is different. My sailboat and its keel are fixed and cannot move in relation to each other, so, because of that, the boat still sails at a bit of a crab angle. But the forces of the hull have to balance with those of the rig, equal and opposite, and that crab angle—the course sailed—is to windward of what it was in Fig. 1. Now, as I admit, the improved angle of course direction may not have a direct 1-to-1 correspondence to the angle of the offset jib to the main, but the concept is possible. Even Tom Speer, whose comments are well taken, allows for the possibility in his post above.

    What you call “negative leeway” is what I call “windway” and yes, I think windway is possible. And yes, if the boat speed has increased, then the Apparent Wind and its beta angle will change accordingly until the boat reaches a steady state condition. What that condition is for any particular boat is anyone’s guess, but I surmise that the direction of travel will be better to windward in most cases. The boat is still tracking to leeward of its own centerline, so it is still generating forces on the keel and rudder, but to the course track is to windward of the rig centerline and to windward of what it was before. In the end, the forces and angles may all be slightly different than shown on these figures, and the trimming of the sails and the use of the rudder to balance forces are all variables that we cannot precisely define with generic diagrams. But these figures show the principle.

    My client for Project Amazon and I considered the concept of windway in relation to the adjustable shape of the keel with its huge faired trim tab and its canting ability, we felt it was possible to generate enough lift to sail the boat with windway. This is very similar to the concept of your gybing daggerboard. Depending on how we set the trim tab, to windward or to leeward, we could put lift on the keel to either side we wanted, and to varying values of lift. In fact, the Around Alone Race committee certified the trim tab as a bona fide emergency rudder after witnessing appropriate sea trials to demonstrate it in Charleston harbor. However, we never got a chance to study and test the science of the adjustable keel shape and windway because of owner’s commitment and preparations for entering the 1998 Around Alone Race, and the sale of the boat immediately thereafter to a particularly poorly funded and inexperienced sailor. The last I heard, Project Amazon was scrapped by that last owner. But that is a whole other story.

    And my point is not so much in absolutes, but it is in trying to spark everyone’s thinking that, just because boats are built symmetrically about a fore/aft centerline, and that we stick the tack of the jib on the bow, it does not mean that that is the best place to put it. Maybe we should move it around. Not once, in all of the documented studies that I am aware of, has anyone attempted to test whether the centerline of the boat is the best place to put the jib. Why should it always be on the centerline? Why should the jib always be well forward of and in line with the main? Why can’t it be off to one side to open up the slot more? Very few people have tried it, and although those that have reported improved performance, no one has studied this concept scientifically—that is, in a properly designed and executed study. If they have, I would really like to know about it.

    I put it to you and all of the readers here that we should experiment with putting the jib tack in different places to generate more force from a given sail area and selection of sails by improved slot shape and improved air circulation around the rig. That is what I mean when I say “thinking outside the box”—the box is our collective mental inertia and reluctance to change the rig outside of normal racing rules—the major racing rules don’t allow you to move the jib tack off the boat’s centerline, but that is a political rule, not an aerodynamic or scientific rule. From what I have seen in my experience, there is a real benefit to putting the jib in different places, as I have described.

    Eric
     
    1 person likes this.
  8. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Looks as though there has been a lot of interesting discussion and material brought to this subject thread. I'll have to sit down and read it thru when I get back to the USA.

    I had wanted to reference some things from the Sail Aerodynamics subject thread, and particularly some of Tom Speers excellent contributions, but a lot of those links and bookmarks are stored on my main desktop computer, and all I have with me in Thailand is a new laptop without a lot of my old references.

    With the quality of the participants I felt you better have some reference links to back up your submissions, and since my older brain is not working quite as good these days, I need to reread things several times before they are 'registered' ;):eek:

    Good thread,
    Brian

    PS: Of course I'm going to be looking for favorable arguments in support of 'jibs' as I'm a headsail sort of guy :p:D
     
  9. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 53, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member


    The diagram shows an AWA of 55 degrees. That would be spinnaker reaching (90-110 TWA) in light or medium wind or beam reaching (75-90 TWA) in heavier wind. I can't image it being upwind sailing with any decent sail boat in any conditions.

    Yes, but the clearly most important reason for this is increased TWA and AWA measured to the boats course not the angle of attack the sails are trimmed to. With higher AWA you have much more drive force compared to heeling forces, which are limited by stability and/or hydrodynamic side forces.

    On most sail boats you must start depowering at 8-12 knots of wind. After that it is not higher force you need, only better efficiency (lift/drag or drive/heel).
     
  10. Paul B

    Paul B Previous Member

    I was going to address the issues with the Wolfson report, but now I see this gibberish and will address this instead.


    Having Apparent wind forward of the beam is now upwind sailing? Who came up with that definition? Your report from the Wolson Unit says something different.

    You could easily have an AWA forward of the beam and tack back and forth forever and continue to lose ground to windward. Do you realize your 55 degrees AWA would be a TWA of more than 80 degrees in most cases? So you are tacking through more than 160 degrees.

    (Note: I was beaten to the punch on this one by Joakim.)


    With an AWA of 55 degrees It would seem the tack should already be to windward.


    To have a clear illustration you do need to be precise about the trim. If you would bother to do so you would find some of your assumptions fall apart. You are making claims that do not hold up when you consider the sail trim requirements.


    No it doesn’t. As you turn down 10 degrees your heading is moving down as well. You may make a bit less leeway, but you are still making leeway.


    I think quite a few people get it. I think the person who doesn’t get it is you.


    No, it isn’t. Relative to the wind the jib is now back to the same place it was in Fig. 1. The tack is where it was and the clew needs to move back to it’s original position. You can’t leave it eased as it was in Fig 2 or it would be luffing in Fig 3. So the sailplan is no different, and the “circulation” and forces would be no different.


    I just can't go on doing this point-by-point. You obviously have 30 years invested into this theory and you are just like a religious zealot. Facts mean nothing. I would think that after so many years of being a proponent of this theory you would have created some sort of coherent model that could be argued. I think the fact you haven't speaks volumes.
     
  11. Grey Ghost
    Joined: Aug 2012
    Posts: 194
    Likes: 9, Points: 0, Legacy Rep: 94
    Location: california

    Grey Ghost Senior Member

    Why don't you post some clear illustrations Paul B.
    A drawing is clearer than so many words.
     
  12. Boat Design Net Moderator
    Joined: Feb 2010
    Posts: 564
    Likes: 162, Points: 43, Legacy Rep: 1004
    Location: www.boatdesign.net

    Boat Design Net Moderator Moderator

    Let’s please try and keep the discussion polite to all other members so the thread can be something everyone can learn from and not sink to insults or personal attacks please. Thank you.
     
  13. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,021
    Likes: 248, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    When posts descend to unwarranted insults and personal attacks, it's time to leave the thread. I contributed an idea, and I explained my reasoning as asked. The discussion was healthy, informative, and fun until now. Jeff, thank you for your reminder.

    Eric
     
  14. Paul B

    Paul B Previous Member

    <removed jab towards another member>

    You did more than contribute an idea. You made a claim that was not true. When asked to provide a real explanation you continued to ignore real issues and provided flawed information in the <removed insult towards another member>

    I have no doubt you will stand down for now, but expect sometime in the future <removed insult towards another member>
     
    Last edited by a moderator: Sep 21, 2012

  15. TeddyDiver
    Joined: Dec 2007
    Posts: 2,614
    Likes: 136, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Ok Paul, you made it again.. being yourself as usual.
     
Loading...
Similar Threads
  1. Roelina
    Replies:
    2
    Views:
    3,051
  2. Earl Boebert
    Replies:
    6
    Views:
    3,593
  3. dustman
    Replies:
    25
    Views:
    3,832
  4. Alexanov
    Replies:
    2
    Views:
    887
  5. tropostudio
    Replies:
    30
    Views:
    5,376
  6. nzl51
    Replies:
    5
    Views:
    1,858
  7. Brian Needham
    Replies:
    21
    Views:
    4,506
  8. massandspace
    Replies:
    11
    Views:
    2,203
  9. Southern Cross
    Replies:
    32
    Views:
    5,597
  10. laukejas
    Replies:
    40
    Views:
    8,535
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.