Jib question

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

  1. BobBill
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    BobBill Senior Member

    What would it take for a jib to improve pointing? Let's say for a 420 with normal main, could the foresail be just a sliver?

    Would that apply to a cat or proa? Obviously, from the question, I am not versed in topic. Just generally curious.
     
  2. Doug Lord
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    Doug Lord Flight Ready

  3. Eric Sponberg
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    Eric Sponberg Senior Member

    You want to know the secret weapon?? Here it is: You should change the headstay attachment so that you can swing the tack of the jib from side to side. This means that you have to fit the boat with either a swinging bowsprit or gin pole, or "bulls horns" as on Hunter's Child. Then, when you are sailing on the wind, move the tack of the headsail to leeward. Every degree of angle you swing the jib tack to leeward, as measured from the jib tack to the mast, you gain that number of degrees to windward.

    So, say on a normal 420 and the conventional tack configuration, you are sailing at 45 degrees to the wind. Now, with the new equipment fitted to the boat, you swing the jib tack 10 degrees to leeward. To get the same lift configuration to the sails as before, you have to point the bow 10 degrees closer to the wind. So now you are sailing to the wind at 35 degrees while your competitors are still sailing at 45 degrees. You'll beat the pants off them on the race course.

    Now, head downwind. Sail deep downwind, deeper than the other guys, because now you can swing your jib to windward--again, for about every degree of swing of the jib tack to windward on the downwind legs, you gain that many degrees sailing deeper. You'll whiz by those guys leaving them in your wake.

    This is all true, I am not making this up. I have tested it myself.

    If it is all true, then why don't we see this happen on the race course? Very simple: It is against all known sailing rules to move the tack of the jib. It cannot move. Period. It would make any one-design class boat in violation of the class rules. The politics of racing rules says it cannot be done. So there. So, are you going to obey the politics or are you going to experiment with nifty ideas that have a proven basis in physics?

    Give it a try. Then report back here to your findings.

    Eric
     
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  4. BobBill
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    BobBill Senior Member

    Eric, very interesting, never even entered my mind...thanks. And it makes so much sense. I believe it will be worth the effort, and I will report. :]
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Good luck. This is a good example of "thinking outside the box." You may have seen on another active thread here with a wonderful quote by Richard Woods who said, "You need lots of experience, because you need to know what the current box size is before you can get outside it." (Link: http://www.boatdesign.net/forums/sailboats/cheap-simple-rig-43209-4.html).

    Good luck, and I look forward to the results of your efforts.

    Eric
     
  6. BobBill
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    BobBill Senior Member

    Thanks. Great quote. I went there...I think I have a good approximation of box size and will be fun. Small boat, so life is easier. Will advise as things perk. + 1 to Rob Denney...geniuses think similarly :] Am reinforced.
     
  7. rwatson
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    rwatson Senior Member

    So Rob Denney ( who copied the idea of baelstrom rigs from years of other developers - not his sole idea at all) designed boats will point much much higher than any other boats ??

    It hasn't happened yet.
     
    Last edited: Jun 10, 2012
  8. BobBill
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    BobBill Senior Member

    I don't believe Eric's comment was meant to "invent" the idea, just explain (roughly to me ignorance) effects and some why's. I certainly can see why not...and even can see how some rigs, maybe crab-claw types, are preferred in some regions.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Eric,
    Do you have any explanatory drawing for what you have described? English is not my first language, so I am having problems with visualizing it.
    It doesn't happen too often, but from time to time it does. :)
    Cheers
     
  10. BobBill
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    BobBill Senior Member

    I visualized a sort of sliding tack up forward, like a traveler...but limited to beam at that point, which would not really be much on my wee boat. And, it seems, complicated to rig, so impractical. Unless I am (again) discombobulated? Simple is best.
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    Daiquiri (and all), see the attached picture which I scanned from C.A. Marchaj's book "Sail Performance, Theory and Practice", Figure 107, page 124. This shows the basic forces on a sailboat, both from the wind and the water. Look at the figure to the left, 107a.

    Note that by definition, the jib and main are located on the boat's centerline, and that the forces on the rig are resolved through a line parallel to the boat's centerline. This line makes an angle alpha-a to the line of apparent wind.

    Now, imagine that the tack of the jib is moved some distance to leeward, no longer on the boat's centerline. The line connectiong the main to the jib rotates some degrees to leeward. As such, the space between the main and jib opens up and assumes a different shape. The circulatory flow around the sails changes, and the resultant forces also rotate to leeward about a like amount. This is just about like falling off the wind, except that you haven't fallen off the wind, but you have effectively rotated the rig so that the air thinks so.

    Generally, as a boat falls off the wind, the forces increase slightly, and that is probably what happens here, except that you still have the ability to reorient the forces back to where they were. To do that, you point the boat higher into the wind so that jib, in space, comes back to its original position, but the boat is now pointed that many degrees closer to the wind, precisely in the direction that you want to go--you are trying to get to windward. Of course, sail trimming of both main and jib is necessary, but the boat is sailing closer to the wind by virtue of the fact that the jib is offset to leeward but the hull isn't.

    The flow around the keel will change as the hull rotates to windward, but the forces will resolve themselve accordingly. I haven't done a full analytical study on this concept yet, but I have seen it in practice, and people who have tested this concept on their boats report that it is real.

    You can manipulate the jib tack off the wind too. When sailing off the wind, the jib tack should move to windward which causes the boat to be turned more downwind, again the direction you want to go.

    So we can gain all sorts of impoved speed made good to windward if we alter the placement of the jib tack. Unfortunately, many racing/rating rules don't allow you to do this. But nothing stops a cruising sailor from doing this, or an open class sailor who would be subject to much less restrictive rules.

    Eric

    PS: That's about the best that I can do today. I have a lot on my plate today and don't have time to produce a better sketch.
     

    Attached Files:

  12. BobBill
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    BobBill Senior Member

    Your help is very appreciated, and that you took the time out from your work (plate) to help.
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    I was just rereading that chapter last weekend trying to visualize just those pictures. The ketch I'm building will have a version of dipping lugs and I've allready reasoned to have the tack points movable, which by the lugsails virtue, is easy. Now this open's new point of view... Maybe a kind of f'c's'le to have a horse for the jib tack, and leave the bouspret just for fun and chute.. :D
     
  14. Paul B

    Paul B Previous Member

    You might really want to reconsider this, lest you lose your standing as "one of the most respected NAs in the world today".

    Here's a hint: Look at the reasons gybing boards work.
     

  15. TeddyDiver
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    TeddyDiver Gollywobbler

    Paul, tell just what you think about the subject and cut the crap.. I know you know a lot so there's no reason for this..
     
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