New design for a self tacking catamaran jib

Discussion in 'Boat Design' started by MichaelRoberts, Feb 3, 2016.

  1. MichaelRoberts
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    MichaelRoberts Junior Member

    I'm liking the rope idea: A continuous loop tied at the clew, two pulleys, one on each side, maybe they are on short tracks for trimming.

    So now we have a hybrid system: A rope self tack, but sheets from the pulleys on the tracks.

    Now we can have a bit of overlap, not much, and can tension the rope loop by pulling the track mounted pulleys back with sheets.
     
  2. rwatson
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    rwatson Senior Member

    Comments found on Sailnet
    "One of the popular myths is that overlapping sails are good for going to weather. Strictly speaking that is not the case. As mentioned above since the leading edge of the sail generates the bulk of the drive and the rest of the sail is generating proportionately greater drag for minimal drive. Most boats can point higher with a 110% (i.e. non- overlapping sail) than they can with their Genoas in almost all conditions. So then, why use Genoas? Well, as I mentioned above, the ability of a boat to go to weather has two components, speed and direction. The greater sail area of the Genoa increases speed but requires the boat to fall off a little bit in order to do so. This results in a greater VMG to windward than would be achieved without the lace Genoa on boats with rigs designed to use Genoas. Modern high performance rigs are generally not designed to use Genoas. If you look at the current crop of race boats, and many of the cruising boats that are evolving from them, the rig of choice up wind is a fractional rig with a non-overlapping jib. The sails are proportioned to work through a wide range of windspeeds from next to nothing on up to very heavy air. The fractional rig allows depowering without shortening sail though a wider range of windspeeds.


    Off the wind they use masthead chutes and reachers but that is another topic."


    Personally, its hard to detect any speed increase with overlapping jibs on ordinary sailboats. With a non -rotating mast, the jib either destroys the the flow over the main, and unless the leech of the jib is paralell to the shape of the mainsail ( rare ) any lift increase is very patchy, with potential for backwinding the main.

    As the previous quote indicates, lift to windward is a 'length of luff' problem.
     
  3. CT249
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    CT249 Senior Member

    Having a genoa up instead of a jib can create a massive increase in upwind speed and height in the right conditions. The best example in Australian racing these days would be J/24s - if you get caught out down-range with the jib in chop you have no chance whatsoever of hanging in there.

    Another example, albeit with many more variables, is racing something like a J/24 against an Etchells upwind in light winds and a bit of a slop. The Etchells, a faster, longer and skinnier boat with a bigger mainsail, is significantly slower than the J/24 with its big headsail on.

    Modern boats normally have some overlap (around 105% IIRC) because it's much faster than zero overlap.

    I thought Tom Speer et al have shown to us time and time again that a jib does not destroy the flow over the main, and that a non-rotating mast is not as disastrous to flow over the main as old sources used to claim?
     
  4. Barra
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    Barra Junior Member

    Exactly CT249.

    Another modern example of a boat with non overlapping headsail is the Seawind 1000xl.

    Having raced against a couple , I can assure you they are much faster in the lighter stuff with their large overlapping screacher unfurled and pulling on the windward legs. Sailing lower and faster.

    A lot of the myth regarding rotating masts being so much better, probably stems
    from boats such as Hobie 16s , where an unrotated mast is painfully slow.

    This has everything to do with the spars bend characteristics and hence sail draft/shape and a lot less to do with the flow around the mast.

    Mast rotation can be a great way of controlling sail draft/shape through varying mast bend on smaller boats.A lot more difficult on larger ones.
     
  5. gonzo
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    gonzo Senior Member

    There is a huge fallacy in the argument of overlapping vs not. An increase of sail area in low winds will produce more speed. However, a larger main with the same foretriangle area would be more efficient. Genoas are only good at beating a measuring rule.
     
  6. rwatson
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    rwatson Senior Member

    Of COURSE a LARGER sail is faster!!!!
    The discussion here is whether the overlap of the jib increases speed ( by affecting flow over the main ?) .
    The question is - If the jib was the same size, but didnt overlap - is there a performance benefit ?
    The sources I have read indicate that you have to have a really sophisticated setup to make it happen, and on the "average" boat, it makes little difference.

    Yeah - because you can fit a larger sail with overlap. Thats about it.

    I would agree with "a jib does not destroy the flow over the main" if you phrased it "a jib does not ALWAYS destroy the flow over a main".
    I would have no trouble demonstrating the flow on a main being destroyed by backwinding from a poorly positioned/shaped jib.
     
  7. brian eiland
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    brian eiland Senior Member

    You got it with that explanation. :!:

    There has been LOTS of discussions about the aero advantages of an overlapping headsail over on the 'Sail Aerodynamics' subject thread, but there are still those people who refuse to believe it.

    So I will repeat a few postings I've saved from that subject thread as well as other subject threads:
     
  8. brian eiland
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    brian eiland Senior Member

    :rolleyes::eek:



    http://www.boatdesign.net/forums/sailboats/code-zero-sails-
    aerodynamic-questions-53291.html#post737374
     
  9. Skyak
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    Skyak Senior Member

    So Brian, where are we? I think I finished off the original question back on the first page. Since then we have digressed into opinions about the value of jib overlap. I completely agree with the T Speer characterization of sail interaction and slot -a 110% beats a 90% by much more than the increased area.

    The overlap naysayers seem to be racers knocking Genoas 130%+
    I can agree that overlap past 110% is not as beneficial as non-overlap area. But adding non-overlap area requires expensive additional structure. And those big Genoas DO add performance downwind. I think racers tend to ignore the downwind value of Genoas because they get more out of spinnakers. But for cruisers, spinnakers are too fragile to set and forget -they pop in no time. A poled out Genoa is the cruiser solution for downwind.

    So are we finished with the digression? Overlap is worth something? Are we going to take a whack at the original request and try some concepts for self tacking of overlapping jib? Or do you accept the most attractive option is a 90% boom and furling?
     
  10. brian eiland
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    brian eiland Senior Member

    Perhaps I should have said, You got it totally correct there Skyak. I wasn't criticizing your contribution.

    My addition to the subject thread was directed at those contributors that continue to condemn and discredit the overlapping genoa. I've certainly seen my share of that condemnation with reference to the two headsails on my aftmast rig.

    I think the OP has been given plenty of info to make his choice on his own at this point.
     
  11. CT249
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    CT249 Senior Member

    Yep. Those who say that overlap only counts because it is extra sail area haven't been looking at development dinghy and skiff classes that measure the full sail area, or allow unrestricted area. Many of them have found that a little bit of overlap is a very efficient way of using their sail area.

    The genoa is also interesting. The dinghy with the biggest genoa is the Flying Dutchman, which gained a lot of speed by going from a short overlap to a big one. It is a pain to tack, but the amount of extra structure is comparatively small, all else being equal, compared to the apparent gain in speed.

    The idea that classes like IRC boats only for go for overlap to fit a larger sail is simply wrong. You can carry more sail in many other ways, and under IRC the overlap is measured. They go for a small overlap because it is the fastest way to use a given amount of sail in many craft.
     

  12. FAST FRED
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    FAST FRED Senior Member

    I'm liking the rope idea: A continuous loop tied at the clew, two pulleys, one on each side, maybe they are on short tracks for trimming.


    The deck line traveler is before the mast , as is the sewn in loop that allows the sail to tack it self and the luff start to pull quickly.

    Only the overlaping portion of the jib needs to be tightened after the tack by the clew. ..

    you can answer the aerodynamic question of weather overlap helps Your boat with simple observation.
     
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