Unstayed Mast Jibs

Discussion in 'Sailboats' started by PI Design, Nov 16, 2006.

  1. PI Design
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    PI Design Senior Member

    One of the problems often cited with an unstayed mast is that jib luf tension is low. But why is this a problem? If the sailed is designed to have a large amount of luff sag, why should it perform any worse than a jib with a tight luff? Due to their very high speed, the asymmetric kite on a 49er operates at an angle of attack similar to most conventional yacht and dinghy jibs, so why aren't there any jibs that look like a 49er kite? Just curious.
     
  2. Crag Cay
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    Crag Cay Senior Member

    As you are in England, perhaps I can be a bit parochial and suggest you take a look at a Sonata that is doing well at the moment in the class at a national level.

    Originally David Thomas designed them with both cap and lower shrouds close to being in line with the mast, but with a fairly stiff top mast so the backstay would transmit some tension to the fractional forestay.

    Then Proctor started to make masts with a bendy tapered top that allowed the backstay to depower the main but forestay tension was lost. So the measurement rules were 'exploited' to move the masts to their extreme allowable forward position, and the shrouds moved aft to theirs. Due to the vagaries of Hunter's building quality, some boats were better able to do this than others as their main bulkheads varied in their position. Cap shroud tension was then wound on (à la J24) to try and get forestay tension.

    Well, this attempt to worship the 'god' of headsail luff tension actually only resulted in boats being pulled apart. The aft angles were so marginal that massive shround tension only ever gave marginal results. So ten years or so ago, Goacher Sails stopped fighting the problem and returned the masts to their 'inline' positions and cut genoas to match the sagging forestay. And they have been unbeaten since. It take a while to get used to the different interaction between the sail controls, but works in a wide range of sea states and windspeeds.

    I think the modern Code Zeros set on their soft furlers probably have similar properties.
     
  3. PI Design
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    PI Design Senior Member

    Thanks for that, it confirms what I was thinking. Basically, it is not imperative to have a boat breakingly high rig tension, it's just another trend that is blindly copied.
     
  4. Crag Cay
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    Crag Cay Senior Member

    Perhaps I need to qualify my statement. I have no evidence that a boat with a really tight foresail luff does not still have an advantage. I guess the thrust of my waffle (can waffle have thrust?) is that if luff tension is not easily achieved, an effecient and fast rig can still be had.

    But as this is not the norm, you have both to find a sailmaker that can work with this and get used to sailing this type of rig. I spent one season with my rig looking perfect. Trim and set were exactly as in the text book, but we went backwards. Sonatas have actually take the notion of 'speed wrinkles' to a new level and it takes a while to believe that wringled and sagging sails can be fast.
     
  5. PI Design
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    PI Design Senior Member

    My statement was a bit sweeping. I can see that a tighter forestay leads to a more upright mast (for mast which need stays to hold them up), which clearly has advantages. However, I am pleased to hear that you can still have a fast boat with less luff tension, albeit there may be some new trimming techniques required. Presumably the leach is less twisted when there is more luff sag, which I would have thought was a Good Thing.
     
  6. Crag Cay
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    Crag Cay Senior Member

    If you hold a straight edge against these luffs you can see the amount of sag that was acceptable even in these winds. Whether this sort of tightness is possible with an unstayed rig, I don't know.

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  7. CT 249
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    CT 249 Senior Member

    There is one major problem with the idea of major speed wrinkles. When the J/24s brought them in, they had to get used to people passing by yelling out "pull some halyard tension on, ya bunch of idiots".

    Sagging the luff does effectively reduce twist and narrow the sheeting angle, which is one reason the J/24s do it. I have a J/24-style No 1 on my old half tonner because I didn't wand to subject the boat to massive rig tensions. Unfortunately, I suspect that because the sail sheets to a narrow stern, the sag means that we end up with too little mid-leach twist. That's one trap.

    Flying 15s (another Goacher class) used to ease rig tension in a breeze to depower (and may still do). The luff sag may have increased, but the overall speed still increased. There must be many other classes where a slight potential loss of performance is more than made up by having a simpler, less-stressed rig.

    The Juan K IMS 50 Crazy Kyote had a jib on a carbon unstayed mast. The few pics I saw didn't seem to show significant luff sag.
     
  8. yokebutt
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    yokebutt Boatbuilder

    On Ocean Planet we literally couldn't see the jib at all from the cockpit without the runner on.
     
  9. national
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    national Junior Member

    I think historically most masts need the tension to help with mast bend. A slack rig can also limit the life of your sails if they flog, and care must be taken not to choke the slot as different sheeting position are needed.
     
  10. Crag Cay
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    Crag Cay Senior Member

    LOL - I know, and these sorts of comments, plus a life time of thinking likewise, had me over tightening halyards for the first season. But one-design racing is a brutal task master and there is no arguing with the results - we were shyte! So in came the crew mantra: "When in doubt, let it out". Some of this change in mind set also was needed with the swap from dacron to mylar sails/ spectra halyards, etc

    The performance transformation was however stunning and the results began to come. And that's what makes it all worthwhile. But I remember looking down at a smart set of cut crystal prize glasses at the end of Cowes Week through celebratory beer swollen eyes and being overwhelmed by a wave of reality. 'Boy', I thought, 'they do look nice - but then they should: they've cost me about two thousand pounds each!'

    So I had a few more beers and by the end of the evening I had firmly driven all such rational thoughts from my mind, and was hanging around the shoulders of our sailmaker telling him how much I loved him and his sails and he could definitely put me down for another complete suit that winter. Yacht racing, you've got to love it.....
     
  11. brian eiland
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    brian eiland Senior Member

    I'm quoting this subject thread on another forum in a discussion of headstay sag. But I had not read through it to the last submission by Crag.....that was great...LOL :D
     

  12. rob denney
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    rob denney Senior Member

    G'day,

    I have used headsails on unstayed masts with ballestron booms (see video at http://www.youtube.com/watch?v=8chR6DAFjGA). I have found that while excess sag can be allowed for in the sail cut, it is only good for one apparent wind strength. As the breeze gets up, it sags more. As it drops, the headsail draft moves way forward and gets very deep.

    Mainsheet tension can keep the forestay reasonably tight fore and aft, but the mast bends sideways, which opens up the slot the harder it blows (good), but also moves the head of the jib lower and to leeward (not so good). The result is a sail with the draft well aft, which does not go upwind well, and when it luffs, flogs.

    I daresay there are solutions with sheeting angles, runners and temporary shrouds, but all add complication to a rig with a primary benefit of simplicity. My fix is to ditch the jib and use a taller wing section mast.

    Eric Hall of Hall Spars has an unstayed rig on a J boat. Reckons it makes better vmg with a jib. He uses runners. He did not make any alterations to the boat apart from strengthening the mast partners. Be interesting to see how much lighter the boat could have been built if it had been designed for an unstayed mast from the beginning.

    regards,

    Rob
     
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