Sheet Stretch

Discussion in 'Sailboats' started by SuperPiper, Apr 9, 2008.

  1. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    Wire halyards have been replaced with ultra-low-stretch braids.

    Here is something that I have never heard discussed: should sheets be stiff or is some stretch preferred? Like a bendy mast, would a springy sheet provide some gust response? Would it allow a sail to ease by a deci-fraction in a gust and then provide that little pump when the air pressure softened again and the sheet returned to its original length?

    What are the characteristics that racers look for in their sheets? Is a low-stretch sheet better or worse than a regular braid?
     
  2. alan white
    Joined: Mar 2007
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    alan white Senior Member

    I'm all for a little bit of stretch to ease the shock loading on hardware. I've got wood spars and old sails, and I use twisted (laid, three strand) lines. I may one day replace those with double braided dacron for sheets, though I like laid rope for it's splice-ability.
    Especially when an unexpected gybe occurs, I think Spectra or other no-stretch ropes act exactly like wires, multiplying shock loads.
    Racers are probably apt to go for the stiffest stuff available, but then their rigging components are very strong and well inspected. I'm no racer so I'd also be curious whether they like a bit of give in their sheets.
    I would add that often times racers do exactly the opposite that safety and good sense would dictate should be done, though I see a lot of people setting their boats up as if their goals were identical with racers.

    Alan
     
  3. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    More About Sheets

    A few threads back - Sail Loading On Rig - is a series of translations from Chapter VI, of Pierre Gutelle's work from "Design of Sailing Yachts" Part II.

    Section VI-II-6 - 'Safety Coefficients' contains a table of multipliers to be used to convert from static design to dynamic conditions:

    - Forestay & Backstay: 3.0 - 3.5
    - Cap Shrouds: 1.9 - 2.2
    - Lower Shrouds: 1.8 - 2.5
    - Halyards: 2.5
    - Sheets: 5.0

    This implies that the load on sheets can vary as much as 500% from the static design condition to the real world condition. This safety coefficient is greater than the other rigging elements. That surprised me.

    What do you figure?
     
  4. water addict
    Joined: Jun 2004
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    Location: maryland

    water addict Naval Architect

    Generaly you want some give in the sheets to keep from blowing out gear from dynamic loads. Need to absorb the energy somewhere, or else stuff breaks.
    I sailed on a 44 footer years back that was originally rigged with kevlar running backs, back when folks were experimenting with various stuff for rigging. The kevlar kept exploding. Went back to spectra and dacron type stuff.
     
  5. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Sheets are generally sized way above working loads due to handling issues. The human hand, in other words, determines the minimum size requirement.
    Even a large boat might get away with relatively small diameter line, which would stretch nicely instead of transferring shock loads to blocks and attachments. The tackle simply increases in ratio. If a winch were used rather than increasing mechanical advantage through tackle parts, size would have to increase at that point.
    Trouble is there's no simple way to have a nice big fat rope in your hand unless the whole 75 ft or whatever is fat too. This makes for more drag and weight when ghosting but sailors seem to prefer a fat and unbreakable rope to a skinny one that's hard to grip comfortably.
    So 500% isn't necessarily a strength requirement, but what happens when ropes are fat enough to grip.

    Alan
     

  6. deepsix
    Joined: Dec 2007
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    deepsix Senior Member

    If your sails are made from a high modulus material, it is best to use high modulus halyards, and in some cases sheets. Dacron sails are far more tollerant of stretch. When a puff hits, the haylard will stretch and the draft in the sail will move backwards causing more heeling force and less driving force.

    I personnaly like a little stretch in my spinnaker halyards, because the sail shape is not strictly dependant on halyard tension and it does reduce the shock loading as the sail fills. High modulus jib/genoa sheets are mostly unnecessary, because the length of the sheet that is loaded is rather short. Compare it to a jib/genoa halyard, if the halyard stretches 0.5% then your sail shape will be terrible, but 0.5% stretch in a short jib sheet is not really a problem.

    Its the usual story if you are looking for the last bit of performance low stretch halyards are very important, but if you sails are blown out, or you are cruising dont waste your money.

    Alot of racers will remove the cover from the line, only leaving the section in the clutch/cleat and where the line is held by the crew. In some cases you can add an additional cover or core in order to thicken the rope so it is easier to handle.
     
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