Thoughts on synthetic rigging pretension.

Discussion in 'Sailboats' started by Gashmore, Nov 28, 2009.

  1. Gashmore
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    Gashmore Junior Member

    I need someone to double check my thinking on shroud pretension.

    Most books on rig tuning say that cap shrouds should be pretensioned to 15-20% of MBL. This presents a problem for synthetics like Dynex Dux. My recent experiments found that when static loads exceed about 10% of MBL creep becomes unacceptable. That got me wondering about the physics of pretensioning.

    I found a section in Dedekam's Sail and Rig Tuning that stated that as the wind load increases the windward shroud stretches and the masthead moves leeward decreasing the load on the leeward shroud. As the sum of the forces must remain the same the tension on the leeward shroud transfers to the windward shroud and the masthead only moves half the distance it would with no pretension. Now, I had a hard time getting my brain around that last part but a simple experiment with weights and springs demonstrates it is true.

    So, ideally,pretension should be half the expected maximum tension on a shroud. Any more just puts excess compression on the mast and any less will allow the leeward shroud to go slack. With rod and wire where safety factors are in the range of 30% to 40% of MBL 15%-20% of MBL is half the expected maximum but in synthetics where the safety factor is more like 20% a maximum pretension of 10% of MBL is more appropriate.
     
  2. gonzo
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    gonzo Senior Member

    Pretension takes the stretch off the standing rigging, or as much as possible. With a material that stretches less, there is no or less need to pretension.
     
  3. RHough
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    RHough Retro Dude

    The pretension is based on the proper sizing of the rigging bit in question. Getting it right with only one shroud per side and no spreader is pretty easy. Each spreader and set of stays makes the problem more complex.

    Do your math and find the maximum load the shroud will see. Size the shroud so that "normal" loads with a safety factor are under the permanent deformation level. IIRC max RM and shroud base serve to calculate the total load. The safety factor is to keep normal loads within the elastic recovery limits of the rigging material.

    Initial dockside tuning is a guess, but a simple method for monos is to use a halyard to heel the boat about 25 degrees. If the "leeward" shroud(s) are slack or the mast is out of column return the boat to upright. Adjust tension as needed and repeat the test.

    Then go sailing and verify that leeward shrouds are never slack and the mast is in column. Make small adjustments until you have enough pre-load on each bit of rig so nothing ever goes completely slack when unloaded.

    Back at the dock. Find a way to record settings. Recheck new rigging at 30 days and 180 days. If everything is right the rig will hold its tune at those settings for years.

    If you find the rig slack, it is proof that you have reached the creep point of your synthetic. Go up a size and re-tune.

    Have a beer.
     
  4. Gashmore
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    Gashmore Junior Member

    That last statement is what I am attempting to avoid. :D

    I have run the numbers several different ways including the Nordic Boat Standard and Finite Element Analysis software and come up very close on the design loads. Settled on the NBS numbers. (The raw results from the FIA may be fine for an all out racer but make me more than a little nervous.) Now I have to determine the maximum pretension to make sure creep doesn't get out of control.

    For example the NBS worked out to 4,050 lb on the cap shroud and 9mm Dynex Dux has a MBL of 24,000 lb so it will have a nice conservative factor of safety of about 6:1 or 16% of MBL. BUT the pretension has to be under 2,000 lb to avoid creep.

    What I am getting at is that with any "new" technology you can't use generally accepted rules of thumb. You really have to understand what is happening, what the rule of thumb was trying to accomplish and adjust to the new requirements. What I have arrived at is that once a factor of safety has been applied MBL is irrelevant. Pretension is related to design maximum tension rather than MBL of the line.
     
    Last edited: Nov 28, 2009
  5. RHough
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    RHough Retro Dude

    YES! The pretension <> design tension relationship is that at design tension the material is within its elastic range. How close to permanent deflection loads your design load is depends on rigging material properties.

    In your case, I would say that the tension threshold that produces creep is the design load. Safety margin will take care of itself. I don't think it is enough to just keep the static (pre-tension) under the creep limit, any sailing done will put the rig into the creep zone and you will have a loose rig after sailing. That is the surest indicator there is that rigging is undersize.
     
  6. Gashmore
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    Gashmore Junior Member

    And that brings in the dimension of time. Dynex is a plastic and by definition plastics creep over time. The elements of time to consider are required service life and ratio of time in port under pretension alone and sailing time under dynamic load.

    Without more data on degradation due to UV and oxidation I am assuming a five year service life. (Probably conservative). The cap shroud is the critical one. In 9mm the design load is about 17% of MBL. )The intermediate and lowers are way over spec.) I want to use turnbuckles rather than deadeyes and a 5/8" turnbuckle seems to be a fair match to 9mm Dux. It has an adjustment range of about 5". Allowing 2" to take up the first year of settling in leaves .75" per year for the remaining 4 years.

    Then for a cruising boat I have to consider the ratio of sailing time to idle time. I am guessing a maximum of 20% actually sailing and (hopefully :D ) less than 5% at the full design load of 18% MBL. My testing however indicates that creep in Dux is exponential relative to stress and linear to time so sailing time has more weight than time in port.

    I will have to ponder that a little as some quick figuring says I will have to reposition the splices at year 3.
     
  7. messabout
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    messabout Senior Member

    Why is it necessary to pretension in such a way as to have tension in the leeward shroud.? Many dinghys set tension so that the lee shroud is intentionally slack, or at least neutral, when on the wind. I presume that one must control mast flop in a bigger boat and also take some measures to keep the jib stay tight but the backstay will usually take care of jib stay tension.
     
  8. Gashmore
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    Gashmore Junior Member

    As the shroud stretches the tip of the mast leans to leeward moving the center of effort of the sail further from the center line creating more weather helm. That can cause serious steering problems in a big boat so you want to keep the stretch in the shrouds at a minimum.

    Here is an example. Say the shrouds of 3/8" 1x19 and 50' long have little tension on it at rest. Each inch of 3/8" 1x19 stretches .00058" for every 1,000 pounds of tension. When the wind picks up, a load of, say, 2,000 pounds is put on the windward shroud. It will stretch about .00058*600 inches*2(thousand pounds) or .70". Now if they are pretensioned to 1,000 pounds and then the windward shroud is loaded to 2,000 pounds, as it stretches the leeward shroud contracts and looses its pretension. Because the sum of all the forces has to balance, that 1,000 pounds of tension transfers to the windward shroud. The windward shroud tension goes from 1,000 to 2,000 pounds and the total stretch is only .00058*600*1 or .35". Half of the un-pretensioned distance.
     
  9. RHough
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    RHough Retro Dude

    Slack rigging creates higher peak loading. Every tack the slack rig goes tight with more or less of a bang. All the fitting have to be oversize or they will soome fail due to the shock loading.

    Tie a bit of string to a brick and pick it up off the floor. Tie the string around your finger.
    Move the brick up and down like a yo-yo, fastest and faster ... at some point one of two things will happen. Either the string will snap or the string will cut off your finger.
    If you put slack in the string first then drop it, one of these things will happen sooner. :)

    Slack rigging is bad rigging.
     
  10. RHough
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    RHough Retro Dude

    I've been thinking about this again ... dangerous I know ... :)

    If you have to limit the working load of synthetic rigging to 10% of MBL, how do the sizes and weights compare to SS wire that is sized so working loads are 40% of MBL?

    For a 0.250" 302 SS 1x19 wire the BML is 8200 pounds IIRC. 40% is 3280. Doesn't that mean you need a synthetic stay rated at 32800 pounds to replace it?

    That would be about 0.500" diameter stay in SK-75?

    0.500 SK-75 is 6.4 pounds per 100'
    0.250 1x19 is 13.5 pounds per 100'

    So about 50% weight savings in the wire at the cost of double the windage?

    What am I missing here?
     
  11. Gashmore
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    Gashmore Junior Member

    There is no 1/2" in Dynex Dux. The closest would be 11mm at 40,000 lb MBL and 3.63 lb/100' or 13mm at 54,000 lb. and 5.57 lb/100'

    The idea of creep under sail got me thinking though. While the reduced tension on the leeward shroud would reduce the creep rate on that side the creep rate on the windward side would increase faster than the leeward would reduce resulting in a net increase in total creep so you have a valid point that design load should not be much higher than 10% or 15% of MBL.

    Given that, in the 58' cap shroud I am studying with a 4,100 lb design load at RM30, 11mm Dynex Dux at 180kN (40,400 lb) MBL would be the better choice. The shroud will weigh about 3.2 pounds. In wire, with a safety factor of 2.5:1, 5/16" (8mm) wire with an MBL of 10,300 lb would just make the cut and weigh about 9.9 lb. That is a weight saving of about 67% for a windage increase of about 37%.

    The total increase in area presented to the wind would be about 126 sq.in. In return you get about 180 foot pounds of additional righting moment and because the 8mm wire would be near its upper limit and the 11mm Dux far stronger than required for the design load, 2" less stretch.
     
  12. RHough
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    RHough Retro Dude

    The trade-off I've seen with the NavTec PBO rigging is that weight aloft and reduced pitching moment more than offsets the increased windage in velocity made good on the course. The NavTec stuff is a replacement item every 3 years the last time I looked compared to rod rigs that have been standing and hold their tune 20 years later. To be honest, 180 pound extra righting moment is a very small percentage on boat with a 58 foot cap shroud.

    This is a 37-42 foot mono? Shroud Load = Righting Moment @ 30 x 1.5 / 1/2 Beam (or 1/2 shroud base if smaller). Cap shroud is 40% of that? So RM30 is only 10250 pound feet? That sounds very low? My 25 foot LWL boat has over 18,000 pound feet RM30. 0.250 302 SS wire at 8200 lb MBL is only a 2.5 safety factor. If I use 316 (7090#) the safety factor drops to 2.1 ... too small for me. Maybe we are not on the same page? Light boat? Wide shroud base?

    It will be interesting to see how the (I assume) uncovered Dynex Dux does after 3-5 years or 30,000 sea miles.
     
  13. Gashmore
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    Gashmore Junior Member

    That was arrived at using the Nordic Boat Standard which I do understand and verified with a Finite Element Analysis program that I don't understand. But if you use the tried and true formula, RM@30 = 63000 Half Shroud base 6.9' so total shroud load works out to ~13,700'. Apportioning that 40% lowers, 30% intermediate and 30% cap, the cap comes out at 4,109 lb. Pretty comforting when the high tech, mid tech and low tech solutions all come out within a couple of percent of each other. :D
     
  14. RHough
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    RHough Retro Dude

    Very good ... it has been awhile since I sized a two spreader rig. 30% cap is lower than what I would have guessed. :)

    Shroud Load = Righting Moment @ 30 x 1.5 / 1/2 Beam is the conservative formula. Glad you checked results more than one way! My experience is that this oldie predicted the rigging issues I had when I replaced lowers with 316 on a rig designed for 302 ... that 15% difference was measurable. The 316 lowers would not hold a tune, 302 did just fine.

    I take it you are using continuous rigging? Did you consider dis-continuous options? Probably too late in the game to change?
     

  15. Gashmore
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    Gashmore Junior Member

    To late for discontinuous. Besides what is attracting me to Dynex is that I am a hard core do it your selfer and rigging Dux is something I believe I can do. It's the principle of the thing. I have brought this 45' boat from two white cedar logs to near launch by myself. Only wish I could whittle up the aluminum spars. :D
     
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