Thoughts on synthetic rigging pretension.

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

  1. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    From what little I know about you, I think that you can make it work in a proper seamanlike fashion if anyone can. I'll wager you have more insight and better records of the process than just about anyone.

    Very impressive from my point of view.
     
  2. jfranta
    Joined: Sep 2008
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    Location: Mesa AZ

    jfranta Junior Member

    First of all thanks for confirming our creep and stretch data.

    Absolutely...

    When I first started researching and testing Dynex Dux over 2 years ago now I had a real problem with understanding pretension and just what to set it at. There are many different processes out there. I also have a real problem with setting pretension based on the strength of the wire which is dependent on the type and size of the wire and oftentimes not what the desinger called for or sometimes not even spelled out by the designer in the first place. Oftentimes cruisers oversize their wire and if the designer sized for stretch this could possibly throw the mast off if pretension is set based on breaking strength.
    What we have come up with for our Colligo Dux rigging (for cruisers generally) is this and it works for just about any rig design out there. Basically you set the tension of the rig while sailing.

    1. Install the rigging. Shrouds and stays need to be sized for creep and stretch per the creep charts on our website. We have equivalent stretch tables for 1x19 wire, rod, and aramid. We shoot for creep less than .2 inches per year based on predicted static loads which generally keeps you out of dynamic load creep issues also and gives at least a 5/1 strength factor of safety, including taking 10% off the MBL for splicing, more often it is higher.

    2. Pretension everything by hand, keeping the mast in column both transversely and fore/aft.

    3. Go sailing with what you would consider full sail in about 7-10 knots of wind and flat water if possible. Tack back and forth and keep tightening the leeward shrouds while keeping the mast in column. The goal here is to end up with the leeward shrouds just loosening in 7-10 knots of wind with full sail and the mast in column.

    The big advantage with this method is that it takes all the guess work out. the loads are what they are, forestay and backstay included which dock tensioning methods don't accurately reflect. Also, I have been very surprised at how low the resulting pretension is on the shrouds when rigged this way, Backstayless rigs excluded. I am convinced that there are alot of overtensioned rigs out there.

    We have rigged some race boats, mini's etc with dux that the owners really want to tighten up so we just oversize the line to allow for the extra static load with low creep.

    We also have rigged a few boats now with low travel turnbuckles (4 inches), one with 2 years on its rig with no movement, a real testament to the low creep properties of Dux.

    John Franta, Colligo Marine.
     
  3. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    No rig that holds its tune is over-tensioned.

    If you don't understand rigging tension, you should not be rigging boats.

    Pretension on a mast/sail combination that requires mast pre-bend is determined by the pre-bend not sailing loads. If you call setting tensions good at the heel angles that a cruising boat (under canvased in general) sees in 7-10 knots of breeze you are going to have slack shrouds in a real breeze ... and shock loads the rig was not designed to take. No wonder you think rigs are over tensioned ... you don't seem to understand the process.

    Rigs should be tuned for maximum heel while sailing ... Full sail to the point where you should have reefed 20 minutes ago angles. Still taught but not lose in the leeward side, mast bend if any still correct, and the mast in column. This is the 25-30 deg heel range for most cruising boats.

    SS wire has a nice wide range of tension within the elastic range ... 0 to about 50-55% of MBL. It would seem that DDux does not. 0 to 10-15% perhaps? As long as the rig is within that no permanent distortion range the rig will hold it's tune. On SS wire when the load into the permanent distortion range the rig no longer hold a tune, it sounds like DDux will do the same if it ever gets loaded to the creep point. 20%?

    If any wire on my boat elongated .2 a year after the initial construction stretch was tuned out ... I throw it away. What % of tension is .2 inch?

    I well designed rig with the proper tune and wire sizes will hold a tune year after year. Rod Stevens used to take 6 turns of his rig for winter storage and put them back in the spring ... rig was tuned every year for many years. This is not magic or rocket science. It is springs under tension and they should never be slack under sail.

    What you say scares me.
     
  4. jfranta
    Joined: Sep 2008
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    jfranta Junior Member

    I certainly don't mean to scare you Mr. Hough.

    You know, an expert is a person who has begun to understand just how much he/she doesn't know about a subject. Understands that the world is gray and full of variables. There are many ways to accomplish a complicated process like holding up a mast, and many things to hold it up with. Also, many ways to tension a rig.
    As I have said to you before, Synthetic Rigging is not steel and needs to be treated differently due to its different material properties. We have struggled both to understand this material with its limitations and to communicate that to people that would value it. And there seems to be alot of people that value it. Check out the list of riggers on our website.

    One thing I have learned is that some people are open minded enough to try new things, take some calculated risks, the benefits outweigh the risks for them and others well, we just have to wait for them to pass on and go away. Human nature I guess.

    In the meantime you should stay away from your local marina because as all the boats start re-rigging with synthetic standing rigging there is sure to be masts falling all over the place.

    John Franta, Colligo Marine.
     
  5. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    Gashmore seems to be taking a very prudent approach. I compliment him for not taking anyone's word for anything.

    My comments above apply to rigging in general. It does not matter what material is used. When boats were rigged to natural fibers, the entire rig was designed and built around the properties of the material. The same holds true for modern rigs. The fittings are designed around the properties of springy SS wire or rod. They are not sized to take shock loading. Slack rigs accelerate wear and failure of things like chainplates, tangs, and clevis pins.

    Have you ever wondered why Navtec went to all the trouble to engineer parallel strand PBO rigging instead of buying of the shelf SK75 braid rope? Call me conservative, but they are rig engineers. They understand the relationships and loads involved.

    IMO any creep in a rig is unacceptable. Creep is a permanent, unrecoverable deformation of the material. SS and PBO do not creep. SK75 does under continuous load over some small % of its MBL.

    You state in your Feb 2008 newsletter:
    Monitor the tension in a shroud as the boat heels. Does the tension in the windward shroud increase with heel? What is the value at 25-30 degrees of heel? I would call a 20 degree heel for 2 weeks or more on a trade winds passage a near constant load, with the load never going as low as the simple pretension and often going higher than the 20 degree constant. If that load is over your "at most" 20% of MBL the rigging WILL creep. The rig will get sloppier and sloppier with every hour.

    This is why I suggested sizing the DDux so RM30 x 1.5 / .5 Beam was UNDER the "manageable" creep threshold. This gives HUGE safety margins when using synthetics so when that PVC tube at the spreader end "Spreader end fittings need to be considered to make sure that chafe will not be an issue. Simple solutions like PVC tubing over the line as it passes thru over the spreader ends works" has ground enough dirt and other crap in to the should until it chafes halfway through "In most cases the line can chafe ½ way thru and still be stronger than the steel it replaces." it will still hold the mast up.

    Sorry if you are taking this personally. I've been around boats too long and seen too many horror stories not to point out marketing BS when I see it. Just because someone buys into the hype and uses a product, that is not an endorsement IMO. Late night TV has hours of "infomercials" and junk science that proves getting someone to part with their money in not an indicator of value.

    You are the only person I know of that is selling uncovered 12-strand SK75 as standing rigging. Either you have vision and the people at Navtec and their years of experience are wrong ... or ... you are selling a product that will serve as a more expensive alternative to SS wire with a shorter service life.

    I think it can be made to work. Keep the design loads under 15% and you still have a substantial weight savings aloft. However the added windage, price, and short service life don't add up for me. The people that have made their name, the standard against which other rigs are measured, don't use braided rope for standing rigging. That has to tell you something ... maybe they know something you don't?

    You make this statement on the site:
    Can you publish here an example of one of your improved brummel spliced shrouds so we can see how your manufactured assembly compares to the advertised MBL of the DDux product by the manufacturer of the rope? How about a statement from Hampiðjan Group stating they approve DDux 75 for standing rigging usage? If they don't, what liability are you exposed to?
     

  6. Gashmore
    Joined: Feb 2008
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    Gashmore Junior Member

    Let me try to get this back on track. I do not believe you guys are all that far apart.

    I have gone back over my collection of overpriced marine technical books and done a quick survey of all the tuning guides I can find and came up with the following.

    All of the tuning guides say to tune in "moderate" winds. Most range from 8-12 knots. A few recommend 14 to 18 knots so John is in the majority here.

    Basic physics says that the pretension should be half the design load and the standard is to estimate shroud loads based on the righting moment at 30 degrees. BUT almost all of the spar and sail makers' tuning advice says that the leeward shroud should begin to go slack at 20 degrees of heel. As the load on the shrouds is directly related to RM and RM is almost linear up to 30 degrees it would seem they are recommending pretensioning to 1/3 of the design load. However, the mast is compressed as tension on the windward shroud increases which accounts for the difference. So RHough is also correct.

    Now here is where I see the conflict. Stainless rigging has been around long enough to develop some proven rules of thumb using standard safety factors ranging from 2.5 to 3.2:1 and based on minimum breaking load. Those rules based on MBL don't apply to synthetic rigging because the design procedure and safety factors are different.

    One established rule of thumb does exist. For load bearing fiber the safety factor must be greater than 5:1. That reduces any rule based on the MBL of wire in half. But the primary difference is that you design synthetics for creep. Not load. Due to the intrinsic properties of the material, if creep is controlled the load takes care of itself.
     
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