rigger 47 Posted August 5, 2010 Share Posted August 5, 2010 ep32 PostPosted: Mon Mar 09, 2009 2:47 pm Post subject: Dynex for prod? I've used 4mm Dynex to stay the prod on the Javelin on recommendation of the local rigging crowd. Unfortunately everything seems to be loosening off after a decent reach down the harbour. I had though that the loads would be low enough for the creep to be immeasurable over a 3 hour period in the boat. Has anyone used anything like it before and have any advice? Do I need to just forget about it and go back to wire? ======================================================================== creative suffering PostPosted: Mon Mar 09, 2009 3:09 pm Hi cep32. I have Dynax all over my current boat and the experts say you never use it anything that has preset loads, such as stays, prods etc. unless you can pre tension it to within 80% of it max load. it is great for high loads where you can adjust the initial creep out of it. for example a kicker, or jib halyard that can be adjusted easily. I had it on a prod and the initial creep, before it loaded up, was alarming. It is very strong but needs good long splices. which are easy to do. ======================================================================== MrWolf PostPosted: Mon Mar 09, 2009 3:48 pm I have it on my prod bobstay, prod sidestays, Cap shrouds, Runners, Topmast backstays. Won't use it on Diamonds or Forestay on a 9m tri. I used it no probs on the Javelin Bobstay. When we make up a Dynex stay or bobstay. Allow 50mm of stretch/creep for each splice and then 3% of the length for stretch. I.E you are going to make the stay in the first instance that much too short. For your Jav you are probably using 3 or 4mm. say 4mm for the example. 4mm Dynex has a break strain of 2 tonnes So to Pre Stretch it you need to load the stay up to just over half of that, say 1.1 tonnes. We apply that load (with a load cell on the line) but it will quickly drop away, just keep applying the load up to 1.1 tonnes. It willl take around 3/4 hour before the load of 1100 kg will stay, then leave it loaded to that overnight. When you let off the load the stay will be the length you wanted it in the first place. When you use the stays they will not stretch at all, unless the load exceeds the load you pre stretched it at. But if your prod is going to see in excess of 1.1 tonnes you should be using the next size up anyway. ======================================================================== cep32 PostPosted: Mon Mar 09, 2009 3:54 pm Thanks for that. The rigging company have offered to prestretch them overnight for me. I'm guessing they will be able to do this at much higher loads. Is it worth taking them up on the offer? ======================================================================== MrWolf PostPosted: Mon Mar 09, 2009 4:00 pm So we just prestretch to just over half the break load. You should not be using a stay that gets close to half its break load otherwise Safety Factor is less than 2 (3 or 4 is most common). the total amount dynex will stretch before it breaks is 6%. If you use a stay that never sees half the break strength then it will never stretch more than you prestreched it to. BUT if there is a rogue load, the dynex still has 3% stretch in reserve, which can be a rig saver. Sure the stay will end up stretched but at worst you will just have to replace the stay. So stay pretty close to just over half for prestretching do not pre load it to higher than that. ======================================================================== Knot Me PostPosted: Mon Mar 09, 2009 7:32 pm I'd say check to see if it is actually Dynex. The name 'Dynex' is being used by many when in fact it's not Dynex at all. In some cases we've seen the name Dynex used to describe a Dyneema SK62 rope, which can look identical to the untrained eye but is quite a differing animal. This is becoming common especially in the Sth Island. I have some 4mm real Dynex on my 930 and I don't get anything like you describe cep32 and I'm running far longer lengths. There are Javs with real Dynex prod strings that aren't getting that either. Something just doesn't seem right. Dynex is a brand name of a Dyneema SK75 rope. A Jav prod shouldn't be able do squat to it after it's been well loaded once. The initial load should pack the splices and deal to most of the non-recoverable PE (Permanent Extension = new rope 'initial non-recoverable stretch', in simple terms) then nothing should happen. What Mr Wolf is doing with his pre-loading is removing exactly this and it's a bloody good idea for the loaded and length sensitive situations like you and he use the ropes for. Where you talk about 'creep' you are actually talking about Elongation or more commonly called stretch. To which there is 3 parts. Without getting too deep they are non-recoverable, immediately recoverable and how's this for a word (which I'll probably spell wrong) hysteresis. That last one is the amount of stretch that will recover over time. The other 2 are pretty self-explanatory. Creep is a differing animal again. Dyneema does get 'cold flow' ('creep' to us mare mortals). This is basically happening at the molecular level of the ropes material when under a big load. Not much you can do about that either if used with small safety margins. Anything below a 5:1 margin is regarded as small when talking ropes, 'Spooky' when talking ropes people hang off. Serious race boats will use smaller margins though but they will replace the rope far more often than a recreational sailor will. If you don't want creep use Vectran. Same elongation numbers as dyneema but no cold flow to worry about. Hence my halyards are vectran rather than dyneema. I'd follow Mr Wolfs advice so even if it is not a real Dynex that should still sort your issue out, all going well. Even if it is a SK62 that is more than enough for a Jav strength wise. SK62 and SK75 are basically grades of the fibre. The bigger the number the bigger the load it will take. While 62 and 75 aren't that far apart the load numbers in ropes using each are very different with 75 being close to twice that of a 62. Almost all dyneemas/spectras are 75 now but there is one NZ manufacturer still running the old school 62. SK80's are now becoming nearly available (read as 'affordable') for us plebs and SK90 should only be a year to 2 away for those with very big Visa card credit limits Smile All easy peasy stuff really Wink ======================================================================== One Extreme PostPosted: Mon Mar 09, 2009 7:50 pm speaking of prods and stays i need to fix my prod and replace the stays as the 3mm wire ones i have now have kinks in them what would you recommend replacing them with? they need to be easily removable for cruising and class racing. ======================================================================== Knot Me PostPosted: Mon Mar 09, 2009 7:54 pm Dynex or another SK75 rope. Easy peasy. ======================================================================== cep32 PostPosted: Tue Mar 10, 2009 7:24 am Anyone know the best place to borrow a load cell? The local rigging shop doesn't have one. I'm in Nelson which cuts down on options but figure there must be some other form of industry that uses them. ======================================================================== Murky PostPosted: Tue Mar 10, 2009 8:31 am Reading this thread from an outsider's perspective, it's looking like it might be good value for money to simply give the "finished" measurement to one of these guys who is familiar with the materials and has the right gear and expertise to turn it out in a way that's going to work first time. ======================================================================== MrWolf PostPosted: Tue Mar 10, 2009 8:58 am cep32 wrote: Anyone know the best place to borrow a load cell? The local rigging shop doesn't have one. I'm in Nelson which cuts down on options but figure there must be some other form of industry that uses them. Its funny that the source of Dynex we use is Hampidjian (spelling wrong) who are next door to Dickson Marine, so it should be easy in nelson to get the good stuff. You can do it without a Load cell. Pull your prod stay tight. (You said it had already been used ?), Lets assume the splices have already taken up. Load the stay up until it is 3% longer. (this would be much safer if the rope was new), ======================================================================== Knot Me PostPosted: Tue Mar 10, 2009 12:17 pm MrWolf wrote: Its funny that the source of Dynex we use is Hampidjian (spelling wrong) who are next door to Dickson Marine, so it should be easy in nelson to get the good stuff. You would expect that wouldn't you. But a un-named not related to Hampidjian or Dicksons person has been known to call his product Dynex if he his asked. And he has a big price advantage so unless you know your ropes well it could be an easy trap to fall into, a few have. Hampidjian make Dynex, not in NZ though. There are plenty of other products just the same as Dynex made by other people. Amsteel and Dynamica are just 2 others also here in NZ. NB: My spelling is wrong also, I only copied Mr Wolfs Smile Don't need a load cell just hang a known weight off it. A Kia Picanto for example is 1000 kg Smile Smile ======================================================================== ScottiE PostPosted: Tue Mar 10, 2009 12:39 pm correct spelling is Hampidjan Just as a few points of clarification - at least this is my understanding - I spent a bit of time researching this a few years ago. The name "Dynex" is the brand name under which Hampidjan sell there Dyneema series of ropes - Dynex 60, Dynex 75, and Dynex Dux 75 (and various cover braided versions of these) Dyneema is the brand name of the particular UHMWPE fibre developed and patented by DSM (Holland base materials science company). DSM sell this fibre in various formats for lots of different applications but grade the fibre as SKXX (the more common grades are SK60, SK75 and SK78). Companies like Hampidjan (Dynex) , Samson (Amsteel-Blue) buy this fibre and develop their ropes accordingly (usually braided cores made from dyneema SKXX fibre). Other companies make all manner of other goodies with dyneema. ======================================================================== Knot Me PostPosted: Tue Mar 10, 2009 1:14 pm Your understandings are correct Scottie. ======================================================================== Link to post Share on other sites
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