Just to emphasise the point, as there were some mentions of “strength equivalence” earlier, that the only factor for sizing fibre rigging is stretch. It should have the same stretch characteristics as the wire it replaces. That typically generates a fibre shroud
that is 3-5 times stronger than the wire it replaces.
On our 55 foot cat with 80sqm mainsail
had 14mm 1x19 SS316 cap shrouds and were replaced by 18mm Dyneema Dux. Colligo has a handy chart https://static1.squarespace.com/stat...quavalents.pdf
. Selecting a heat set line minimises the fibre stretch and provides higher strength (e.g. SK75 Dux MBL spliced 40.6ton, SK99 MBL spliced 38.5 ton).
Fibre is also subject to creep, which is non-recoverable elongation in response to constant tension. The lower the ratio that the tension is to the breaking strength of the fibre the less the creep. Colligo has a table for that too, https://static1.squarespace.com/stat...eep+Chart.jpeg
, but it is also dependent on how big your dynamic loads will be. We upsized our shrouds based on stretch (16mm) to reduce the creep.
Finally there is pre-tensioning, to remove the constructional stretch of all the splices. Effectively, you’ve loosened and opened the fibres where you’ve done the splices, so now the fibres need to be pulled back together. The tension required is about 1/3 the working tension.
Note that a rig’s static tension is usually between 10-15% of breaking strength (wire). This will be somewhere around 2-5% of the dyneema breaking strength. A rule
of thumb when sailing is that the leeward side should be just soft when fully powered up (that is, just at the time you should reef) on a close reach or close hauled.
We pre-tensioned at about half of the recommended tension, so had a fair bit of movement over the first day. Our turnbuckles started out with threads 300mm extra long and that was gone after 3 tensionings (600mm stretch in a 17.9m shroud with eye splice top and bottom). Then we cut the excess lengths and shortened another 100mm over the next 3 days. 6 months on we’ve brought in another 6mm, so we’re pretty much there. The other way to do it with existing turnbuckles would be to use lashings between the bottom terminator and the turnbuckle top pin, until you can finally get the pin through the bottom terminator. Smaller boats or rotating masts can use lashings instead of turnbuckles.
Keep trying with Colligo. John Franta is on CF and albeit busy is very helpful. Try calling him.