I have a daysailer (Chrysler Mutineer 15) and Catalina 22, and I've thought about re-rigging each with synthetics. I'll use my C-22 rigging for all examples here, approximated to:
- 15' lowers and 30' uppers.
- Standard rigging = 1/8" 316 SS. Breaking strength ~1670 lbs (304 would be slightly higher).
- Regular working load of cap shrouds: ~240 lbs (~15% of BS)
- (Scale as needed for your own boat.)
I think I'd spec 5/32" / 4mm heat-set Dyneema, with a BS of ~4k lbs (one size up from the normal 1/8" SS). So that would load the shrouds at ~6% of BS. Maybe that's overly conservative.
Dyneema has many advantages, and I won't reiterate them here. But we do have some complexities - especially about getting the shroud lengths correct and keeping them there. We have at least 5 issues (I've tried to use standard names for these where I can find them; correct me if there's a better name and I'll edit this post).
2 Issues during Construction:
- Construction Stretch: Lines may experience a one-time stretch when initially loaded. I believe this is mostly handled by the manufacturer's heat-treating process (heat while pre-stretching the line). Note that not all Dyneema is heat-treated. I think we probably want to spend the $$ for heat-treated line.
- Splicing Set: Our splices will extend as they're tensioned for the first time. I'm not sure just how long this takes (minutes? hours?) IICU, professional riggers might have a winch system integrated into their rigging bench to hold a line at a % of its rated load for awhile to properly bed splices. I have a 3200-lb 2-speed trailer winch allocated for this purpose when I get there. Maybe I'll get a load cell too - recommendations welcome.
And we have 4 issues while in use:
- Elastic Extension: The line will stretch when loaded, and contract when unloaded. This happens quickly - seconds perhaps? Essentially instantaneous? Not sure.
- Viscoelastic Extension: Additional stretch / contraction that takes place over a longer time period. Hours? Days? I'd love to know.
- If I'm reading correctly, Marlow (Fig 1) indicates that Elastic + Viscoelastic extension total ~0.5% at 10% of breaking strength.
- Creep: Over a long time period, loaded lines will permanently enlongate. This happens at the molecular level, and releasing the load doesn't reverse the process. This has been a big obstacle to synthetic rigging in keelboats.
- Even while the boat is sitting idle at the dock, the rigging is stretching, so it has to be re-tuned regularly. And, eventually, we run out of turnbuckle travel. Thus the frequent use of lashings in place of turnbuckles.
- I think this is mostly eliminated by DM20 (see the post above); even for a moored boat, and especially for a trailer sailer that will be rigged at most a few weeks per year.
- Temperature Response: Unlike most materials, Dyneema actually extends when cold and shrinks when hot. So the rigging might be slack in the morning and tight in the afternoon. So far as I know, there's not much to do about this except to watch it and re-tune as appropriate.
Personally, I think I'm no longer worried about creep - newer fibers have made that less of an issue. For my daysailer, I might try SK78. She's only rigged for a few hours at a time, a few days per year. And I have about 3" of available adjustment for 20' shrouds, so I think I can safely ignore creep at 0.5% per year of time under tension. For our C-22, I might be able to get away with SK78, but I'll go for DM20; my loosely-considered advice is to do the same. DM20 is more expensive, but not
that much more.
Elastic and Viscoelastic Extension are more troubling to me. Imagine we've arrived at a destination and started to rig.
- Since our last trip, elastic and viscoelastic extension have both recovered, so the shrouds are ~0.5% short (uppers ~1.8"; lowers ~0.9").
- To step the mast on a C-22, we have to disconnect the forward lower shrouds.
- We will usually use some sort of winch or block and tackle to pull the mast upright. That will stretch the uppers and rear lowers (recovering the between-trip shrinkage).
- But the forward lowers must be reconnected by hand. So we need enough travel in our turnbuckles or lashings to connect to the chainplates and then to re-stretch them ~0.9".
- My standard turnbuckles have about 2" of travel. So if I spliced my lines to the perfect length, I still have about half the travel for rig tuning. But if I'm off by only 1/2" on the shroud length, I'm down to about 1/2" of turnbuckle travel. Maybe longer turnbuckles or lashings are in order for those.
And the temperature response is an open issue in my mind. I'll have to think more about that one... Comments or real-world experience welcome!
