Do not use common 'snap shackles' on tether lines !!!!!!!! They have the tendency to 'unlock' when the shackle becomes 'wrapped' by the tether. The 'wrapping' can cause the shackle to open unexpectedly.
Only use 'locking' carabiners, or specialty double-action / locking 'safety shackles' designed especially for tethers.
http://marine.wichard.com/fiche-A|WICHARD|7006-0203010201000000-ME.html
Can't be stressed enough.
Snap shackles are actually one of the most dangerous "connectors" known to man, and I've removed them ALL from my boat.
Years ago, before I knew better, I used one to haul myself up my C25 mast in a bosun's chair. After working a bit, I looked at the shackle, and it was completely OPEN!!!
![Eek! :eek: :eek:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Eek! :eek: :eek:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
The ONLY thing keeping me up there was the friction of my legs around the mast!!!
Almost before I could tell my crew below that I needed help, I realized there was nothing they could do to help, so I just slid down the mast to the boom.
Good thing it was "only" a 25 foot boat, but it was still pretty "high up there" for me.
Of course we all now know better and use the correct knots/carabiners/D shackles and backup halyards, but that significant moment stays with me.
Snap shackles might be good for spinnakers, but I can't think of any other valid use for them on a boat.
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Centerline jacklines for tethers: I'm not sure I understand this concept, since many of us with dodgers really can't envision the routing. I use one on the starboard side, by habit because years ago reefing was done on that side. We now have double line reefing from the cockpit, but I still use one jackline.