A few thoughts, responding to several posts:
* I do use rope jacklines as I find them easier to work with and last longer in the sun. I do not run them on the deck, so they are not an underfoot hazard. The type of rope (stretch) must be matched to the boat; there is no one-size-fits-all.
* I prefer webbing for tethers. That is where I got visual confusion (just me perhaps) and webbing is lighter.
* I can't imagine cutting a tether loose and then fetching the swimmer. Very likely fatal at night. Simpler (and tested at speed in MOB drills) is to tie a second line to the harness before you cut the tether; then they drift behind the boat and are easily retrieved. A lift can also be used at that point.
* 3' can be too long for some smaller boats, particularly on the side decks. Who said you needed to be able to stand? Either custom or tie a loop to shorten.
* Some clip the 2nd leg to their harness d-rings, deactivating the quick release function. Oops.
* Some harness/PFD combinations have made the quick-release IMPOSSIBLE to reach once inflated (Practical Sailor Testing).
* Test hang from the harness to check the fit. Is it on the lower ribs (can't breath)? Will it slide off over the foulies? How's the crotch strap? Maybe this is the time to shorten the tether!
This is all very helpful to me, since I have zero experience with offshore, harnesses, or tethers. But reading these comments helps me "think through" the experience of falling overboard on a tether - something that I hope I never have to actually do.
I had not thought about attaching a messenger line to an overboard passenger before cutting loose from the tether. That practice needs to be permanently etched in my mind for future reference. I also need to make sure to have such a line available in the cockpit in rough weather, and maybe at all times.
In rough conditions I would crawl to the foredeck on my boat, especially since I need to go to the cabin roof with no sidedecks. So a shorter tether is likely more appropriate. Shortening the tether also reduces the fall distance, might prevent the PFD bladder from inflating (a good thing, since it will interfere with reboarding or cutting loose), and could eliminate the need for elastic to take up the slack. Now I need to decide whether to just make something up custom and save my money on the fancy Kong tether.
Clipping the 2nd loop to the harness does not completely eliminate the release-under-load benefit of the hinged shackle, as long as you remember to first release the 2nd loop first (while it's not under load), then release the hinged shackle last. But remembering to do that "in the heat of the moment" is questionable. And since those large clips require some strength and dexterity, it might not be easy to do while hanging from a boat, and while obstructed by an inflated bladder (if you're dangling in the water).
I'm starting to see some potential benefit to a short course that might take me though these scenarios.