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Hi again,
No offense taken; sometimes I take too much for granted. So let's back up a bit.
The reason we have any sort of jackline or tether or lifeline is that, at some point, sailboats were losing too many crewmembers. Even on big ships, in storm conditions, some form of lifeline would be rigged, if the captain had any sense. The best lifelines would be at least chest-high, because it is difficult to fall past these. But it's not practical to make stanchions this long on small craft, so what we get are those knee-high things. Better than nothing, and they've stopped many a sailor from washing overboard, but obviously we can do better.
That's where on-deck jacklines come in. The early ones were of wire or rope, but it was found that these rolled underfoot, which is why they are now usually webbing. But in order to attach yourself to these you need a tether that reaches comfortably from your harness to the deck. And this tether is then long enough to allow you to fall completely over the lifelines and into the sea, where you might find it difficult -- or impossible -- to get back aboard before you drown. Again, these jacklines are better than nothing, but I believe we can do better still.
Hence the chest-high alternative. The height of the middle portion, where one is most vulnerable, is much better at keeping you inboard. The shortness of the tether means that even if you managed to get past the jackline you couldn't fall far. And the shortness also means that if you are thrown across the deck you can't build up as much speed, making for a dramatically decreased shock load when you fetch up.
To make this work best you need a double tether. I don't see this as a deal breaker, considering the advantages.
Of course, there is a lot to be said for at least pretending that there are no lifelines or jacklines aboard, so you don't get careless.
I hope this make the system clearer.
Fair leads,
Brion Toss
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