The Jon Line...

[Scuba Cobra]

What's a good length to make the line?

I think this is one of those things that comes down to personal preference. With that said, mine is 5' long. I can always shorten it if needed by tying an overhand knot in the line.

 

[JohnnyC]

Get a garvin hook and a bolt snap (or snap shackle if you might need a breakaway). Splice a brummel with some 12-strand Dyneema through the bolt snap, splice a whoopie through the ring on the garvin hook. Clip it to your scooter loop. Now you have an adjustable jon line that you don't have to tie knots in to shorten. It's helpful in serious current instead of taking slack ahead of you so that you can try and shorten a non-adjustable by tying a knot one-handed. Carrying a bunch of stages/deco/bailout makes for a pretty decent surface area for moving water to act on.

Dyneema (and other ultra high molecular weight polyethylene-based rope) is designed for use in marine environments. In fact, cruise ship mooring lines are made of UHMWPE ropes, typically 12-strand, like the Samson branded Amsteel Blue. An alternative is aramid-based like Tech-12, again a Samson rope. However, many manufacturers make similar 8-strand and 12-strand UHMWPE, liquid-crystal polymer (Vectran), and Aramid-based ropes that would all serve equally as well.

Interestingly enough, you can actually buy pre-made whoopies from hammock retailers, as they have embraced it as one of a handful of standard suspension configurations.

 

[Celt]

Handy in a strong current, otherwise I’ll let the upline run through my hand in a swell, if the swell is big I’ll plan the last stop at 6 meters. I would never tie myself off to an anchor line or mooring line.

 

[Akimbo]

@JohnnyC

Good thoughts, but less words -- more pictures and links please. Not everyone who is trying to learn knows the jargon.

 

[JohnnyC]

Sure. I just figured people wouldn't be that interested.

Here's a garvin hook:
Highland Garvin Hook

Here's a pre-made whoopie sling:
Amsteel Whoopie Slings (single)

I'm assuming everyone knows what a bolt snap looks like, but maybe not a snap shackle. Here's a snap shackle, and take note it's really only if a quick disconnect is necessary, and they come in many flavors:
RONSTAN 2200 lb. Stainless Steel Snap Shackle | West Marine

If you want to make your own, here's a locked brummel:
Brummel Eye Splice

And here's a whoopie:
Whoopie Sling

You can see in the second half of the video how you can adjust the length of the sling.

 

[Billy Northrup]

10 ft, longer than a swimmer with fins trying to kick me. FWIW, I had allot of numbness and loss of feeling in my right arm for about 3 weeks last year from hanging on to a line.

 

[flymolo]

Thanks all this is very helpful

 

[Centrals]

There is nothing to lose to have one in the pocket. Absolutely essential. 3m should be long enough.

 

[hroark2112]

Whoopies and hooks and knots...this might be getting PG rated quickly!!!

Thanks for the links!! I was just getting ready to reply to ask for them when I saw them.