It's something of a rarity outside of the tech diving community.
Jon Lines are as simple as they are brilliant.
What is a Jon Line?
A simple piece of 3’ long nylon webbing with some form of karabiner in one end and a hook the other. It’s dirt cheap to make, weighs nothing, and can make deco or safety stops a breeze.
Originally, Jon Lines were used exclusively by technical divers, for doing prolonged decompression stops, sometimes in strong currents. But over time, they have been adapted by more and more recreational divers, too.
Some divers prefer to make their own, but Jon Lines can also be purchased commercially from most dive shops. The commercial ones will typically come with its own carrying bag that can be attached to a BCD D-ring.
Why do I need a Jon Line?
While ascending from a dive, and needing to do a safety or deco stop, scuba divers sometimes have to face strong currents, making it difficult to maintain the desired position near the boat.
The typical way of handling this is for the divers to hold on to a downline or anchorline to make sure they can stay on position. But for popular sites, or large dive teams, the line can get very crowded.
If five or maybe ten divers all need to maintain the same depth for any given time. There’ll be quite a competition for space on the line to hold on to. And holding on in a strong current can get quite exhausting, especially in very strong currents or for prolonged stops, or both.
Enter the Jon Line
A diver will take out his or her Jon Line when starting the ascent or the stop, attach one end to himself, and clip the hook on the other end to the line.
This allows for the diver to move freely up and down the line, but constantly at the same distance to the line itself. This removes the strain of grasping on to the line, and allows for more divers to be holding on the same line at the same depth.
Things to note when using a Jon Line
A few important things to remember about using a Jon Line: when not used, it needs to be securely stowed away in a pocket or similar. So it doesn’t drag along the bottom or the reef, and so you don’t risk entanglement.
Also, the hook on the end should be large enough allow any vertical movement on the downline to not yank the diver up and down. This could be problematic from a DCS point of view.
The Jon Line is only intended to keep the diver in the same position in relation to the downline, not to tie him or her off on it. The depth is still supposed to be maintained by you using your own buoyancy skills.
The hook or karabiner that attaches to the downline should also be easy to remove if the diver suddenly needs to free him- or herself from the downline.
Have you ever used a Jon Line?
With the advantages, the ease of use, and the low cost of a Jon Line, it should be standard equipment for all divers who regularly dive from boats.