A Jon line (also known as Jonline) is a short cord divers use to secure themselves to the main anchor line during safety or decompression stops. The purpose of the line is to relieve the diver from having to hold on to the anchor line, which is often dangerous or unreliable in rough weather where the line constantly jerks upwards and downwards, making maintaining a level depth stop extremely difficult. The Jon line also helps when several divers use the main anchor line at a safety stop, and getting a place on the line is difficult. The Jon line allows you to distance yourself from the other divers hanging onto the anchor line while maintaining a level deco stop.
The line is typically around 1 m (3 feet) long and equipped with a clip at each end. One clip is fastened to the diver’s harness, and the other is used to attach the line to the anchor line. This relieves the diver from holding on to the anchor line during the decompression stop. The line consists of nylon straps or bungee cords, and the clips include complex steel clips or carabiners. The line is often compact and easily carried by divers on their BCD D-clip or pocket.
The Jon line is named after the device’s inventor, Jon Hulburt, who discovered that a short line made decompression stops in rough water and currents much easier while diving the Andrea Doria.
The benefits of a J-line extend well beyond decompression stops, and divers use these cords to hang equipment from the dive boat or a drop tank at the safety stop level. Divers have also been known to use the line to secure themselves to the ship when several divers are trying to get up the ladder, and getting a purchase on the rocking boat is dangerous, and the current is too strong to float free. Divers can hook one end of the line to the ladder and the other to their BCD and float at a distance while waiting for the others to hand over their equipment and get aboard.
The Jon line is one of those simple and fantastic contraptions that, once you start using it, will leave you wondering how you ever lived without one.
