Michael Guerrero
Contributor
It's probably been said above, but I use a Jon Line.
Best Way to Conduct Deco Stops at 20' When Waves and Currents are Present?
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It is very difficult to hold depth +/- 1' at shallow decompression stops with 6'+ swells. You absolutely want to ride the swell so your depth doesn't vary and pound your ear drums. Unfortunately momentum and gas expansion work against you regardless of how stable you are in calm water.
I do most of my deco dives suspended - slightly negative buyant - on my large smb. Most of the time, I launch my smb @ 60 feet, so that it is nicely fully inflated at the surface. I never used 100% oxy, rather 50% Nitrox.
I'm not doing 166ft deco dives with O2 in the first place, take way too much gas to get you and an OOA buddy up to 20ft.
You could have done the whole deco on EAN50 and saved yourself the bouncing around shallow.
You could have done the whole deco on EAN50 and saved yourself the bouncing around shallow.
There are two separate issues;
1. Decompressing in heavy swell
2. Safe decompression gas.
1. Decompressing in heavy swell.
The technique is heavily dependent on local diving practice and length of decompression.
In the UK, dive boats do not generally " anchor in" to the site or wreck, they, basically act as chase boats. The practice is in part to short tidal windows for diving. For short decompression dives then the vast majority of UK divers would deploy a DSMB and just float down stream of the site with the chase boat following (it does take skill to track pairs of dives who have all terminated the dive at different times).
For longer dives return and ascent up the shot is often dictated (shot - line from site to surface with buoy), the dive boat still does not anchor into site. Often a trapeze is connected to the shot with a quick release and tag system ( this is a bit hardcore). As divers descend they "tag in", as they ascend, they "tag out", if you're is the last tag, you release the trapeze, allowing it to drift free, followed by the chase boat. The effect of tide is immediately minimised, but the whole group are kept together. An added benefit is emergency gas on the trapeze, and other divers to assist in the event of a problem.
Jon lines are used to " tie in" to the shot and trapeze.
Jon lines allow the diver to float neutrally at the correct depth with out being pulled up and down by swell. Even in tide without swell it is much more comfortable. It significantly reduces fatigue in the river and injury to your arms and shoulder. An added benefit, is that if you do "tox", you are tied in and will not sink to the seabed. It gives your budddies the opportunity to rescue you.
2. Optimum or safe decompression gas.
The optimum (richest, fastest) is not always the safest in non ideal conditions. Whilst some seem to dictate 100% is the" only correct decompression gas" , they are not taking account of the conditions and additional tox risk. 80% doesn't add significant time to the decompression, but does reduce the o2 clock and give some depth flexibility. Similarly 50% is very flexible.
When using a computer rather than hard tables or ratio Deco, then decompression depth can be adjusted if swell is excessive, as long as air calc's have been conservative.
The techniques you are based on a mix of local practice, conditions on the day and good sense. Sometimes it's just better not to get in the water.
Gareth
1. Decompressing in heavy swell
2. Safe decompression gas.
1. Decompressing in heavy swell.
The technique is heavily dependent on local diving practice and length of decompression.
In the UK, dive boats do not generally " anchor in" to the site or wreck, they, basically act as chase boats. The practice is in part to short tidal windows for diving. For short decompression dives then the vast majority of UK divers would deploy a DSMB and just float down stream of the site with the chase boat following (it does take skill to track pairs of dives who have all terminated the dive at different times).
For longer dives return and ascent up the shot is often dictated (shot - line from site to surface with buoy), the dive boat still does not anchor into site. Often a trapeze is connected to the shot with a quick release and tag system ( this is a bit hardcore). As divers descend they "tag in", as they ascend, they "tag out", if you're is the last tag, you release the trapeze, allowing it to drift free, followed by the chase boat. The effect of tide is immediately minimised, but the whole group are kept together. An added benefit is emergency gas on the trapeze, and other divers to assist in the event of a problem.
Jon lines are used to " tie in" to the shot and trapeze.
Jon lines allow the diver to float neutrally at the correct depth with out being pulled up and down by swell. Even in tide without swell it is much more comfortable. It significantly reduces fatigue in the river and injury to your arms and shoulder. An added benefit, is that if you do "tox", you are tied in and will not sink to the seabed. It gives your budddies the opportunity to rescue you.
2. Optimum or safe decompression gas.
The optimum (richest, fastest) is not always the safest in non ideal conditions. Whilst some seem to dictate 100% is the" only correct decompression gas" , they are not taking account of the conditions and additional tox risk. 80% doesn't add significant time to the decompression, but does reduce the o2 clock and give some depth flexibility. Similarly 50% is very flexible.
When using a computer rather than hard tables or ratio Deco, then decompression depth can be adjusted if swell is excessive, as long as air calc's have been conservative.
The techniques you are based on a mix of local practice, conditions on the day and good sense. Sometimes it's just better not to get in the water.
Gareth
2-3 waves is not heavy waves (of course the period matters too) but 2-3 is standard mellow diving in NC. Secondly you say there was current. Which direction was it running? Thirdly, why were you going to make contact with the anchor at line at 20 ft (as I read it). why not go up it loosely before then. If you buouancy is dialed in your arm hand might be going up and down a couple of feet but no reason for you body not to be at the average. Could always tie on with a short line so it bounces and not so much you.
I can’t agree. They buoyancy characteristics of an SMB actually work against you when your objective is to rise and lower with the swell. There is very little buoyance/unit of height to an SMB so you get a Jojo or slingshot effect, especially with short interval swells. It only slings you up and you aren’t heavy enough at the end of a dive to sink back down. The line and diver are not negative enough to counter the inertia of the water or buoy.
That is why the best shape I have found for the buoy is a fat tear-drop like a cherry buoy. Displacement increases rapidly as the buoy submerges a small amount, but still provides a little shock abortion. It is also why the rig needs 20-30 Lbs of weight. The cherry buoy can be tied to the boat with enough slack that boat movements don’t transmit down the line if the boat is drifting or the current is mild.
SMBs are great for signaling but aren’t very helpful at holding depth in sloppy seas, unless you are seriously overweighed during the dive. Obviously you can’t carry a cherry buoy and 30 Lbs of lead with you, but a chase boat can drop one for you near your SMB.
Michael Guerrero
Contributor
Or a lift bag.
Ah, loss of buoyancy control during a gas switch! This is a not uncommon problem that comes up in teaching tec. Be re-assured - you are not alone, and many (many) divers go through that stage of development, as they work through the task loading that is required for competent deco diving. We try to intentionally task load tec students during training to head off just this kind of issue down the road.
And, this was probably a core element in the situation that developed. Ocean diving, with swells and current, is a bit different. As a result of your experience, I bet you will be trying several different approaches (different mixtures, use of a jon line, etc.) going forward. That which does not kill us makes us stronger (Nietzsche). We all make mistakes, and ideally we continue to learn from them. All of my tec training years ago was done in fresh water - no currents, no swells. The first time I jumped in the ocean for a deco dive was essentially a continuation of that training. I won't say it was problematic, but I don't think it was necessarily pretty, either.
I use a jon line for most ocean deco dives. On of the reasons I do is that my preferred deco configuration is 2-gas, with 50% and 100%, the 20 ft stop is (of course) the extended stop, and I find that divers often 'stack up' at that 20 ft depth. If there is no current, it is fine to hover away from the down line. But, more often than not, this isn't the case, at least in dives off the NC coast, and I like to have a jon line, to stay away from the crowd (especially the breather guys who seem to take up a lot of space
). Another option which I like is for the boat to hang a weighted, marked, deco line over the side, away from the primary tie-in line. Several posters have suggested that the use of 80% as a single mix is a possibility, and I agree - you start your deco a bit deeper than with 100%, and may not have as much of an issue with the swells. But, there is still the possibility of 'stacking', if a number of divers enter the water within 5-6 minutes of each other, are using similar gases and have similar run times.All in all, you did reasonably well under the circumstances. You had a 'good' learning experience, and can put some lessons into practice the next time out.
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