When to use different bailout strategies
In a previous post I talked about different CCR bailout strategies that are available for us to use but when would we chose one over the other and if necessary how might we combine strategies to optimise our dive.
If we think back to the easiest of the plans, individual bailout, this is straight forward and simple. Each diver carries their own bailout you just need to check your numbers to make sure you have the right gasses and sufficient volume, then off you go diving.
The downside is that the deeper or further you want to dive the more each diver will have to carry. It won’t be long before you could be met with the law of diminishing returns. Some fellow cave diving friends wanted to go a little further in a particular cave system. They planned their dives and were encouraged by their initial success to add another bailout cylinder and go a little further. With each dive came more equipment and they realised their progress was getting slower. They went back a step, left a few stage cylinders behind and started to get experience using DPVs in this cave system. They had already done a Cave DPV course and had used DPVs in cave systems before so again initial progress was good. Then they started to add in those extra stage cylinder and a few more on top, having realised their bailout needs had increased due to the greater distances being covered using DPVs. With all the extra stages slowing them down they thought to add spare scooters to the ensemble just in case a DPV failed. They were not getting not much further in to the system than when they first started out with their rebreathers, a couple of stages and old fashioned fin power!
In their case they would perhaps have been better served planning their dive using staged bailout rather than trying to carry it all. Their original thoughts against doing this were the time it might take to set everything in place, the additional time to then retrieve it all after the main dive and their lack of practice and therefore confidence in knowing how and where to stage their bailout gasses.
Before you start using staged bailout gasses you must be able to ensure you can get back to them. If you are not able to guarantee this then you need a back-up to the staged cylinders or you need to choose a different strategy. In many circumstances but certainly not all, a cave is a reasonably safe bet for getting back to your staged cylinders.
The trick is to look at swim pace (or DPV speed), gas consumption rates, decompression obligations, cave passage configuration and then carefully plan where you will stage your gas. When you drop cylinders take the time to check your travel pace back out and see if it matches your estimates. Take account of the fact that you might be swimming or scootering wearing more cylinders than during the set-up dive if you have to pick them up on the way having bailed out due to an unrecoverable CCR failure. Remember that using a DPV has the capacity to put you a long way back in a cave and if the DPV fails, has your team practiced team towing skills or do you have a spare DPV that can be towed or possibly staged, if so how far is the swim or tow back to it?
So outside of the cave environment is there anywhere else we might consider using staged bailout cylinders? The open ocean is not really the best place to leave cylinders. Sadly there have been instances of divers being injured through leaving cylinders on the sea bed as part of their plan, only to find they have either been moved by the ocean currents and tides or they simply couldn’t find their way back to their gas. Not enough gas, omitted decompression and injured divers was the end result.
We could use the drop cylinder approach. Using what has now become the accepted norm in tec diving of using a yellow surface marker buoy to signal some kind of gas emergency, a dive boat can drop cylinders down your SMB line to you. The cylinders need to be set up first on their own sturdy line with a flotation device attached to the other end and ideally the signal needs to be sent to the surface at a deeper depth than that at which you will access the gas, just so the thing doesn’t hit you on the head when the boat crew drop it down your line to you!
This really needs to be discussed with the skipper and crew and practiced, although some boats I have used are very familiar with this system and will even ask before the diving commences if anyone onboard has any drop cylinders, how they want them rigged and how they would like the boat to deploy them if needed.
Remember you will still need to carry enough gas to get you to a point where you can access the drop cylinder.
It is just a personal opinion but I do think that with practice and when you know the boat, skipper and crew this can be a really good option but I do tend to limit the depths at which I will consider this to no deeper than the Normoxic CCR range. I still need to carry a bailout I can access at the deepest point of the dive but if the bottom time is not excessive there are many normoxic level dives from which you could bailout and decompress on your off board trimix stage. Yes you will be in the water a lot longer than with your deco gas but if the boat takes longer to get to you than expected you're probably not going to get in to immediate difficulties.
Deeper than normoxic or shallower range dives I would choose a different option.
You can have similar strategy to the drop cylinder system if you have the luxury of support divers to help you. By using support divers you can plan how much gas you ned to carry for the deeper sections of the dive and arrange to meet your support team who can exchange cylinders with you for more depth appropriate mixes.
The use of staged gas, the drop cylinder system or using support divers can be combined with the concept of individual bailout (definitely with the drop cylinder system so that you know you have your own cylinder on the boat if you need it) or on deeper more complex dives, especially involving a larger team than just you and a buddy, can be combined with the concept of team bailout.
As discussed in a previous post (Bailout Strategies), team bailout gives us the capacity to spread the gas load amongst the team. Yes, we have to be presumptuous and say that we don’t think all the rebreathers in the whole team will all fail on the same dive but personally, I think that is a reasonable stance to take given the track record of reliability most modern rebreathers now have. Couple that with rigorous pre-dive checks, a comprehensive pre-breathe and monitoring of all the CCR systems under water we should have some trust in our rebreather to do its job.
I did own a CCR that had done a significant number (well in to four figures) of dives. A statistician friend told me that whilst it’s track record of reliability was proven, it did not make it any more or less likely to fail the next time I used it. That gave me some food for thought and was a reminder of the need to go through all the pre-dive things we do with CCR with no less diligence just because I have been doing it for some time. It does also give me trust in my rebreather. Not blind faith but a lot of trust.
I guess I also thought that whilst I too had a track record of reliability I was no more or less likely to fail the next time I went diving, so every dive incorporates a mental pre-dive check of myself!
So, in addition to thinking about what gasses you are going to take (I think that will be a topic for a future post) and how much you are going to take, have a think about how best to manage all these cylinders!
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