Dive planning advice

[Simon Richards]

I have been reading some of the other threads here, and there seem to be some very experienced divers willing to offer advice. I wonder if anyone can offer some advice on how my wife and I can do a dive which we have been meaning to do for a while?

For a couple of years we have been exploring a chain of caves in a certain area in Mexico - they are typically shallow (4-6m), and appear to be one long old cave which has collapsed in a number of places along its route. We have found a couple of cenotes about 800 feet apart, pointing at each other along a known fracture orientation, I have freedived them in open water, and seen cave leads in both. I suspect that they will be connected by a straight shot of cave at about 4-6m. Typically that series of caves has passage heights of 2-3m, and passage widths of 3-20m, and no detectable flow.

For planning purposes, gas consumption for each of us is about 90 minutes per 80CF at that depth, and we would normally swim at about 50 feet per minute, maybe 40 feet per minute average when surveying (but the surveying time can be much longer, as it depends a lot on conditions and how long we can make each shot). So 800 feet would take 16 minutes without surveying, and may take 20 minutes or more surveying, in both cases about 1/4 of an AL80. We can lay the line going in and then survey back out, but I don't like that because then our exit may take longer than our entry. Alternatively as a team we can lay the line and survey it as we go in, then turn round and have a nice pleasure dive coming back out, maybe looking for other cave leads, sketching some passage cross-sections and so on. We will probably only dive this cave once, so it would be nice to document it as well as we can on that dive.

The problem we have is access: both cenotes are about 1.5 miles through very rough jungle from anywhere we can get the truck. The landowner is a friend and he is willing to help carry gear out to the caves. Alternatively it would be possible in principle to use horses, but that would require a lot of work on the trail first, several days at least, and then we would have to find some horses ...

The tanks I have are single aluminum 40s and 80s, single 80s with Beuchat Y valves, and double 80s. All tank valves have their burst disks permanently plugged, and all tank O rings have been substituted with O rings of a higher hardness than the ones usually sold by dive shops/dealers. I have a range of equipment setups, including some very minimalist/lightweight setups for access through caves/jungle/on rope, and including backmount single, doubles and sidemount.

I don't think getting two sets of doubles out there would be feasible without the horses: I can't ask the landowner to carry a set of doubles 3 miles, my wife can't do that, and I doubt I can manage a total of 6 miles walking with doubles in a day. I guess I could go solo with a single set of doubles, but even then I am not sure I could carry them that far, and they aren't fully redundant so I would want a stage with me if I were to do that. (Also backmount is not so great for solo diving in my view - all of the stuff is behind your head.)

I had also been thinking of sidemount. If I did that solo then I would have to lay the line on the way in and survey on the way back out, meaning that my exit might be slower than the entry, which I don't really like. If I did it sidemount on my own, I wouldn't feel very happy about using the 40s - just not enough margin for my comfort for that sort of dive, and 40s empty really quickly if you have a leak, so I would really want to take two single 80s out there for a solo sidemount dive. I guess a compromise would be an 80 and a 40, breathing the 80 to half before turning, but I am never super happy if my exit is likely to take longer than my entry.

Therefore I have been thinking about both of us diving, using single 80s with Y valves. If we surveyed on the way in as a team, I'd be just about happy to dive these to thirds (because we can reasonably expect our exit to be quicker than our entry), giving us 30 minutes, say a planned 1,200 feet penetration while surveying at 40 feet per minute, or 900 feet at 30 feet per minute. We aren't guaranteed to make it to the other cenote like this, but there are never any guarantees with exploration anyway. My wife could probably carry a single 80 herself, or I could ask the landowner to carry her 80 for her, and wife could carry the (minimalist) dive gear. I'm not worried at all about burst disks or tank valves failing because those have been dealt with directly as described above, and a small leak is unlikely to be a problem. A catastrophic regulator/HP failure would be a problem, but unless that happened close to our furthest point, we could reasonably expect to get away with it, as long as we don't go through any tight restrictions on the way in, and we are unlikely to do that as we aren't in to exploring tight shallow cave (tight shallow cave is normally unstable here ...)..

I just haven't been interested in doing this dive solo - we have plenty of other caves we have better access to which we can explore as a team with backmounted doubles, and its more fun to dive together. At the moment my favoured option is to go as a team with single 80s with Y valves.

Obviously I am past the stage of being concerned about whether I am seen in big doubles or a single tank (I'm going to be in a 3mm wetsuit and snorkelling fins anyway just to keep the equipment weight down). And I won't be looking to set a new personal penetration record in this cave - I am fairly certain it will not go more than 800 feet because we have the cave "boxed in" by other caves we have found. We will anyway be limited by the amount of line we will be taking - I am thinking of taking a single 1,200 foot reel. And no-one will be holding guns at our heads either literally or figuratively, so we can do it whatever way makes most sense.

Advice and opinions would be appreciated.

Cheers,

Simon Richards

[REastman]

Sidemount...

[Webmaster]

The first things that come to mind would be to either get more people to help carry gear, or make multiple trips and stash tanks underwater at the sink.

[Simon Richards]

Well I posted this because it is a genuine dive we have been wanting to do for a while. As a result of reading another thread, it occured to me that single tanks with Ys would be an option for this dive, and I hadn't thought of that as we have them but don't normally use them. I am genuinely interested in what people think or suggest (but I'm not interested in agency stuff or rehashing old arguments).

If I didn't strongly suspect there was cave there, I would normally just take a mini sump diving setup and see if the thing goes or not. If it did, then we would return with more gear. But I suspect that this does go, so I am willing to skip that step.

If we go to sidemount with 80s, then that either means a solo dive, or taking 4 80s out there. In diving terms, it can't be a mistake to have more tanks. But getting them there means enrolling helpers (not so easy where this is) or multiple trips, realistically 3 days (one to take 2 tanks, one to take 2 more tanks and dive and bring them back, and one to bring back the remaining two tanks). The cave can't be more than about 800 feet, so 3 days work for a 40 minute dive seems rather a lot (I realise people do worse for sump diving).

Faced with that, the single 80s look rather efficient, and the dives would be done within 1/4s of them. I realise that some people might just do this solo with sidemount 80s, but as it is most likely going to be the only dive in the cave, only one of us could do that.

Does the singles idea seem so bad compared to the other options?

Cheers,

S

[Webmaster]

Well, I suppose it ultimately boils down to what you're comfortable with but personally I would prefer to have some redundancy. You could go with an 80 and a 40 apiece if you think you could haul it.

If it came down to a single 80 or 2 40's, I'd rather go with the 40's.

[Simon Richards]

Well, I suppose it ultimately boils down to what you're comfortable with but personally I would prefer to have some redundancy. You could go with an 80 and a 40 apiece if you think you could haul it.

If it came down to a single 80 or 2 40's, I'd rather go with the 40's.

You might be persuading me about the merits of this - its an interesting case to consider because the weight is about the same. Normally my thinking with very small tanks is that they aren't conducive to buddy diving - if you have a problem, you need to leave immediately, really there is no other action you can take, and it doesn't leave you a lot of scope for hanging around to help a buddy, or even thinking about them.

But these tanks are not so small for this shallow a dive. We are talking about a planned 20CF or so penetration. With single 80s each, that means to 1/4s, with 60CF left each at the turn point. If there is a delay or whatever coming back, we have 20 mins plus to fix it, not a problem if we stick to big enough cave and make sure the tie-offs are good. If there is a catastrophic gas loss, we will need to air share out. The singles are not truly redundant, even with the Ys (and neither would doubles be), so we have to assume that whoever has the gas loss might not shut down quick enough, so all their gas might be gone. Then we have to get two of us out from 20CF penetration with 60 CF while air sharing. There's probably going to be a delay and an increase in our breathing rates from fear. We can tolerate a total gas overrun of 50% - say a 30% increase in breathing rate and a 20% reduction in speed from the air share, something like that. Tight for a furthest point failure, and you wouldn't want to do the analysis from thirds or with tight cave.

With an 80 and a 40 each there is no problem, that's plenty of gas for the proposed dive.

With two 40s each, we have the great advantage that they are truly redundant. So the analysis of an exit delay is the same as for a single 80 - we have 20 minutes plus margin, and we have time to buddy dive properly. The analysis of a gas loss is better than for the singles - we assume that one tank is completely lost, so one diver has 30CF and the other has 60CF to exit, for a total of 90 CF - 50% better than the singles case. In fact we shouldn't then need to do anything before exiting - the worst diver has 30CF to exit from a 20CF penetration, so they can tolerate a 50% overrun. So you wouldn't need to go to an air share, you would just make sure the affected diver got the second tank closed off (and maybe even ditched), put them in front (no remaining redundancy, and scared) then head for the exit. They might make it on the gas they have, or they might need more gas a little before the exit. However by this time you will be much closer to the exit, and away from what would probably be a total visibility loss where the problem originally occured.

Maybe I am over analysing this, but I think that from a gas perspective, by using independent 40s you have the same total margin in the event of a catastrophic failure as (notionally) using either single 110CF or double 55CF each, because in those cases each diver would start with 110 CF, use 20CF going in to leave 90CF each, then you would have to assume total loss of all gas by one diver, so that the team has a total of 90CF left to get out with.

If my sums are right, then a team sidemount dive with 40s looks OK from a diving perspective, and better than a single 80. From a transport perspective, there isn't much in it in terms of weight - I think the two 40s are a little heavier than an 80, but probably only a pound or two. A single 80 is a little easier to carry in the jungle, because you can put it on a backplate, which is a little more comfortable than two 40s in a backpack. (Double 40s would be possible but wouldn't make any sense - if you manifolded them they are heavier than a single 80 and they are not truly redundant, and independent backmounted 40s sounds awful.)

If we were willing to dive to thirds, then we could theoretically dive a 40 and a 19 each - however that just doesn't seem like a whole lot of gas to be going cave diving with, even shallow. In the event of a furthest point gas failure it seems unlikely we would make it out - total gas is 60CF at the furthest point after the failure, but three separate tanks each with 20CF. Gas management would be a bit complex coming back out if there were any delay/breathing increase - we would possibly end up buddy-breathing the final tank ... (frightening thought!). I think if we were willing to dive thirds, I would rather do that from singles or doubles, so that at least all remaining gas could be shared without buddy breathing.

The only fly in the ointment is that my wife doesn't sidemount dive yet ...

I am over analysing this of course ... who could make such a fuss over a 20CF penetration? But it just surprises me how many possibilities there are. I think that by now I would just say that to me the dive is OK to 1/4 of single 80s or sidemounted 40s, that a furthest point gas failure could be tight with single 80s, but is much better with sidemount 40s, and go do it.

But any other thoughts are welcomed.

Cheers,

S

[DeWayne]

The decision to do this dive on a single 80 should come from your own experience (have you ever completed such a dive safely?) while you say screw agency politics. From what you have presented I would opt for the 80's and sling a 40 as a safety to offer some redundancy. The ideal would be as Marbry first suggested, attempt to put together a team of sherpas to get the cylinders you need to the cenote and back. As you decline one plan in favor of another, you need to keep asking yourself "am I comfortable with the trade-offs and risks that this option comes with." I would have no problem doing this dive with a single 80 and a slung 40 if I was confident that my buddy could do the dive with this setup. The politics that emerge from the various agencies can provide you with little more than guidelines by which to plan; it is you doing the dive so you will have to make the final decision as to what is used. I would use whatever the logistics allowed me to use to see/survey as much as is safely possible; if the cave goes and holds promise you can always come back with more help.

[Simon Richards]

The decision to do this dive on a single 80 should come from your own experience (have you ever completed such a dive safely?) while you say screw agency politics. From what you have presented I would opt for the 80's and sling a 40 as a safety to offer some redundancy. The ideal would be as Marbry first suggested, attempt to put together a team of sherpas to get the cylinders you need to the cenote and back. As you decline one plan in favor of another, you need to keep asking yourself "am I comfortable with the trade-offs and risks that this option comes with." I would have no problem doing this dive with a single 80 and a slung 40 if I was confident that my buddy could do the dive with this setup. The politics that emerge from the various agencies can provide you with little more than guidelines by which to plan; it is you doing the dive so you will have to make the final decision as to what is used. I would use whatever the logistics allowed me to use to see/survey as much as is safely possible; if the cave goes and holds promise you can always come back with more help.

All sensible words. I don't let the agency politics determine how I dive. I have plenty of experience for this dive, I have done lots of this stuff, including a couple of sumps after dry caves. In one low dry cave I timed the transport runs - it was more than twice as fast to get a 40 through a cave than an 80, so two trips with a 40 are quicker than one with an 80 in that particular cave - its the other way around in the jungle - an 80 on an AL backplate is great. In the case of that particular dry cave, sidemounted 40s are the way to go without doubt, in the jungle, 80s are interesting.

Despite having some experience, I didn't actually know the best way to do this dive I am talking about - we have been sitting on that cave for over a year (also my wife has been having our baby ...). As a result of that other thread, I started thinking about the advantages of a single 80 with a Y, which I have two of, and I think it has lots to offer in the case of this dive - its pretty simple, but its not fully redundant so it needs a team. But we want to dive as a team anyway, so that's fine.

My wife has less experience and the sidemounting is not for her yet. So if we were going to do this dive right now, I think I would do what you suggest, which is single 80s each and if we can get it out there, maybe one 40 between us which would transform the analysis of the worst case, and make it a little better than the sidemounted 40s.

I enjoy having a sensible discussion of dive planning, and that is particularly interesting in the case of exploration dives or dives with transport logistics (e.g. jungle or sump diving). Someone often has a better idea, or at least an idea worth analysing. The first interesting idea is the idea of the single tanks, which came from the other thread - I don't normally think of these, but they do have some attractions for some simple dives. The second, very good, idea is that of topping up with a 40. The third thing is that although I instinctively don't like the idea of buddy diving with sidemount 40s, I am now convinced that it is OK for this particular dive.

I would offer some personal thoughts though:

(1) The aim has never been to get as far as we can on one dive - we have a target of about 800 or so feet, and want to plan for that dive. As you say, if the cave goes we will can back with a re-thought plan. The issue is one of deciding what size "steps" to take in exploration - its not worth cutting a big trail until we know the cave is going somewhere, so we need to take a first step. In this case I am willing to skip the very initial stage of a mini sump-diving setup, but not go right to a full trail, horses, and double 80s.

(2) We have never really thought about diving to thirds on this dive - I have done that occasionally, but not very often even on sightseeing dives. If I could drive up to the cave I would just take doubles and as many stages as I need to do the dive comfortably and have done with it.

(3) The equipment choice isn't really between singles and doubles - because neither are fully redundant. It is between backmount team diving and sidemount (which is fully redundant), either solo or as a team.

(4) Our final decision is probably going to be different from how Marbry would do it, because it is going to be affected by my wife's training and experience level. She is more comfortable and safer using backmount, but she has practised carrying a stage with that for redundancy (not to extend penetration). Whether or not there is a knob in the middle of the backmount setup is not really a big issue either way - there is no guarantee that in the event of a serious gas problem she is going to get any valve shut off quickly enough to save any gas in the affected system, and we have to plan as if she won't.

Cheers,

S

[Simon Richards]

I was also thinking of a pit cenote I dived with a friend last year. This involved an 80 foot vertical drop to water, and we were looking to see if there was cave at the bottom. We set up a hold rope (with a big float on the bottom), a lowering/haul rope, and a climb rope. We dived sidemount 80s. First we lowered one tank, then I went down to the water and transferred gear from the haul rope to the hold rope while my friend lowered the rest of the gear. Then my friend put his vertical gear on, came down, the dive was done, and we reversed the sequence for exiting. The timings from the trip report were as follows:

7:00 am Leave Puerto Aventuras
9:30 am Arrive Valladolid
11:00 am Arrive Rancho XXXX
1:00 pm First team member descends
1:45 pm All equipment for first team member in water
3:00 pm Second team member & equipment in water. Dive starts
4:00 pm Dive ends; first team member ascends
~5:30 pm All gear out of water, second team member ascends
~9:30 pm Arrive Puerto Aventuras

This was our first attempt at this sort of thing, so we were not fast. We did 6 drops and 6 hauls (4 tanks, two harness/wing/light combos). In retrospect we could have got this down to 2 drops and 2 hauls and saved a lot of time (including my 2 hour wait in the water) by lowering two complete single tank setups ...

Cheers,

S

[Moonfuzzy]

How you do this dive depends on what you feel comfortable with. Although I have heard of several areas (Jug Hole, Woody's Room) being explored with a no-mount y-vale 80 I don't think I would feel comfortable with that in a cave.

With the set up you are talking about I would probably try my best to use sidemount. There would be no choice about solo or not - not a chance that Gary would miss the dive or I would let him dive something that sounds this cool without me!

If you could get one more person to help with gear you could each carry an 80 (strapped to your back?), and the stronger people could carry the extra stuff.

If not that then I would go for a Y-valve 80 for each diver and at least one 40 as reserve "oh sh*t" gas between you - or one each.