Stuartv - The plan looks pretty good to me.
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I would highly resist complicating the dive with lots of extra gear. My only comment on the plan is why GF 50/85? More often it is GF 30/85 which will have a little less bubble stress.
GF 50/85 is the result of these things: Watching the presentation by Simon Mitchell that was posted in the thread about Decompression Controversies. Given what GF actually are, it has never really made much sense to me to use such a low GF Lo as most people use. You're trying to keep your tissue pressure gradient below 30% of the M-value at your first deco stop? Why so low? 50 still seems low, actually, but 50/85 is what I have adopted as my standard settings for easy, low stress dives. Which brings me to my second point. This dive is expected to be warm water with good viz and an easy ascent (given the superstructure and anchor chain I'll have to ascend on). If, for example, it were cold and low viz and a free ascent, I would use more conservative settings. I add that the warm will be warm (by my standards) but I would still do the dive in my drysuit.
Since you mentioned 21/20, I assume you're Trimix certified?
I will have my Helitrox cert by the time of this dive. So, no on Trimix. And thus why I will not have any training on handling multiple deco bottles. I talked to my tech instructor yesterday and he was firmly against me attempting to use multiple deco bottles until I get proper training on that. But, he said he thinks it's fine to plan to use a single AL80 for deco gas.
It sounds to me that this trip does not work as you do not have a buddy.
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Deco'ing out on back gas for an hour or more is not a solid plan. Wasn't there some bit about limiting factors? Often the limiting factor is the consumables, but it can be your bladder, boredom/paranoia threshold or the cold.
TBH, you should like you are trying to justify a dive you should not do:
1 - no buddy
2 - wrong kit (40 isn't enough)
3 - not an ideal time of year
4 - possibly too big a jump, especially given 1-3
So, perhaps I should just plan to do it solo? I will have my SDI Solo Cert in about 3 weeks. That cert is not for tech dives. But, between my tech training and having formal training for solo diving of any type, I would feel comfortable doing it solo. I've already done a number of wreck dives as part of a photo/video contest where I may as well have been diving solo. I had two buddies that were mostly off doing their own thing anyway.
A 40 may not be enough (though I think it is, with the right dive plan), but I will carry an 80, if that is what is needed.
I don't see what difference the time of year makes. I will dive in a dry suit. Conditions could be too rough to dive any time of the year. If I plan to dive solo, then the time of year making it hard to find a buddy is moot, so what difference does the time of year make?
I've been to 160' before (on EAN23). And that was a dark, low viz, somewhat cold dive. The "jumps" here are (possibly) carrying an 80 instead of a 40, and being solo. To offset that, I would have some Helium to keep my head more clear, and the water would be warm (and me in a dry suit) and better viz. Is that really "too much"?
Oh, and regarding my bladder and boredom threshold: I am a very experienced long distance motorcycle rider. I am a member of the Iron Butt Association, which is a real thing and you can only become a member by having a certified completion of a qualifying endurance ride. My longest single ride so far is 1100 miles straight through, in 21 hours (from Tallahassee, FL, to Sonora, TX). I have a lot of experience with being cold and sedentary, holding my bladder, and occupying myself with mental gymnastics.
I'm not trying to justify a dive I should not do. I am trying to work out a plan for a dive so that I will be safe. If that requires a relatively short bottom time and relatively short deco plan, so be it. I certainly don't mind being conservative in that way in the planning. But, "don't do it at all" is a bit too conservative, I think.
