This isn't "Heat", this is just Discussion! And I am glad for it, it helps me crystallize my thinking and in some cases gives me new food for thought.

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1. It will be a huge PIA to put foam in those places, especially sheet foam.
Not really! I already have access to the hull from basically everywhere, and am putting in the first layer today.

2. It will be expensive to put in that pour in place stuff.
If I use this stuff, it will only be in small areas between sheet foam edges. As it is open cell, if I use it it will only be in areas which will be glassed over so that no water can get in (and stay in).

3. It is unlikely that an Ariel will get holed in such a way that you will want to stay on the boat.
3.a. If you already sealed off the anchor locker, that is going to be your most bang-for-your-buck. That is the most vulnerable part, the front. That is the part that runs into things like containers, other boats, bouys, etc.
I realize that there isn't much out there which will actually 'punch a hole' in an Ariel hull. Maybe if you just happened to strike the corner of a floating shipping container at the right angle, that would do it. But how water gets in won't really matter too much, if it is getting in. That's what the +flotation is for: Prepare for the worst, hope for the best. Agreed about the anchor locker/stem, but in the frame of the just quoted philosophy, I am paying particular attention to any part of the hull which when moved through the water would be diverting it away from flowing straight back (or: any part of the hull viewable when you stand in front of the boat). Also I keep in mind that it is rare for an Ariel to sail unheeled, and so areas above the at-rest waterline are probably just as likely to take an impact as those below it.

4. Even if it does float after getting holed, why do you want to save the hull? (combined with)
5. A good 3-person life-raft is probably cheaper and less frustrating than any floatation-addition-project.
The idea is that the boat will be it's own liferaft, floating with *some* freeboard, even if only 8-10". If it can do that, then it can be fixed enough to be bailed out and used to gain the safety of land for further repair.
Electronics etc in a survival situation are not usually _needed_, so theoretically as long as I am alive and have the basics, and a way to get back to shore, I am much better off than if I were floating around in a rubber raft with a small bag of 'survival tools', hoping that someone finds me before too much time passes. I will have basic electronics (VHF, GPS, flashlights, etc) in handheld waterproof versions which can take exposure to water. I will not have an EPIRB, because I believe this: since I am the person who is taking the risk to go to sea, I don't want anyone to risk their life to try and save mine. I have no children, noone besides myself who is dependent upon me; rescuers likely do. So I assume the mantle of responsibility now, before ever leaving shore, to see that I am "self insured" when it comes to my own survival. Of course, YMMV, and that is up to you, just as this decision is up to me.
Most everything will be stowed in top-loaded watertight lockers inside the boat, with the exception of what I (prior to setting out) decide is necessary to repair a hole in the hull, a "fix it" kit. Should the worst happen, I would try to fix it, and then bail the boat out. I will have at least one HIGH capacity manual bilge pump onboard. After bailing, a large part of what was in those lockers will hopefully be fine. If not, then I will still have more stuff to "MacGyver" with.
I think that positive flotation is also probably *much* cheaper - materials will total less than US$500, and time would be spent refitting the boat anyway, even if I weren't going the +float route. That said, this isn't about saving money, it's about what I think is the best way for me to continue sailing as long as possible, even if I encounter the worst situation imaginable for a sailor.

6. In a small boat, the space taken by the foam is going to be "expensive" in terms of usability. Better to install the air-bag-floatation things. They stay small until called upon.
I am a minimalist when it comes to "things", and so require less storage space than if I were otherwise. (Except maybe for books, but they float, and can be dried out - lol). Also, I have learned from backpacking and kayaking - sports which have much in common with cruising and living aboard a small boat - that before setting out, you will wind up "needing" as much stuff as you have space for, yet once on the road, you find that much that was "needed" goes unused. Compared to my backpacks (1.15-3.76 cu/ft) or my kayak (5.78 cu/ft), the Ariel carries a ton of stuff. I actually revel in the "large" space for stuff I have on my Ariel! I am not 'normal' in this, and people are as a rule somewhat amazed that I feel this way, especially when they come aboard. But I have a place to sleep, sit, cook, and stand, and it's all out of the rain, with no setup needed - and that is almost luxurious to me. Sure, she'll be loaded down when I leave for long passages, but mostly with food and water.
As far as float bags - I did do some serious thinking about them, and checked out all the products I could find on the market to see what they offered, but eventually came to the conclusion that tanks and tubes and bags can all leak, can all fail at what they are intended to do. Foam cannot - it just sits there, holding air all the time. I like that simplicity, and in the context of what I am doing, it makes more sense.

7.Do many other long-distance ocean voyagers do this mod?
I am not aware of any statistics about that, though I would assume that most don't. Most probably rely on liferafts (or dinghies which can serve as one, but that is a much smaller number), as that way is, as you said, much easier, involving less thought and construction. My objections to depending on a liferaft are already stated here, my boat is too small to stow a liferaft-capable dinghy on deck (besides which, I want my decks clear), and I don't want to have to deal with keeping a dinghy-in-tow during a storm.
I do know these things: just a few days ago I was aboard a 52' aluminum sloop, hull/deck made in France, fitted out by the 4X-transAtlantic owner in Canada, and he'd made it to have positive flotation, so there may be more of them than is reported, too.
My friend Neal Peterson, 2x OSTAR and 2x BOC racer who has rounded all the Capes, knows and approves of what I am trying to do. He's been in situations where his boat was about to sink, or would have, had he not acted quickly and properly. He hand-pumped his boat 20 minutes of every hour around the clock 2/3 or so of the way across the Atlantic after being hit by a Russian freighter. He understands what I am trying to do, and it is stories like his among others which are part of the reason I am doing it this way.
There is a European company named "Etap" which builds well-regarded boats that have positive flotation from the factory. I am sure that it is much more expensive to construct a boat that way, a cost which would have to be passed on to the buyer, and I would bet that price alone is *the* significant factor in why fewer boats are made this way.

---not raining on your parade, just giving you something to think about, and maybe save you some time so you can get to making passages sooner.---
I don't think that at all! I appreciate your taking the time to discuss it with me.