Hey amigo,
Yeah, the idea, whether using a dedicated bothy, using a bivy, or improvising your poncho or tarp by cocooning up in it, is all the same: can I retain enough heat generated from my body or multiple bodies (or artificial means i.e. candle output or heated water bottle) in a layer of trapped or slowly exchanged air to stave off hypothermia. As with any VBL idea, it's based on the notion that even if you end up wet, mainly from condensation and humidity from your own respiration and insensible perspiration, you will be able to adequately thermoegulate... enough to keep you alive until you remedy your situation or it is remedied for you.
I think the differences come down to ease of achieving complete protection from the elements and the durability or waterproof/water-resistance of the specific materials employed. And what scenario you think to apply it in. The worst case scenario I'm envisioning I described earlier, but basically amounts to alone, no contact, injured enough to render me practically immobile, and storming like a mofo. I'm not looking to spend the night or multiple days in comfort, I'm looking for the most efficient way to prevent hypothermia. And I need the minimal level of extra crap to do that; on a typical day hike, I'm unlikely to carry a stove or build fires for heating liquids, don't carry chemical heating packs, and would not be inclined to use a candle if I didn't think I could vent well enough (and I wouldn't be carrying a lantern either so flame is also a concern but less in my mind if you're aware. If you're injured enough to be passing out or falling asleep, your shelter better be able to keep you warm enough without a candle). So I need a shelter that will stand on its own in thermoegulation that only requires my body heat to keep me alive in te expected temperatures (already wearing insulating layers is a given in my mind, that's your first line of defense).
So, I feel that while yes the other options will work, I feel like as you said earlier, if you try to fit these things into performing this specific job, they're solutions that reach but don't quite make it all the way and fall short.
Tarp: It's a big plane that you need to force into an enclosed 3d structure that will prevent air and cold external water from reaching you while allowing normal respiration. You need to make sure you have a tarp large enough to cocoon yourself properly which means more weight and bulk that you have to decide to bring on every day hike no matter if the weather is fine when you planned to be out. I don't think many plan to be in survival situations, and like you said: will you trust your life to hoping the weather conforms to predictions? Temps and lack of rain may be fine for the 6-8 hours or so I'm out in the day, but did I bring the "right" stuff while remaining light and mobile for the day hike, not anticipating having to camp out or having incurred an injury serious enough to prevent self-evacuation?
Poncho: Used to be my go to, but in the case of injury, it suffers the same as the tarp above, with the added benefit of having a hole in the middle of it. So not only do you have to prevent warm air escaping and cold air/water entering the plane sides, now you have to contend with a hole smack dab in the middle as well. Sure it'd work, you can sit on your pack, hunch down and assume sitting fetal, wrap the poncho around you and keep the hood playing nice or cinch it up all together, in which case you might as well be in a bothy.
Bivy: A properly designed one with a purpose built hood or large enough to fashion a hood, fixes everything about the previous two, except for the one hole that is still exposed to the elements. I think in essence a full rain suit would basically do the same as a bivy, if it trapped enough air and allowed you to go fetal for warmth conservation. The only negatives would be that the jacket isn't connected to the pants and your shoed feet would be exposed to the cold and/or wet.
Bothy: Fixes everything above. You pull a material 3d bubble over you. No fuss. Warm air is trapped in the bubble, since the only hole is flat against the ground. The same goes for wind and rain getting in - it doesn't, not if you keep the hole right to the ground. It is light, and no added weight for unnecessary material because it is already shaped for you. Since it is light and compact, you are more likely to have it on you when you actually need it, no matter the conditions you set off in... because sometimes despite your planning and expectations, stuff just happens. It takes the bare minimum effort on your part to make it work. You just unstuff and pull it over you, more comfort can be had by tweaking once you're inside. If you have mangled limbs, you don't need to maneuver yourself with wrapping or moving your body into a bivy, all you need to do is weight down the seat with your body and get the bubble over you.
One caveat on the bivy vs bothy, something like the blizzard bag I don't really count in terms of how much warmth is provided. The blizzard bag seems like it is made of cells in the material so it traps heat much better than a simple fabric layer. I do think that for one, the Blizzard Bag would provide more warmth because its basically a waterproof wind proof light/medium sleeping bag.
Now the one area I can see differences being blurred is in the specific materials used. Then you're into the standard weight vs material properties debate (PU coated nylon vs silnylon vs cuben etc).
Anyway, yeah, fun discussion, I look forward to doing the bothy experiment :thumbup:
