Originally Posted by frankfrank

The science of weightingdown a swim ring in a body of water would actually be very interesting, and I don't know what the answer to this is.
One thing for sure, get a 500pound person who sits on one of these rings, and manages to entirely submerge it. I would think the amount of weight it would take to submerge it, would be the amount that the cubic volume of the (nowsomewhatcompressedandreducedinvolume) tube would weigh if, instead, it was filled with water.

I believe you are correct. I think the amount of weight it takes to submerge a ring is the weight of the water it displaces. But when you are on a swim ring, some of your weight is on the water directly. Your legs and your butt and your arms are in the water.
I think I've calculated the water volume of 30, 36 and 47 inch swim rings, and probably posted that calculation on this forum (Hello Alan, yeah, I'm a dork! lol). I can't square that with what I've observed, because it's not obvious how much of that weight is born by the inflatable and how much is buoyant in the water. But by observation, someone under 200 pounds can sink a 30 incher sitting upright on it, but someone bigger than that won't sink it laying across it with much of their body floating on the water. By observation, it takes 350 pounds to sink a 36 incher, though it probably displaces less than 350 pounds of water (I don't remember and I'm too lazy to do that calculation again). And I've seen someone weighing 750 pounds not sink a 47 inch ring, though if I recall it displaces less than 600 pounds of water.
If you google the volume of a taurus you'll find a calculator you can plug numbers into, and you can find the volume in cubic inches (or mm) of a swim ring, and then you can multiply that by the weight of a cubic inch of water (also googlable). Remember the calculator takes radii and not diameter.
I just did that for a 47 inch swim ring, which I believe inflates to 43 inches with a 15 inch inner hole, approximately, and displaces 540 pounds of water. But I've seen someone bigger than that not sink it, because some of their weight is in the water and not on the ring.
Inflatables that you sit in a hole work on this principle. Some of your weight is born by your own buoyancy.
Quote:

is the pressure on a submerged ring ever more than what the waterequivalent of the ring would be?

And by that, if you mean, should a 755 pound person stress a swim ring more than a 350 pound person, given that 350 pounds will sink it, too, I do not know. I suspect the answer is yes, but not by as much as you'd think. I can just tell you that it LOOKS like it should pop. Which is the fun of swim rings. The surprise when they don't pop!
Possibly the extra stress once it is submerged comes from the depth of the water it is in. I just googled and you can calculate the pressure in pounds per square inch according to depth. But I can tell you that it doesn't go far under water. When she's got it trapped under her belly, basically under her hips, laying on it, it is maybe 18 inches under water. She's pretty buoyant, even without an inflatable under her. She can float with it under her belly without a lot of help from me to stabilize her. It goes farther under water if she sits upright on it, but then it very much wants to scoot out from under her.
EDIT: Unless I've miscalculated, a 36 inch ring, which is actually only 32 inches inflated, with a 13 inch hole, only displaces 180 pounds of water. I have one inflated to check my numbers. I don't have a 47 inch, so I've just guessed those numbers. Also guessing at 30 inch numbers, 27 inch outer diameter, 11.5 inch inner, displaces about 100 pounds of water. It takes a person almost twice as heavy as the displaced water to sink a swim ring, because of the buoyancy of the person. Sitting upright on it sinks it the most, as more of the body is out of the water.