Hacker News Reader

Swimming was quite common in ancient Mediterranean area societies. They even did a lot of breath hold free diving, hunting for sponges and shellfish.

Modern swimmers probably are better than anyone in history. An optimal front crawl stroke technique isn't intuitively obvious at all. There have been significant improvements just in the last few decades.

https://en.wikipedia.org/wiki/Front_crawl#History?wprov=sfla...

Don’t know about “optimal”, but apparently the front crawl itself isn’t too novel.. from that wiki page:

> The "front crawl" style has been in use since ancient times. There is an Egyptian bas relief piece dating to 2000 BCE showing it in use.

When I was a kid we had an above ground pool that was about 3 feet/~1 m deep. One day in the shed I found a 6 foot/2m length of hose that looked to be about the same diameter as the tube on the my little snorkel. Ah ha! I can attach this to my snorkel, tie the other end to a pool float and breath under water like a diver!

My idea did not work out as planned. At the surface I was able to breath through the hose but once I tried to "dive" down I found that drawing a breath was difficult. At the full depth I could not draw a breath. I told my father later on (who was an engineer) and he explained the physics of water pressure.

That was dangerous due to the risk of hypercapnia: "increased CO2 levels could lead to unconsciousness which, if happening while in the water, can result in drowning.". Note that our breathing is regulated by the amount of CO2 in our bloodstream, not the amount of oxygen. There is a fantastic explanation of the danger here: https://blog.daneurope.org/en_US/blog/are-full-face-snorkeli... There are safety regulations to limit the volume of air inside a snorkel (the dead space).

"Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, or the smaller the tidal volume of breathing, the more this problem is exacerbated." —https://en.wikipedia.org/wiki/Snorkel_(swimming)

> When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again

For a long snorkel, you would want to exhale through your nose.

> This would be about as useless underwater as one of those little one-liter bottles with integrated mouthpiece that scams show being inflated with a bicycle pump...

These irk me too! The pictures never mention that the hand pump in them, isn't a bike pump at all, it's a high-pressure pump that you will have to pump for nearly an hour. I see these constantly in the "suggested products" when shopping for any diving kit on amazon.

IIRC, the average air consumption is about 1 cu. ft./ min. at atmospheric pressure. Assuming the bladder is not pressurized, at a guess it could hold ~2 cu. ft., so maybe a couple minutes worth of air. If you could manage to submerge, that would be substantially longer than the average person can hold their breath, maybe enough to be worth it. (Notice the "if".)

Strictly speaking every 10m under water increases pressure by one bar. So elastic bag 10m under water would take up 2x less space.

It’s more complicated than that because the bag can be deflated unlike a scuba tank so you’re benefiting from normal atmospheric pressure. Assuming the scuba tank was filled with normal atmosphere and this bag was about the size of someone’s torso, it’s probably much closer than 200x.

First you don’t normally do ultra slow deep breathing with scuba gear which can increase the amount of oxygen extracted. Next the volume of scuba tanks isn’t that high normally and you lose reserve capacity. My guess is up to 5 minutes of air in a bag is probably possible which could be a quite a bit if their pearl diving or something.

On the other hand it could simple be an inflatable bag useful as a flotation device.

None of that is even close to correct. Are you a scuba diver?

Not regularly and only very shallow so my recall of the details isn’t great. But I do recall asking about even slower breathing and the response was it extracted more oxygen but wasn’t recommended due to the risks of disorientation. I think the 2x was in relation to failing to do slow deep breathing though.

As a sanity check 6l of air per minute x 5minutes is 30l which is about the size of your torso. Assuming that bag started that size and got completely emptied it’s about right. The volume would decrease rapidly with depth but the available oxygen per breath would increase to compensate.

Unless you have something specific to correct?

That's about the size of my torso, but I'm fat as hell. I suspect the average ancient swimming warrior was in better shape.

Unless your short your estimate seems odd. The torso is about 1/2 the volume of the average adult and people are close to the density of water of 1kg/l or 2.2lb/l. So 30l would be the expected torso volume of someone weighing ~60kg or ~130 lb.

Edit: “47% trunk and neck” so 40% torso would be 75kg and 165lb seems reasonable as a smaller person would also need less air.

30L is 8 gallons. The average torso is somewhat longer than a five gallon bucket, but of course it's much slimmer than a five gallon bucket. An eight gallon bucket, if it existed, would be larger yet. I would question both the assumption that a torso is half the volume of a human and the further assumption that the density of humans is uniform. Obviously the thorax, which contains the lungs, is less dense than the legs, which just have muscles and massive bones with a thin layer of fat.

Sorry it took a while to respond; I got rate-limited this morning.

Ok in anatomy the torso is shoulders to anus. Various other places use shoulders to top of hip bones. https://en.wikipedia.org/wiki/Torso vs https://www.wikihow.com/Measure-Torso-Length

I never realized it was ambiguous, but the image shows roughly chin to balls so whatever term you want for the longer one. https://twitter.com/FedeItaliano76/status/143529525743702835...

You're just so far off that I don't even know where to begin. Either the person you asked was completely confused, or you didn't understand what they told you. As a starting point I would recommend reading the US Navy diving manual.

https://www.navsea.navy.mil/Home/SUPSALV/00C3-Diving/Diving-...

Can confirm - I went on a bunch of dives with a former Navy Seal and he used less than half the air on any given dive than the rest of us (who were all only regular or advanced certified) did. His explanation was that he'd just mastered the meditative mindset/breathing technique to use as little air and energy underwater as possible.

Mindset and breathing technique is certainly important in minimizing open circuit gas consumption, but it's not the only factor. Some people just have a naturally higher tolerance for CO2 loading (the instinct to breathe is driven more by increase in CO2 levels rather than lack of O2). Perfect buoyancy control helps a lot since you're not wasting energy on depth keeping. Equipment configuration should be streamlined to minimize drag. A high level of physical fitness also allows you to keep your breathing under control during periods of exertion, like finning against a current.

But I've also seen former military divers who had relatively high gas consumption. Some units mainly use closed circuit rebreathers or surface-supplied gas where breathing rate doesn't matter.

That’s true, but irrelevant in this case as in the short term people are O2 limited not CO2 limited which is why the world record for holding ones breath goes up by breathing pure oxygen.

It’s also one of the reasons I was asking for examples as I assumed you where making assumptions that didn’t apply.

For the vast majority of people that's just wrong. The urge to breathe is driven primarily by increasing blood CO2 levels (hypercapnia). You can learn to tolerate higher levels but to reach the point where pre-breathing pure O2 makes a difference takes extensive dedicated apnea training.

Anyone can extend breath hold times at least a little by hyperventilating first to drive down CO2 levels, thus surpressing the urge to breathe. That can be dangerous underwater as it becomes easier to go hypoxic before the urge to breathe gets overwhelming.

https://pubmed.ncbi.nlm.nih.gov/17274316/

I am pretty well aware of what applies here.

> For the vast majority of people that’s just wrong.

>That can be dangerous underwater as it becomes easier to go hypoxic before the urge to breathe gets overwhelming.

Again true on both counts but irrelevant, there is no reason to suppose people in 800BC where average divers. Pearl diving goes back to ~2000BC so demonstrating a very long tradition of people pushing these limits and it’s exactly the people pushing such limits that might try bringing an air supply underwater.

Sure this depiction is almost guaranteed to be a floatation device, but you build something like that and spend your life around water and people are going to try stuff.

Perhaps you could begin by explaining what's wrong, rather than citing a book (or actually not even a book, rather a page linking ten or so publications), with no indication of what part to read. I am of the opinion that you are the one who doesn't know what he's talking about.

> The volume would decrease rapidly with depth but the available oxygen per breath would increase to compensate.

> Unless you have something specific to correct?

Firstly, how would you bring the highly buoyant bubble of air underwater, in a controller manner?

But let's say, for the sake of argument, that you could. Even though the volume of the bubble would indeed decrease with depth (e.g. it would halve at 10m), the volume of each breath would remain roughly unaffected by depth. This is the reason scuba divers deplete their air supply faster the deeper they go.

The amount of available oxygen per breath is irrelevant. Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[*].

[*] Interestingly, there's a point at which oxygen becomes toxic. The circumstances in which this happens is well outside the parameters of this discussion, however. https://en.wikipedia.org/wiki/Oxygen_toxicity#Underwater

> Even at sea level the exhaled air is about 16% oxygen (down from 21%). Giving the body access to more oxygen per breath won't make the air supply last longer[*].

-Incidentally, in the survival suits we wear when helicommuting in the North Sea contain a rebreather device - which is simply a small bag in the suit's lining with a mouthpiece and a valve.

The idea is that when getting underwater is imminent, you take a few deep breaths. When you exhale into the bag, you can then draw the spent air back in for another gulp or two of air.

The idea being that in a couple of breaths you'll either be in the clear or dead - so a couple of extra breaths is all you realistically need. (Ever the cynic, I suspect part of the reasoning behind it is to keep you busy and un-panicked in your last few seconds alive, but anyway...)

I am not saying it’s practical or their going to spend time at significant depth. The utility might have been a simple game of who could stay under water longer. That said, spending noticeably longer at 5-10m with an air filled bag weighted down by stones is a very long way from oxygen toxicity or the bends.

As to available oxygen, removal of CO2 is major limitation in the long term and that’s limited by partial pressures so having more available oxygen doesn’t help in the long term. However, the world record for holding your breath is 22 minutes on pure oxygen so if we are talking a 7 minute vs 12 minute dive CO2 is going to be irrelevant.

I used to be a scuba (SCUBA) diver, and yes, that is not so incorrect as you imply. See my post a little higher suggesting that the bladder could contain a couple cubic feet of air, which would give you a couple-three minutes.

The real problem is diving, rather than floating on top of (or more likely underneath) the bag, as others have pointed out.