Open Letter to Non-Divers Who Have Watched 47 Metres Down

47 Meters Deep is a survival story about how two sisters went shark diving in Guadalupe, Mexico, and how the cage went unhinged and they dropped to 47 meters, survived attacks from the Great White, profess their love for each other, and eventually one died and one rescued. One moment they were having the time of their life, next, their lives in peril. Sounds epic? It is.

Being a movie, it means that it serves to entertain your senses. Being a story, it means that you need a protagonist and antagonist. Being a Hollywood movie, you have a Hollywood ending.

Dear non-divers, this is what we want to tell you if you have watched the movie. Spoilers ahead.

 

1. Size of Great White Sharks

Taylor, the man who operates the shady cage-diving business, remarked that some Great Whites could go up to a size of 28 feet (8.5 meters), that is much larger than what has been verified by records. The largest Great White Shark recorded was only 20 feet or so.

2. Depth and Pressure Relationship

The sisters were wearing full-faced masks with a communication unit (We shall not delve into the prowess of the communication unit, let’s just talk about scuba diving first) when the winch of the cage broke and they started to drop drastically in depth. As you go down further in depth, the higher the pressure builds up. Here is an illustration in numerics.

Every 33 feet of salt water = 1 ATA of pressure

  • Pressure a diver experiences = water pressure + 1 ATA (from the atmosphere)

Total Pressure at Standard Depths*

Depth / Atmospheric Pressure + Water Pressure / Total Pressure

0 feet / 1 ATA + 0 ATA / 1 ATA

15 feet / 1 ATA + 0.45 ATA / 1 .45 ATA

33 feet / 1 ATA + 1 ATA / 2 ATA

40 feet / 1 ATA + 1.21 ATA / 2.2 ATA

66 feet / 1 ATA + 2 ATA / 3 ATA

99 feet / 1 ATA + 3 ATA / 4 ATA

*this is only for salt water at sea level
Credit to thoughtco.com

Air spaces start to compress with an increase in depth. The air spaces in your ears, sinus, lungs and all other air-filled cavities in your body gets compressed as pressure increases. To combat this, we were taught to equalize the pressure as we descend and to descend in a controlled manner. The sisters were not descending in a controlled manner, they were dropping like crazy, leaving insufficient gaps of time to equalize. Upon reaching the bottom, at 47 meters, Lisa had a bloody nose, but they were wearing full-faced masks. The air in the mask would have been regulated with every breath so it would have been the ears that would have “popped” instead.

3. Air consumption

Every scuba diver knows that the air that you have in your tank is finite, meaning it runs out at some point. Question is, which point exactly? That knowledge serves to help you to plan your each and every dive so that you can emerge alive at the end of a dive. It is common sense to deduce that the longer time you spend underwater, the more air you will use. However, the film has conveniently ignored the fact that changes in depth are also relevant to the rate of air consumption.

The compressibility of gasses is also an important consideration for divers as it affects how long a diver can stay underwater. Scuba regulators are designed to deliver air to a diver at the same pressure as the surrounding water pressure. That means that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amount of air as two breaths at the surface. Obviously then, a tank will only last half as long at 33 feet as it would at the surface.

A tank that would last 1 hour on the surface would only last 1/3 as long, or 20 minutes, at a depth of 66 feet, etc. So at 47 meters (154 feet), the pressure is 5.67 ata, assuming a regular rate of breathing and no change to surroundings, they would have lasted about 1/6 of the time with a supposedly full tank. Let’s not forget that they were supposedly left with less air after staying a while at 5 meters, they were not calm, and both were moving about quite a bit, trying to dodge attacks from the sharks. With all these factors, I’d say that the maximum bottom time they’d have left with is, 15 - 20 minutes?

In reality, one might have asphyxiated while trying to get out of the cage.

4. Decompression Illness

Nitrogen gas, comprising of almost three-quarter of the composition of air, dissolves and accumulates in our tissues. Nitrogen is an inert gas, unlike oxygen, which is used up by the body. When a diver surfaces up from depth, the pressure decreases and the dissolved nitrogen starts to want to leave your body, the excess nitrogen comes out as air bubbles and they start to expand in the process of decompression. Try shaking a can of coke (building pressure) and then opening it up(releasing pressure), you see the compressed air leaving the can as bubbles fizzing out from the drink. Same theory at work. If you ascend too fast, the excess nitrogen bubbles do not have enough time to leave the tissue (off-gas), hence, when you do deep dives over a certain period of time, you need to do “decompression stops”.

The Captain told Kate to descend again to 47 meters and wait to be rescued because they would get the bends (Decompression Illness) and DIE when she said she has 55 bar left in her tank. So running the risk of DCS (the most fatal being Arterial Gas Embolism), or die for sure due to the lack of air to breathe, which one would you choose? Taylor emphasized that “nitrogen bubbles could form in your brain and you will be dead” and insists on a decompression stop at 20 meters when they have stayed at the bottom for a good half of the film. Seriously??? Captain Taylor’s obsession with decompression stop is real.

5. Sharks are attracted by blood and love eating humans

That is a a very common misconception about sharks, especially about Great Whites. It is true that they are one of nature’s top predators and are at the top of the food chain with a rare number of competitors like Orcas etc. Humans, however, are not their desired prey. Do we need to keep our distance from them? Yes, that is a good idea, as with all wild animals right? But by depicting them as blood-thirsty predators all the time, I’d say that it is a wrongful and unfair depiction.

To conclude, it is an absolutely illogical film that conveys wrong information about scuba diving and Sharks. First and foremost, accidents in diving do not happen as frequently as traffic accidents, to say the least. And if you really want to do cage-diving, seek out a reputable operator first! And here are two women who seek to redress the grievances of these beautiful creatures. Meet Ocean Ramsey and Kimi Werner.

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