47 metres (154 feet) is considered a deep dive and only to be dived by experienced scuba divers
47 Metres Down is a fantasy movie about two sisters who end up 47 metres down with great white sharks. As a result of this scuba diving movie, people ask: “Can you survive 47 meters underwater“
If you are researching the answer to this question because you would like to scuba dive, it’s important to understand about surviving 47 metres underwater. You can you survive 47 metres underwater but to do so you need to have the necessary training and experience as a scuba diver. To survive a deep dive to 47 metres down you must follow decompression stop limits or carry out decompression stops on your ascent to avoid getting decompression sickness.
How deep is 47 metres in feet?
- A depth of 47 metres in feet is 154 feet.
- A dive of 47 metres in yards is 51.4 yards.
- Diving to 47 metres in miles is 0.0292044 miles.
- A depth of 47 metres in kilometres is 0.047 kilometres.
7 metres or 154 feet deep is the equivalent of the following:
- The length of 2.6 of the longest recorded whale sharks (record whale shark is 18 metres or 33 feet).
- 7.7 of the longest recorded great white sharks stacked head to tail (Longest recorded is 6 metres or 20 feet).
- 7.7 giraffes piled on top of each other (tallest giraffe is 6.1 metres (20 feet tall).
- 10 metres (30 feet) taller than the Leaning Tower of Pisa, which is 57 metres (187 feet).
- 5 metres deeper than Tower Bridge, London is tall, which is 42 metres (138 feet) high.
- The same height as the Eschenheimer Tower in Frankfurt, Germany which is exactly 47 metres (154 feet) tall.
How far is 47 metres underwater?
47 metres underwater is 154 feet or 51.4 yards and the equivalent of 0.0292044 miles. It would take you 42 seconds to walk this far walking at 2.5 miles per hour.
But how far is 47 metres underwater when compared to everyday items?
- 3.13 times the average length of a UK garden which is 15 metres (49 feet).
- 1.5 times the average length of a US garden, which is 104 feet (32 metres) and equates to 10,871 square feet
- Just under the length of an Olympic swimming pool, which is 50 metres (164 feet).
- Similar to the width of an American football pitch which is 53 1/3 yards wide (48.8 metres; 160 feet).
- Just under half the length of a football field, which are 100 metres (328 feet) long.
How long can you dive for at 47 metres?
You can dive for 8 minutes as a no decompression stop dive to 47 metres down using BSAC tables. PADI dive tables have a maximum depth of 140 feet (42.6 metres) which gives a no-stop dive time of 8 minutes.
Is 47 meters down a true story?
47 Metres Down is not based on a true story and the same is true of its sequel 47 Metres Down: Uncaged. If you watch 47 Metres Down as a scuba diver you will know how ridiculous the movie is, but even the writer Johannes Roberts refers to some movies as preposterous.
Roberts puts it like this: “For me what works about both movies is that they’re actually, as preposterous as they are, you know, they’re movies“.
He goes on to say in an interview with Roberts, “There was a wonderful three-page thesis someone wrote on the first movie about the inaccuracies of the diving stuff. And yeah, sure! But you know, if you went down 47 meters in a cage to the bottom of the ocean, with a tank, and you were an inexperienced diver, you would probably last about three minutes before you died or ran out of air.”
More about the film 47 Metres Down?
The film 47 Metres Down has came in for major criticism from scuba divers for the mistakes in the dive scene. The criticism is around how realistic the underwater scene is. How true is the story or not?
The underwater scene when Mandy Moore (Lisa) and Claire Holt (Kate) sink to 47 metres down is certainly not realistic. The underwater sequence lasts for nearly an hour.
A dive time at 47 metres would probably require much more air than the dive tank they went down with. Even with the second tank that was sent down this would still not be enough air to last for 60 minutes at 47 metres. See below why scuba diving at depth requires more air. But in the explanation below this assumes a ‘gentle‘ relaxed dive, without much strenuous exercise.
In the film 47 Metres Down, Lisa and Kate are far from relaxed and they are also put through their paces too. This meant they would be ‘drinking‘ their air supply very very quickly.
More Reading: Is it dangerous to scuba dive?
You also see below how 28 minutes at 47 metres requires a total of 21 minutes of decompression stop time. But in the film the bottom time is nearly double this time. The decompression stop time would more than double the stop time of a 28 minute dive.
In the movie 47 Metres Down Lisa is told to do a deco stop for 5 minutes at 20 metres (67 feet). With such a short decompression stop for such a long dive at 47 metres she would definitely get decompression sickness.
With the length of dive and resulting amount of nitrogen build-up she would have suffered from severe decompression sickness (or the bends). If she wasn’t taken to a hyperbaric chamber she may have even died.
I’m sorry to spoil the movie for those of you who enjoyed it. As long as you apply real escapism to the movie, then who cares about the true facts of scuba diving to depth?
Trailer to the movie 47 metres down
How realistic is 47 meters down?
The movie 47 Meters Down is completely unrealistic. This includes how much air is consumed in 60 minutes at 47 metres; how fast Lisa ascends to the surface is unrealistic with only 5 minutes decompression stop time allowed.
Please read on to discover the alternative ending to the movie 47 Metres Down.
Can you survive 47 metres underwater?
Scuba divers can survive 47 metres underwater, but unless you restrict your dive time to 9 minutes your dive will turn into a no-stop decompression dive meaning you must stop to decompress on your ascent. 60 minutes at 47 metres would take several hours to surface safely with long decompression stops.
Diving to 47 metres is 7 metres beyond the PADI recreational diver limit of 40 metres.
Scuba diving to 47 metres (154 feet, 51.4 yards) is considered a deep dive.
I’ve dived to 47 metres (154 feet, 51.4 yards) on many occasions and deeper. This means a dive to 47 metres carries more risks. See below for the risks associated with scuba diving to 47 metres and deeper.
All scuba diving carry an element of risk, but the risks and the dangers of diving can be minimised. Any dive deeper than 30 metres (100 feet) is a dive only to be considered by an experienced and well trained scuba diver.
This means a scuba dive to 47 metres (154 feet) with PADI is not an option.
Why I dived to 47 metres, plus?
So you may now be asking ‘how is it that you’ve dived to 47 metres plus?
Why dive deeper and why dive to 47 metres?
Any dive should have a purpose. The same goes for diving deeper as there should be a purpose for the dive. Diving deep merely for the ‘bragging rights,’ is a fools game.
So what might there be at 47 metres down? In the UK there are a huge number of wrecks to dive. These wrecks are as a result of two world wars and the UK are a shipping country.
With that there are a huge number of ships that either sank due to bad weather or were sunk in battle during one of the world wars.
More Reading: Truk Lagoon deep wrecks sunk during World War II
The problem is, when a ship sinks, the wreck sinks where it is in the ocean. That might be at 10 metres (30 feet), or it may be at 47 metres (154 feet). If the wreck concerned is of interest and it happens to be at 47 metres, then it gets dived at that depth in the UK.
This same concept applies in other parts of the world too. There are many great ship wrecks at depths that scuba divers visit. Some of which are beyond PADI’s recreational limit of 30 or 40 metres.
If you need reassurance about scuba diving, let’s take a look at the risks that are specifically associated with deeper diving. But also I will include ways to mitigate those risks.
Risks associated with scuba diving to 47 metres and deeper
Deep diving is relatively safe as long as you follow all the rules and procedures. But I thought it best to cover the risks associated with diving to 47 metres.
Risk #1 of diving to 47 metres underwater – risk of running out of air
The first risk associated with diving to 47 metres is running out of air.
The deeper you dive the greater the water pressure. With this increased pressure air gets compressed. So the deeper you go the more your air will get compressed, which is Boyle’s Law.
Your lungs are air sacks which require more air to fill them at depth than at sea level. For example, your lungs require twice the volume of air to fill them at 10 metres than at sea level.
As a result you breathe more air the deeper you dive. So you will go through your 200 bar (2,900 PSI) air tank much quicker at 47 metres (154 feet) than you would at 10 metres (30 feet) or less.
In essence: The deeper you dive, the less time you have to dive with the same amount of air in your dive tank.
The risk therefore at 47 metres is of running our of your air.
How to minimise the risk of running out of air
To minimise this risk of running out of air at 47 metres make sure you check your air contents gauge more regularly. But having said that I suggest this is not too different to diving at any depth. But as your air will be consumed faster at depth, you are advised to keep a more focused check on it.
You must also make sure you have enough air to get you back to the surface. Which will require more air from a depth of 47 metres than it would be from 30 metres.
But you also need to take account of decompression stops too where relevant. See below.
More Reading: 28 tips to conserve air when scuba diving
Risk #2 of diving to 47 metres underwater – risk of nitrogen narcosis
At sea level and on land you are subject to what’s known as ‘one atmosphere’ of pressure. This is known as atmospheric pressure (one bar). This pressure amounts to a weight of 10,000 kg m–2, or 14.7 pounds per square inch.
More Reading: What are the advantages of diving with Nitrox
Sea water is 800 times more dense than air
Sea water is approximately 800 times more dense than air at sea level. This means that underwater pressure increases by one atmosphere for every 10 metres of descent below the surface. For example, at 10 metres (100 feet), the pressure is now two bar or two atmospheres.
If you dive to 47 metres down (154 feet), which is roughly 50 metres (164 feet), the pressure would be six bar or six atmospheres. This is six times the pressure of the pressure on land.
This results in a higher concentration of nitrogen at depth than it is at the surface. Which at 47 metres this is roughly six times the concentration.
Nitrogen represents about 78% of air. Nitrogen breathed at atmospheric pressure has no effect on us as it’s an inert gas.
However, as you go deeper this changes and nitrogen becomes intoxicating. The effects are similar to nitrous oxide inhalation.
This progresses from euphoria to drunkenness to confusion and sometimes to unconsciousness. However in some scuba divers dysphoria (a state of unease) and paranoia occurs. This can lead to panic with some scuba divers.
I have suffered from a bad dose of nitrogen narcosis, which funnily enough was at around 47-48 metres. To find out more, please read this article about how to overcome panic when scuba diving.
The risk of diving to 47 metres is therefore suffering from nitrogen narcosis or the narks.
How to minimise the risk of nitrogen narcosis
The first and simplest way to avoid nitrogen narcosis is to limit your scuba diving to no more than 30 metres (100 feet).
This is actually what I have done. I rarely dive deeper than about 25 metres these days. I no longer dive on deep wrecks to 50 metres or so. That’s not to say that you can’t or shouldn’t dive deeper.
The second way to minimise this risk is to build up your dives. Don’t dive straight to 47 metres or whatever depth it is you are planning to dive below 30 metres.
By slowly building up the depth of your dives during a season, you build up a bit of a resistance to the effects of nitrogen on your body. However, this isn’t a guarantee, as was shown by what happened to me.
Consider using nitrox, which has a higher percentage of oxygen in your air. But with a 32% mix of oxygen you are limited to a depth of 37 metres (121 feet). You will need to take more training for diving using nitrox.
Alternatively, you use mixed gases. Using mixed gases at depth changes the composition of the air you are breathing. This is technical diving, which is a type of diving that I’ve not done myself. So I won’t explain this anymore in this article.
However, having said that the technical divers will probably say; using trimix gas at 47 metres would be wasted as it’s expensive.
Risk #3 of diving to 47 metres underwater – Decompression sickness
The next risk that’s associated with diving deeper is the risk of decompression sickness or DCS.
Decompression sickness is caused by a scuba diver coming up too fast from a dive. The cause of decompression sickness is as a result of the dissolved nitrogen being released too quickly from your body’s tissues on ascent.
This released nitrogen forms bubbles. It’s these bubbles that can cause major problems. These problems can be as minimal as a skin rash or a skin bend, to the extreme of a fatality.
The deeper you go the faster and the more nitrogen dissolves into your body. Once the amount of nitrogen dissolved in your tissues goes past a certain limit the dive becomes a decompression stop dive. Which means you must stop on your ascent to increase your decompression time.
However, if you want to limit your dive to a no decompression stop dive, you need to limit your bottom time accordingly. For example, at a depth of 47 metres diving on the BSAC Table A you have a total of 9 minutes no decompression stop time at this depth.
If you exceed the no-stop dive time, you fall into what’s known as a decompression stop dive. This means that in addition to using your ascent from your dive as decompression time, you will also need to incorporate certain decompression stops too.
For example, if your dive time at 47 metres went to 28 minutes, this would involve two decompression stops. One at 9 metres (30 feet) for three minutes and a second decompression stop at 6 metres (20 feet) for 18 minutes.
The important thing to note is that you not only need enough air to complete your bottom time of 28 minutes, bearing in mind your increased air consumption due to the depth of the dive, plus you’ll need enough air to get to both decompression stops and to carry out the stops.
On top of that you then need to arrive at the surface with your reserve air, which is generally accepted as 50 bar (725 psi).
The risk is therefore decompression sickness due to not decompressing properly on your return to the surface. The risk of which is increased due to how quickly decompression time sets in at these deeper depths.
For example, I remember getting towards the time to ascend from a 50 metres dive (164 feet), and at the time we had over 20 minutes of deco time. But for some reason I couldn’t release my delayed surface marker buoy.
The delay was only a minute or so but this added a significant amount of time to our decompression stops. At depth things can go wrong quickly. So be prepared and don’t dive tired and be cautious.
How to minimise the risk of decompression sickness
The easy way to avoid decompression sickness is to follow the correct ascent rules of scuba diving. You must not ascend too quickly for a start. But you must also carry out decompression stops when necessary. Furthermore use a dive computer to calculate bottom times, ascent rates and decompression stop times.
To reduce the risk further, you are better to only dive no stop decompression dives. However, this will limit the amount of time you have on the bottom.
There’s not much you can do or see on a wreck in 9 minutes. This is especially true if it’s a particularly large wreck.
More Reading: Is scuba diving more dangerous than skydiving?
47 Meters Down movie alternative ending
The film 47 Metres Down, might have had an alternate ending. But the alternate ending might not be Hollywood at its best or at their most exciting.
The alternative ending might be that sisters Kate and Lisa:
- Carefully plan their ascent from 47 metres down
- They would carefully checking their total bottom time.
- Their dive planning would include a dive computer.
- With a correctly calculated bottom time and carefully planned deco-stops, they would ascend slowly to the surface.
- On their way to the surface they would stop at the various decompression stops, for the right amount of time.
This alternative ending to 47 Metres Down may please the scientists and the die-hard scuba divers who understand decompression and decompression stops. But for those who just want to enjoy a thriller or a horror movie, it probably doesn’t really matter.
For pure escapism it’s sometimes okay to stretch the bounds of science.
The final step in the ‘less-exciting‘ 47 metres down alternate ending would be for Kate and Lisa to carry out a safety stop for three minutes at 5-6 metres (16-20 feet).
Safety stops are precautionary stops scuba divers do for safety. But for Hollywood actors and a in film where the characters are escaping from sharks, safety stops are not likely!
I hope this article doesn’t spoil your fun of watching the movie. But if you’re genuinely researching the concept of scuba diving to 47 metres for the purpose of being safe when diving, then I hope this article has helped.
More reading: Is it dangerous to scuba dive with sharks?
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