# What is the buoyancy limit for swimming?

I have read in various books, sci-fi, fantasy or modern day with supernatural beings, doesn't matter. In some of them, the characters killed their enemies by drowning them, because they were too heavy or dense.

# Question:

I want to know what the limits of buoyancy are for humanoid organisms with a weight of 200 to 250 kg and a height of 250 cm. They cannot drown instantly, staying above water, like a human.

• I have added the tag "magic" to your question as you mention supernatural beings. It would also be good to think of how to better make this question relate to world building. As a separate point of possible interest some creatures (and I am an example of this) can change from positive to negative buoyancy by breathing out. – Slarty Oct 4 '17 at 13:31
• Reminder to close-voters: The problem cannot be solved if the OP is not made aware of it. – Frostfyre Oct 4 '17 at 15:23
• This is unanswerable without a time/energy constraint. Fit and slightly sub-buoyant humans can tread water or with care swim all but indefinitely, but dolphins can tail-walk with their body effectively entirely out of the water, at which point you could eliminate all the blubber and end up with a very dense creature - for a very short time duration of "swimming". – Chris Stratton Nov 26 '17 at 20:41
• While the question shows a severe lack of understanding of the topic of buoyancy it is still a fine question. Thus I upvote and vote to reopen. It needs work, but it doesn't need to be put down. – dot_Sp0T Nov 27 '17 at 9:29
• @Erick Silveira: Buoyancy is really irrelevant to deliberately drowning someone, because to do that you either hold them under water, or toss them overboard wearing the proverbial cement overcoat. – jamesqf Nov 28 '17 at 4:23

It's fairly easy to get an idea, using A=F/M to see how fast things sink.

Water will create an upwards force of ~10N/L minus the weight (mass * ~10); hence at 1kg/l (roughly human density) the force is 0, or neutral bouyancy.

So if your item has a density of 2kg/l, then that's going to counteract the 10N upwards force twice over, and give a downwards force of 10N at a mass of 2kg - 5m2^-2 downwards. Clearly once you're sinking at more than a few ms^-1 hydrodynamics is going to kick in, but you're looking at the need to swim up very hard just to keep up.

• At 1.01 kg/l you're looking at a very manageable 0.1 ms^-2
• At 1.1kg/l though you're already looking at 0.9ms^-2.

A big consideration here is how your creature breathes - a person wearing scuba gear (or with gills) could probably get around OK at 1.1kg/l underwater, whereas trying to breathe on the surface would probably get into difficulty very quickly.

One thing that no one else has yet brought up is that density amongst organisms is governed in no small part by the fact that most organisms are mostly water. So the chances of having a very dense creature are fairly small.

• I just made an answer twice as long and half as informative while you were posting this. Nice! – Paul TIKI Oct 4 '17 at 14:21
• I understand the accounts after a bit of search, xd and I understand what you mean with them all, thank you. – user43339 Oct 4 '17 at 16:46

Some of the terms you are looking for are density and buoyancy.

The quick and dirty explanation is that the more dense something is, it will naturally sink below less dense material and float on top of more dense material.

Buoyancy, in layman's terms, is related. This is what allows a ship that is made of steel, which is much more dense than water, to float. Buoyancy is kind of like relative density. A ship floats because all of the mass is spread out over enough surface area to make the ship, as a whole unit, less dense than the water.

So now for the animals. I am a 117 kg Animal and I float fairly well, that's because I am fat and therefore, as a whole, less dense than the water. My son is a very muscular 90 kg Animal with almost no body fat. He is, as a whole, more dense than the water and is not buoyant. Hippos are so very mobile in water because they are fairly close to being the same density as water. They are slightly buoyant. Tigers, being heavily muscled, are less buoyant but are very strong, flexible, and agile. Therefore they are very strong swimmers. (incidentally, hippos are pretty fast on land too, and they are NOT friendly. They kill a lot of people.)

They animals you intend to drown aren't likely to make it easy for you unless they have the following characteristics:

1. Negative Buoyancy. They are more dense than water, and not by a trivial margin. This is a potential problem for large, very strong creatures.

2. If they are flexible and agile enough they might be able to overcome the negative buoyancy. If they are strong, but with limited mobility, they are going to have a hard time in the water.

3. They would have evolved in a place that has no major bodies of water, or at least nothing deep enough to be a persistent threat of obstacle.

Apologies to any biologist tearing their hair out over this very simplified version.

• I may be mistaken, but I think hippos (at least adults) are negatively buoyant; they essentially move by bounding off of the bottom. – Matt Bowyer Oct 4 '17 at 14:26
• @MattBowyer could very well be true. The ones at the zoo here seem to float easily, but their enclosure has a fairly steep incline that isn't visible unless you get there right after they fill it (twice a day, those guys are kinda nasty), Either way, they are pretty close to neutral buoyancy judging by the way they move in water. Surprisingly graceful – Paul TIKI Oct 4 '17 at 15:35
• @MattBowyer You're half right. [Can Hippos Float?] "Given their size, one might answer “no” because hippos seem too heavy to swim. But that’s wrong. A hippo’s fat makes it buoyant enough to float very well. On the other hand, if you said “no” you are quite right. Hippos do not really swim. Their method of travel is very simple: they walk underwater." Link – user41674 Nov 28 '17 at 17:50

# Hippos can't swim, elephants can

Two main factors, density and strength of paddling.

The fundamental aspect of buoyancy for a creature is overall body density, usually close to water but it varies (most animals are mostly water). It relates to body fat, muscle mass etc. Lung size affects it as well, bigger lungs means you can take a bigger breath and that helps you stay afloat.

Swimming is an entirely separate matter to buoyancy. Swimming is about having decent sized paddles available relative to the amount of work they have to do. Should your creature be high density, a natural sinker, it will have to have decent paddles to stay afloat first, then to gain movement relative to the water. Ducks (and other water birds) don't have this problem, they float, but need strong paddles to dive underwater.

It's also personal

As a species humans have reasonably sized paddles (hands and feet), a trained strong swimmer can hold their own weight and a considerable amount of extra mass. A weak swimmer will struggle to hold their own weight. Roman soldiers were said to have trained to swim a river in full armour.

Whether your character drowns is up to the combination of these two factors.

There isn't really a density limit when it comes to swimming. Humans are too dense to float with empty lungs and we swim just fine, cats have it even worse than we do in some ways because they have denser muscle and a higher proportion of muscle by weight than a human.

The issue is that denser creatures must use more energy to maintain a particular position in the water column, otherwise they simply sink. For example many sharks sink if they stop swimming. Sinking is really bad if:

A. you're over the abyssal plains and the bottom is literally miles down below you

or

B. you need to breath air. But in shallow waters anything with gills is fairly safe.

So there isn't really a limit to either the mass or density of a swimming creature that swims all the time. What there is, is a limit to how heavy a creature designed to walk on solid ground can be and still swim reasonably well. This is not a hard limit though, it greatly depends on the exact body structure on the creature in question.

Take as an example the difference between a human swimmer and a feline. Humans have pretty flat hands and feet without extendable toes and only vestigial webbing between their digits. Feline paws are cupped and have webbing and oiled fur between their toes.

In short cats get a lot more thrust per stroke than humans do when swimming, so humans have to put in much more energy per kilogram of mass to stay afloat than cats. A cat the size of a person may be considerably heavier yet have an easier time of swimming.

• Thanks, all the answers were a big help to me, yours especially about the felines method of swimming. – user43339 Oct 4 '17 at 16:50
• Some paragraph breaks would be nice… – JDługosz Nov 27 '17 at 2:06
• I formatted your answer a bit, but I wasn't sure about "extendable toys". I only know of the translation where "toy" means "something you can play with". At first I thought you meant "retractable claws", but you mention "toys" later again where it looks like you meant "toes". A little link to wikipedia or somewhere else to look up what you mean with "extendable toys" would be nice. – Secespitus Nov 27 '17 at 9:24
• @Secespitus "toes" not "toys", my apologies. – Ash Dec 3 '17 at 11:27

Well as you mentioned there isn't really a weight limit to swimming. An adult blue whale can weigh up to 300,000 lbs. As long as a creature is not overly dense there isn't really a size limit to swimming.

Now density on the other hand will very quickly kill a creatures ability to swim. It is hard to put a limit on it without looking at specific animals but, they'd lose the ability to swim fast. Keep in mind humans and most other mammals are equally dense to water, giving you "weightlessness." Any extra weight caused by increased density will just mean you need to exert that much more upward force to stay afloat. A normal human would not be able to swim with 10% increased density, as this is like (worse than) swimming with a lead weight equal to a tenth your weight.

• Yeah but have known others people perspective about the issue can bring more wisdom to me about the problem and thank you I will use your example of whale in my novel, maybe a hybrid between a shark and a blue whale, need to think about it. – user43339 Oct 4 '17 at 16:48
• "A normal human would not be able to swim with 10% increased density, as this is like (worse than) swimming with a lead weight equal to a tenth your weight." Enh...plate armour could weigh up to 50 pounds, which is certainly more than 10% of a fit person's weight, and it has been proven that you can, in fact, swim in it. Not for long, but it can be done. – Keith Morrison Nov 28 '17 at 20:38