That sinking feeling..

Ok you physicists!

Your character has just fallen overboard! Assuming they've enough weight on them to pull them down, how fast do they sink? Obviously, one sinks faster than one falls, but is the difference big enough to affect the rules of D&D?

Thanks,

J from Three Haligonians

Three_Haligonians said:

Ok you physicists!

Your character has just fallen overboard! Assuming they've enough weight on them to pull them down, how fast do they sink? Obviously, one sinks faster than one falls, but is the difference big enough to affect the rules of D&D?

Thanks,

J from Three Haligonians

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Hmm, interesting notion.

D&D is not a "reality engine"... and I'm not sure this subject would come up enough in my games for me to hash out how it works. Basicly if a Fighter in Plate falls into a 20ft spiked pit that has 15ft of water volume... the Fighter takes "falling" dmg as normal, has the same chance to hit the spikes... and is now in danger of drowning (assuming he isn't strong enough to swim/climb his way to the surface). Now if someone that wasn't encumbered fell into the same pit, I'd say they go down 5-10ft before they bob back to the surface on thier own.

actualy, if i have this correct, you actualy will sink slower then you fall. as you sink you begin deplacing water, when you hit the equal amount to your weight you will float there. for very 33 feet or 10 meters water become 1 atmosphere dencer, increasing the weight of the water. so efectivly you should fall slower for evey 10 meters or so. add in the swiming factor and figguring out numbers can be a real pain

As I belive it:

Water has a pretty high density (for a fluid anyway) of 1000kg/m^3 air has a density of 1.29 kg/m^3. Cutnell and Johnson 3rd ed.

I think that no matter how much armor a char is wearing (unless they have the strength to wear armor made of hydrodynamic ton weighing things) they will "fall" or sink MUCH slower than they would fall in air. Sure they wont bob up with normal armor, but they will be descending at a rate of feather fall.

Now for the case of spikes at the bottom of a water pit:
IF they were actually able to hit they wouldn't get the "strength" bonus for hitting a falling character.

It sure would suck to fall in the ocean with armor on, and you would most likely die because by the time anybody reacted you'd be too far down to swim to. That being said, the fighter has a chance because this is fantasy afterall. They can attempt to swim (with a x3 armor check) so if they are lucky in rolls and have quick acting friends they can easily survive.

Now to answer the origional question:

Terminal velocity of a human is 56m/s (124 mph) that's at a density of 1.29 kg/m^3. Now I'm gonna jury-rig this result so bear with my math if you disagree: with just dividing the Terminal velocity of air by the ratio of water you get an insanely small ammount of speed. Part of why things don't sink as fast as they fall is buoyancy. A fighter with lungs fulla air would not have enough to float, but they would still not go very fast.
If the fighter was also just sitting there not trying to stay above, I would venture to say they would descend at 15-20ft every three seconds (move action).

After writing and researching all of that, I said to myself I wonder what the SRD has to say about that:
SRD:
Falling into Water

Falls into water are handled somewhat differently. If the water is at least 10 feet deep, the first 20 feet of falling do no damage. The next 20 feet do nonlethal damage (1d3 per 10-foot increment). Beyond that, falling damage is lethal damage (1d6 per additional 10-foot increment).

Characters who deliberately dive into water take no damage on a successful DC 15 Swim check or DC 15 Tumble check, so long as the water is at least 10 feet deep for every 30 feet fallen. However, the DC of the check increases by 5 for every 50 feet of the dive.

There you have it...

Wow

majustismp15 said:

After writing and researching all of that, I said to myself I wonder what the SRD has to say about that:
SRD:
Falling into Water

Falls into water are handled somewhat differently. If the water is at least 10 feet deep, the first 20 feet of falling do no damage. The next 20 feet do nonlethal damage (1d3 per 10-foot increment). Beyond that, falling damage is lethal damage (1d6 per additional 10-foot increment).

Characters who deliberately dive into water take no damage on a successful DC 15 Swim check or DC 15 Tumble check, so long as the water is at least 10 feet deep for every 30 feet fallen. However, the DC of the check increases by 5 for every 50 feet of the dive.

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This is a different topic though. The OP wanted to know how fast someone falls through water (or sinks).

This SRD quote indicates how much damage (or effectively, how fast in terms of damage) someone falling through air takes before hitting water.


Water has to get a chance to slow down a character (hence, the reason for the 10 feet deep water requirement for falling into water from a considerable height rule), but that aside, I suspect that no actual damage should be taken for actually sinking in water. I agree with you that it pretty much should be like a feather fall spell.

Without taking into account increased water density, the 60 feet per round of a Feather Fall spell would seem like a good rule of thumb.

How would Freedom of Movement (spell or Ring of Freedom for example) come into play? Since you are supposed to move freely through water, if you fell (lets say) 50' into water, which would happen:

1) Once you hit the water, you stop falling and can then move however you want, only taking damage for the 50' fall.
2) You fall "through" the water until you hit the bottom, becuase the water does not affect you and take falling damage for the whole distance (50' + water depth).
3) Something else.

JonnyFive said:

actualy, if i have this correct, you actualy will sink slower then you fall. as you sink you begin deplacing water, when you hit the equal amount to your weight you will float there. for very 33 feet or 10 meters water become 1 atmosphere dencer, increasing the weight of the water. so efectivly you should fall slower for evey 10 meters or so. add in the swiming factor and figguring out numbers can be a real pain

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No, water, being a liquid, does not increase in density under pressure as it's incompressible. 30 gallons of water weighs the same at the surface as it does 10,000 feet down.*


Gases however do act like this, but this is unlikely to come up in game unless you are falling through the atmosphere of Jupiter. If that happens to you drowning is not your problem...


* Technically water at the bottom of oceans tends to be slightly denser than surface water, as it's usually colder and more saline. If it wasn't denser it wouldn't be at the bottom, but it's not by orders of magnitude

Space Coyote said:

How would Freedom of Movement (spell or Ring of Freedom for example) come into play? Since you are supposed to move freely through water, if you fell (lets say) 50' into water, which would happen:

1) Once you hit the water, you stop falling and can then move however you want, only taking damage for the 50' fall.
2) You fall "through" the water until you hit the bottom, becuase the water does not affect you and take falling damage for the whole distance (50' + water depth).
3) Something else.

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#3

The Sage ruled about 5 years ago that Freedom of Movement is a choice. Hence, #2 does not occur unless you want it to.

#1 does not occur since Freedom of Movement does not defy gravity.

Basically, you would take the damage for falling the 50' and then you would sink, just like without Freedom of Movement.

Wilphe said:

No, water, being a liquid, does not increase in density under pressure as it's incompressible. 30 gallons of water weighs the same at the surface as it does 10,000 feet down.*

...

* Technically water at the bottom of oceans tends to be slightly denser than surface water, as it's usually colder and more saline. If it wasn't denser it wouldn't be at the bottom, but it's not by orders of magnitude

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Water compesses. If water did not compress, the oceans would be ~36 meters deeper.

The compression, however, is not great. The water in the deepest part of the ocean compresses about 5% as compared to surface water. In order for iron to float, it would have to compress 800% (i.e. take 1/8th the volume of surface water). Gravity (on Earth and presumably in a game environment) is not great enough to do that and so I agree with you that it does not make sense to take water density into account on the OP's question.

But, ocean water compression is not just based on salinity and temperature. It is also based on pressure. Not just because pressure directly affects temperature, but because pressure also directly affects density. In fact, ocean water density is not measured. It is calculated (and quite accurately) based on measurements of temperature, pressure, and conductivity.