dealing with water pressure

[flamewolf393]

I have a character that is of the sea spirit folk. They can breathe water and have a natural swim speed. My class replicates both air and water benders from avatar, effectively giving me near perfect control over both air and water. Being able to both push the water back and effectively casting the air bubble spell, I should be able to go as deep as I want to shouldnt I?

My group says that water pressure would just compress any air bubble (despite it being magical), and would also overcome direct control of the water.

I need both RAW rulings and popular opinion on this issue. I designed this character specifically to deal with this issue, yet apparently they dont think it should work at all.

[Cog]

If you want a rules-as-written answer for your homebrew abilities, it'd be helpful to provide the rules in question.

[SamsDisciple]

Since this is a home brew class there won't really be any RAW but as for using water bending and air bending to dive deep I am of the opinion that the deeper you go the harder it will be to maintain the bubble. In the avatar d20 home brew system it does bending based off of skill checks and so if I were dming I would increase the dc for your air bubble by the depth you dive to. So in other words you don't have infinite depth right off the bat but you can go a heck of a lot deeper than most.

[Phelix-Mu]

I don't know anything about water-bending or air-bending aside from the basics of what those terms imply; aerokinesis and hydrokinesis. Beyond the rather silly part of the debate dealing with whether physics allows x or y, you should be able to compensate for air compression by simply adding more air to the bubble.

Furthermore, because "air" isn't what your character is bending, rather more likely controlling pressure gradients in order to create flow in certain directions (in the form of bursts of pressurized air...commonly thought of as wind), you should have a pretty good control over the pressure of the air around you.

Finally, even if you can't control the air pressure, you should be able to create a localized area of solidified water to channel the pressure around the air bubble. Solidified water is simply compressed water, and forms the basis of methods such as walking on water, water shields, and other basic methods of a typical hydrokineticist.

[SamsDisciple]

I completely agree with your analysis of what is going on when the character is manipulating the elements but I believe that power/skill are still relevant when dealing with deep sea diving. Walking on solidified water is one thing, letting a colossal giant walk on water is a whole different ball game. As water pressure rises the forces placed on your bubble reach the point that it's comparable to that colossal giant mentioned before slowly applying its full weight on a puppy. The powers do work but I would require skill checks/ caster level checks/concentration checks once you pass a certain point.

[NichG]

If you care to, you can probably convert from damage to a certain amount of pressure you can resist with a little bit of physics and a few assumptions about how mushy people are.

Lets use a falling object as the example to get out a pressure measurement. The act of stopping a falling object by being under it causes damage equal to 1d6 per 10ft fallen per 200lbs. I am now going to switch to metric to avoid the lbs-force vs lbs-weight headache.

The momentum of the falling object (rock, whatever) as it impacts is p, and p^2/2m = energy = mgh. So the momentum is 700 kg m/s.

Pressure has units of force/area, so we have to estimate both how long the falling object takes to crush its victim and also what their cross-sectional area is.

As a sort of first approximation, lets assume that the fall is arrested in about a tenth of a second, and that the 'target' is a 1/3 meter radius circle cross-section (so it has a surface area of about 0.35m^2. That would imply that the pressure-per-1d6-damage is about 20000 N/m^2. This is about 0.2 atm. It turns out that water pressure increases by about 1atm per 10 meters.

So based on this very iffy calculation, you can counter-act the pressure of water by an effective depth equal to 2 meters * '# of d6's worth of damage you can directly deal with water pressure by e.g. crushing someone with it'.

Which is actually surprisingly little. But I guess thats why casters burn or freeze or dissolve or shock people to death rather than trying to crush them with Mage Hand?

[sleepyphoenixx]

By RAW only creatures with the aquatic subtype are resistant to pressure (aquatic aberrations, elementals and outsiders are immune). Stormwrack has the rules on page 11.

Since you don't have the aquatic subtype you get no special resistance to water pressure from your race.
Unless your class explicitly mentions it i wouldn't assume it does either since real world physics and the rules have only a passing relationship at best.

If you need to dive deeper than 100ft there are a few spells and a domain in Stormwrack that give immunity to pressure damage.
If you need the ability all the time either get a magic item, get access to the spells/domain or (since your class is homebrew) add the effect to your class abilities if your DM allows you to. It's hardly a gamebreaker.

[Sliprunner]

I would like to make the comment that I am the poor unfortunate DM in this case. Surrounded by a completely insane party. So I apologize for this situation appearing here.
Regardless, I would like to clarify the situation. I am using modified Stormwrack rules, with the pressure depth extended to 250ft (for non-aquatic) and fort saves every 30 seconds (5 rounds) after that.

It should be clarified that Flamewolf here had been directly told on three occasions that his idea (of simply not being effected by pressure) would not work without the Aquatic subtype. I was lenient on him and didn't have him make a fort save until after 25 rounds. Due to the fact his a spirit like creature (though he doesn't have the subtypes). When he failed the fort save (and took pressure damage), he begun to ask if he could simply create the air bubble around him, while already at 500ft depth. The way it was explained to me, from the player, was simply to replace the water around him with an Air bubble. After he was already suffering the effects of pressure.

It will be said, that both me (As the DM) and other players had said no to the concept as he explained it. Being that he had tried to handwave it as "Magic can do whatever it wants", so I will clarify again, this problem happened AFTER he was at depth, and AFTER he had been told before that it wouldn't work. so again, apologizes that this appeared here.

Edit:As note, the "Air Bubble" ability he dose have, is to allow one to breath while underwater.

[Phelix-Mu]

Well, seems like the lesson here is to plan ahead and be cognizant of what your homebrew ability does and doesn't allow.

That said, homebrew often leads to some confusion on these points, and internal imagery about what a given character can do via Rule of Cool or otherwise often doesn't mesh perfectly with what is communicated between irl individuals.