can you blast through a wall of Shape Water using Fireball?

[Plane]

M185 mentions you can use Shape Water to create a wall which stops fireballs.

A finite amount of water stopping potentially hundreds of burning damage (if you have enough magery to pump in that sort of energy) strains belief which made me wonder how we might do something like assign HP to that water which if depleted means your wall can't stop more fireballs.

The 20 gallons you get with a basic 1 energy occupies two hexes so it seems like 10 gallons per hex a fireball would need to blast through...

A gallon weighs 8.3 pounds, so 83 pounds per B557's chart on Diffuse objects would be 32 HP.

Probably the big saver would be the cap of 2 HP you can take from most Burning attacks, so if you wanted to destroy that 32 HP you're probably better off throwing 32 fireballs that do 1d each, than a massive 100d fireball.

M75's Explosive Fireball is probably the ideal thing though since as explosive (along with AE or cone) it does full injury.

I guess in theory if you "kill" something made of water you may not atomize it, just change it from "diffuse water" to "diffuse steam" and spread it out over a much larger area.

Almost like giving the "Body of Swarm" enhancement to a water elemental but you give it a "Scatter" enhancement which can't be turned off.

[Pursuivant]

Originally Posted by Plane

A finite amount of water stopping potentially hundreds of burning damage (if you have enough magery to pump in that sort of energy) strains belief which made me wonder how we might do something like assign HP to that water which if depleted means your wall can't stop more fireballs.

Your rules seem simple enough that they'd work on the fly. They're certainly good enough for magic.

If you wanted to be really fussy and scientific, you could figure out joules of energy per die of fire damage then work out evaporation rates using the enthalphy of vaporization equation.

The idea is that you're not just blasting through water, like a bullet through DR, but instead forcing the water to undergo a phase change which requires a lot more energy.

[Varyon]

If I'm not mistaken, GURPS Magic, unlike most of GURPS, allows for absolute effects, so a wall of water defeating any arbitrarily-powerful fireball fits with that. Sure, you can make an argument against it being realistic (as you note, sufficiently powerful flame can get through water, thanks to evaporation), but this is magic, and simply the idea that "water trumps fire" may be enough. Of course, you can also work out any number of methods for making that wall of water no longer be an absolute defense against fireballs; I'd suggest playtesting to make certain it's balanced (offhand, because Shape Water's primary purpose isn't to block fireballs and the like, I'd be inclined toward roughly equal amounts of FP, maybe even putting Shape Water at a disadvantage - so if you're using a 3 FP Shape Water to block a fireball, it will block a 3 FP - 3d burn - fireball, but anything larger will get some damage past it, say).

[malloyd]

Do remember spell descriptions were all written for the 3e rules where fireballs are not going to exceed 3d. You need a rule for blasting through walls of water to more or less exactly the same extent you need a rule for blasting through *human targets* (mostly a few inches thickness of dirty water after all) and injuring people standing behind them, which isn't going to happen with a 3d attack enough to be worth the mess of trying to write a rule for it.

If you must have one, I'd just give the wall of shaped water a DR value vs. fire (a flat DR 20, or 4 per inch, or whatever depending on how much detail you feel matters), call it total cover with that DR, and move on.

[whswhs]

I've looked this up in GURPS Magic. It says that 20 gallons of water will make a wall that blocks a fireball. It also offers the approximation that 1 cubic foot of water is 8 gallons, which gives 20 gallons equaling 2.5 cubic feet. That protects an area 1 yard x 2 yards, which is 3 feet x 6 feet, or 18 square feet. Dividing gives 0.13888 ... feet thick; multiplying by 12 inches/1 foot gives 1.666 ... inches.

The maximum fireball that's legal in GURPS Magic is 3d.

What I would propose is that 1 inch of water has DR 10 against flame damage. A thickness of 1.666 ... inches would be DR 16 (rounding down). That would totally stop a 1d or 2d fireball; there would be a small chance that a 3d fireball would get through, doing 1 or 2 points damage, slightly more than an Ignite spell but not as much spending a turn in an ordinary fire.

[Plane]

Originally Posted by whswhs

What I would propose is that 1 inch of water has DR 10 against flame damage.

Ablative? Not sure why we shouldn't just use HP.

Also given that I can stab a pond a million times with a knife there's probably something beyond Diffuse that we need to reflect how it would resist Impaling or Cutting attacks...

You'd almost need Insubstantiality to get complete damage immunity.

[Eric Funk]

Originally Posted by Plane

You'd almost need Insubstantiality to get complete damage immunity.

Note that in Magic 4e Resist Fire explicitly states that only level 1 Resist Fire is required to protect from all combat-type Fire spells... (p.M74) so absolute protection is possible.

[Plane]

Originally Posted by Eric Funk

Note that in Magic 4e Resist Fire explicitly states that only level 1 Resist Fire is required to protect from all combat-type Fire spells... (p.M74) so absolute protection is possible.

Of course like Malloyd mentioned that was probably written in 3e when we had 3d fire damage caps, as opposed to now where w/ enough energy+magery there's no limit.

A fireball which theoretically reaches "volcano" or star/nuke should in theory take the 2nd or 3rd levels too, though I don't know how many dice that would be.

[Lovewyrm]

Hmm, well if the fireball explodes upon impact with something then yeah the fireball is blocked, I'd say.

Isn't that how magic fireballs tend to work?
You throw a magical projectile that explodes in a fireball at the destination?
Thus, it stands to reason that the fireball impacting the water wall is a collision, even if the fire the ball releases would be enough to vaporize the wall.

[Plane]

Originally Posted by Lovewyrm

Isn't that how magic fireballs tend to work?
You throw a magical projectile that explodes in a fireball at the destination?
Thus, it stands to reason that the fireball impacting the water wall is a collision, even if the fire the ball releases would be enough to vaporize the wall.

Ultimately it comes down to the function of GURPS damage types and whether to apply B408's Overpenetration rules. If fireballs are TB burning like a laser they would use them

If they're not, they would be excluded, the same as crushing damage (ie you cannot throw a stone through a rice paper wall to hit someone on the other side, in GURPS terms, legally speaking, AFAIK)

I guess Cutting is also excluded, so no whipping a throwing axe through a wall to hit someone either I guess.

It seems strange to me though, if an axe can chop through platemail to sever a limb why can't you throw a spinning axe through a wall to sever a limb?