Decanter of Endless MATH

[thriceborn]

Does anyone know the calculations for the force of water relative to the diameter of the stream? A friend and I had the thought process of taking a decanter of endless water, and putting an adamantine nozzle on it, and making the nozzle tiny, therefore trying to recreate a 'water sword' is it possible? And if so, what are the calculations required?

This is just for fun, not meant for actual game play, unless the DM allows it.

[Icewraith]

Actually, sort of.

Based on the cross-sectional area of the original decanter and the diameter of the new cross section, if you can determine the flow rate of the original you should be able to calculate your new flow rate.

You might have to work backwards and calculate the power or energy involved, and then be able to determine your energy output in a one-round time period, and from there estimate your force.

Remember though that you'll probably need to specify a range over which this works if you wish to quantify damage, since it should drop off with increasing distance. It might work better as a jetpack or engine of sorts.

In terms of finding the relevant equations, Google is your friend since I don't have the time or patience to dig out my fluid mechanics book and do all the work for you. :p

Kill ALL the catgirls.

[Saidoro]

if memory serves: V1A1=V2A2 where A is the cross-sectional area of the pipe and V is the velocity of water traveling through that section of the pipe. Not that using the decanter on full blast requires a DC 12 strength check to remain standing even without this enhancement, you may want to look into mounting it on something sturdier than a person.

[TuggyNE]

Unfortunately, this won't work well. I was going to say it was because you actually need a slowly-tapering nozzle (in order to efficiently convert fluid pressure to velocity) starting out at a foot in diameter, the result being probably at least ten feet long and rather heavy. However, the real problem is that you only have a certain amount of kinetic energy to work with, and anything you do to increase speed also decreases pressure (due to Bernoulli's Principle), and even drops some energy to friction. Granted, it might become more efficient at dealing damage, but not massively so.

I suppose if you don't mind an exotic reach weapon that does maybe 2d4 bludgeoning and water damage with a standard action to start it, it'd be fine? (Weight: 15 lb, 20x2, two-handed.)

[holywhippet]

What I could see happening is the DM asking you to make a really high DC strength check to keep hold of the decanter. If you fail, it flies from your grasp and skips off towards the horizon. Eventually the party becomes wanted persons due after the authorities scry out who unleashed the decanter.

[Icewraith]

Right, but in this instance the pressure is technically coming from the weight of an infinite plane of water that is on the other side of the tiny portal in the decanter.

Now that I'm thinking about it (curse you distraction!), if there's no local gravity to speak of wherever the other end of the portal is, the flow rate is dependant on the local current and its angle to the opening of the portal, plus the effect of any gravity leakage from the portal.

If there is local gravity, the pressure should be equal to the effective depth of the portal plus gravity leakage plus the effect from any strong local currents as mentioned above.

Do remember that whoever's holding this will have to deal with the increase in force. If you can make the friction on the inside of the nozzle negligible (permanent grease?) that will reduce losses.

Also, while the force varies by the distance from the decanter, you may have more success determining the momentum, since you can determine that directly from the flow rate and weight of water.

[thriceborn]

What I could see happening is the DM asking you to make a really high DC strength check to keep hold of the decanter. If you fail, it flies from your grasp and skips off towards the horizon. Eventually the party becomes wanted persons due after the authorities scry out who unleashed the decanter.

Already considered, an adamantine turret. It would only be useful for a standing structure, or a battlefield where we know we'll be fighting a day in advance. It would allow us to put it on a rotating axis but keep it within workable device. And the DM already said if we can make it work, math and all, and it gets lose, it's going to wreck massive destruction, if not a TPK, so the most work will be in keeping it completely steady and immobile, except on that one axis.

[Erik Vale]

Unfortunately, this won't work well. I was going to say it was because you actually need a slowly-tapering nozzle (in order to efficiently convert fluid pressure to velocity) starting out at a foot in diameter, the result being probably at least ten feet long and rather heavy. However, the real problem is that you only have a certain amount of kinetic energy to work with, and anything you do to increase speed also decreases pressure (due to Bernoulli's Principle), and even drops some energy to friction. Granted, it might become more efficient at dealing damage, but not massively so.

I suppose if you don't mind an exotic reach weapon that does maybe 2d4 bludgeoning and water damage with a standard action to start it, it'd be fine? (Weight: 15 lb, 20x2, two-handed.)

You mean, a Reach Falchion without the crit range, extra weight and a start up time?

Already considered, an adamantine turret. It would only be useful for a standing structure, or a battlefield where we know we'll be fighting a day in advance. It would allow us to put it on a rotating axis but keep it within workable device. And the DM already said if we can make it work, math and all, and it gets lose, it's going to wreck massive destruction, if not a TPK, so the most work will be in keeping it completely steady and immobile, except on that one axis

.

Sounds like your done. Personally, I would just place the Decanter in a 'box' of imovable rods. When I want to use it, activate the rods and then the decanter. The problem will be aiming the thing as most opponents can move, so perhaps if you could convince him you could activate both at once in the same action (Both activted on the same command word?)

[TuggyNE]

Right, but in this instance the pressure is technically coming from the weight of an infinite plane of water that is on the other side of the tiny portal in the decanter.

There's a problem with this. The Decanter isn't created with, say, a gate spell, but with control water, which doesn't suggest any particular connection with the Plane of Water at all.

Also, if you had infinite pressure to work with, the last thing you want is to narrow the opening!

You mean, a Reach Falchion without the crit range, extra weight and a start up time?

Loosely, yes. (Also, does extra damage against some opponents at the cost of less against others.)

[Kelb_Panthera]

Can't help with the pressure math but for mounted weapons, I've always been a fan of the immovable bipod. Take your bog-standard bipod, replace the legs with immovable rods.

[Gralamin]

Its worth noting that the math for the decanter doesn't make much sense. This thread on enworld is much more thorough on the matter then I.

[Delwugor]

It would not be a very effective use of an adamatine nozle. Below indicates the math based on simple hose nozzle as a model
For the following d indicates dectanter and s indicates "sword"
Volumetric flow shows that the velocity would increase
Vs=VdAd/As

Bernoulli's Principle (simplified) shows (P is pressure, p is density)
Ps+(1/2)pVs² = Pd+(1/2)pVd²
Ps=Pd+(1/2)p(Vd²-Vs²)
or using Vs=VdAd/As ends up with
Ps=Pd+(1/2)pVd²(1-(Ad/As)²)

The physical description is nozzle increases the speed (and travelling distance) of the water by decreasing the pressure.

So a nozzle on a Decanter of Endless Water becomes a water cannon not a water sword. As a GM I would say there is no practical benefit (in game) for doing that or at the most increase the gyser length to 40 or 60 ft while still keeping the same d4 damage.