# Thread: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

1. ## Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Hi all, I'm looking for some numbers on how various engines and rockets based on the Decanter of Endless Water work.

*About how much thrust does a Decanter of Endless Water set to Geyser produce?
*How does that compare to the thrust a Jet Ski Produces?
*If you take a generic D&D sailing ship, about how many knots could a Decanter of Endless Water on the rudder get you up to? Assume you are using one of the basic ships from Arms & Equipment Guide or Stormwrack, if it matters...
*Has anyone actually done the math of a Decanter of Endless Water + Riverine Chamber with a nozzle with a Permanency'd Wall of Fire in it? What sort of thrust does that produce? How does that compare to different things in our world that produce certain amounts of thrust?

EDIT: What if you ignore the 20 feet thing? So it has, say, a 1.5" diameter nozzle outputting room temperature fresh or salt water at 300 gallons a minute, however much force and whatever velocity would the water be going, and how much thrust would it generate if it is doing that?

EDIT EDIT: I guess the unit I am looking for is Newtons or Pound Force on the Force side of things, or maybe Newton-Meter or Foot-Pounds on the Energy side of things... I'm not entirely sure what is most appropriate to describe how useful at pushing something this is..? Am I also looking at the Velocity of the water exiting the Decanter?

EDIT EDIT EDIT: Okay! Space Shuttle Main Engine is 1.8 Meganewtons. That helps!

http://www.enworld.org/forum/d-d-leg...ess-water.html
But my eyes kinda glaze over when I read them. I found a few different numbers..
Someone said 255 newtons... Someone said 115 newtons... I don't know what's right...

2. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

The problem (or saving grace) with the Decanter is that its physics are actually ill-specified.

The force it generates is equal in magnitude to product of the density of water, the volumetric rate of water emission, and the velocity from which it is emitted from the nozzle.

F = dp/dt = dm/dt * v = rho_water * dV_water/dt * v_water

Unfortunately while the density of water is known and the second is given at 30 gallons/6 sec., the emission velocity is unknown, because "20ft. long stream" doesn't tell us anything, really.

If we assume in the best case that 20 ft. long means that an upward-directed geyser attains a height of 20 ft. before it starts to fall, then the emission velocity is sqrt(2*g*h) = sqrt(2*32*20) = 25 ft/s.

So the force is about 33 pounds. Not very impressive. It would take 4-6 Decanters to make a useable rocket jet pack for a human. A gnome, on the other hand, might only need 2.

3. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Specifically, starting from post #29 (calculates energy output of wall of fire)

And then here:
http://www.giantitp.com/forums/showp...5&postcount=18

is exactly what you want. I calculated the thrust from a 1D approximation of a De Laval nozzle.

This sort of things comes up every now and again. =P

4. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

For what it's worth, wall of magma can also be made permanent, and is easier to work with than wall of fire as far as making a contained engine. Wall of stone, stone shape, and hardening is all you need to encase the wall of magma+decanter, riverine isn't worth the price.

5. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

What if you ignore the distance bit?

Just say, it has a 1.5" diameter nozzle, and it outputs room temperature fresh or salt water at 300 gallons a minute.

Whats the amount of thrust then?

6. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by Gavinfoxx
What if you ignore the distance bit?

Just say, it has a 1.5" diameter nozzle, and it outputs room temperature water at 300 gallons a minute.

Whats the force then?
300 gallons / min = 1363.82756 litres / min = 22.7304593 litres / s

22.7304593 litres = 22.7304593 kg
and occupies 22.7304593 dm^3 = 0.0227304593 m^3 of space

diameter: 1.5 inch = 0.0381 meters

height of cylinder to acheive volume: 4.98434836 meters

Hence, speed of water is 4.98434836 meters / s.

Momentum of water generated is 113.296528 kg m / s.

7. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by jseah
300 gallons / min = 1363.82756 litres / min = 22.7304593 litres / s

22.7304593 litres = 22.7304593 kg
and occupies 22.7304593 dm^3 = 0.0227304593 m^3 of space

diameter: 1.5 inch = 0.0381 meters

height of cylinder to acheive volume: 4.98434836 meters

Hence, speed of water is 4.98434836 meters / s.

Momentum of water generated is 113.296528 kg m / s.
Uh. How many *checks* Newtons is that, and how does that compare to other things?

8. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

According to DnD wiki, the steam is 1ft wide, so I guess 1ft diameter without a stopper.

Originally Posted by jseah
300 gallons / min = 1363.82756 litres / min = 22.7304593 litres / s

22.7304593 litres = 22.7304593 kg
and occupies 22.7304593 dm^3 = 0.0227304593 m^3 of space

diameter: 1.5 inch = 0.0381 meters

height of cylinder to acheive volume: 4.98434836 meters

Hence, speed of water is 4.98434836 meters / s.

Momentum of water generated is 113.296528 kg m / s.
Not exactly.

0.0227304593 m^3/s

(1.5*0.0254/2)^2*pi area

velocity*area = volume/time
velocity = volume/area/time

puts velocity at 19.937m/s (I may have rounded a bit). I think you forgot to convert from diameter to radius, since you are off by a factor of four.

From there, we need momentum generated per second (since it keeps generating water, momentum over a given period of time is what matters).

That would be, at 22.7304593 litres / s=22.7304593 kg / s

Force = mass/time * velocity = 453.168N

So, aside from converting from diameter to radius, and units at the end, your is correct.

That said, those 453N are sufficient to lift someone that is 46kg (101.7lb). Not too impressive. Without a stopper, cut that value by a factor of 64 (since 12in diameter is 8x as large, so 64x the surface area)

9. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

That's the same number in Newtons. 113.296528 N

From wikipedia:
The force of Earth's gravity on (= the weight of) a human being with a mass of 70 kg is approximately 686 N.

Thus you need slightly less than 6 decanters to lift a 70kg person.

Originally Posted by demigodus
puts velocity at 19.937m/s (I may have rounded a bit). I think you forgot to convert from diameter to radius, since you are off by a factor of four.
Yes, I am out by 4x. I forgot to halve the diameter.

10. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

So it produces 453 Newtons? One of the threads I linked to said stuff from 100 some odd newtons to 2000... can someone help me figure out what they were doing in that thread???

11. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by Gavinfoxx
So it produces 453 Newtons? One of the threads I linked to said stuff from 100 some odd newtons to 2000... can someone help me figure out what they were doing in that thread???

the only calculations done were in the 5th (I think... not good at counting) post. That gives mach 45 water blast. The problem is, he has 1L = 1m^3, so he is actually off by a factor of 1,000. It also uses a diameter of 4cm compared to 3.81 that we used based on your stopper, so he should be getting around 15m/s, where as we were getting around 20m/s for speed of water.

Anyways, the source of error is him accidentally setting 1L=1m^3. The rest are just slight differences in assumptions, but that is why his numbers are way off.

Looking over the second link atm. This one I might actually have to look through physic equations to check all they did.

12. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

I'm also totally confused with the

Newton-Meter or Foot-Pound
vs
Newtons or Pound Force

thing. What DO I want to answer 'how good is this thingy at pushing a boat?' question?

13. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by Gavinfoxx
I'm also totally confused with the

Newton-Meter or Foot-Pound
vs
Newtons or Pound Force

thing. What DO I want to answer 'how good is this thingy at pushing a boat?' question?
Newton-Meter and Foot-Pound are energy.

Newtons and Pound force, are force. Force is a unit for how much something is being pushed. Energy is a measurement of how much total pushing (over a time interval) something has received, or is capable of giving. You want Newtons or Pounds.

Also,

Lets go in order

Post #9: http://www.enworld.org/forum/1638728-post9.html

treats water as having density of 1kg/m^3, is off by a factor of 1,000, so his answer should be F=450N, rather then 0.45N

also, he goes from having diameter of 4.5cm, to a radius of 4.5cm (typo I assume), putting off by a further factor of 4. Not sure about his water container height thing. I can pull out my physics text book and check if you really want

post #11: http://www.enworld.org/forum/1638759-post11.html

makes the assumption that water, if shot straight up, would go 20ft in the air, as per item description. Figures out area of nozzle from there. While we assumed area of nozzle, and ignored that 20ft thing cause it made 0 sense.

He got 4172 lb-ft, which, no matter which definition of pound you use (mass or force), is NOT a unit of force.
4172lb = 18,558N for reference though.

Problem is, he miscalculated velocity. He got 100ft/s, which would, with 32.2ft/s^2 gravity, cause the stream to hit 155ft. For comparison, the actual water velocity would be ~10.8m/s=35.6ft/s

Second problem is, he confused the pound (mass), with the slug (another empirical unit of measurement for mass). Which caused him to be off by a factor of 32.2.

Adjusting for those, the actual force given his assumptions is:
~46.1lb ~= 205N

Post 13: http://www.enworld.org/forum/1638825-post13.html

differing assumptions, however, given his assumptions, his math is correct as far as I can tell.

Post 19: http://www.enworld.org/forum/1639087-post19.html

fixes the wrong velocity problem in post 11, doesn't fix the pound-mass != slug problem

will look at the rest later. But basically the 200N~500N range seems to be the most consistent/realistic (if you want pounds, type "convert xxx newtons to pounds" into google, replacing xxx with a number)

14. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

At least in terms of propulsion, everyone here is thinking the wrong way. The Decanter of Endless Water is a magical item, ergo we cannot assume that it produces the same amount of force behind it as it does in front. In fact, we are given a strength value to hold the decanter - 12 Strength. If we assume that that is exactly how much strength is required to produce an equal amount of force on the decanter to hold it still, that value is the amount of propulsion a decanter would produce by itself. A character with 12 Strength can push 130*5=650 lbs, so the decanter produces 650 lbs of force behind it.

Note: I am assuming that a strength "check" equates to a person trying to produce the strength required to produce the same amount of force as a creature with the check result as their strength score. This seems like a weird definition, so if someone knows another one that makes more sense, then I'm all ears.

15. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

strength check of 12, is someone with strength of 14 or 15 taking 10. Their max capacity is 175lb, and 200lb respectively. Double that if stuck. The 5x increase would be pushing along the ground, in which case since most objects have a coefficient of friction less then 1, that would be around 2x to 4x rather then 5x, I would say.

However, since using this item doesn't cause one to stagger, lose dex to AC, etc, lets go with max carrying capacity. That puts us at

175lb (778N) to 200lb (890N)

Which, honestly isn't all that far off from the estimates derived. Off by about a factor of 2, which given the amount of estimation, and assumptions made, isn't that bad.

16. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

So basically.. a SINGLE one of these can make a ship maybe move reaaaalllyyy slowly?

Of course, it would be significantly more if you put one or more of them into one of those Riverine chambers with a Permanencied Wall of Flame in it, AND actually put some effort into figuring out the best size and shape for a nozzle (probably via Divinations, if the advanced types of math to do that hasn't been invented yet...)?

17. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by Gavinfoxx
So basically.. a SINGLE one of these can make a ship maybe move reaaaalllyyy slowly?

Of course, it would be significantly more if you put one or more of them into one of those Riverine chambers with a Permanencied Wall of Flame in it, AND actually put some effort into figuring out the best size and shape for a nozzle (probably via Divinations, if the advanced types of math to do that hasn't been invented yet...)?
F=a/A, where F=force, A=area of nozzle, and a=some constant.

Or basically, "best size and shape" for the nozzle translates to whatever produces the smallest area. The issue is how hard can you glue on the nozzle. If the decanter normally has area Ao, and you are reducing it to area A,

P=F/A, Fg=(Ao-A)P=(Ao-A)F/A=a(Ao-A)/A^2

constant a can be determined experimentally, the rest is basic algebra. Fg is the force on the nozzle, that your "glue" has to overcome. Find a way to make a tough enough "glue", and you could have one of these proper a flying ship.

But yeah, building it as part of a complex system would be more efficient.

18. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Using it as a direct propulsion method is probably the most wasteful. Instead, combine with magical heat source to create a steam rocket with indefinite delta-V.
Probably won't get off the ground, but teleporting to geosynchronous orbit will take care of that. Combine with a bottle of air and an item that casts create food and drink in a pressure vessel, with a bag of holding as a radiation shelter, and you got a spaceship that can take you anywhere in a solar system.

19. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Let's propose a 5' square wall of magma, shaped into a cylinder, made within a slightly larger cylinder of stone made from wall of stone. Turn that cylinder sideways and you're basically looking through a small magma tunnel. You'll need a decanter that has a narrow neck and wide mouth, so that you can then enclose another wall of stone around the neck, so that you're sealing one end of the tube with the decanter's mouth trapped in it, aiming out the other end. The body of the decanter will be outside the engine, and therefore presumably able to respond to command words when you want to shut the engine off. On the other end you of course make a nozzle. Assuming the ship were already airborne via multiple castings of suspension, and the engine was only needed to provide horizontal movement, how quickly would this move a 150 ton caravel? Hardening could be used on the stone to make it even harder than adamantine, so you could really pressurize the nozzle if that matters.

I also was told a turbofan might be more optimal, since it might not necessarily need a decanter of endless water? Having air blow through a Darrson's chilling chamber and into fiery furnace spells from Shining South would probably be the best way to accomplish this from my minimal understanding.

As for figuring this out in character, I'd say a knowledge: architecture and engineering is most appropriate.

20. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

So it would be ideal to make the Decanter itself out of Riverine, and have it actually have a nozzle as part of the Decanter itself?

Alternately, you could not have a nozzle on the Decanter, make it out of Riverine, and then you just build another Riverine pressure container that holds the Decanter securely and lets it be part of a heat producing device? Integrate the nozzle into the entire assembly?

21. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Riverine is not necessary anywhere - and thank goodness, it's expensive. The wall of magma bonds with existing stone and doesn't melt it, per the spell text. Making the decanter out of ordinary iron and casting hardening on it causes the magma to never damage it, because it never can penetrate its hardness. Casting hardening on the stone system can make stone harder than adamantine, and therefore you could use a nozzle to generate a huge amount of pressure, which if I'm understanding things correctly would cause more thrust to be generated.

22. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

What I was talking about was using Riverine *because* the nozzle is at the very very end of a somewhat different assembly than the one you described, Hirax.

And the fact that, inside the thing I was talking about, water is being pumped in, and steam is being generated, at a constant rate no matter what the internal pressure gets up to!. The idea was to set up a device where the steam is going to be under ridiculous amounts of pressure, and use Riverine so that the thing doesn't explode, and that this causes it to eventually spew something out the end at extremely high amounts of thrust.

23. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

It doesn't matter whose design is being spoken of, riverine still isn't going to be a meaningful help over hardened stone, especially as far as price/performance, if I'm understanding what's being written above. But that's a pretty big if.

24. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Okay. Lets say you have an airtight, hollow container, except for a hole at the end. This container has a Wall of Fire inside of it, hot enough to make water from the Decanter into steam before ti reaches the opening, and the Wall of Fire is Permanencied. At the far end of the container from the hole is mounted one or two Decanters of Endless Water. Now, if you turn the Decanters to Geyser, they pump a standardized amount of water (which gets turned into steam) into the container, regardless of how high the pressure is in the container. Also, the higher the pressure in the container, the faster the steam spews out of the container. Make sense?

As near as I can figure out, there is little to no limit of the amount of steam that can be put into the container. If the entire contraption (decanter and all) is made of Riverine, the psi could go astronomically high, and it would still not stop adding steam to the container.

If someone could do the math on something like this, and figure out how big the nozzle on the end of container has to be to get a 'safe' equilibrium where the pressure doesn't keep increasing, and the thrust for that, I would be very much obliged!

I think that is some of what the steam things in other threads did.

http://www.giantitp.com/forums/showp...5&postcount=18

That one in particular.

25. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Right, I'm know it can get higher pressure, what I'm not convinced of is that it's needed. While nice for TO, it costs 2,000GP per pound, if you can avoid that cost, I say do it.

Also, based on 2d6 versus 2d4 damage for being within 10' wall of magma is hotter, which makes it a better candidate.

26. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

At this point, you'd think there'd just be an outright magic item for this sort of thing...

27. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

There is, the coldfire engine from Frostburn. Sadly, it's prohibitively expensive. Gate zeppelins from the Planar Handbook (or MotP?) are also pretty cool.

28. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

A decanter of endless water could probably power (at least in part) a water-wheel for milling or for power generation. For that matter, enough research could use a permanent wall of fire for a perpetual motion steam engine. Don't stop small, we need spelljammer!

29. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Originally Posted by Hirax
Right, I'm know it can get higher pressure, what I'm not convinced of is that it's needed. While nice for TO, it costs 2,000GP per pound, if you can avoid that cost, I say do it.
For a rocket, you do want higher pressure.

Higher pressure means your rocket generates higher exhaust velocities (due to being a smaller nozzle).

Also, you sort of underestimate the power output of wall of fire. Even assuming a horrendous 10% efficiency, 1 circular wall of fire at minimum CL can boil the output of 130 decanters.
And with further estimates and handwaves, generates something like 2.1 MN of thrust. Yes, meganewtons. It can lift 200 tons off the ground.

30. ## Re: Physics help / Catgirl Killers needed: Decanter of Endless Water engines?

Another thing you can do is drop the water from a great height to increase the power delivered by it. Imagine a tall smokestack-like pipe jutting up from the deck of a sea ship. At the top of the pipe is your decanter, set at 30 gallons per round. At the bottom you put a paddlewheel, which powers a propeller, etc.

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