The RW Physics of the Decantur of Endless Water

[reanjr]

I can not believe that I am actually posting here but...

since it is MAGIC would it be okay to say that the decantur opens a hole to the elemental plane of water and water comes out with a force for those in front of the geyser but produces no foce for those behind (or holding) the decantur? In a sence the water pushing out of the hole "pushes back" (that whole equal an opposite force thing) into the plane of water, bypassing the person holding the decantur.

I'd say that would be a VERY reasonable assumption. And given this assumption, it would be the water pressure that pushes the water out. If the thing were put under water, the pressure would even out and create 0lbs. (N) net force. This is probably the nicest solution (if not wholly accurate in every case, given different substances and distance from sea level).

One would, of course, have to rule that the portal is one way or there would be a problem when a PC decided to use the decanter as a portal for escape.

 

[ARandomGod]

First:
Is non-sensical. If it is really a 1 foot diameter stream 20 feet long, it would take 111 gallons to 'fill' this stream. Which is over 22 seconds. It just makes no sense whatsoever.

For purposes of calculating this, I assumed that the relevant info was the 20' long stream, and the 30 gallons/round.

Just to throw some doubt into this, I'll add some arbitrary descriptions of the magic, and how it could be potentially not nonsensical.

The stream is not composed entirely of water. Indeed, there is a lot of air, and the "water" couple be considered very, very wet air ... sort of gyserlike. However, there is at least 30 gallons of water each round in this "gyser" of moistened air. It really is one foot in diameter.

After 20 feet, the gyser looses it's magical properties, all acceleration ends abruptly, and the water falls to the ground as it's magically created kenetic energy is instantly stripped from it. The air also looses kenetic energy, and disperses normally. Note that secondary effects of kenetic energy are not lost, IE any air, water, or other substance that the kenetic energy was transferred to maintains that energy. Note also that subsequent water falling onto the water that has been stripped of it's kenetic energy might add energy to this new, ground stream of water, and that there is 30 gallons per round of water being added as long as it remains in the same general area.

 

[strongbow]

I would like to thank all the human computers for their calculations, but would like a layman's explanation of the most probable computations. Assuming that the decantur loses its magical property after 20 ft is a good assumption. As I pointed out in the beginning, the rate of flow of a firehose is about 120-150 gallons per minute, with the decantur output being 300 gallons per minute. I have seen some good calculations (in the sense that they look pretty to the untrained eye), but my eyes have glazed over a bit. Are Scion's approximations about as good as you can get with an item that has "bogus (read: magic)" physics? If so, that is what I will use.

I will revise my first post to add some solutions, but I need to know what will be good to add. Thanks

 

[ARandomGod]

I would like to thank all the human computers for their calculations, but would like a layman's explanation of the most probable computations. Assuming that the decantur loses its magical property after 20 ft is a good assumption. As I pointed out in the beginning, the rate of flow of a firehose is about 120-150 gallons per minute, with the decantur output being 300 gallons per minute. I have seen some good calculations (in the sense that they look pretty to the untrained eye), but my eyes have glazed over a bit. Are Scion's approximations about as good as you can get with an item that has "bogus (read: magic)" physics? If so, that is what I will use.

I will revise my first post to add some solutions, but I need to know what will be good to add. Thanks

Well, IMO, what I would go with is based of this quote from above:


"The geyser mode is the only one that produces any significant recoil, and it's not very much. The Strength check for a wielder to avoid being knocked down is only DC 12, which is equivalent to an opposed check against a character with Strength 14 who takes 10."

I like that, it neatly ignores the question of the amount of water while giving out a good amount of recoil, which is what you're using to propell the boat. Each one is equal to a strength 14 person. But you can pile on as many as you want, and each additional one would multiply the base speed.
"It's a 14 decantur powered boat!"
I'd say you could pile on WAY more of those than you could of actual 14 strength men.

 

[glass]

Maybe water from the decanter in geyser mode magically disappears after 20 ft.

This would make more sense and would also make it more difficult/expansive to feed a city or flood the world, as you would have to use the slower settings.


glass.

 

[strongbow]

I'm with glass on this one. It makes more sense that the water magically disperses after 20 ft, to prevent complicated trajectories and distance calculation with where the water lands.

 

[glass]

I'm with glass on this one. It makes more sense that the water magically disperses after 20 ft, to prevent complicated trajectories and distance calculation with where the water lands.

I anyone needs an explanation as to why it disapears with one setting but not with the other two, maybe in geyser mode it partially shifts the area in front of it to the plane of water. The force is just that of normal water currents there.

glass.

 

[jasper]

Ok let see the time to fill various things.
For an oval swimming pool the calculation is Len * width * avg depth * 5.9
For an rectangle swimming pool the calculation is len * width * avg depth * 7.5
A 16 by 32 foot pool with an 8.5 deep end and 3 shallow end holds about 22,080 gals
If the decanter has the rate of 30 per round = 300 a minute = 18,000 an hour = 432,000 gallons a day,
Then it would take 1.226 hours or 74 minutes to fill the pool
For 10 by 10 by 10 room it would take 25 minutes to fill
For a 20 by 20 by 20 room it would take 3.33 hours or just enough to enjoy one of lord of rings before you drown.
If want to fill the Empire State building
424 * 424 * 1250 * 7.5 (this max volume not true ) 3901 + days. (I could not find where the dimensions of the breaks were)
After lunch I post how long it takes to fill the map in dmg.


Okay it was a long lunch
The sample dungeon is 170 squares making 1700 * 1700 * 7.5 * 1 foot deep = 21,675,000 or
50.17 days. This does not take in account the depth of the various stair ways. And if the water was able to enter all parts of dungeon.

 

[Gavinfoxx]

Sorry to bump something so old... but what was the ultimate answer to this thread?

I have a variation on this question...

Ignoring the 20' bit, a 1.5" diameter (no particular shape) nozzle is outputting room temperature fresh water at 300 gallons a minute. How much thrust (in Newtons, and I think... Pounds Force? Or Newton-Meters or Foot Pounds? I'm so confused...) is it generating? How fast is the speed of the water leaving the Decanter? Is there any more information that needs to be described to solve this problem? How about a saltwater nozzle?

Bonus: What's a rule of thumb for how fast a raft or small boat using this as a propulsion method is likely to be able to get using this device? A larger ship? Exactly how useful is this motor?

 

[Greenfield]

The basic problem with all these calculations is that the description in the book is inconsistent with anything actually possible.

The only way to get an actual 1 foot wide stream of water is to have a 1 foot wide opening. And it's a decanter, not a bucket.

Also, the idea that a spray of water will cease moving after exactly 20 feet, no matter which direction it's facing is, well, a fantasy.

Now there are a few things that can be agreed upon.

1) At Geyser setting it's producing 5 gallons per second. That's 40 lbs.

Does 1 count as "a few"? Because that's the only actual number you can possibly count on. Or can we even agree on that 1? The Decanter has an option to produce salt water, which will have a different weight.

How different? Depends on the salinity. Dead Sea water is about as saline as water can get at that temperature, but as temperature changes so does the ability of water to hold salt in solution.

Sea water weighs approximately 8.552 lbs per gallon, though even that is variable based on which part of which sea you're taking it from. (The Mediterranean is a shallow, warm water ocean so evaporation plays a big factor. It's saltier than water from the mid Atlantic.)

So we have a couple of different values for the M part of the kinetic energy formula, but no way to calculate the V part. The speed that the water leaves at, in terms of actual velocity, is going to depend on the size of the opening, and if we take the description at its word, that somehow the opening size changes to a foot, you'll have so little action/reaction going there that the only reason for the Strength check would be to hold to weight of the now bucket-sized decanter full of water.

If it was possible to move 5 gallons per second through a pinhole (it isn't, fluid dynamics make it impossible), you'd need a release velocity in the thousands of miles per hour. (Water jet cutters push half a gallon a minute through such a hole, and it comes out with enough force to cut steel.)

At 80 psi (common pressure for household water systems) you can move 5 gallons per second through a 1/2 inch opening. At higher pressures you can get up to 21 gallons per second through that same opening. More than that starts to generate backpressure turbulence, and you actually begin losing flow rate.

Did I mention that hydrodynamics is weird?

So the answer is, as usual, that applying physics to a magical fantasy game is a bad idea. At a minimum you need more data than the game system is providing, and you probably need to ignore some of the flatly impossible information. (Such as a foot wide stream going fast enough to carry 20 feet, but that only carries 5 gallons per second.)

The in-game answer? Whatever the DM says.