Shape water paddle boat/ship

[Hilary]

I want to know if it's possible to use shape water cantrip to power a paddle boat or ship. I envision making the water flow up to the top of a large paddle wheel and then filling one of the cells. Gravity pulls on the water at that point, turning the wheel. Then the water is carried back to the top again.

If the entire operation is sealed, could the spell caster make the water flow 'up' in the return trip? Or would it need to be carried back up along a ramp?

The weight of the water of even casting worth of the cantrip is significant.

[JackPhoenix]

The movement option is instantaneous: it'll happen once, and that's it. The caster will be able to move it up, then it it flow down due to gravity, and stays down, unless he casts the spell again.

And if it's in a sealed container, it can't be targetted in the first place. The caster must see the volume of water he wants to affect, and even transparent container won't help, due to general spellcasting ruler about total cover and targetting.

[Tiadoppler]

I want to know if it's possible to use shape water cantrip to power a paddle boat or ship.

If your DM says so, then sure.


I envision making the water flow up to the top of a large paddle wheel and then filling one of the cells. Gravity pulls on the water at that point, turning the wheel. Then the water is carried back to the top again.

If the entire operation is sealed, could the spell caster make the water flow 'up' in the return trip? Or would it need to be carried back up along a ramp?

The weight of the water of even casting worth of the cantrip is significant.

If you're carrying the weight of the water with you, then the water weight is added to the boat's weight.

Shape Water can move 4 tons of water 5 feet in 6 seconds, or roughly .5 mph. Assuming 100% efficiency (hah!), continuous castings of the Shape Water cantrip can move 4 tons at .5 mph, or 8 tons at .25 mph, or 20 tons at .1 mph.

Let's imagine you have a ship that weighs 4 tons (probably 30 feet long, carrying 6-10 people). If you had a spellcaster using Shape Water to simply push water at the stern, I would allow that ship to travel at up to .5 mph. That's probably not enough to get anywhere reliably, because natural winds and waves will also be affecting the course of the ship.

Now let's imagine you have a ship large enough to carry a sealed paddle engine that uses 4 tons of water and a spellcaster to provide motive force. Your cantrip is still providing the same amount of thrust, but the ship will have to be much heavier to support the weight of the engine, probably a minimum of 20 tons, 4 tons of which is the stored water. That means that your engine will only be able to provide (at most) .1 mph of movement, which is not enough to overcome any sort of wind or wave movement.

[Hilary]

One casting of the cantrip shape water controls 7798 lbs of water. I am not saying use that to push the boat. I am saying move that into one water wheel on the boat and allow gravity pulling on that to turn the wheel.

https://drive.google.com/open?id=1ejKHx8TJejt6ULNwFZZPmbVr6QxcaJma

You can even have one wheel to provide the power, and a second or second and third wheels to move the boat, with a gearing ration to make the drive wheel spin faster.

A dozen spell casters casting multiple cantrips could add a lot of water to the drive wheel.


***

If the water being controlled can flow up a tube, a caster could target the water, move it up the tube, then out into wheel. The guy next to him would move an other batch of water right behind his. This would make a continuous flow up the tube. (assuming you can make water flow up a tube). I get that you can't make it flow up in free space. But it doesn't specific horizontal only. And it doesn't mention any angle of climb at all.

Even one of these 5' cubes of water each round would provide a lot of drive force. Multiple casters make for multiple 'cubes' of water.

[Tallytrev813]

Should work with the correct contraption - basically the reverse of a steamboat. Instead of steam pushing the wheel, the flow of water down (Gravity) would push it.

Essentially, you'd need a large container above a wheel of paddles, you'd move a 5x5 cube of water into the container, it would flow down, and push the paddles. The paddle-wheel would, at its bottom 1/4, dip into the water. The wheel would turn due to the push of Gravity+Water flow, the turning of the wheel would move the boat.

Im not sure how FAST it would be, but it should, in theory, work.

[Hilary]

Right. The speed would depend on the height. But the force of gravity is the same. Using the one gravity water wheel to turn turn a drive wheel would allow for a gearing ratio. The force of tones of water falling should be plenty to increase the speed ratio by 2 or 3.

It would require constant spell casting and the casters will need breaks. But I have to think the force of multiple tons of water has to be at least equal to whatever force an early steam engine could manage.

The real point of contention would be restrictions on moving water with the cantrip.

[Tiadoppler]

Shape Water (under many interpretations) allows you to lift ~8000lbs of water 5' up over the course of 6 seconds.

Assuming that (campaign setting) has the same gravity as Earth, your DM allows you to cast Shape Water on water that's inside a machine, and your engine is perfectly efficient at capturing the gravitational potential energy of lifting a 5'x5'x5' cube of water 5', each casting of the cantrip produces about 54000 Joules over 6 seconds, or 9000 Joules per second.

9000 J/s is roughly 12 horsepower (still assuming a 100% perfectly efficient energy transfer).

A 12 horsepower motor weighing at least 8000 lbs (because your ship still has to carry the water) is not going to go terribly fast, no matter what gearing you use. And that assumes that the weight of the materials forming the water wheel is negligible.



Now lets assume your DM allows you to use multiple spellcasters to run the same 8000lb engine, and still assumes perfect efficiency. If you have 10 spellcasters constantly casting Shape Water, you can have an 8000lb (+ weight of 10 people) engine that produces 120 horsepower.

You still need a ship that can carry 8000lbs of engine, so let's say the ship weighs in at 40,000lbs. The engine is 20% of the ship's weight (that's a lot of weight for the engine, but let's assume that a sufficiently skilled shipbuilder could make it work. No real room for cargo though.). 120 horsepower in a ship that large is still quite low.

1200 horsepower is probably going to be decent in a 40,000lb ship. So we need 100 spellcasters continuously casting Shape Water on a single 5'x5'x5' cube of water inside a perfectly efficient and ultralight engine. But spellcasters have weight too. 100 spellcasters weighing (they're all really slender, because we haven't brought food for them) 100lbs each adds another 10,000lbs to your ship, making it slower.




The cantrip Shape Water requires that the spellcaster be within 30 feet of the water in question. Assuming that each spellcaster is stationary on a single floor and takes up a 5'x5' square, and that the engine is also a single 5'x5'x5' cube, that suggests a limit of (13x13 - 1) 168 spellcasters, producing a measly 2016 horsepower, and weighing 12 tons.

But wait! By having multiple levels of spellcasters (perhaps hung from the ceiling of a giant engine room in safety harnesses, or sitting on the rungs of a ladders), we can increase the number of spellcasters to (13x13x13 - 1) 2196. In that case, the engine produces 26352 horsepower and weighs 100 tons (not including the ship, or framework for people to sit on surrounding the engine).

This is exciting because it's actually more efficient (in terms of weight) than large modern marine diesel engines!! It just requires an cube shaped engine room 65 feet across, and the permanent services of over 2000 spellcasters.



Calculations:
Horsepower = 12hp x spellcasters
(Minimum) Weight of engine = 8000lbs
(Average?) Weight of spellcasters = 100lbs x spellcasters

Weight of Engine+Spellcasters = 8000+100xspellcasters

Power:Weight ratio starts very low (1hp per 666lbs) with 1 spellcaster but gets excellent with many spellcasters (approaching 1hp per 8lbs) using the same engine.


TL;DR:
Engines using Shape Water and a heavy internal water supply are best utilized in large ships, using hundreds of spellcasters. If the number of spellcasters is below 10, the engine will not produce enough thrust to maneuver the vessel in any weather condition other than perfectly still and calm. Depending on the rarity of magic in your campaign setting, hiring hundreds or thousands of casters capable of a specific cantrip may range from extremely expensive (2gp per day per hired spellcaster) to totally unviable (individual price per cantrip casting, or 10gp+ per day per hired spellcaster).

[Trampaige]

Shape Water

So the second line of the spell is "• You cause the water to form into simple shapes and animate at your direction. This change lasts for 1 hour."

Lacking physics knowledge, I'll ask in a simple way. What if you animated the water into a simple shape and had it spin to interact with gears? The 1 hour duration means significantly more automation potential.

[Segev]

So the second line of the spell is "• You cause the water to form into simple shapes and animate at your direction. This change lasts for 1 hour."

Lacking physics knowledge, I'll ask in a simple way. What if you animated the water into a simple shape and had it spin to interact with gears? The 1 hour duration means significantly more automation potential.

This is a good approach, but I will offer one that's even simpler and works with any floating vehicle: create a surfing wave behind you.

Shape the water into a wave that causes the water level behind the boat to be higher than the water level in front of the boat, and then have that shape move to stay behind the boat as the boat is pushed out of it. You've got a water shovel. This obviously works better with smaller boats.

[Tiadoppler]

So the second line of the spell is "• You cause the water to form into simple shapes and animate at your direction. This change lasts for 1 hour."

Lacking physics knowledge, I'll ask in a simple way. What if you animated the water into a simple shape and had it spin to interact with gears? The 1 hour duration means significantly more automation potential.


Using that second option, the water must stay inside the 5'x5'x5' cube, but it's certainly unclear as to how much power it can produce. I would not rule that the caster can tell the water "go inside the turbine and spin rapidly enough to produce ten thousand horsepower for the next hour". Although that option doesn't necessarily violate the text of the spell, it certainly violates the spirit of the spell, the design principles of cantrips, the described limitations of the spell, and any semblance of game balance.

How much energy does a single casting of Shape Water impart on the water? For the purposes of doing work, I'd treat the maximum power output of the spell to be the maximum stated change in gravitational potential energy listed (~54000 Joules). If you animate something over the course of an hour, it can produce a steady 1/50 horsepower.

Can you order Shape Water to repeat a motion for an hour? The RAW is unclear. You animate the shapes at your direction, which implies that you have to keep directing the water's movement if you want it to move.



In terms of using Shape Water to move a ship, 12 horsepower is actually a decent amount, and perfectly suitable for a cantrip. By pushing the hull of a small (less than 4 tons) ship, you could move at about 1/2 mph. That's not an unreasonable speed for docking in a safe harbor, or maneuvering slightly in an emergency, but it's not really enough to fight a sustained current or wind. Which means that 1 spellcaster with 1 cantrip isn't going to replace sails or steam engines single-handedly.

Which is good. It's a cantrip, and a very versatile one, but unless your DM cooperates with some very questionable rulings, it's not going to really break anything in your world's economy.

[Sigreid]

I think it may be easier to use shape water to create a dimple in the water and let your boat be carried forward as the water flows into the area.

[Crgaston]

So if you can move 125 cubic feet of water 5 feet "instantaneously," what if you were to move it through a funnel with a, say, 2' diameter mouth and a 6" diameter exit?

Attach it to the back of a reinforced canoe?

Water jet propulsion?

[Hilary]

OK. I agree that the math is not on my side.

The water on the ship creates too much weight.

Last question:

Can more than one spell caster affect the same volume of water with shape water? Can they stack their effects so as to make that one 'unit' of water move twice as fast?

"You instantaneously move or otherwise change the flow of the water as you direct, up to 5 feet in any direction."

I take that 5 feet to mean D&D speed, since it lists it as a 'change' in the flow.

So, with multiple casters affecting the same 5' cube of water, could each of them add 5 feet to it's speed?

Or when multiple casters try to affect the same target, does only one of them ever produce an effect? (-aka- the effects never stack)

Thanks for all the help.

[Segev]

OK. I agree that the math is not on my side.

The water on the ship creates too much weight.

Last question:

Can more than one spell caster affect the same volume of water with shape water? Can they stack their effects so as to make that one 'unit' of water move twice as fast?

"You instantaneously move or otherwise change the flow of the water as you direct, up to 5 feet in any direction."

I take that 5 feet to mean D&D speed, since it lists it as a 'change' in the flow.

So, with multiple casters affecting the same 5' cube of water, could each of them add 5 feet to it's speed?

Or when multiple casters try to affect the same target, does only one of them ever produce an effect? (-aka- the effects never stack)

Thanks for all the help.
Yes.

Mechanically, they'd each act on their turn, casting shape water to move it. If the DM wants to rule that it all sloshes back down after each movement, have each one Ready to cast it immediately upon the completion of the last one in sequence, so whoever goes last in initiative order actually goes first, and then they chain it.

[Crgaston]

Is this the 5e version of the peasant rail gun?

[Segev]

Is this the 5e version of the peasant rail gun?

I suppose it COULD be, but you can also frame it in a reasonable fashion.