Rain-powered transport

The plain of Soggi experiences a constant rainfall. Can a rain powered vehicle be built according to the following conditions?

  1. The plain of Soggi experiences a constant rainfall of 10 cm per day

  2. Flooding doesn't occur because the plain is floored with a porous volcanic rock that naturally drains into the Big Wet Lake.

  3. The inhabitants have smoothed the rock in places to make roads and other flat areas

  4. Wood is plentiful but non-vegetable materials (including metals) are expensive and have to be imported.

  5. In this pre-industrial society, stationary rain-powered water-wheels are in common use to power mills and other basic machinery predominantly built from wood.

  6. The vehicle is to be made from a wood very similar to oak. Minor metal components can be used but minimally to avoid expense.

  7. The vehicle must fit into a 3 metre cube to satisfy local laws. There are no draft animals in this world.

  8. This must be a go-anywhere vehicle that does not require any special infrastructure beyond the existing flat-and-level roads - no digging canals or building railway tracks or regularly spaced "charging stations"

Someone has the bright idea to power a moving cart by harnessing rain-power. This will consist of a water-wheel that drives the wheels by a system of leather belts.

The inventor decides that the minimum useful speed is walking-pace (say 1.5 m/s) and that the vehicle must be able to carry at least him (he weighs 60 kg) but preferably a much heavier load.

The vehicle will travel over levelled volcanic rock and has a similar configuration and technology to this one from the 1800s.

enter image description here


Reality check: Are there any obvious flaws in this plan that makes it highly unlikely to work


Some people are suggesting alternative inventions which I find fascinating to read. However they aren't true answers because I'm asking solely about the vehicle and conditions specified above.

In-story justification: This guy wants to set up a door-to-door collection and delivery service. He needs to use existing roads and driveways. He can't start digging canals or building large engineering projects. See condition (8) above which wasn't added by me but with which I agree.

  • 1
    $\begingroup$ whats wrong with a boat? Water wheel powered boats have existed since medieval times. the boat had water wheel with a large axle, a rope was wound round the axle and the turning of the water wheel pulled the boat upstream, downstream was just a matter of removing a pin and letting the axle free spin. boats take less energy to move than a cart as well, so you can move more for the same force of water. $\endgroup$ – John Nov 22 '18 at 14:08
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    $\begingroup$ +1 for 'the plain of Soggi'. That gave me a good chuckle. $\endgroup$ – Smeato Nov 22 '18 at 15:37
  • $\begingroup$ He's just one guy. He wants to use existing roads - maybe start a door-to-door delivery service. He can't start digging canals. $\endgroup$ – chasly from UK Nov 22 '18 at 16:04
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    $\begingroup$ "The Foggy land of Soggi has ground that's rather boggy"? $\endgroup$ – Chronocidal Nov 22 '18 at 16:18
  • $\begingroup$ Yeah. With a very large extended inverted pyramid catchment area you might get enough mass to drive a cart at a crawl. Maybe 0.5mph. no good for speed but maybe for bulk transport of non-perishable goods. Canals in the UK were used for this purpose. $\endgroup$ – Richard Nov 23 '18 at 0:32

14 Answers 14


Yes, there are flaws. You don't get enough power.


Rain terminal velocity is about 10 m/s.

10 cm/day of rainfall means you will get on average 1.1 μm/s, or 0.0011 $l/s\cdot m^2 $.

The average power delivered by that rain is given by its weight times its velocity, thus, remembering that 1 liter of water weight about 10 N, we have that the average power is 0.1 $W/m^2$.

To move horizontally just 60 kg overcoming the rolling resistance of the wheels and the bearings, you would need to provide about 1.5 W (assuming wooden wheels with radius 1 meter), corresponding to about 15 square meter in the ideal case of 100% yield, more than the 9 you have available.

In the extreme case where you want to move 60 kg at 1.5 m/s of constant speed up you would need 900 W, meaning that you would need, a collecting surface of at least 9000 $m^2$, which is way larger than the 9 $m^2$ that you have.

And, mind, this is the estimate just for moving 60 kg. If we add to the calculation the mass of the vehicle and its payload the required surface further increases.

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    $\begingroup$ I'm not sure what you mean by "against gravity", its not moving upwards. Assuming perfect axels gravity shouldn't effect you. It's power vs air resistance that surely matters? The rolling resistance part makes more sense to me but where does the 900w come from? $\endgroup$ – Richard Tingle Nov 22 '18 at 20:08
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    $\begingroup$ You're using the wrong formula for power. Power = energy/time. Kinetic energy = 0.5 * mass (not weight) * velocity squared. 1 litre of water has a mass of ~1 kg, so power per m^2 would be 0.5 * 0.0011 kg * (10 m)^2 or about 0.05 W/m^2, half what you've calculated. $\endgroup$ – Geoffrey Brent Nov 23 '18 at 5:23
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    $\begingroup$ In a water-wheel arrangement, there's also potential energy to consider: if you catch 1 kg of water at 3m above the ground, you can extract another ~ 30 joules from lowering it to the ground. This makes for another 0.03 watts per m^2, so the final answer is close to your 0.1 W/m^2, but your method for calculation is incorrect. Multiplying by 9m^2 gets us about 0.7W total. Where does the "about 1.5 W" estimate come from for rolling resistance? $\endgroup$ – Geoffrey Brent Nov 23 '18 at 5:28
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    $\begingroup$ @GeoffreyBrent: Energy = mass x acceleration x distance. Power =Energy /time. Force = mass x acceleration. Therefore, Power = Energy/time =(mass x acceleration) x (distance/time) = Force x velocity. The formula used above is correct. $\endgroup$ – nzaman Nov 23 '18 at 6:53
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    $\begingroup$ @PeterLeFanuLumsdaine, I have rearranged the content to have a more linear explanation. $\endgroup$ – L.Dutch Nov 24 '18 at 4:18

As has already been described, you can't get enough power from using the rain directly on an individual vehicle.

Hence the only way to proceed must be to use the rain indirectly.

We're going to build rain gathering header tanks all around the village each one will drive a waterwheel linked to a continuous belt along a stretch of road. A vehicle wanting to pass along that stretch of road will hook itself to the belt and ring the bell for the tank to be opened.

This approach removes the size limitation from the vehicle power supply.

You're still going to have limited power making climbing hills difficult. It's going to be well worth hooking vehicles into the system on the decent as well as ascent. That way you get the funicular effect of the descending vehicle helping to pull the ascending vehicle up the hill, while also controlling the decent speed and reducing the risk of runaway vehicles.

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    $\begingroup$ This is a cable-car system. $\endgroup$ – Jasper Nov 22 '18 at 22:37
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    $\begingroup$ As an expansion: You can just put a roof over the road, collect the rain, and drain it down via waterwheels powering the belts. That should supply ample amounts of power along the way, as long as there are not too many vehicles on the road at the same time. $\endgroup$ – cmaster Nov 22 '18 at 22:37

Just skip Wheeled stuff. [EDIT: apparently that is against OPs requirements, but I like my answer so it'll stay]

Replace your roads with perfectly level canals and "power" them by only having one end supplied with water at any time.

You need two between each city (back and forth) and each city needs a very large area to power its canal (unless you use small canoes only), but its viable, and once the canals are set up pretty cheap.

  • $\begingroup$ This is good. However the inventor wants to be able to go anywhere around his local village - even off-road if the ground isn't too rough. $\endgroup$ – chasly from UK Nov 22 '18 at 15:59
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    $\begingroup$ @chaslyfromUK That is information you should probably be adding to the question, in your list of bullet-points - the "rail-free, go anywhere without prior infrastructure" requirement invalidates the vast majority of the answers posted which are based on a more general concept of "vehicle" $\endgroup$ – Chronocidal Nov 22 '18 at 16:16
  • $\begingroup$ In my OP I stated the conditions clearly. The invention is a wheeled vehicle and fits into a 3m cube and is driven by a waterwheel. If you re-read the conditions and following paragraphs thoroughly I think they exclude many answers. (Although to be honest I enjoy reading the alternative inventions). $\endgroup$ – chasly from UK Nov 22 '18 at 18:02
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    $\begingroup$ Sorry, looks like I overread that part in the question. But since anything wheeled seems kinda impossible (see other answers) I ll still leave this here, maybe OP changes his conditions. Thanks for pointing it out though @chaslyfromUK $\endgroup$ – Hobbamok Nov 23 '18 at 10:19
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    $\begingroup$ @chaslyfromUK, the thing with asking a question to which the fundamental answer is "no", is that you get a lot of workarounds, some of which can be quite inventive, but all of which ignore one of the question requirements. $\endgroup$ – Separatrix Nov 23 '18 at 10:40

Put the cart in a river and let the flow of the river carry the cart. If the river is shallow enough the cart wheels will touch the riverbed.

The river is fed by the rain, and concentrates water from a comparatively large area into a small stream to give you the watts per square meter you need.

This is unidirectional, but being able to go anywhere was not in the question requirements.

  • $\begingroup$ Interesting but the inventor wants to travel along existing roads. He's just one guy, he can't start start redirecting rivers. $\endgroup$ – chasly from UK Nov 22 '18 at 16:01

This answer has been invalidated by requirements added to or clarified in the question after posting, but is being left for future web-searchers to whom it may be useful.

Use a system based on a Funicular to store water in raised reservoirs and convert it to mechanical energy. You can use this to drive carts along rails, like a cross between a cable-car and a tram/railway.

Since you rely on the reservoirs refilling between carts, this is more likely to be a scheduled system for transporting goods at scheduled times, rather than a public ad-hoc transportation.

  • $\begingroup$ Thanks for the idea. In fact it is quite possible they already have something like this. As I said before any editing, "In this pre-industrial society, stationary rain-powered water-wheels are in common use". I didn't exclude them having other devices such as funiculars with largely fixed machinery. The inventor just wants to come up with his own personal more flexible and compact transport vehicle. That's why I set the parameters so clearly. I showed a picture of the type of cart and I explained that the water-wheel was connected to the wheels of the cart. $\endgroup$ – chasly from UK Nov 22 '18 at 20:52

It won't work, no way

[This is a repost of a previous, wrong and therefore deleted answer. Orders of magnitude are hard. The deleted answer had accumulated a number of upvotes, showing that orders of magnitude are difficult for others too.]

To fix ideas, let's assume that the vehicle is supposed to work by catching water on its roof and directing it to fall on a waterwheel. Let's compute available power.

  1. 10 cm of rain per day means 1.16 µm per second.

  2. Suppose the catchment area is 3 m by 2 m = 6 m².

  3. This means that 6 × 1.16E-6 = 6.94 grams of water will be caught per second.

  4. Say that this water falls 2 meters: since energy is weight times distance, it results in 9.6E-3 × 9.81 × 2 = 136.25 milliwatts.

If 136 mW are quite obviously not enough to power the vehicle; powering the vehicle from rain directly won't work.

Note: L.Dutch had the idea of using the kinetic energy of water droplets to power the waterwheel; see their answer for the computation. It may give more power than trying to catch water and make it fall on a waterwheel, it still is a pitiful amount of power per square meter.

  • But wait! We can charge the reservoir!

    Let's borrow an idea from Mr. Musk's electric cars. We don't have to power the vehicle all the time: we can charge a reservoir over a period of time and use the accumulated energy at a higher power.

  • Let's assume that we accumulate water for 10 hours; the total accumulated energy will be 136.25 mW * 36000 = 4905 J.

  • This is enough to give us 1 horsepower for about 6.6 seconds. Still too little.

  • Let's make the charging area larger. We can catch water over a larger area, and then pour it into the vehicle's reservoir. Let's say that we want the vehicle to have 4 horsepower for 1 hour; this comes to about 11 MW of energy.

  • To get that amount of energy from water falling 2 meters we need some 546 tons of water. This is a ridiculous amount; there is no way to make it work: the required catchment area would be enormous, the reservoir on the vehicle would be a hundred meters tall, and the vehicle won't be able to move under its own power.


Two ways to make it work:

  1. A wide catchment area at either end of the road collects rainwater and forces it through a Pelton wheel. The wheel drives a chain of belts from one end of the highway to the other. So, you now have two belts on either side of a road, rotating in opposite directions. Your vehicles will latch on to the belt in the direction it wishes to travel and be carried along. The size if the catchment and the flow rate determines the maximum load that can be moved, and the diameter of the turbine wheel determines the speed.
  2. Same Pelton Wheel as before, but this time, it is used to charge a battery, which can then be used to drive a motor, moving the car.
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    $\begingroup$ OP mentioned pre-industrial technology - I don't think option 2 is a valid answer under this aspect. $\endgroup$ – Inarion Nov 22 '18 at 14:37
  • $\begingroup$ This is one guy. He can't start digging up roads. I'll add a bit of rationale to the OP. $\endgroup$ – chasly from UK Nov 22 '18 at 16:09
  • $\begingroup$ @chaslyfromUK: He won't be digging up roads, he'll be making wide area reservoirs. This is essentially the same as AlexP's answer above $\endgroup$ – nzaman Nov 22 '18 at 16:12
  • $\begingroup$ Have a look at the note I added to my OP. It doesn't contradict anything I said to start with. ;-) $\endgroup$ – chasly from UK Nov 22 '18 at 16:17
  • $\begingroup$ @chaslyfromUK: Not doable $\endgroup$ – nzaman Nov 22 '18 at 16:59

Can you collect the rainwater in a cylinder and light a fire under it turning it into steam? If the rainfall is constant, you could potentially have a steam powered vehicle which doesn't need to be refilled from water towers as it could simply be left in the rain to refill it's water tender. Of course you would have to make some of the components from metal and others could potentially be non-combustible material such as stone. By 1800, the likes of James Watt and Richard Trevithick had created rudimentary steam engines which were just beginning to be commercially useful.

Another option, if you have a world where there are many mills powered by waterwheels, you could use these are stationary winding engines so you'd attach a rope to your cart and have the action of the turning wheel pull the cart to the mill. Once it arrives there, disconnect it and attach it to the next mill. Assuming you are delivering to locations which have proximate mills, this could be an option.

The only other thing that springs to mind would be a rain powered impeller so rain falling on the vehicle is channelled to a drain point, this drain point contains an impeller driving some sort of motor to charge a battery. If it's raining all the time, this would be constantly charging. The cart left in the rain for a while would increase it's battery charge. Problem is, for this to work you'd need metal for the windings in the motor and the charger and it isn't exactly pre-industrial.

  • $\begingroup$ This doesn't really answer the question, but I like the idea of a refilling steam engine. $\endgroup$ – Trevor D Dec 13 '18 at 22:22

I'm going to assume other posters got the value of 900W correct.

As a comparison:

  • An untrained adult can put out about 100W of power when riding a bike.
  • A competent commuter could be doing 150-200W
  • A race sprinter can do well over 700W but only for short periods.
  • Team Sky riders can do 400W for the duration of their race.

At an estimate, your cart would need 4-6 people pedalling through a shared geared axle to drive a roadwheel. Getting up to speed would be the most effort, but once its rolling then speed maintenacne of 5 km/h or 3 mph will have a lower power input requirement.

Something vaguely like this but with less beer. http://isuwestlake.com/images/pedal-pub2.jpg


  • significant climbs are going to be very slow, because of the load on the cart.

  • Your cart won't be particularly aerodynamic, so more than 5 km/h will be exponentially harder due to wind resistance.

  • Metal requirements - ideally you'd have metal bearings and races to decrease friction, and a metal chain and cogs and axles for strength. However hardwood wooden chain/gears and bearings certainly have existed.

  • 6 people on a 3 metre long cart will use a lot of load space carrying the people around. I'd guess less than half of the deck will be available for load.

Another variation on this is to use the people as draught animals, and have them tow the cart on traces and shoulder harnesses. Perhaps a punishment for convicted criminals?


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    $\begingroup$ Unfortunately, the 900W figure is bullshit. The correct value is 0.307W. See my comment on LMD's answer for the derivation (worldbuilding.stackexchange.com/a/131097/37815). $\endgroup$ – cmaster Nov 23 '18 at 22:23
  • $\begingroup$ In what sense are these suggestions rain-powered?? $\endgroup$ – Peter LeFanu Lumsdaine Nov 23 '18 at 23:24
  • $\begingroup$ @PeterLeFanuLumsdaine good point - the only real advantage here is they all work perfectly well in the rain, and at OP's required technology level. $\endgroup$ – Criggie Nov 24 '18 at 1:33
  • $\begingroup$ @cmaster I used 900W as an input for the power required to move the cart at walking pace, and a way to get that power output using things available in OP's world and technology level, and not overly affected by rain. $\endgroup$ – Criggie Nov 24 '18 at 1:35

Other posters already showed that it seems to be impossible with the given requirements.

However, when able to alter those requirements, those would be my proposals :

  • Altering the streets to decrease friction - have your vehicles "glide" on a water film(similar to a phenomenon known as aquaplaning). For that purpose, you would probably have to slightly adapt the streets, however.

  • Besides, if it was possible, I would suggest your character to add a hot air balloon to it's vehicle; it's quite simply to build & attach, and you can travel by wind - as your salesman doesn't really care where he goes next, that should be fine - and if the wind is moving you the wrong way, just make your balloon stick to the ground and wait until wind direction changes ;)

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    $\begingroup$ Unfortunately, the 900W figure is bullshit. The correct value is 0.307W. The equation is simply 0.1m/d * 3m*3m*3m * 1000 kg/m^3 * 9.81 N/kg / 24 h/d / 3600 s/h. The result of this product is 0.307 Nm/s = 0.307 W. $\endgroup$ – cmaster Nov 23 '18 at 22:21

Would storing the energy temporarily be acceptable? Since there is plentiful wood, you could harvest the rain and use a steam engine to convert the energy of the rain in to tanks of compressed air. Which can be used to power a vehicle, similar to the one you pictured.

  • $\begingroup$ How can a steam engine convert the energy of the rain? Steam engines use a burning fuel to produce heated steam. Rain is not a fuel. $\endgroup$ – L.Dutch Nov 24 '18 at 12:35
  • $\begingroup$ Interesting idea but in this pre-industrial society, producing and storing compressed air would be very difficult if not impossible. Just making the containers and valves would be beyond them. This is pre-industrial so the steam engine isn't really a thing yet. $\endgroup$ – chasly from UK Nov 24 '18 at 12:43
  • $\begingroup$ I see why using compressed air is an issue, but I'd like to mention that the first commercial steam engine was patented in 1712 while the industrial revolution was from 1760 till 1820/1840 (depending on which resource you take). So steam enigines are pre-industrial in fact. They are the origin of the industrial revolution, but that makes them pre-industrial. @L.Dutch, the trees (which are plentiful available) are used as fuel to heat up the collected rain. $\endgroup$ – user007 Nov 25 '18 at 13:10

Edit to answer your question: There seems to be no obvious flaw in your idea, if you consider that your story takes place on a planet different from earth. This does not mean you can disregard the laws of physics, it means you can change constants that are specific to our planet in your math.

To be more specific:

After finding the equations you find most suitable, alter the constants that you want, so that enough energy can be sufficiently be harnessed.

Your story is obviously not situated on our planet, which means that you are free to modify:

  1. gravity
  2. liquid that the rain consists off
  3. density of materials for construction
  4. mass of the creatures, and the materials they are interested in carrying.

Dont let our planet restrict you, unless you are set on the story playing out there.

If you do want the planet to be as earth-like as possible, play around with changing the constants minimally. Some should give you more likely vehicles, with less modification then others.

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    $\begingroup$ The OP is asking if the exposed idea has flaws or not. You are not answering that question. $\endgroup$ – L.Dutch Nov 24 '18 at 20:37
  • $\begingroup$ I believe it is understood from the context that I cannot see any obvious flaws, and that none of the other answers have pointed any out either.... Unless the OP modify his list of demands(yet again) to include: played out on Earth. I may even come back and add some math if he commits to keeping his list constant $\endgroup$ – Stefan Karlsson Nov 24 '18 at 20:50
  • $\begingroup$ Your response has been flagged as low quality. In this instance, I am sure, because of quality. You begin by commenting on others' answers. You continue by going on about constants and equations and gravity and what the rain is made of. You're doing basically everything except answer the query. You are new here so I'd recommend two things: first is to check out the help center and tour, where you'll see how to answer a SE query; then edit your response to actually address the query as it is. And for preference without whining about the edits. The OP is free to edit their own question at will! $\endgroup$ – elemtilas Nov 24 '18 at 22:39
  • $\begingroup$ I have edited the answer, but I think it has made the answer as a whole larger and uglier than it has to be. This was unnecessary, and I suspect the reason this happened is that my point of view invalidates some of the other posters answers. $\endgroup$ – Stefan Karlsson Nov 25 '18 at 7:35

Okay, so water from above wont give you enough energy to get your vessel moving? Than combine it with something, that might offer a more lasting impression, like... Sodium (Na).

While that might collide with your "no much metal" rule and maybe with your expected level of technology, you still could put (handwave) your inventor or your society into a situation where the availability of an element that reacts in the same (or even more exothermic) manner with water would be possible.

But I have to admit that I cannot offer any calculations about how much thrust that could apply, nor what it would do to your vessel... at least, there would be enough water around to put out the fires....


This is actually a very similar reason as to why the sparks from a welder don't hurt. Individually they carry very little energy, just like a rain drop.

So you can't use the power of a falling raindrop to power anything beyond erosion.

As for a reservoir, now your fuel weighs more than the power it generates. So no matter what, the plan doesn't work. Water simply doesn't carry enough energy to do much of anything. Your cart would be better off with pedals and having the water collect in a small jug to quench the thirst of whoever is doing the peddling.

  • $\begingroup$ But a water wheel (rather than a paddle-wheel) has a number of reservoirs that fill up on one side and overbalance it. Progress may be slow but it will be inevitable. $\endgroup$ – chasly from UK Dec 13 '18 at 22:30
  • $\begingroup$ @chaslyfromUK Same problem, the wheel with water weighs more than the power it would generate. If the vehicle was just the wheel itself, it would work on a perfectly flat road. $\endgroup$ – Trevor D Dec 14 '18 at 14:03

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