# I Fluid mechanics question

1. Aug 31, 2016

### FlemishDuck

I'm working on an idea in product development, though being still a fairly new student in this i still am left with a lot of questions, perhaps anyone can aswer?

Say that i have a closed can with pressurised water lying in a pool of water. My question would be what the best method would be for this can to proppel itself forward using the pressurised water inside the can. Lets also not take into consideration such things as weight or flotation, consider that the height where thrust will be delivered is so near the surface that the pressure there is negligable.

Would the best method of proppulsion be simply a waterjet? Or would it be better to convert to the movement of a screw?

Last edited: Aug 31, 2016
2. Aug 31, 2016

### A.T.

If the can is just a cylinder, no.

3. Aug 31, 2016

### Nidum

If you release even a very small amount of water from the can the pressure inside would drop almost immediately to the pressure in the pool .

Making a propulsion device the way that you propose is not a realistic proposition .

There are other ways of making simple propulsion devices for use underwater though - what are you actually trying to do ?

4. Aug 31, 2016

### Staff: Mentor

The first thing to do is to look at similar devices which have already been built. In your case it sounds a little like a jet ski or a water rocket.

5. Aug 31, 2016

### Staff: Mentor

Or historic designs of torpedoes.

6. Aug 31, 2016

### FlemishDuck

So a jet of water would still be more effecient for propulsion than anything else? (comming from water pressure as energy)

So in what way ius it best used? I know the velocity of the watter passing trough the exit will increase as the exit becomes narrower and i guess you also have to consider local losses.

I would like to know how much i could proppell that can forward irrespective of the limited supply of pressure. I'm a student who has completed courses in fluidmechanics among others and i cna work with pressure or the Bernoulli energy equasion for ex, but i don't have the knowledge of dynamics to to calculate how much i would need to propell a light material of my choosing with the amount of pressure i can work with. Is there a way to know how much thrust i need in the velocity of the water that exits into the jet for a given volume/weight/shape with "drag"?

Quite right ... and yet i want you to go from the idea that this particular can provide's a constant amount of pressure until the moment it runs out completly.

Good tip, that will be usefull for me.

Torpedo's had engine's afaik. This can can only use the water pressure stored inside.

Last edited: Aug 31, 2016
7. Sep 1, 2016

### Nidum

It is not going to happen .

To use water as a propulsion medium for any really practical purpose you basically have to pump it continuously using energy from a separate source .

If you have to stay with the idea of the prepressurised can of water then you will need to think about using a spring and piston or gas pressure to maintain the water pressure .

Last edited: Sep 1, 2016
8. Sep 1, 2016

### FlemishDuck

It's a theoretical question, and i rather simply know how to solve it regardless of what happens.

9. Sep 1, 2016

Staff Emeritus
Which is it?

In any event, it won't work. There is no momentum transfer, so there is no force, so there is no motion.

10. Sep 1, 2016

### FlemishDuck

Thats right. No offense but "how to maintain pressure" was not my question.

You mean that regardless of the amount and size of water pressure inside and the lightness of the material it wouldn't propell forward a bit?

11. Sep 1, 2016

Staff Emeritus
You don't get a force unless water exits the can. To the extent that treating it as an incompressible fluid is a good approximation - and it is - it won't.

12. Sep 1, 2016

### cjl

I'd be inclined to say you'd get the best efficiency by powering a small turbine or piston engine, and using that to power a propeller. That having been said, unless you have a source of compressed air or something to maintain the pressure inside the container, you'll barely get any energy out of this, since energy storage is related to both pressure and volume change. Since water is very close to incompressible, the volume change is very small, and thus, the available energy in pressurized water is very low.

13. Sep 1, 2016

### bsheikho

The problem is that water doesnt have elastic characteristics similar to gas. therefore It wont store energy at a different pressure. To think of it, I'm not sure if a still body of water can have more pressure unless it is located at higher height (stored as potential energy).

Basically Boyles Law wouldnt apply

Alternatively: if you heat up the can, and evaporate the water, you'd end up with some gas, which can compress.

Now, If you have pressurised air in with the water, then now your talking. The compressed air will store your energy, if you merely had a can of compressed air, it would do the job. You could even attach a turbine on the exit side of the hole, such that the air being pushed out would spin the turbine giving some more propulsion I suppose. Not too sure.