SpudFiles

I was thinking of making some kinda of super water cannon, something along the lines of backpack mounted PVC tank > largeish diameter hose > modded sprinkler valve > maybe a footish of 1/2" pvc. i'll probably use about 60-80 psi. any reason that this shouldn't work? or any suggestions?

A ball valve will do, you don't need a sprinkler.

The reason it won't work well is because you forgot to add a nozzle.

60-80psi is unnecessarily high. You shouldn't need more than 40.
Too much pressure will result in turbulent water flow (more like a mist than a stream) and a painful close range jet.

This is the workthrough of the one I made. It started quite complicated, and I got very far before realising I could create an excellent cannon from only 4 parts. It's definitely better than any I've ever owned.

psycix wrote:The reason it won't work well is because you forgot to add a nozzle.

what shape would it have to be? like just a hole in an endcp or something?

Like... a nozzle. A hole, but smooth. Click here to see what I mean.

If you can, find a real firehose nozzle.

Biopyro's right about the pressure. In addition, if you do some tests you'll find that doubling the pressure would increase performance less than doubling the diameter of the nozzle.

Conical shaped nozzles (like some fire hose nozzles) are best as they have the least losses. But holes in endcaps aren't bad if drilled cleanly. Most everything I use is a hole drilled in an endcap.

De Laval nozzles are for gas flow, specifically for trading pressure and temperature for velocity. Don't try something like that water as you won't see any benefit.

Using sprinkler valve would actually reduce your performance. Opening time is not important when the duration of the shot is on the order of seconds. Ball valves offer unobstructed flow, are readily available, and inexpensive. They unfortunately are not as easily adapted to triggered operation as other approaches, but I have not found this to be problematic.

you'll find that doubling the pressure would increase performance less than doubling the diameter of the nozzle.

That's because when diameter doubles, sectional area doubles twice...

I think my point was clear, but I understand what you're saying. Look at area then. At steady state the flow of an incompressible fluid through a water gun follows an equation like Q = C * A * sqrt(dP / rho) where C is a constant, A is the nozzle orifice area, dP is the pressure differential, and rho is the fluid density. The influence of doubling the nozzle orifice area is greater than that of the pressure in terms of the flow rate.

Personally, I was thinking of copying the prototype of the Super Soaker 50. There was a picture of it in either Scientific American or Popular Mechanics, pictured next to the re-released Super Soaker 50.

The other design I really want to try and reverse-engineer is the water cannon they have at Schlitterbahn. They seem to have a piston mounted in the barrel and use a fairly wide nozzle to create an effect not unlike an airzooka (more accurately a vortex gun, but different terminology than we use). It ends up firing an almost coherent ball of water (which has quite a bit of KE to it, if you ask me) about 30 feet up to the decks of a fake ship, which tells me that on a level plane it will get a good 50-60 feet on a high angle lob. It was dang impressive, and really freaking fun.

That would be incredible seaking, but can't imagine what it might look like? Surely in two such different fluids it wouldn't work?

D_Hall already made a beast one, long ago.

http://thehalls-in-bfe.com/UberSoaker/ubersoaker.html

It just looks like an M1919 With a barrel the size of a softball gun. It's quite impressive. And even if that's NOT the effect it uses to achieve a water ball, it still works, whatever they did. And well enough to send a 180 pound teen staggering back four feet from the impact.

SEAKING9006 wrote:It just looks like an M1919 With a barrel the size of a softball gun. It's quite impressive. And even if that's NOT the effect it uses to achieve a water ball, it still works, whatever they did. And well enough to send a 180 pound teen staggering back four feet from the impact.

is this what you speak of

VERY close to it. But a bit longer on the beak and it had a barrel shroud. Not to mention an actual 1919/Maxim/M2 type grip+trigger. There was also a release valve under the aluminum shroud on the controls that you use to clear a jam.

But I'm sure they offer different models. So yes. That IS what I refer to.