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Car Mounted Water Cannon
April 9, 2006 11:43 PM   Subscribe

I'm building a water cannon that shoots out from the grill of my car. Range is lacking, and I (probably) don't have the funds for a more powerful pump. I need a better nozzle, possibly one utilizing a blowoff valve to build pressure. Advise.

Here's what I have to work with: I'm powering the cannon with a 7 PSI carburetor fuel pump, which fits to 3/8 in. hose. I ran wiring thru the firewall and tore into my center console to affix an awesome red push button trigger below the center arm rest so that the passenger (or driver) can casually crook a finger and fire the cannon. For the hell of it, an orange LED on the dash lights up during firing unless the fuse is blown. The jet fires out of the center of my grill.

Everything is installed at this point, but the jet only shoots about 12 feet! We were hoping for more like 30 feet, although I didn't bother with calculations when I started throwing this together. The nozzle is makeshift - just the nozzle from a windex spray bottle fixed onto the end of the injector hose with superglue and sealant. The nozzle aperature is basically a pinhole, and the path of the water through the injector hose terminates abruptly in a wall with this tiny hole. I'm thinking that the cannon would benefit from a slightly larger nozzle aperature and a taper into the nozzle as opposed to a sudden termination. Will this significantly increase range? What sort of household items, such as pens, can be fashioned into good nozzles? I'm also thinking of using a blowoff valve to build pressure before the cannon can fire - the pump seems heavy duty enough to survive working against some back pressure. A solenoid valve would also work, although more wiring and another button would be required. A more powerful pump would help, but funds are lacking. Help me improvise and scavenge my way to finishing this project.
posted by Derive the Hamiltonian of... to Technology (29 answers total)
Cool idea, and we need pics.

Regarding the BOV idea... You won't be able to install a blow-off valve if you're car is naturally aspirated. They only work on vehicles with a turbocharged engine. They also face up, tend to look like the bell of a trombone at the end, and would be very difficult to re-route the blow-off from. But damn, if it wouldn't be cool to fire water out of my car at every shift... Except that it'd come back on me.
posted by disillusioned at 11:57 PM on April 9, 2006

disillusioned; I meant capping the "cannon" hose with a blow off valve so that it will not fire until the pump has created high enough pressure. Obviously, the BOV would have to release at about the maximum pressure of the pump, and the pump might not be strong enough to start with...

Sure, pictures will be forthcoming if this turns out awesome. I haven't checked out Projects yet, maybe it would be appropriate to post the finished product there. In the meantime, if you want to use your imagination, I drive a red 94 Volvo 940 wagon with a bunch of celtic knotwork and scrimshaw type stuff that my girlfriend and I have painted on the interior and hood for fun.

We want 30 foot range so that we can spray drunk frat boys and sorostitutes standing/vomiting in the middle of the street after the bars get out. They usually won't move until you've nearly run them over. Otherwise, lots of innocent squirting of friends and people in convertibles.
posted by Derive the Hamiltonian of... at 12:18 AM on April 10, 2006

I have nothing to say, this not being my area of expertise, other than this is super-cool and I lol'd at "sorostitutes".

Although thinking about nozzles, I imagine you could get something cheap and good at the hardware store, like the flanged nozzles they use for the gas hook-ups for bunsen burners and the like?

Seems like this might help.

I wish you the best of luck, and am looking forward to updates.
posted by exlotuseater at 12:47 AM on April 10, 2006

Some points that came to mind:

1. It's probably pretty hard to aim one jet of water using the movement of your whole car. Maybe a spray? More than one jet? You could have a cool "sweep" if you had equidistant jets across the front, but it'd be loads more work.

2. I'd look at incorporating an air compressor, a la Super Squirter. Compressor setups can be had inexpensively if you're patient. I don't know if you have the room for significant compressed air and water, but it's a thought. Sounds like a fun project.

Or, you could just buy one.
posted by RikiTikiTavi at 1:11 AM on April 10, 2006

I remember reading an article about the Super Soakers. Apparently, it's essentially impossible to shoot a coherent stream of water past a certain point, no matter how much pressure you use. They had to do a lot of engineering to get their guns to work even as well as they do, and from what they were saying, they're right at the physical limits of a water stream.

If a Super Soaker does what you want, it should be possible with your car, but you will probably have to tinker.

Have you thought about using compressed air or CO2 instead? You could just avoid the whole pump-in-the-car problem. This would involve some cost for filling the tanks, but unless you're doing a lot of spraying, it might actually be cheaper. (I have no idea what the pumps cost.)

Just a thought.
posted by Malor at 3:23 AM on April 10, 2006

Attach a small spare tire air compressor (powered from a cig lighter, how convenient can you get?) to a tank full of water.
Get creative for a fitting to attach it to your tank (hardware stores carry such things cheeeep)
Pressurize to whatever safe point you're happy with (your connections can let go quite forcefully if you overdo it).
If you can't rig a solenoid of some kind to release the pressure (which would be cooler), try a water faucet or something similar.
Just remember, the lower the resevoir gets, the more pumping the compressor will have to do.
Good luck.
posted by IronLizard at 4:05 AM on April 10, 2006

A compressor is a good idea, but... The last time I tried to use one of those little 12 V car adapter compressors for a somewhat similar application, it seized and died way too quickly. I originally steered clear of compressors because of the perceived expense for a good one and a strong tank and lines to contain the pressure.

Compressed air or CO2 doesn't really appeal to me for some reason, but I could be sold on a sufficiently cheap and effective setup.

My prototype used an old windshield washer fluid pump and achieved about 10 feet range, so I thought that if I picked up this carburetor pump, which generates about 16 x the PSI, I would be good. The range is only slightly better, so obviously something else is going on besides pressure... Maybe I'm at the cohesive jet limit for the nozzle or pressure I'm using, as Malor suggests.

I have tons of space under the hood, this car is big. Expense is the primary issue.

Really, we'll almost exclusively be spraying people we know, but fall down drunks in the middle of the road will definitely be fair game for a quick squirt.
posted by Derive the Hamiltonian of... at 7:42 AM on April 10, 2006

I did the same thing with the rear winshield washer sprayer on the rear hatch of my 1976 red Volvo station wagon. The car had a seperate tank for the rear sprayer. I replaced the pump with a bigger pump from a bus. I went through a couple of different nozzles but eventually settled back on the original nozzle, just widened out a little with a needle. The nozzle sent a stream of water to my right that was the perfect balance of letting the victim know they were getting wet without actually soaking them. Good times.
posted by leftoverboy at 8:07 AM on April 10, 2006

A189Nut - did you miss the "note" at the bottom of your screen?

Here, I'll repeat it for ya: note: Ask MetaFilter is as useful as you make it. Please limit comments to answers or help in finding an answer. Wisecracks don't help people find answers. Thanks.
posted by UnclePlayground at 8:21 AM on April 10, 2006

fyi: I had a VW Thing that used the air pressure of the front truck spare tire to spray the windshield washer fluid. You would think it would drain the spare but it didn't.
posted by linklog at 8:33 AM on April 10, 2006

er front truck spare tire = front TRUNK spare tire
posted by linklog at 8:41 AM on April 10, 2006

Are you firing it while moving? Obviously that is going to cut range. Also you are going to end up taking most of the strike on your own car. Have you tried shooting backwards? Sideways?
posted by Pollomacho at 8:46 AM on April 10, 2006

As you've discovered, it's hard to get a mechanical pump that can get the pressure high enough to get good distance. It can be done, but it gets expensive fast because the seals and tolerances are much more restrictive.

At the level you're willing to spend, compressed air is probably your best option. It's really quick response (in the sub-milisecond range) and develops good pressure. Pressurize your resevoir and actuate with a stopcock valve or solenoid. The CO2 cartidge idea is a good one. The only moving part then is a valve. By the way, plastic soda bottles wrapped with tape make great pressure vessels.
posted by bonehead at 9:20 AM on April 10, 2006

Pollomacho: Maneuvering at about 5 mph, the jet has a range of about 12 feet. Yes, by the time I'm doing 15 mph, the jet is starting to arc back onto the hood, but we're really interested in having a front mounted cannon. I ran some spare wiring into the console when I did the first button, so the car is already wired to have another button firing a sideways jet, which we plan on doing eventually.


A189Nut: If squirting people with my car is the incident I finally get arrested for... meh. It will be fairly amusing if that is the only thing that ends up on my record. Lawrence is not the kind of town where I would face assault charges for a squirt gun.
posted by Derive the Hamiltonian of... at 9:22 AM on April 10, 2006

OK, can anyone think of something I can scavenge a solenoid valve from? An old water heater maybe...
posted by Derive the Hamiltonian of... at 9:40 AM on April 10, 2006

I had a VW Thing that used the air pressure of the front truck spare tire to spray the windshield washer fluid. You would think it would drain the spare but it didn't.

IIRC, the VW's windshield washer quit operating after the tire's PSI dropped below a certain level and wouldn't draw air from the tire after it reached that point. I owned several Beetles and buses in my youth and always rotated the washer jets to the side of the car so I could spray people as I drove by.

I'd go with the 12v tire compressor option. It would be a simple matter to hardwire one in behind the grille, just be sure to fuse it so you don't burn the car down.
posted by buggzzee23 at 9:40 AM on April 10, 2006

Those 12v Compressors are depressingly weak for the kind of stream you're tlaking about. I have a squirt gun (former fire extinguisher) that takes 90-100 PSI and shoots a pencil-thick stream of water 30-40 feet.

The (tiny) 12v compressor that I have took 15 minutes to fill a tire tube to ~60psi, and it seemed like it was close to burning out after 1. I wouldn't imagine it'd ever charge my squirt gun enough to be useful.

You're going to need a more serious compressor (maybe stored in the trunk) to get the kind of pressure that you'd need to shoot 30 feet. You'll probably need upwards of 50psi, depending on the width of your stream.

I admire your inventiveness. Don't be surprised if you find your car destroyed with baseball bats one morning if you spray the wrong drunk though.
posted by Four Flavors at 9:51 AM on April 10, 2006

Will you post the video of the squeegee guy flying through the air?
posted by weapons-grade pandemonium at 10:10 AM on April 10, 2006

At the level you're willing to spend, compressed air is probably your best option. It's really quick response (in the sub-milisecond range) and develops good pressure. Pressurize your resevoir and actuate with a stopcock valve or solenoid. The CO2 cartidge idea is a good one. The only moving part then is a valve.

Not a bad idea. Depending on how much room you have in the engine compartment (or if you're willing to use some trunk space and run a hose) you could get one of these portable compressed air containers. They're about the size of two bowling balls and you can fill them up with a compressor of whatever type. There's 12V DC compressors you can get as cheap as $12, perhaps you can get one that you cycle on and off as pressure drops. They're slow as all hell but this doesnt seem like a rapid-recharge weapon, particularly since you have a limited water supply anyway...

Once you have that you could simply toggle the valve open and shut and have the output from the tank feed into the (airtight) water receptacle. The output nozzle can have it's outtake at the bottom of the bottle and it can work like a sports squeeze bottle does.
posted by phearlez at 10:40 AM on April 10, 2006

1. Your nozzle is killing your pressure, and not in a good way. The tiny pinhole is creating a whole lot of backpressure, eating most of the energy from your pump (bad, you want the energy to go into increasing water velocity). A better designed nozzle will help a lot here.
Can you borrow (permanently) a nozzle from an old supersoaker? That would help. They are slightly larger, but I'm fairly sure they taper nicely.

2. I'm not sure your pump is suited to your application. (Looking at flowrates of typical carburetor pumps) You're just not getting any water out of it! As others are saying, use air pressure as your "pump", resulting in much higher flowrates. You can (and should try) a better nozzle, but assuming your pump does 19.2GPH (maybe, got off the 'net somewhere), you water velocity just isn't that high.

Enjoy the experimenting. I recommend an air based system!
posted by defcom1 at 10:42 AM on April 10, 2006

Small appliance repair stores and/or junkyards are good places to look for solenoids. I recently got a dozen or so ex-dishwasher ones for less than $20 per. You'll want to find something that works with a 12V DC system though---I'd phone a few autoparts stores and ask. You know NAPA guys are going to be totally into this.

For nozzles, check out the gardening section of you local hardware store. You could do much worse than using one of the old-fasioned needle-valve nozzles like this. You can probably pick one up for a few bucks.
posted by bonehead at 10:53 AM on April 10, 2006

Thank you defcom1!

Pressure and nozzle aren't the only significant factors, you also have to consider the amount of water moved by the pump (mass flow, I guess). The windshield washer pump isn't capable of much pressure, but it moves a lot of fluid compared to the fuel pump.
posted by Chuckles at 11:44 AM on April 10, 2006

I'm leaning towards a compressed air tank that I can just fill up at a gas station, a reinforced 2 liter soft drink bottle, a garden hose sprayer nozzle, and a solenoid valve. I'll just wire the 12 V switch circuit that actuates the pump into the solenoid instead. Alternately, a few attachments (Schrader valves? to plug CO2 cartridges directly into the reinforced coke bottle. Thanks all. Further advice or comments would be great.

What the hell am I going to do with this $40 pump? grrrr....
posted by Derive the Hamiltonian of... at 11:47 AM on April 10, 2006

What the hell am I going to do with this $40 pump? grrrr....
posted by Derive the Hamiltonian of... at 11:47 AM PST on April 10


A more complicated circuit, which would allow you to continue to use your pump. Pump from a reservoir, through your pump, to a sealed tank. Put a check valve at the inlet, so the pressurized water doesn't flow out. Then, a second line, through a solenoid, to the nozzle. Now you "charge" your system by putting water in the reservoir and running the pump. When the pump can pump no more, you will have a tank of pressurized water (Make sure there is air in the tank to pressurize)
If you've got a nice big reservoir tank, this should give you a little bit of a stream to play with. Still.. it's only at 7psi. Adding an air compressor, you could "boost" this pressure though... LOL - this is building a fire truck to get a supersoaker.
Good luck with the soda bottle. (And don't blow anything up if you're not expecting it. Pressurized air contains a lot of energy!)
posted by defcom1 at 12:14 PM on April 10, 2006

You can get solenoid valves from Home Depot, rather like this one. They are intended for use in automatic sprinkler systems. I can't remember if they're really cheap or just afforadable. They are ideal for your purposes.
posted by breath at 1:08 PM on April 10, 2006

Oh, and if you want to get more elaborate than a soda bottle, you could get together some PVC pipe and fittings and chemically weld them together to make an airtight chamber of virtually any shape and size. Very easy to do, and they take pressure up to the hundreds of PSI.
posted by breath at 1:10 PM on April 10, 2006

Where are you guys guying your cheap 12v compressors? Mine's been up to 120PSI several times and I regularly use it to test/clean fuel injectors at 40-80psi, hasn't failed or balked yet. (It's a MAXAir brand, for truck tires cost about 30). I made the adapter by drilling a hole lengthwise thorugh a bolt that I'd cut the hex-head off of.
posted by IronLizard at 1:53 PM on April 10, 2006

we did kind of the same thing on mud bog truck but used this and a selinoid and modified tip shot like 30 feet
posted by starvinron at 12:45 AM on April 11, 2006

I had a 1973 VW bus in which the windshield wipers worked off a pressurized tank (no pump) that was hidden in the front bulkhead, just in front of the passenger's feet. it had a standard tire valve and every few months you just gace it a shot of air pressure. It might be readymade for you -- check at a wreckers.

or, look into boat bilge pumps. A 1000 gallon / hour pump is the size of two fists and can obviously deliver a lot of pressure. New, only about 50 bucks, works off 12 volt, and already wired for a dashboard toggle switch.
posted by Rumple at 10:29 PM on April 14, 2006

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