Build a Water Mortar II : Design Alternatives




Introduction: Build a Water Mortar II : Design Alternatives

About: My name is Carl, I'm an engineer. I like to build things and solve problems. I like learning how other people build things and solve problems. I like to laugh, and look for the humor in any situation. I...

This is a followup to my Build a water mortar instructable. It shows some design alternatives tried this weekend.

Step 1: Piston Cap Alternative

I had some great feedback on the original instructable and wanted try out a suggestion by thinkahead for a different way to construct the piston. The suggestion was to get rid of the wooden plug at the upper end of the inner pipe, substituting a cap at the lower end instead. I also wanted to try filling the inner pipe with expandable foam. Here are the ingredients.

Step 2: Trying the End Cap

First thing I did was remove the wooden cap and put the end cap on the lower portion of the inner pipe. This worked pretty good. You do have to do a "priming" step initially where you suck water up, point the mortar at the sky, and push the air out, but then you're in good shape and things function pretty much the same as before. I like this better than the wooden plug overall.

Step 3: Trying the Foam

I added the expanding foam to the inner pipe and let it set for a couple hours, then sanded the excess off flush. I set it aside for a while more but couldn't stand waiting the whole 8 hours you're supposed to for a full cure! :)

This type of foam starts curing when exposed to the moisture in the air, so the outer layer hardens quickly, but the parts that are buried take a lot longer. When I tried it out I got a couple good shots, then water was able to make it down past the cured foam to the part that was still uncured. The uncured foam worked its way out and fouled up everything resulting in a total loss. Sigh...

This clearly didn't work out like I expected. The idea still has merit, but I'd use a two-part expanding foam if I did it again, so that the cure is not dependent on exposure to air. On the other hand it works okay without the foam, so try it that way first.

Step 4: New Inner Pipe Design

Here's a different inner pipe idea. Experimentation in the PVC aisle revealed that the end cap for 1 and 1/4" pipe would fit fairly well into the 2" outer pipe. I decided to try cutting an o-ring groove into the end cap.

First picture shows the end cap and the tool I fashioned to hold it. The tool is made from a couple of discs cut from 3/4" with a hole saw. The discs are threaded onto a 1/4" x 20 bolt, and a nut is used to tighten them down. Next step was to wrap the discs with masking tape until the end cap fit snugly.

Second picture shows the end cap and tool assembled.

Third picture shows tool mounted on drill press, ready to turn the groove. Doing the groove this way was a little easier than the original approach, not sure if you could pull it off with a hand drill though.

Fourth picture shows the fit test with the finished groove. The pointy tool in the foreground is a dental pick from a surplus store. It works great for picking o-rings out of grooves.

Fifth picture shows modified end cap glued onto pipe. Note that with this approach we've done away with the wooden plug (though we still had to use them to make the tool).

Sixth picture shows the fit test of the new inner pipe. Looks good!

Step 5: New Mortar Design

Since I was trying out a new inner pipe design, I thought I'd try a couple new outer pipe ideas as well. I made the outer pipe longer, to hold more water, and I added handles at the base, to help apply more pressure. Here's the new design compared with the original.

The handle is a 2" PVC cross, with a couple short 2" pieces. The cross is attached to the base of the outer pipe. The inner pipe runs through the cross and into the outer pipe.

Step 6: Trying It!

Mixed reviews on this design iteration...

It's impressive looking.

It's unwieldy for little guys, the original design is more practical for them.

The handles help, but getting maximum pressure on them while staying on target is a challenge.

I'm going to try shortening the outer pipe to bring the handles up higher and see if that makes it easier for an adult to control, maybe an under-the-arm thing...



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    26 Discussions

    Thanks for putting this information together, particularly your specifics the O-ring size and suppliers. I use water guns similar to the water mortar on rafting trips. They're handy not only for squirting each other, but for rinsing off sandy or muddy boat surfaces. I made some changes to the fabrication and design that work quite well - better than the commercial water guns I own. First, I used a 1 5/8" hole saw wrapped with tape until it's a snug fit into one end of the 1 1/4" pipe coupling. Put the hole saw into a drill press and you have a very easy lathe setup to cut the O-ring groove into the coupling. Once the groove is cut and tested, glue a 1 1/4" plug into the grooved end of the pipe coupling. You now have a piston to glue onto the inside tube. Take a 2" cap, mark and punch the center, then cut a 1 3/4" hole with a hole saw. This is your retainer that will keep the inside tube straight and prevent the gun from coming apart. Slide this retainer over the inside tube with the dome facing the handle end of the tube. Install a cap on the inside tube. Grease the ring and install this assembly on 2" pipe. Use screws or set screws that are just long enough to secure the retainer to the 2" tube. The retainer can then be removed to later service the O-ring as needed.


    for aiming why not have a hose or tube attached to the nozzle, and have one kid pumping whilst the other holds the hose and aims?

    I like it. We need to identify a readily available source of rubber donuts, any suggestions?

    Big rig and auto stores carry some for shock absorbers. You may have to grind to size.

    I'm picturing rubber bushings? I replaced some of those in the front end suspension of my car, had to do some grinding/sanding to get the aftermarket parts to fit, not fun. I think if we manage to find exactly the right thing from an auto parts store it's going to be more than we want to spend. All the guys out there with lathes are laughing at us right now... :)

    Also you may find in the alternative that layers of sheet rubber from cut up pieces of inner tube will work and if not custom molded donuts from polyurethane liquid rubber marine sealant.

    The challenge with all these ideas is that we want something that's going to fit pretty close to the part we want to turn, and we want the threaded rod going through it to be in the exact center. I like the idea of casting something up, we don't have much volume to fill, so a few dollars of material ought to do it. I can't come up with a good way to center the rod during casting, though. I have this nagging feeling that there's something readily available that would be perfect for this, we just haven't put our finger on it yet!

    Casting are no different than sheet rubber when it comes to centering. Its a simple matter of using a lathe.

    Okay, got it: a rubber stopper! You can get them from home improvement stores and drill your own hole, or scientific supply houses have them with a hole already in them.

    I've used "piston cups" from McMaster-Carr ( before. They wouldn't be good for this though because they seal too tight. The friction reduces performance noticeably when the force applied is relatively low (as in arm powered water guns).

    Check McMaster-Carr out for parts if you haven't before. They have almost anything you can think of. You might be able to find some real "rubber donuts" on there.

    Here's something you might want to consider though: . This is a pretty easy to build piston water gun.

    Thanks for the pointer to McMaster-Carr. Now, if only I knew what to search for... :) Nice design, I'm going to borrow some of the ideas and try scaling it up.

    The product you illustrated here is available in the plumbing section of Home Depot. I bought a few 3 inch plugs like this when I was replacing the floors in my home. When you remove a toilet, you have this big stinking pipe that lead to the septic tank. This plug stops the smell. I will recommend against using channel-locks/pliers to tighten the wing-nut that comes with it. They break. Go ahead and spend $0.10 to buy regular nuts while you are there. (This illustration here does not show wing-nuts.)

    When I saw your first posting I thought about using "Great Stuff" foam instead of the rubber gaskets. (I hate special order stuff when I can avoid it.) My thought was: 1. Take a section of the larger pipe about 6 to 8 inches long and put the end cap on it. 2. Line the inside with a garbage bag. 3. Fill about 3/4 of the way full of foam. 4. Insert the smaller inner pipe into the foam. 5. Let it cure 24 hours. I know... It's torture. I end up eating 1/2 my beef jerky before it finishes drying too. We're guys. We hate waiting. 6. Remove the inner pipe to find your perfectly fitted piston.

    I want to see the kid use the one with the cross-bar like a pogo stick! You may have to drill the hole in the cap off center to keep him from shooting his own face though.

    DO you really need NSF-PW (Pressure rated) stuff on this? Its just a water shooter, not a Pneumatic Spud Cannon

    1 reply

    Good observation. I started from a table of PVC pipe sizes and looked for combinations that fit close enough for an o-ring seal. This is the first combination I hit that was readily available in the PVC aisle, I'm sure there are others.