Introduction: Build a Baking Powder Submarine

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About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

I have long wanted to build a baking powder submarine from commonly available materials. I had one of these in the 1950s when Kellogg's gave them as a prize in cereal boxes. Those were a little over two inches in length. A couple of years later l saw a much longer one in a store window. Mr_o_uk got me thinking about it again with his Baking Powder Diving Submarine Instructable, but his is a concept submarine he has not actually built. There are some practical problems I had to solve before my concept would be a working bathtub submarine that goes to the bottom, rests there, rises, discharges a bubble of CO2, and sinks to the bottom again in endless cycles until the baking powder is depleted. I have done that in a way someone reading this Instructable can replicate.

Step 1: Materials and Tools

The bulk of the submarine is made from a gray PVC nipple nominally 1/2 inch by 6 inches. (The actual internal diameter of the PVC is a tiny bit more than 9/16 inch.) 

All materials--
  • 1/2 x 6 gray PVC nipple
  • Wine bottle cork (synthetic [plastic], not real cork)
  • Sheet metal screw
  • Axle cap for a 1/2 inch axle
  • 5/8 inch internal diameter (nominal) braided rubber hose (actual internal diameter measures about 9/16 inch
  • Hot glue
  • Finish nail
  • Clear finger nail polish
  • Baking powder (not baking soda)
Tools
  • Fine tooth saw
  • Sandpaper
  • Screwdriver or nutdriver
  • Drill and bits
  • Dremel tool and burring bit
  • Grinder and wheel
  • Knife
  • Hot glue gun
  • Side cutter pliers

Step 2: Saw Threaded Ends From the PVC Nipple and Fit the Conning Tower

Saw the threaded ends from the PVC. One will be discarded and the other will become the conning tower on the submarine.

Place a piece of sandpaper over the remainder of the nipple. Move one of the threaded pieces over the sandpaper until its contour fits the contour of the PVC. See the second photo.

Step 3: Prepare to Attach the Conning Tower to the Submarine Hull

The conning tower on a submarine is usually a bit forward of midships. Decide where you would like the conning tower to be on your baking powder submarine and drill a hole for a #6 or #8 sheet metal screw in the PVC. The screw does not need to be longer than 3/8 inch, but a little longer is not a problem. Turn the screw into the PVC, but do not seat the head of the screw. Leave it a couple of turns from seating fully. The screw head will provide a good grip for the hot glue that will hold the conning tower in place.

Step 4: Make a Ridge Inside the Conning Tower

Hot glue may adhere to PVC well enough, but a recessed physical surface the glue can flow into before it hardens is more certain, especially if something will become a play toy for children. I used a burring bit on a Dremel tool to make a recess channel around the inside of the threaded piece that will be the conning tower and made it on the fitted side that will join the hull of the submarine.

Step 5: Attach the Conning Tower

Hold the conning tower piece over the screw head and deposit enough hot glue to cover the screw head. Make sure the hot glue has reached a very warm temperature and flows well. Wait for it to harden, or set it aside where it can harden undisturbed. (One end of my nipple was deformed into an oval. I used this for the conning tower. In retrospect, I could and should have used the threaded end that was not deformed. But, it all worked fine.)

Step 6: Make Bulkheads

Some bulkheads will be needed in the submarine to keep air and water separated. By hand I ground away the circumference of a plastic wine bottle cork using a power grinding wheel. Go slow and take it in steps so the finished product is as round as possible. I ground the cork down until it was just a little larger in diameter than the inside of the PVC. (I found this process went better when I rotated the cork into the rotation of the grinding wheel rather than with the rotation of the grinding wheel.) 

See the second photo. It was also easier if I ground back on the cork about 1/2 inch at a time. Then I cut pieces off of the reduced portion of the cork to make buttons about 1/4 inch thick. Just a little less than 1/4 inch is good, too. In the end, you will need five of these buttons. (One for the top of the conning tower may need to be a bit more than 1/4 inch thick.)

Step 7: Conning Tower Ballast

You want the submarine to stay upright in the water. The PVC conning tower makes it top heavy. Correct that by grinding down the circumference of the wine cork and inserting the reduced portion into the conning tower. Trim the cork flush with the top edge of the conning tower. That was enough to keep my submarine upright, but not so much that the submarine cannot be made to sink to the bottom of a vessel filled with water later.

Step 8: Find the Balance Point for the Hull

Use a round rod, like a drill bit, to find the approximate balance point of the submarine with the conning tower in place. Mark the bottom of the hull where the approximate balance point is. (When I made this photo I had not yet added the conning tower ballast from the previous step.)

Step 9: Make a Big Hole in the Bottom of the Hull

It is time to make a hole in the bottom of the hull to accommodate the baking powder chamber and bell cap that seals it and holds the CO2 bubble until the submarine surfaces. Use the largest bit you have to make a preliminary hole. I had a 1/2 inch bit. I used a Dremel tool and a straight burring bit to enlarge the hole so it would fit the rubber hose I bought, which measures 13/16 inch on its outside diameter. See the second photo. The fit as shown is not airtight, but I will seal that later.

Step 10: Fit and Cut the Hose

I have pushed the rubber hose into the hole made for it in the hull of the submarine. In my hand I am holding a 1/2 inch metal axle cap. These are used on children's wagons and tri-cycles to keep the wheels on the axle shafts. I got a package of two at Lowe's in their small parts bins in the hardware section of the store. (For people outside the USA, Lowe's is a large building supply chain of stores, but you can find axle caps many places.)

An axle cap looks like a man's hat. Cut the rubber hose just below the brim of the hat as it is shown in the photo. That allows enough hose to glue into the hull, to leave a chamber for baking powder, and to receive the axle cap for a good seal.

Step 11: Improve the Glue's Function

Just as with the conning tower, I want to add something the glue can really grasp for a good hold when children play with this. I drilled a hole on each side of the rubber hose near one end and threaded a finish nail through the hose. Then I cut the excess nail away on each side and ground the pointed ends flush with the outside of the hose. The nail inside the walls of the hose will be encrusted with hot glue.

See the second photo. With the rubber hose pushed into the hole at the bottom of the hull and the end with the finish nail in first, cut two wine cork buttons. Stuff one into each end of the submarine hull as far as each will go. A dam is needed to keep hot glue from flowing too far inside the hull. Squirt hot glue into the hull through the open end of the rubber hose. Take your time so it can flow into all of the nooks and crannies. Add enough hot glue that the finish nail is encased in it. Do not overfill, lest there be no room later for baking powder between the hot glue and the axle cap. Give it time to cool fully.

Step 12: Make a Full Seal

Fill the exterior joint between the hull and the rubber hose with fingernail polish. Let it harden. If the joint between the hull and the rubber hose is not sealed, CO2 from the baking powder could escape, or water could leak into the air ballast that will be discussed in coming steps.

Step 13: Drill the Axle Cap

The axle cap needs a hole in its center. How large the hole is determines how much water reaches the baking powder to make a bubble, which determines how fast the submarine is able to rise and fall in the water. The Spudmarine is a baking powder submarine from MAKE Magazine that used no cap over the baking powder and it cycled fairly rapidly. 

I chose this design because it is what I remember from the free miniature diving submarine in my breakfast cereal. I made the hole in the axle cap 7/64 inch in diameter. In practice, my submarine rises after about 30 seconds, sits on the surface for 10 or 15 more seconds, discharges a bubble and sinks to the bottom. But, that was in a shallow bowl about six inches deep. If I were using this submarine in a fish tank, travel time to the top would be longer and the time spent at the top before discharging a bubble would likely be lessened.

Step 14: Air Ballast

The hot glue used to cement the rubber hose into the hull effectively sealed the front of the submarine from its rear section. Although it may not be strictly necessary, I decided I wanted air to flow freely between the two for purposes of balancing the submarine along its length. I used a drill bit that is long enough to do the job, but not large enough in diameter to interfere with anything I have installed inside the submarine. Remember there is a steel screw that holds the conning tower to the hull. If the ends of the submarine were a clock face, I chose to drill a hole at either 10 o'clock or 2 o'clock. Blow through the hull when finished to know if the hole went through. Then hold your hand over the opposite end of the hull and blow. There should be no indication air is escaping anywhere. If air is escaping, determine where and seal it with hot glue or fingernail polish. 

Notice that in the photo the axle cap is inserted into the rubber hose. It makes a nice snug fit and a good seal.

Step 15: Tuning the Air Ballast

Make two buttons from the wine cork. Insert one into each end of the hull. Begin with them so they are just flush with the ends of the hull. Place the submarine into a bowl of water. You want to be sure the conning tower remains upright. You also want the submarine to be as near to level in the water as possible. It will probably float on the surface of the water. Gently nudge the buttons to push each just a little farther into the hull. Push each no more than 1/16 inch at a time. You want to find the point when the submarine is able to sink on its own. It should remain near to level at all times. It should not sink too rapidly.

If you happen to push one of the buttons into the hull too far, drill a small hole into the button and thread a wood screw into the button. Use a pair of pliers to pull it back out. If the hole is big enough for water to get inside the hull, use a new button or seal the hole with a toothpick or some fingernail polish. 

This step is the trickiest and the most important of the whole project. But, it is not difficult if you work carefully.

The wine bottle opener we use is electric and it makes a hole through the entire cork's length. The hole seems to seal, but we have found wine drippings in our refrigerator when bottles are stored on their side with the cork in the bottle. I plan to use a little fingernail polish on the ballast buttons to insure that water does not seep into the hull later, but I will need to wait until the submarine is dry after testing and initial use.

Step 16: Fill'er Up!

The tip of a table knife makes a good tool for loading baking powder into the submarine. Fill the cavity in the rubber hose to leave about 5/16 inch empty below the top edge of the hose. Insert the axle cap so there is a cavity for a bubble under the submarine as if the cap were sitting on a man's head like a hat. (Some baking powder may blow out through the hole in the axle cap.)

Put the submarine into a vessel of water, turn it on its side to discharge any bubbles trapped under the metal cap, then let it sink to the bottom and wait. As I mentioned before, my submarine rises after about 30 seconds, sits on the surface for maybe 15 seconds, and a bubble escapes from the side. It does not roll over on its side like I remember from the cereal box submarines. Then it sinks to the bottom and goes through the cycle again. I have found the action of the baking powder in the water is a little aggressive at first. Sometimes my submarine surfaces and discharges a bubble, but the baking powder is working so aggressively that the submarine does not sink again. I have had to tip it to release more of the bubble, sometimes twice. Then the baking powder settles down and the submarine rises and sinks as it should. Perhaps I need to experiment with tampng the baking powder more tightly.

It would be possible to paint the submarine yellow in honor of The Beatles and their 1966 hit song. That would be whimsical. I would not want to risk disturbing the ballast, though.

Here you can find a lot of information about baking powder submarines , especially if your submarine does not work properly.

The second graphic shows a cutaway drawing of the various parts of the submarine, just for clarity. I would suggest cutting the screw at the base of the conning tower shorter so there is not so much need for hot glue holding the rubber hose in place. The way the hot glue is shown in the drawing shows an excessive amount of glue. That part really does not correspond to what I actually did quite so much.

Step 17: Video

Here is a link to a video I took to show the submarine in action. I purposely filmed it in silhouette because I got a sharper image with fewer reflections. You can also see the bubble form under the submarine, and discharge. The submarine sat on the bottom of the glass bowl for almost two minutes before there was a bubble large enough to lift it. This video gives you an idea of how the submarine works. Clicking on the photo will not take you to the video. Click on the orange hot linked words in the first line in this step. The video is about a minute and a half in duration.

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