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The 1950s were a golden age for the curious kid builder of all things balsa, plastic, Erector, and Lionel. Everything seemed to cost $1.90. I loved building models with my friend Merle down the street.

The Revell Company had the great line of Highway Pioneers plastic car models; you had to melt the axle with a hot knife to form a little blob of plastic that held the wheels on. Balsa wood airplane kits were common; some slipped together without glue and could be instantly tossed into the air, others had propellers you’d wind with a rubber band. There were wooden battleships with metal pins for cannons, decals, and other little details.

We had a soft spot for boats, since we lived a block from Lake Michigan. We made weird-looking boats from driftwood, which we’d pull along the shore with a long tree branch and string.

One day, Merle and I fished one of those free plastic submarines out of the bottom of a cereal box. Up and down it went in a small container of water with only a pinch of baking powder to create an air bubble for buoyancy. We loved it.

Fifty years later, I wondered if I could advance the design of that baking-powder sub and inject a little more fun by adding an electric motor. The answer is an unequivocal yes! Follow along as I share some model building ideas. You might learn some new skills along the way.

Sub Systems

The Kid Sub dives and ascends like the classic cereal-box toy subs, using baking powder to create bubbles for flotation. But unlike those toys, this one’s got forward propulsion from an electric motor, rudder, and dive planes, so it can perform powered dives, ascents, and turns, like a real submarine.

The circuit’s voltage is so low (1.5V) that immersing the switch and wires does not impair the sub’s function or present any danger. What does have to be waterproofed is the motor, which is sealed in petroleum jelly and beeswax.

Like a real submarine’s ballast tanks, the bubble chamber can contain water, making the sub negatively buoyant, or air, displacing the water and making the sub positively buoyant.

The powder tube contains the sub’s secret chemical mixture for vertical propulsion: ground-up Alka-Seltzer tablets.

The resulting air bubble forces water out of the bubble chamber, increasing the sub’s buoyancy.

The sub rises to the surface, relieving water pressure on the air release lever. The lever falls forward and unstops the hole in the bubble chamber, releasing the air bubble. Water rushes back into the bubble chamber, making the sub negatively buoyant. Submerged again, the air release lever is pressed back to its closed position by the water pressure caused by forward motion. Repeat.

Parts:

Soft drink bottles, plastic: NOS Energy Drink, 22oz (3) NOS has discontinued their blue bottles, but you can use any bottle that’s similar size with thick plastic walls, such as SoBe tea or elixir, 20oz (2)

Plastic knives (6) Dixie MEDIUM weight knives or with at least a 1/8″-wide spine at the back of the blade.

Propeller, one inch Dumas, (other small 1″ plastics boat propellers ok) 

Motor, DC, 1.5V, 7/8″ diameter. Any motor will work as stock varies from these companies: American Science & Surplus sciplus.com or Electronicsurplus.com

Wire, insulated, 26 gauge, 2′ total length

Blue or Transparent RTV sealant, 1 tube available in auto parts stores

Cyanoacrylate (CA) glue aka super glue

Baking soda to accelerate the CA glue

Zap Goo flexible adhesive aka Zap-a-Dap-a-Goo

Epoxy, two-part such as J-B KwikWeld

Urethane foam, two-part expandable such as Silpak SP 328-4. Dow, Gap and Crack ( Great Stuff ) looks good too but I suggest anyone using any of these products experiment with them first to see what quantities to mix.

Mold release (Mothers) Carnuba car wax as a mold release for the urethane foam and found it works perfectly, However, be sure the inside of the mold has no edges and apply ample wax and let it dry sufficiently (to where it is dry to the touch) Silpak R-80 with a Preval Sprayer to apply it is an alternative.

Liquid styrene glue Testors Liquid Cement for Plastic Or the non-toxic version 

Battery holder, 1×AAA McMaster Carr #7712K16

Square tube, brass, 1/8″, 0.014″ wall thickness, 12″ length K&S Engineering #151

Rod or wire, brass, 0.052″ diameter, 12″ length Detail Associates #2512 or similar

Rod, brass, With an ID to match your motor shaft’s OD

Acrylic tube, clear, 3/4″ OD × 5/8″ ID, 6 foot length cut to size McMaster Carr #8532K16

Hidden hinge Sonic Tronics #SOT130 Wholesaletrains.com

Machine screws, brass, 00-90×1/2″ (7) Walthers #947-1126

Hex nuts, brass, 00-90 (7) Walthers #947-1250

Battery, AAA

Switch, SPDT toggle, micro-subminiature Allied Electronics #G12AP

Plastic tubing, clear, ¾” OD × 5/8″ ID × 6″ length Grainger #4VXH9

Plastic tubing, acrylic, clear, 2½” OD, 0.65″–0.70″ wall thickness You only need about ½” total length.

Silicone rubber, two-part for moldmaking, such as Oomoo #25 or #30

Flexible tubing, 1/8″ OD, 1″ length such as aquarium air line, heat-shrink tubing, or covering from electrical wire

Corks, #5 (2) Fisher Scientific #07-781H or similar

Alka Seltzer tablets

Monofilament fishing line, 20yds

Coin, 25 cent or metal or plastic disc of the same size

Beeswax sold in hardware stores

Styrene sheet, 2″ x 1-½× 0.030″ thick  Plastruct #SSS-103

Petroleum jelly

Bolts, stainless steel, ¼-20: ½” (2), 1″ (1), with hex nuts (8) for ballast adjustments

Decals, self-adhesive, 1″ or 2″ letters such as Hy-Ko Products #MM series

EPDM sheet, about 3 × 3″ this is the same plastic used in blister packaging

Nylon plug, threaded, 3/8″

Test tube and cap, McMaster Carr #7012A44 and #6946A16

Corks, Size 6, McMaster Carr #9566K2

Sub-miniature toggle switch, Mouser Electronics # 633-G12AP

Tools:

Soldering iron and rosin-core solder

Hobby knife with new blades

Drill and small drill bits

Stepper drill bit for stern cap hole

Cup and pan to melt beeswax

Band saw or coping saw

Files, small

3/8″ × 18 NPT tap

Steps

Step #1: Clean and cut the bottles.

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  • Not just any plastic bottles will do. The NOS bottle looks like a submarine and has adequate thickness for the cutting and handling necessary to build the Kid Sub. The SoBe bottle has the thickness and long neck needed for the bubble chamber.
  • Remove the bottle labels. If necessary, heat with warm water to soften the glue.
  • Cut 2 circular paper templates, 1½" and 3" in diameter, and trace around them to mark the main holes in the top and bottom of the sub’s main NOS bottle (the hull) as shown.
  • Make a hole! Using a hobby knife, cut all the holes in the hull, following the diagram.
  • CAUTION: When cutting thin plastic, the knife can jump or cut too fast if you’re not careful. (My thumbs still have scars from many a knife slip when I was a youngster.) Always cut slowly and away from your fingers, using a sawing motion.

Step #2:

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Cut off the top off the second NOS bottle, following the diagram. I used a small band saw. This bottle-top is used as the alignment and mounting bracket for the small 1.5V DC motor.

Step #3: Install the motor

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  • Prepare the motor for waterproofing, using electrical tape and Zap Goo as shown in the diagram. Spin the shaft while the Zap cures, to prevent it binding.
  • Solder two 26-gauge wires to the motor’s contacts.
  • NOTE: Most small DC motors have no required polarity. If the propeller spins the wrong way, you can simply swap the positive and negative battery connections and the motor will spin the other way.

Step #4:

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  • Mount the motor. Drill a hole in the center of the NOS cap slightly wider than the motor’s shaft, and test-fit to be sure the shaft can turn freely.
  • Then mount the motor in the NOS bottle-top, packing the motor shaft and the cap with petroleum jelly, as shown in Step 2 and 3's diagrams. The motor I used is 1½" long and 7/8" in diameter. If yours is narrower, improve the fit by wrapping it with more electrical tape.
  • Seal the motor. In this low-voltage system, water will cause no damage to the wires or their electrical activity. The motor, however, needs to be watertight. Melt the beeswax and pour it over the back of the motor to seal it completely into the modified NOS bottle as shown in Step 2's diagram.
  • CAUTION: Beeswax can catch fire if overheated or spilled on your stove, so heat it in a double-boiler arrangement as shown to make this procedure safe.
  • Glue the motor assembly into the stern of the sub’s hull as shown in Step 2's diagram, using RTV (room-temperature vulcanizing) rubber sealant.
  • NOTE: Don’t use common bathtub caulking to assemble any part of this sub. Bathtub caulking deteriorates when submerged in water for any length of time.

Step #5: Build the bow planes.

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Cut the diving planes from 2 plastic knives as shown in the diagram. Make the curved ends match the hull curve where the planes will be mounted.

Step #6:

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  • Cut a 3½" length of 0.052" brass rod, thread it through its holes in the hull, and glue the bow planes to it as shown in the diagram, using CA super glue accelerated with a dash of baking soda.
  • Cut a 3/8" ring from the 2½" acrylic tube, then cut it in half. Glue one half to the bow planes as shown, saving the other half for the next step. (All plastic-to-plastic gluing can be done with the liquid styrene glue, aka model cement or plastic cement.) This bow plane control handle allows 2-position adjustment of the bow planes, in conjunction with the front ballast bolts.

Step #7: Build the rudder assembly.

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  • The small ridge on the back of the knives makes them an essential component for the rudders.
  • Cut the remaining 4 plastic knives as shown in Step 5's diagram to make the rest of the attachments. Carefully cut the small holes in the attachments, using a new sharp-tipped hobby knife knife to ensure accurate alignment.
  • Cut notches in the remaining acrylic half-circle as shown in the diagram to make the rudder control.

Step #8:

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  • Assemble the rudder, following diagrams from Step 7 and this step. Be sure the rudder has a small gap at the bottom so it can swing back and forth without touching the lower rudder attachment. If necessary, install a small spacer of 1/8" tubing on the rod.
  • Glue the upper and lower attachments to the sub hull with RTV sealant; it dries flexible, and can be disassembled if the alignment is incorrect.
  • Note the unique rudder control device; you can rotate it to 5 positions to set up the sub for straight-ahead or left or right circular movement.

Step #9: Install the propeller.

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  • Any small propeller will work in this project. I used a two-blade propeller because I had one, and in the end, it worked.
  • Cut a bit of brass rod the same diameter as your motor shaft, and epoxy it into your propeller. Before epoxying, you can bend the end of the rod to prevent it from breaking loose and spinning free, but this precaution may not be necessary with the low-speed, low-torque motor you'll be using.
  • Connect the motor shaft and propeller shaft using a short piece of 1/8"-diameter plastic air tubing or heat-shrink tubing. Glue it if necessary. I used air tubing and discovered that it held in place with no glue required.

Step #10: Make the foam ballasts.

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  • Ballast typically means weight that decreases buoyancy, but sailors also speak of “positive ballast” — objects that add buoyancy or flotation. The Kid Sub won't float correctly without 2 foam ballasts installed inside it. You'll cast these in urethane foam.
  • Cut the top off the third NOS bottle and use it to make a mold, as shown in the diagram. The bow float must fit around the sub’s battery compartment, so the mold has a 2½" length of ¾" plastic tubing epoxied into the center.
  • Apply mold release to your mold. Use more than you think you’ll need. The urethane will hold fast if too little release agent is used.
  • Cast the foam about 1" deep. I chose 2-part urethane foam but you can also use spray urethane foam that’s meant for filling gaps and cracks in homes. Two-part urethane is simple to use, but it needs some testing first. There’s a time element in mixing it, and it expands after you pour it. Test it first to see how much you'll need to make these two small bow and stern floats.
  • When the foam is set, stick a knife into it and carefully pull it out of the mold. One good casting can be cut in half and you're done.
  • Glue the foam ballasts into the bow and stern with RTV sealant as shown in the 2nd diagram.

Step #11: Make the battery tube.

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  • Drill a hole in one cork just big enough to pass the AAA battery holder’s 2 wires. Assemble the battery tube from the corks, the 25-cent coin, and ¾" plastic tubing, then pack the battery and holder inside with petroleum jelly as shown in the diagram.
  • The battery chamber fits in the sub’s nose, held in place by the bottle cap; the quarter keeps it from falling inside the sub. It doesn't have to be completely watertight, as this low voltage isn't upset by water contact. I try to seal the chamber as best as I can, and I just don’t worry about a little water leakage.

Step #12: Build the bubble chamber.

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  • The bubble chamber is the heart of the Kid Sub. Here, a chemical mixture combines with water to form the carbon dioxide bubbles that make the sub rise to the surface.
  • The diagram shows the configuration of the bubble chamber. The left photo shows the basic components of the chamber while the right photo shows the assembled powder tube.
  • The styrene bubble baffle dimensions are 2" by 1½". The plate is elliptical in shape to fit at the diagonal shown in the drawing and slotted in the center to account for the test tube to fit through it. The angle of the plate is not critical as the gas will collect successfully at the upper end of the plate no matter what the angle. Styrene sheet is easily cut with an X-Acto knife after drawing the shape on the sheet. You can snap the styrene without having to cut all the way through it.

Step #13: Grind the Alka-Seltzer tablet.

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  • The secret formula for our sub is sodium bicarbonate (baking soda) and citric acid, available in that fizzy little Alka-Seltzer tablet. Powder it well using a mortar and pestle.
  • Keep your powder dry. Baking powder is made of an alkaline agent such as sodium bicarbonate (baking soda) combined with an acid salt such as tartaric acid. When combined by wetting, they react to release carbon dioxide (CO2) gas. Baking soda alone won't react in water. The Kid Sub uses Alka-Seltzer tablets which, like baking powder, contain sodium bicarbonate and an acid, in this case citric acid.

Step #14: Add the air-release arm.

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  • Carbon dioxide bubbles pass through the holes in the side of the powder tube to accumulate at the upper front of the chamber. The forward movement of the sub keeps the stopper closed over the bubble chamber exit hole. When the air release arm reaches the surface, it falls forward and releases the trapped carbon dioxide bubbles, allowing the sub to sink again. The essence of this sub is that it will use the carbon dioxide produced by the Alka-Seltzer powder to propel it vertically up and down in the water. Various views of the completed bubble chamber are depicted in the photos.
  • This chamber is made from the upper portion of a SoBe soda bottle. The electric motor pushes the sub forward. The mechanism at the front of the chamber, the “air-release arm”, moves forward and backward allowing a small silicone rubber stopper to release the trapped carbon dioxide so the sub can sink. You can cast the small rubber stopper from Sugru or Oomoo, which is a two-part silicone mold material. You can cast any small button- shaped object that fits on top of the hidden hinge to make the stopper.
  • The holes for the air release arm machine screws must be carefully drilled through the square tube. Also, a slot must be cut in the top of the air release arm to allow the pressure plate to be held firmly. The pressure plate is pushed backward when the sub is under power. This is how the carbon dioxide bubbles are held in the bubble chamber until the sub surfaces. Notice the hole in the bubble chamber in front of the stopper. This hole can vary in size but should be about .020” and countersunk. The stopper must be aligned to match up with this hole.
  • I have designed the sub to operate for about 1 hour on a single filling of 3 (powdered) tablets packed into the powder chamber. Lightly pack the powder into the powder tube in the bubble chamber using a wide pencil. Completely wet the sub by setting it in the water, turning it upside down and right side up, watching specifically to see that air bubbles are being released. I attach a 20' length of cheap clear monofilament fishing line to the SoBe bottle cap for safety, switch on the motor, and then let it go. If problems arise, you can pull the sub up with the fishing line. The actual testing of the air bubble system and the flotation was all done in my 50gal aquarium at home. If you are running the sub in a swimming pool, you can release it without the fishing line. Just be ready to hop in the water to fetch it.

Step #15: Align and test.

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  • The final testing should be done somewhere where you can set up the balance of the sub with the motor off. The final setup of the sub will rely on the precise balance it is able to attain as it slowly settles into the water. The balance is achieved by adding ballast weights along with the ballast foam
  • When the balance is correct, the sub should be able to sit level in the water, front- to- back and side-to-side. This is time consuming, but it must be done. Add a little weight here and add a little water there to get the balance right. The goal is to have the balanced sub sink as slowly as possible to the bottom. This allows time for the bubbles to accumulate, enabling the sub to surface without going too deep.
  • Notice that there is a small space in the bubble chamber above the bubble baffle and below the bottle cap. This small airspace is where you make the final trim necessary to have the sub sink slowly. Make sure the sub floats until you add a little water to this small airspace so it sinks slowly. Make sure the SoBe bottle’s white ring on the neck projects above the NOS bottle. Once you establish that the air release arm moves without restriction, you can glue the SoBE bottle in position with the RTV. Glue the bottom of the SoBe bottle to the blue NOS bottle with the blue RTV that matches the sub.

Step #16: Use It.

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  • When the bubble chamber fills with air, the sub rises. Upon reaching the surface, the air release arm falls forward enough for the air to escape and for the sub to sink. This sequence should repeat for 1 hour or more. Keep in mind that the sub only weighs 7oz and must be operated in very still waters, preferably in a swimming pool.
  • Clean out the bubble chamber after each use, as the Alka-Seltzer dries hard and can clog the bubble chamber if it’s left partially full after use.
  • I've found that my method of sealing the motor works very well and I've submerged my sub many times with no damage to the motor. To prevent the petroleum jelly from melting, don’t run the motor for too long when the boat is out of water, or let the boat sit in the sun too long.
  • So, by now you should be holding your completed Kid Sub which you have successfully built by following this step-by-step, enjoyable, creative, and inspiring process. For me, I wonder if my energy to build the Kid Sub and write of its birth here now is just an excuse to revisit those days along the Western shore of Lake Michigan where I grew up. I think back on those summer evenings when the warm Western breeze blew East and created that calm on the lake, from Milwaukee to Door county. And, I ask myself whether my urgings to write this exciting piece were propelled by the image of those five scruffy kids with disheveled hair and worn-out clothes, jumping, laughing, and jostling on their way home (after a day of play) on those warm summer evenings, holding tree branches in their hands extended over the lake, with cotton store string pulling toy boats on that calm water which we loved to see, and certainly feeling that life, indeed, is exuberantly and exhilaratingly wonderful… In my mind, I walk to the calm lake with the Kid Sub in my hand and undyingly enjoy the lakeshore once again.
  • IMPORTANT: The sub may take about 2 minutes to gather enough carbon dioxide bubbles to surface, running forward all the while; therefore, it might get caught on underwater obstacles you cannot see if the water is murky. I suggest you connect a fishing line to the sub, which will cause a small amount of drag but will allow you to retrieve the sub if something goes wrong.

Michael Wernecke

Michael Wernecke presented at the Maker Faire in San Mateo, Calif., in May 2007. He creates Alfa hull kits and welcomes questions about his sub. ocean_tech04@yahoo.com.


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