Das Benchy! A Fully-Functional Benchy Submarine

Das Benchy is a fully functional submersible which works with the help of CO2 gas produced from the reaction of baking powder in water.

How does Das Benchy work?

A reaction chamber on Das Benchy holds baking powder and is completely sealed except for a small hole located at the bottom of a removable cone section. When initially placed in water, Das Benchy sinks.  After a few seconds, the baking powder reacts with the water, forming a CO2 gas bubble in the chamber.  As the bubble grows in size, it eventually provides just enough buoyancy to rise Das Benchy to the water surface. Once it resurfaces, it tilts to one side, releasing the bubble from the cone & causing Das Benchy to sink again.  As it sinks, it continues to produce more gas and the process repeats itself.  The entire reaction will last several minutes, causing Das Benchy to dive and resurface repeatedly on its own until the baking powder fully expires.

Some Chemistry

Baking powder is made up of 47% (by weight) mono-ammonium phosphate, 43% sodium bicarbonate (baking soda) & 10% corn starch. When mixed with water, the compounds react, producing carbon dioxide gas.  IMPORTANT NOTE:  Baking soda & baking powder are not the same!  Baking soda will not react well on its own in water, but baking powder will, as it already contains the acid needed to produce CO2 gas.

How was Benchy modified to allow it to work like a Submersible? 

First, the requirements:

1. Das Benchy must retain as much of its original Benchy design, appearance & form factor as possible;

2. Das Benchy needs to have the correct buoyancy so that it barely sinks in water;

3. Just the right amount of gas needs to be generated, which allows Das Benchy to rise to the surface in an upright position;

4. Upon reaching the surface, a sufficient amount of gas needs to be released to allow Das Benchy to sink again to the bottom; and

5. The cycle needs to repeat itself several times.  

After several iterations, I came up with a working benchy submarine remix. It basically amounted to removing a lot of volume from the original Benchy design to give it the right weight & buoyancy.  Using the correct filament type & infill was also found to be important. Finally, a baking powder reaction chamber was added, including a cone section which screws onto the Benchy hull to form a perfect seal, thus preventing any leakage of resurfacing gas. The cone & chamber sections were designed to be removable from Das Benchy so that anyone can experiment with different shapes, weights & materials, etc. without having to reprint the chamber.

Das Benchy Remix Observation Details

The following observations were made during the remix process:

• The bubble chamber needs to be centrally positioned on Das Benchy, and it needs to be aligned along a vertical line through its center of gravity.  In other words, the weight distribution at the bow & stern, including port & starboard, needs to be balanced for Das Benchy to rise in a level & upright fashion;

• Das Benchy needs to be hollow with an open hull to allow for additional weight & buoyancy distribution;

• Das Benchy needs to tilt to one side as it re-surfaces to allow the release of the CO2 bubble from the chamber to re-enable it to sink;  

• The top deck requires equally spaced openings to allow any trapped air bubbles to
  escape from Das Benchy's open hull, which could cause it not to sink. These openings also help in the control of weight reduction & distribution: 

  • In order for the bubbles to escape the deck, it must overcome the water's surface tension.  It was determined that holes greater than 1.6mm in diameter would fulfill this requirement;

  • Example: For a deck area of 484 mm2, the total hole area should be no less than 32.3 mm2; i.e., 7% of total deck area.

• The material used to print and the size of Das Benchy will also affect buoyancy and how long it takes for it to sink & resurface.  With a density of approximately 1.04 g/cm3 for ABS and 1.34g/cm3 for PLA, a Das Benchy built entirely of ABS may not sink, but a Das Benchy built entirely of PLA might not float; depending on infill, the size & shape of Das Benchy & the size & shape of the bubble chamber.  Therefore, it was determined that Das Benchy should be made of a plastic whose density is close to, or slightly less than, that of water but that the lower ballast should have a density which is more than that of water: 

  • Ideal material for Das Benchy main body would be polyethylene @ 0.96g/cm3

  • Suitable alternatives:

    1. Polystyrene has a density slightly more than water @ 1.06 g/cm3;

    2. ABS is slightly better @ 1.04 g/cm3;

    3. Others:

       1. PLA @ 1.24 g/cm3           Too high

       2. PETG @ 1.27 g/cm3        Much too high

       3. ASA @ 1.07 g/cm3          OK but ABS is better

  • As a final note, the average density of Das Benchy, including the sealed chamber & any lower ballast, should be greater than that of water to allow the benchy to barely sink;

• To permit the water to enter the lower bubble chamber & yet not permit too much loss of baking powder, the hole size in the cone section needs to be optimized.  I found that 3mm was a good choice;

• The insertion of the chamber cone should provide some compression of the baking powder so to render the powder more susceptible to uniform wetting & a steadier generation of gas;

• The volume of the bubble chamber should be large enough to produce a suitable bubble to make Das Benchy rise.  For example, a Das Benchy made of ABS weighing a total of 10 grams requires a lower chamber capacity of approximately 1356 mm3 (11x11x11 mm), for it to hold an adequate amount of gas;

• The total sub weight, weight distribution, sub size & shape, design & location of chamber, holes, aesthetics, etc. all affect weight distribution & buoyancy; therefore, all are critical in designing a working submersible Das Benchy.  Buoyancy was controlled in 3 different ways & their combination forms a design which allows Das Benchy to float, sink & remain in the upright position when placed in water:

  1. A sealed air chamber ballast (or % infill) above the Benchy deck which is fixed in volume;

  2. A CO2 gas bubble chamber located beneath Das Benchy which allows it to rise in water when the bubble is formed & then sinks once the bubble is released from the chamber;

  3. A lower ballast whose density is greater than that of water located beneath Das Benchy's center of gravity & located below the bubble chamber.  This could be incorporated within the bubble chamber or cone section itself.

• Optional:  Additional ballast could be placed below Das Benchy which has a density greater than that of water.  This could be in the form of a rod printed in PLA.  The ballast positioning should be adjustable in the longitudinal direction to allow balancing the weight distribution along the body of Das Benchy.  It also should be adjustable in the off-center position relative to the longitudinal axis of Das Benchy to allow it to tilt to the side once it breaks the water surface so that it can release the gas bubble.

Print Settings

• Supports:  Only required for bubble chamber

• Resolution:  0.2mm 

• Infill: 

  • 50% - for Das Benchy body

  • 100% - for bubble chamber

  • 100% - for cone section

  • 100% - for washer

• Brim: 10mm for Das Benchy body only

• Filament brand:  Doesn't matter

• Filament material: Important!  Do not substitute! 

  • ABS for Das Benchy body & bubble chamber

  • PLA for Cone section only; the reason is PLA is heavier than ABS

  • FLEX for washer.

• Filament color:  Doesn't matter

• IMPORTANT NOTES:  

  • Keep infills as recommended above; otherwise, your Das Benchy may not sink or float properly!

  • Follow filament material recommendations for each part listed above in order to get a working Das Benchy submersible!

Assembly

• Slip the FLEX washer into the top portion of the Cone;

• Making sure that the notches on the bubble chamber & Das Benchy are both aligned, slide the chamber through the bottom hull.   Push the chamber all the way in so that the chamber threads are completely flush against the hull bottom;

• Now screw on the cone section into the bubble chamber. 

How to Use

1. With the cone section unscrewed, place a small amount of baking powder (not baking soda) into the lower portion of the bubble chamber, making sure that it is below the threads & filling it only ¾ ways with a loose charge of powder.  Use the end of a bread knife as a spatula to facilitate filling.  Try not to spill too much powder outside of the chamber.  The chamber will hold approximately 1 gram of baking powder;

2. With the chamber ¾ -full, wipe off any excess powder that may have fallen outside of the narrower bottom portion of the chamber;

3. Now fill the remaining volume of the chamber with water.  Make sure to fill it to the very top of the threads, then seal the chamber by screwing in the cone section.  Make sure the cone is screwed in snugly against the bubble chamber to provide a good seal;

4. Place the benchy in water at a depth of at least 200mm.  A tall, wide-mouth vase works best. Tilting it sideways, shake it several times in the water, then let it go;

5. Das Benchy will initially sink & after several seconds resurface, release a bubble, then sink again.  It will do this multiple times until it runs out of baking powder.  

Troubleshooting

• Das Benchy is not sinking.  

  • There is air trapped somewhere in the assembly.  Hold the Das Benchy upside down in water & shake it several times to get rid of any trapped air;

  • There is not enough baking powder in the chamber to weigh down Das Benchy.

• Das Benchy will not resurface.  

  • Try using a different brand of baking powder.  During my testing, I found that some brands worked better than others.  “Magic Baking Powder” by Kraft worked the best;

  • Make sure that the FLEX washer is installed inside the cone section;

  • Check that the cone is screwed on snugly;

  • Check that there are no bubbles leaking from the top of the cone section.  If there is, replace the washer or the cone section;

  • Check that the hole under the cone section isn't blocked;

  • Try adding less powder in the chamber to see if that helps & don't forget to top up the remaining chamber space with water.

• Das Benchy sinks & resurfaces but only a few times.

  • Try adding more powder in the chamber to see if that helps.  Again, don't forget to top up the remaining space with water.

How to Refuel

• Unscrew the cone from the chamber & rinse out any remaining propellant from both parts;

• Shake & blow out any water from the chamber & cone;

• Note:  Any water left in the chamber or cone before refueling may cause Das Benchy to malfunction!

• Refuel as before. 

Have fun building & playing with Das Benchy. Please hit the like button if you think this is a cool design & thanks for your support!