Ballast Submarine

We have done this project as part of our Under the Sea theme, where we study fish and other underwater life and ocean exploration.


Plastic bottle, Two rubber bands, Quarters and Nickels, Waterproof tape, Straw and modeling clay or silly putty.


  1. Drill / cut a hole in lid of plastic bottle.
  2. Put a bendy straw in the hole. Seal around the opening with silly putty or tape.
  3. Punch two or three holes in one side of the water bottle.
  4. Add weights. Adjust as needed: The weight wants to be enough that the sub will sink as it takes on water. (We found it worked well to tape 4 nickels or 4 quarters together. We put the quarters next to the bottom holes and the nickels next to the top holes.
  5. Place the “sub” in a tub of water – let it sink, but keep the end of the straw above the water. Once it sinks, blow air through the straw – this forces the water out, so the sub will float back to the surface.

This experiment relates to how submarines work. When they want to dive, they fill ballast tanks with water to increase their density and sink. (In your model, the water flows in through the holes.) When they want to re-surface, they pump compressed air into the tanks (you blow it in with a straw), which forces the water out. Since air is less dense than water, the sub rises.  This experiment is not a perfect re-creation of that… since they’re not watertight… the water bottle has big holes in the side, the second you stop blowing on the straw, the water rushes back in and the sub sinks.

If you’re up to the challenge, you can make a sealed submarine with a water bottle, balloons as air tanks, and syringes or pumps to fill the balloons. See the video at

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