3-29 spudmarine

make fun

Editor’s Note: Bob Knetzger has been making fun stuff all his life — first as a kid with his trusty Vac•U•Form and Thingmaker toys, then as a professional toy designer at Mattel, and for the last 35 years as an independent toy inventor. He’s been sharing playful projects with Make: in the Toy Inventor’s Notebook for years and now those projects have been gathered from the pages of the magazine into one delightful book, Make: Fun! The book is available now!

The “Diving Spudmarine” is just one of the many fun projects packed into this book. It’s an easy DIY version of the classic cereal premium toy, the baking powder-powered diving submarine. Instead of plastic, this one is made out of potato.

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Project Steps


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First, cut a potato into a ¾” diameter cylinder, about 3 inches long. Or, just use a small fingerling potato about that size. Use a piece of ¼” brass tubing as a plug cutter. Press the tube all the way into the potato to make three through-holes along its length. This will reduce the mass of the potato for better diving.


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Enlarge the bottom of the center hole to make a flared conical opening. This will create an air chamber for the bubbles. Next, make a periscope. Cut a thin piece of wood about ½” by 1″ and drill a ¼g hole in the center. Cut a 1″ long piece of ¼” wood dowel and insert it in the hole.


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Place the periscope into the middle hole in the potato.


Time to test: place the Spudmarine into a tall pitcher or vase of water. If it floats, cut off some of the periscope and try again. If it sinks FAST, trim off some of the potato and try again. If it sinks very s..l..o..w..l..y, it’s ready! The sub should be just ever-so slightly heavier than neutral buoyancy for best diving action.

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Remove the sub and shake it dry. Pack some baking powder (not baking soda!) into the bottom of the center hole. Use some more wood dowel to tamp it in tightly.

Gently lower the sub back into the water and let it sink, then watch it bubble … rise … breach the surface and burp its bubbles … and then sink again! How many times will your sub rise and sink before you have to reload the powder?