How do you build a self-submersible boat?

[Monom]

So I've been thinking for a long time now about making a self-submersible boat out of a plastic bottle without electronics. It should resurface by itself after a few minutes. I was thinking that it might increase its volume. By slowly dropping weights down the bottle and opening flaps on the sides of the bottle. But weights could also slowly escape from the bottle. Anyway, I'm looking for ideas and thank you in advance to anyone who pays attention to me.

 

[berkeman]

So I've been thinking for a long time now about making a self-submersible boat out of a plastic bottle without electronics. It should resurface by itself after a few minutes. I was thinking that it might increase its volume. By slowly dropping weights down the bottle and opening flaps on the sides of the bottle. But weights could also slowly escape from the bottle. Anyway, I'm looking for ideas and thank you in advance to anyone who pays attention to me.

Welcome to PF.

Do you mean like these baking soda powered toy submarines?

https://www.teachersource.com/product/diving-submarine

 

[anorlunda]

Do you mean like these baking soda powered toy submarines?

Wow! I had forgotten about those. They were great toys.

 

[Baluncore]

But weights could also slowly escape from the bottle.

Buoyancy all comes down to total vessel density. You might include an internal water ballast tank, with sufficient internal air pressure that it will drive the water out through a small bleed-hole, as used for drip irrigation.
A pack, or a block of salt, will dissolve in the water over time, until the vessel floats. But that will not repeatedly sink and surface, like the CO2 bubble produced by baking soda.

 

[Monom]

Willkommen bei PF.

Meinst du wie diese mit Backpulver betriebenen Spielzeug-U-Boote?

View attachment 301357
https://www.teachersource.com/product/diving-submarine

Ich habe schon über Backpulver nachgedacht, aber es kam mir nicht in den Sinn, es auf diese Weise zu verwenden. Vielen Dank!

[Translation via Google Translate by a Mentor]

I've thought about baking soda, but it never occurred to me to use it that way. Thanks very much!

 

[Baluncore]

You can get a more rapid reaction if you mix a dry acid powder with the baking soda.
Water will then initiate the reaction.
https://en.wikipedia.org/wiki/Sherbet_(powder)#Ingredients