If you are tried of building things that fly, why not try a submarine like [DIYPerks] did? As you can see in the video below, the key is to control buoyancy, and the mechanism used is impressive. The sub has two giant syringes fore and aft to compress or decompress water. The plungers are now 3D-printed actuators that travel on a lead screw. Two high-torque motors and some batteries sandwiched in acrylic disks make up the rest. This is a big vessel — you won’t be trying this in your bathtub and maybe not even your pool unless it is a big one.
Of course, everything needs to be watertight. Instead of trying to waterproof a power switch, this sub uses a reed switch so that a nearby magnet can turn it on. Not an original idea, but we always think it is more elegant than seals and potting compounds.
The best part, though, was the modification of a servo control board to close the loop with a linear slide resistor instead of a rotary pot. A clever hack and seems to work well for this application. The downside is the RF control. As real submarines know, 2.4 GHz signals don’t go very far underwater. Like a real submarine, this one floats an antenna array on the surface so it can receive control signals from the shore. It also sends video up in real time.
A bunch of water pumps work as thrusters. The submarine’s hull is a tube that is the same diameter used for irrigation pipes so the sub can use off-the-shelf parts meant for that purpose.
The sea trials were done on a lake with a friend’s much smaller submarine. There were the usual issues to work out, but in the end, the addition of some ballast and preventatively adding some seals got everything in working order.