Underwater topedo.

The topedo is made with cavities for ballast.  The chamber for ballast is approx. 150 cm3/cl.

Balancing is important. Too little ballast will not dive, but will search for the water surface. Too much ballast will sink. Theoretically, the topedo has an equilibrium point in fresh water of 322.7 grams. 

Use a cement-based plaster/ concrete/ tile adhesive etc. as ballast (A density on approx. 2400 kg/m3). 

Do not fill the entire chamber with ballast or it will likely sink.

Fill the topedo with some ballast and then test how it floats or sinks in a bucket of water. Ideally, only a small part of the topedo is above the water.

When the ballast is done, then glue the topedo to avoid drawing in water.