Japanese Experiments. Studying Ships

Why does a piece of iron thrown into water sink immediately, while an iron ship floats on the surface? How does a submarine dive and rise to the water's surface? What is a catamaran and why is it more reliable... than a regular ship?
This scientific educational set will help a child gain knowledge about the density of various materials and understand why some objects sink in water while others do not. They will even be able to design their own little ship and launch it in an aquarium, bathtub, swimming pool, or on a river - anywhere! This toy vessel will never sink, as it is made according to all the rules of shipbuilding!
The set includes an underwater motor, propeller, fastening tape, fin, screw propeller, colorful instructions.
Material: polymer materials, metal elements.