Floating ships made out of iron...

For thousands of years boats and later on ships were made out of wood, because experience showed that wood floats.
If you submerge a piece of wood completely under water, the buoyant force is much higher than the gravitational force of the piece of wood. The emerged resultant force is directed upwards. The piece of wood is pushed upwards until its buoyant force and its gravitational force are equal. The piece of wood is only partially submerged in water. This is the reason why solid wood floats on the water's surface and therefore boats made out of wood as well.

Why is it possible that wooden boats can sink after all?
The boat sinks if...

• the buoyant force is higher than the gravitational force of the boat.
• buoyant force and gravitational force are equal.
• the buoyant force is lower than the gravitational force of the boat.