Why don't cargo ships sink?

Back to the misconceptions of young minds about floating and sinking: heavy things sink and things made of metal sink.  How then do cargo ships ladden with tonnes of cargo stay afloat?  We tried to build our own cargo ships.  The criteria was that the ship must float and that the cargo should not mix together.  Here's the list of the cargo each ship was to hold.

In pairs (to allow for conversation to confirm ideas) the students planned what they were going to construct.

Students are often paired to ensure that they have someone to discuss their thinking with.  If a student can discuss outloud, they can often hear their own errors.

Each pair was given the same amount of plasticine.  That's an important bit for scientific study: only one thing should ever change in an experiment.

The designs were very different!

Every one of the above designs sunk!  Do you know why?  The students were given a second chance to modify their original design, and then they tried again.

What did we learn in the end?  If the plasticine is thin and the edges are up, the ship would float.  This was then related to the cargo ships.  The weight of the metal ships is distributed over a large surface area allowing a heavy metal ship to hold a cargo and still float!