The critical element in a subordinate clause or idea is wishing to make the reader want to continue reading. It is here where the use of complex sentences and a subordinate conjunction can help compel the reader to continue. The author uses subordinate clauses to vary up sentence structure and ideas, being able to do more in standard sentences than "subject + verb" as the fundamental layout in sentences.
In the sentences above, I would consider setting up a subordinate conjunction that can being out the idea of being "jammed" or packed into a tight situation:
Since they were jammed together as sardines, the ships became overcrowded.
I used "since" as my conjunction early on in the sentence in order to highlight the "jammed together" element. This also allows the overcrowding issue to become more important as explaining it because it is a result of "jamming together as sardines." It is here where there is a greater sense of understanding in the sentence, bringing out more in the verb and greater variety in its topic.