Complex sentence: Gamle Hellesund had its point of highest tension in the 19th century, in the so-called "era of sailing ships", it was a lot of maritime traffic along the coast, and ships from many countries visited the outports.
Split into simpler sentences: Gamle Hellesund had its point of highest activity in the 19th century during the so-called "era of sailing ships". There was a lot of maritime traffic along the coast, and ships from many countries visited the outports.