Merchant vessels were not designed for combat. They were slow lumbering vessels designed to have a large capacity for transporting goods but at the sacrifice of manoeuvrability.
In Black Seas there are many scenarios that necessitate the use of merchant vessels. Scenario 4 – Blood & Plunder! (Page 41 Black Seas rulebook), see two opposing privateer fleets attempt to claim the contents of merchant vessels for their own, whereas Scenario 9 – Landing Expedition (Page 46 Black Seas rulebook) sees merchant vessels loaded with troops attempting to cross hostile waters and disgorge their loads onto a contested island.