When did warships become unmistakably different from merchantmen?

When did warship design become distinct enough to prevent confusion with merchant vessels?

The separation became clearer in the ironclad era. Large wooden sailing merchantmen could still resemble contemporary warships at a distance.

Wooden sailing warships and large merchantmen shared rigs, galleries, and many hull proportions. A distant observer could have difficulty distinguishing a large East Indiaman from a fourth rate. The move into the ironclad era made the separation between warship and merchant design more conspicuous. That places Pulo Aura's identification problem within the technology of its period.