How to get a Ships-of-the-Line?

All the countries other than the Spanish can spawn a ship of the line.

These conditions under which SOLs can spawn:

• Ships of the line are only spawned by non-Spanish nations; in fact, the Spanish will never spawn any Frigate class ships.
• Ships of the line are only spawned when any non-Spanish places a bounty on the player of what appears to be no less than 30,000 gold pieces.
• Like all other New Warship -classified ships, Ships of the Line are spawned along trade routes that have been recently terrorized by pirates, enemy nations, or the player. For example, attacking merchant ships traveling from City A to City B will eventually spawn a New Warship traveling from City A to City B, though there is no guarantee of the New Warship being a Ship of the line.
• Ships of the Line will only spawn from Wealthy cities. City population also seems to be a deciding factor, as well as national prowess, ie a stronger nation is more likely to send out multiple Ships of the Line. The game, however, provides no hard data on this; ergo, there is no way to accurately confirm this theory.
• Ships of the Line seem to be the most frequent in 1680, uncommon in 1660, and rare in 1640. There are, as of yet, no confirmed sightings of any Ships of the Line in 1620 and 1600. It is unknown whether Ships of the Line can actually spawn in those eras. Ships of the Line do not behave like Pirate Hunter -class ships: they are simply sent out from one city to another, will display no preemptive hostility towards you, and must be attacked first.

I have had most success causing ship of the lines to spawn by finding a wealthy city and camping outside of it, attacking any trading ships that pass by.