The Ship of the Line can be "Summoned". Play 1660 . Start your game at low difficulty. You can increase it each wealth share-out, if you like.
*Target all English ships in the St. Kitts/ Nevis / Antigua area. Try to capture or sink them. Don't blockade so closely that no ships come out of port, run a figure 8 around the islands if need be.
*Let the new governors and smugglers alone because they increase the wealth, and wealth seems to increase the chances of Ships of the Line being spawned.
At first the English will start escorting merchants, and sending Pirate hunters. Keep up the pressure.
They will start escorting with frigates, then you'll probably see a large frigate as an escort. It's great to capture it. What you don't want is a fleet full of broken down ships that move too slow to engage the Ship of the Line.
When it appears, it will look just like a frigate, but on a different scale. It will be labeled NEW WARSHIP. It will most likely be on the edge of your field of view ( maybe coming from or going to Barbados ) Remember, it carries 48 guns, and it takes a fair amount of punishment. You probably want to avoid trading broadsides with it. Rake it and board it, or dismast it so it surrenders.
Once you have your SOTL you can make it your flagship and go to port and share out the loot and play the game according to your priorities.
Or maybe you'd like to try for additional SOTL captures. If you can keep up the pressure, you will continue to see them. Good Luck!