The easiest way, short of cheating, of getting a Man o' War is thus: once you capture a colony, man your ship and wait for your colony to get attacked, preferably by the French or the English, though I'm not 100% sure the other nations won't send a MoW. When you get the message that your colony is being attacked, you'd better be close by. Sail to that port in the world map and wait until it's daytime before you hit F3 -- I find naval battles much better when conducted in daytime. Then join the battle. You may have to try a few times, loading from the save point just before you hit F3 to enter the battle, before you're lucky and the invading force includes a MoW. If you get really lucky there might be two MoW (off topic, the plural form of Man o' War is Men o' War) -- this happened to me once. Most often it's one or two class one ships, or a couple class two vessels and the rest would be class 3, usually corvettes.
Once inside the battle, don't attack anybody until you see your prey's crew massacred by your fort's cannons. Then sail quickly to it and board it. You have to have good fencing skills, enough crew to satisfy the minimum crew requirements for the desired ship and at least one free officer.
This strategy has worked really well for me and at one point I had four MoW in my squadron. But the upkeep of for such monsters is... monstrously high. Just paying the crew and the many officers I like to keep in my retinue amounted to a few hundred thousand. And if the ships get damaged in a battle, you'd better be a millionaire. If you want to be a millionaire, read below
I hesitate to offer this as a "tip" because it's borderline cheat. It's a bug in reality, but I'm sure many of you will frown upon it.
I've noticed a bug when I capture high class ships (I can't remember which one it was), but when I clicked the Get all goods button, instead of all the conquered ship's cargo being transfered to my ship's hold, the goods in the former started to grow exponentially as I kept clicking the button. At some point the ship was holding millions of units of many goods. At this point don't dump the ship. Capture it if you have a free officer and sail to the nearest port. Sell as much of the most expensive commodity you're carrying and you'll be an instant pirate millionaire. Since stores have a maximum capacity for every type of goods they sell, you won't be able to sell everything in one store. But selling as much as you can in the first store will make you rich beyond belief, so don't get greedy. Dump the rest or just sell the golden goose ship. Just keep in mind that if you keep sailing with that ship in your squadron you'll eventually run out of food and even though you buy more food and your ship's hold shows enough food for many months, out in the open seas you'll get the message that you're out of food and your pirates will start eating each other.
I'm sure this bug will at some point get fixed, but enjoy it while you can
chepati