Oh it's simple, destroy an enemy ship by ramming it.

If you are going to go after this achievement I recommend having the realism settings toned down (You should have the AI difficulty set to easy, and maybe also have the damage difficulty set to easy as well.). Go start a new game if you have to.

I also recommend doing this on type-XIV uboats as they are faster and heavier than type-II uboats.

They are unarmed and unarmored, but they're rather heavy. There are better options.

Just, why? They have several main guns that can (and will, very quickly) make short work of your boat. They're also much faster and heavier than you.

They have bigger guns and more weight compared to destroyers. It should be pretty obvious as to whether or not you should ram one.

No.

Not going to happen. Putting their planes aside, they are larger and heavier than destroyers, and are often found in a large convoy. You're probably not going to be able to sink all of the escorts in a large convoy. Even if you do manage to do that, their weight means that they're likely going to shrug off your ram.

They are lightly armed, have next to no armor, and are much lighter than you. They'll sink very quickly once you ram them. Unfortunately, they're way too fast and will easily avoid you in a ram.

That leaves us with corvettes, and they are the perfect target. They are lighter than destroyers and slower than you. Do keep in mind though that they have a main gun in the front.

In this guide, we will ram a corvette.

You should find a convoy or a task force near the coast. A good place to look for them is in the English channel.

Then I want you sink every escort, except for one corvette, with your torpedoes. You do not have to sink transports if you are in a convoy as they are unarmed. After saving that corvette for last, it would be a good idea to wait for it to no longer be alarmed before you begin ramming it.

As for what corvette you should save for last, an Isles-class corvette is preferable because it's lighter and slower than a Flower-class corvette.

Also, pray that there are no planes nearby.

Remember when I said corvettes have a main gun in the front? Well guess what, they don't have another one in the back. You can see where we're going with this...

First save the game, just in case you mess up the ramming process. You'll then get behind the corvette while underwater. Once you are behind it, go flank speed ahead. When you get spotted, surface the boat. The corvette shouldn't be able to fire back with it's deck gun because you are behind it, stay behind it so it doesn't get a chance to fire it's deck gun. You can use your deck gun to slow down the corvette, but don't over do it or else you'll sink that corvette.

The corvette should sink after you ram it. Do note that this will cause a lot of damage to your boat and open up many leaks, so make sure you have plenty of spare parts.

Now, go and enjoy your achievement.