From the Depths - How to Build a Stealthy Submarine
11Several factors influence submarine stealth in FtD.
1. How far away you are from the enemy.
2. The SIZE of your craft. Smaller is better.
3. What your submarine is made of, or at least what its surface is made of. Rubber and stone are good, metal not so much.
4. What kind of detection equipment you are using. Active sonar is detectable to enemies, passive sonar is not.
5. Whether you have wireless transmitters on your boat. Wireless transmitters can be detected.
6. I believe your propulsion methods also matter. Ion is silent, but it is also expensive and less powerful so I usually just use propellers.
7. I have not proven this or seen independent confirmation of it, but I believe depth also matters. Deeper is better.
Additionally, though this is not a matter of stealth but sort of its opposite, the use of sonar simulators on harpoon torpedoes can misdirect any torpedoes that can find their way in your direction. The simulators can have a much larger sonar signature than your boat.
If you can keep far enough away from your enemies (you can go as fast or faster than them) and have these factors covered, your submarine will be almost impossible to hit. Additionally, since you are not focusing on armor but on not being hit to begin with, you can build your submarine cheaper than an equivalent surface vessel. My Dory submarines can take out enemy vessels costing ten times as much or more, simply because the enemies can't manage to hit them. If they do hit them, such as with particle cannons, well these submarines are cheap enough to just spam at any enemy in large numbers.