Building submarines instead of the good ol' regular battle ship has it's ups and downs.

Pros:

• Enemies have a harder time detecting the submarine and placing shots on it.
  
• Almost impervious to cannon shells.
  
• If the submarine is deep enough, it becomes missiles and torpedo proof.
  
• Has lower cost of resources compared to a ship.
  
• Can be steered to avoid rockets and torpedoes.
  
• Can attack both air and naval targets.

Cons:

• Steering needs some practice to get used to it.
  
• Lower visibility when steering underwater.
  
• Harder to build (still relatively easy but it can get claustrophobic when working on is small submarine).
  
• Weak armor.
  
• Weapons can't be laser guided and need to rely on Sonar and IR camera.
  
• Torpedoes can be ineffective if the enemies are dropping Sonar decoy.
  
• Weak in shallow waters.

Overall, by building a submarine you trade heavy ship armor with stealth and maneuverability. The submarine prove itself to be highly effective as it is impervious to missile, torpedo and shells.

Every type of ship has a specific shape and layout. You will need to fit both AA and torpedo weapon systems engine, stabilization system, Ammo boxes, resources boxes, ammo processors and propellers while conserving that submarine shape. Space is highly limited depending on how small the submarine is.
 
 
The layout of the vitals should look similar to the following on a large submarine:
 
The layout of the vitals should look like the following on a small submarine:
 
 
 
It is important to keep in mind while building the submarine that only the least important vitals should be located at the back. Generally, whatever is found in the middle of the submarine is less likely of being destroyed. This is where you want your engine and ammo boxes. If the submarine is large you can combine vitals together like I just did in the large submarine picture where I combined ammo boxes and the stabilization system.

As FTD doesn't have ballast to change the weight of the submarine so it sinks or float, you have to rely on propellers under the submarine that will push the nose up and down. Both lines of propellers can be put at the bottom so that you don't have a propeller on the top that breaks the look of the submarine.

Here's the line of propellers at the front and the propellers at the back:

 
 
 
The rear propellers need to be set on reverse so submarine note can be pushed up or down  
 
 
 
To move left and right, you need a line of propellers that are at the same level as the center of mass. This will prevent the submarine from falling on it's side when turning. The propellers in the red rectangle is where the propellers line should be.
 
 
 
If you submarine float, replace some metal blocks with some of the heavy armor. This will make it heavier and thus make the submarine sink. Although don't add too much weight as you want your submarine to sink really slowly. Too much weight and it could cause major annoyance.

Your submarine will want to capsize but fear not, jet stabilizers are here to save the day!
 
You can find these here:
 
 
You need to place the jet stabilizers like this (along the submarine on the inside) so it stabilize it from falling on it's side
 
 
The amount of stabilizers that are required to keep the submarine from capsizing can change based on the weight. This means that trial and error is needed for optimal result but usually having 5 to 6 stabilizers on each side of the submarine is recommended by 4 out of 5 doctors.
 
 
The front and back movement of the submarine needs to be adjusted by balancing the weight so it's nose doesn't sink forward. This is tricky and will never be perfect but can be brought close enough by spending a good amount of time balancing the weight afterward. On the dozen of submarine I made, none of them ever had a perfect balance and always were front or back heavy.

As a submarine is quite small in size, it's hard to build an engine big enough to power it fully. Both electric and gas engines requires a lot of space to generate enough energy for the submarine to function properly. Though steam power can be highly compact and produce enough energy with a single 2 meter piston.
 
 
The benefit of the steam powered engine is it's small to power ratio. Small engine like this can produce over 700 energy units at max power:
 
I recommend on having a steam engine that produce around 500 energy units but as always, depending on the amount of propellers your submarine will have, this number might be higher.
  
 
As a quick example, these are the engines that powers both my small and large submarine.
 
 
As you can see, there's no unique shape to steam engines and this is what makes them the best. They can be as small as you want and just perfect to fit unused areas.

The weapon systems is composed of frontal torpedo and upward AA rockets. There are already a lot of guides which explains how to make missiles system and I encourages you to find one. I'm lazy
 
 Torpedoes should have a 5 meter long ballast and a torpedo sonar on the tip. If the torpedo is underwater when it hit the enemy ship, it will have a large damage multiplier.
 
 For the AA rockets, you need to tip them with an IR camera. This will allow the missiles to follow target that are straight above and about 130 degrees on the side of the missile tip.
 
Both weapons need to be fired manually as there's no real way to make a AI work the submarine. (AI system is still work in progress)

This is my first guide and it won't be perfect but I hope it will help you understand how to build a submarine. If you have any questions, ask them in the comment section bellow and I will try to answer them as best as I can and might change the guide to cover the questions accordingly.
 
Any positive feedback is welcome.

Edit : I've put the small submarine up on the workshop so y'all can observe and analyses how it's made. Also you can just use it and blow ♥♥♥♥ up :)
Enjoy
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1256503052