1) Build a small platform of 600mm or normal ballast armour.
2) Place an Angled Boiler (or some small boilers to make it cheaper, but take up more of the percentage bar, and aircraft carrier engines are better for smaller submarines) on the platform. Move it around to take up as little space as possible
2b) [Optional] Expand the platform and add a second engine.
3) Cover ALL sides of the engine in 600mm (or 200mm armour, it will make the submarine less armoured but it weights the same as 600mm lightweight and makes building a sub much more cheap) lightweight armor as engines won't work underwater.
4) [Optional] Add various sculpted blocks to increase the sub's hydrodynamics.
5) Add some (preferably 2-4) propellers to the back.
6) Add some (preferably 3-6) propellers to the bottom - orient them with the poles facing DOWN.
7) Test it - increasing the throttle should cause it to dive and drive forward.
8) Add ballasts as needed, you want to be able to dive and rise to the surface easily. If it "dolphins", add fins to the sides of the sub, near the front. This will make the water hold down the front.
9) Add rudders.
After you have a basic sub built, you can add weapons - just note that only the Underwater Torpedoes work when submerged! Any guns should be put on top so you can rise from the water and use them as needed so you at least have some form of firepower when engaging ships when surfaced.
10) Modify ballasts as needed as you want the submarine to balance well with the new cargo.
11) Test the sub in Drill>Battle - see if you can kill other ships without being seen!
12) [Optional] You can always give your submarine a cool paint job, like blue for camouflage.
13) Use it in multiplayer!
14) Note: On multiplayer on the map morning sea and evening sea, subs can't be seen! Using the depth gauge block, the minimap and some practice will assist in underwater navigation.
While the above method (using downward propellers to 'push' the submarine underwater) is easier to control, this faster but more risky alternative is possible to do
The concept of this alternative is to make your sub front-heavy. This will cause the ship to submerge if limited speed is applied (from the rear propellers). Increasing the speed will then cause the rear to be pushed down, resurfacing the sub.
General theory: No speed, float on surface (albeit at a strange angle). Small speed, dive down. Full speed, slowly resurface. This can sometimes become difficult to control if engaged in combat, and a depth gauge is essential.
Another technique is to use a level submarine and rely on the inherent forward movement to sink. The main idea is that at slow speeds, the submarine does not sink. At higher speeds will the submarine sink.
The submarine should, ideally, be as close to the density limit (1.0) as possible. You will need to calibrate the descent rate as best as possible; if the front is too heavy, you will dive into the death-zone (under the red floats of the depth gauge), and if your front is too light, you won't be able to stay underwater (you will bob back up).
The two methods were tested on identical submarines (same engines, shape, rear propellers, etc...). While the first method provided an average speed of approximately 20 knots, the second method provided an average speed of 50 knots.
If you are inexperienced in submarine handling, it is recommended that you use the first method to begin, and upgrade your submarine to the second or third method later on.
Step-by-Step Submarine Tutorial
This is a tutorial showing how to construct the SSV Banshee, a moderately complex submarine which will go 51.9 knots submerged, and is armed with four underwater torpedo tubes. It is somewhat difficult to control, but with practice, it is very effective and can be modified easily.
In the following screenshots, each picture will represent a one block high layer of the submarine, with the first picture showing the lowest layer. To identify which blocks are which, they will be colour coded as follows:
White: Hull/Sculpted Hull
Red: Heavy Ballast
Blue: 600mm Armor
Lime Green: Underwater Torpedo Tube
Purple: 1/2 Ballast
Yellow: Gas Turbine Engine
Dark Green: 400mm Armor
Black: Already Completed Layer
If built correctly, the submarine shown in the tutorial should slowly sink when at full throttle. If the throttle is reduced to neutral or reverse, the submarine will tilt drastically forwards and then begin to rise fairly quickly.
For general use, this submarine should stay at full throttle until both depth gauges are just below the surface, at which point you are nearing the "death zone", the depth at which you will instantly be sunk. To prevent going into the death zone, you should go closer to the surface. To do this, put the throttle to neutral for a moment, and then back to full as soon as you see the top of the yellow/orange part of the rear depth gauge appear above the surface. If done correctly, this maneuver will cause the submarine to lean forwards, then angle up towards the surface, leveling off and beginning to descend again just before it reaches the surface. Executing this without surfacing may take a little practice, but it's worth the effort. Never surface in full reverse, or the submarine will fly into the air and then flip over and sink.
To sink more quickly, use the same maneuver as used to rise, except instead of staying at neutral, move the throttle to neutral or even full reverse for only a moment, before going back to full. Rinse and repeat until the desired depth is achieved.