Post it on the workshop and we can take a look at what you already have and what's missing.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2164181800 <-- Deleted

Quite a sublike sub. Main thing, sub is built sideways, go to load menu, click that rotation icon in the bottom right, confirm. load it. Go to build, confirm that the arrow is pointing in the same direction as the front of the ship. Make sure all your propeller still have the correct orientation.
Ships don't need detection unless using guns/lasers and turrets, so you can get rid of that unless you want it for aesthetics. Noting torpedoes first cause it's the first thing I noticed.
Torpedo:
- It's long, not a bad thing more so for torps than missiles, long torpedoes work fine, missiles not so much
- Fins place near the frontmost/rear most sections where they apply most turning power, your missile will fly slower with fins near the front though.
- Too much regulator and fuel, generally the base 40 second is fine for torpedoes, though bump it to 80 if you want longer range
- Adjust the thrust per second so your thrust duration is equal to the lifetime at least. No point in leaving spare fuel around when your missile expires. That or use less fuel tanks.

Sub Stuff:
- It's quite a bit buoyant, alloy is very buoyant, convert some of them to metal, especially the ones near the bottom. That or convert more stuff to lead near the bottom.
- Install some air pumps, preferably compartmentalized, this way you can maintain depth without having to rely on propellors
- When you do install air pumps you need some General PID (not the AI version). Input set to "Altitude above mean sea level", Outputs select "Airpump fraction control", Turn on "Enable fake set point", tweak the PID settings per section below.
- PIstons aren't air tight apparently, had to seal off the conning tower to get it no leak.

PID Tuning:
Start with gain at 0.01, integral at 1, derivative at 0.1. Watch the graph you ideally want the green line to follow the red, if it starts bouncing too much general 2 fixes, lower gain or lower derivative time. For subs the rate of gain and interval doesn't matter terribly much, but generally you want to tune gain and integral to be responsive enough but not enough to go over.
These are starting guidelines, sometimes settings may end up completely the opposite.

Misc:
- No material storage, fine unless you need to repair, collect materials, or run ammo processors
- No ammo processors, fine though you might run out of ammo
- Remove thruster balancing, it causes more issue than it solves

Massive thanks! Imma try and recreate this from instructions first, and use this as a "physical" reference if i get stuck, I'll post here when i succeed or run into anything else.
Thanks for all the advice and, well everything LOL

Okay this is working great now, it holds the depths i ask, the pitch and roll are also stable.
General PIDs worked great, and after a little bit of youtube diving i figured out a couple other things.

But now what i really need to know is how to with the press of a button change my altitude PID's "fake set point" so i can change depth without entering a menu every time.

Any suggestions?

Shift + N to name your PID, you'll use this for later.
Place down an ACB
Go to the Adv Stim Trigger tab on the far right and click on a hotkey you want to use
Go to the Slave Action tab, click General purpose PIDs under AI
Under Select action to take toggle Turn on/off fake point value, set that to what you want. Alternatively there's also Set point value change if you'd rather increment it or decrement it.
Under Search pattern in Filter affected components type in the name that you placed in step one

Test it out at a seat near a complex controller block, repeat for your other setting.

Hmmm I followed all the steps but the "Set point value change" seems to only set it TO the value specified. Rather then increasing it by or reducing it by the specified value.

Originally posted by Blast (AKA LostAmI):

Hmmm I followed all the steps but the "Set point value change" seems to only set it TO the value specified. Rather then increasing it by or reducing it by the specified value.

So it seems, seems like a useless feature then. There's probably a use case but I can't think of one. In which case you'll just need to bind preset points then. There might be a way to do it with lua, though you'd probably need to find someone more experienced with it or search. I don't know how up to date the API documentation on it is.

Well, we tried and thats what matters. Thank you for your advice. I'll post here if i find a way to change the value on the PID value with a button. Again, thank you so much!

for changing fake point on PIDs you have to have an ACB for every different value that you want
EX. 2 ACBs on 10 second timers on a range of 2 on either side of the altitude PID with the action set to change fake point to say 100 and the other set to 200(these can be whatever you want). this works well for many of my aircraft as it makes them harder to hit.

you can set the trigger to whatever you want

my GP-PID doesn't seem to be responding to my ACBs.
I have the GP-PID named, the ACB's are filtering and say they are controlling it.
I have the ACB's set to "set fake point value" each with different values. A key set for each and yet, when i press those buttons the value is not changed and the Altitude does not change.

Now in this instance we are talking about a submarine, so they are -values -20 -50 -90
the GP-PID is controlling the ballast tanks/pumps

Should work fine, make sure you're in range of a complex controller. if the ACB's close to you, hover over it, otherwise go into build mode and hover over it. Press or hold down the button, if it's bound properly it should turn green, likewise you can do test in ACB control panel.

If all else fails feel free to repost it, but ACBs should be able to control fake set point properly.

Bingo! I totally forgot the complex controller! That was silly..

So we now have "preset" depths.
Now if only i can find a way to increase and decrease the value by 10 i could just use two keys

In the GP PID Editor, go to adv controller options, bind a key green for ascend and bind a key red for descend, this apparently'll change the fake set point every time you press the relevant buttons. Who knew.