Simulation is not just to simulate point and shoot, then hopefully boom, but to simulate the actual experience of the WWII sub jockey. Did you realize that from Pearl to Japan is over 3,600 miles? The real submarines had fuel problems too, just as you are.
Some have complained that the mast heights give incorrect ranges. The real ID manual was rife with errors. Real skippers took it for granted that without radar ranges they were likely to waste more than half their torpedoes.
Don't even ask for a rundown on the American torpedo scandal. That's search material and check out Ducimus'
A Brief Introduction to the Pacific Submarine War.
That's only the lightning survey. Don't assume that any problems in the game are due to defects. Most likely they reflect reality in a most uncomfortable way.
Mods: I have to say that mod #1 is Ducimus' Trigger Maru Overhauled. I am playing a U-Boat campaign without it (it wasn't available yet when I put to sea) and I am continually surprised by the details I took for granted that were actually part of TM. One of our struggles is always to leave map updates on or turn them off. I contend that leaving them on with Trigger Maru most closely reflects the actual plotting situation WITH RADAR AVAILABLE. If you don't have radar yet, even the TM plot is too precise, but it is preferrable to no map updates in my opinion. I'm talking about realism only, not ease in playing SH4!
A trip over to the realm of the Pacific Environment boys is not required--but it should be! Since the late Dave Bunnell, leovampire, produced Reflections on the Water, this has been the greatest thing to ever happen to SH4. And it has nothing to do with gameplay at all! It is strictly an environmental mod to make the ocean behave more realistically and beautifully, the clouds, the sky, the colors and reflections of everything are immeasurably and indescribably more beautiful than the original. Danger, you may spend too much time on the bridge at 1x time compression when you should be 2048x headed for dangerous waters.
Another mod I'm playing without that I miss terribly is Natural Sinking Mechanics. In the stock game, you torpedo an empty tanker, the toughest ship on the ocean to sink, and on torpedo #2, the ship explodes and sinks like a stone in less than a minute! In R/L it wasn't unusual for one of those suckers to inhale 4 torpedoes and cruise over the horizon without even slowing down. Natural Sinking Mechanics fixes this, giving you a much more realistic sinking model. You won't appreciate it until you're forced to play without it as I am right now. Just as the battle comes to a climax, the little hitpoints counter guy in your target says that's enough. He hits the nuclear scuttle charges and you get a very pretty but stupid explosion no matter how inappropriate. Check out Natural Sinking Mechanics.
And finally Run Silent Run Deep took apart all the enemy ship movements, removed them really, and substituted historical shipping patterns and warship deployment for the entire war. It also removed the unrealistic missions and replaced them with missions that make sense. Warning: it replaces the shooting gallery atmosphere of stock SH4 with the frustration in finding targets (sometimes) of the real war.
Remember, a real part of war is frustration. SH4 and these mods definitely model that frustration. Wait until you encounter (actually they will encounter you. You don't actually ever have to LOOK for trouble

) Ducimus' evil ASW aircraft, which can see you at periscope depth or perhaps somewhat below. You're making an approach and BOOM!!!! If you're not hit, you know that merchie out there now knows where you are and is taking evasive action. You aren't the hunter any more, you are the hunted. Forget that approach you set up over the last several hours, you're in survival mode now! Good luck pal...........
So: you have what it takes? Welcome aboard. Here's a Momsen Lung. You may need it.