Why ships moored in ports often cannot be identified?
 

I mean after I get close to about only 500m from a stationary large ship (a 20k ton troop transporter confirmed in log after its sunk) and raise my periscope to observe it clearly, still the game doesn't realize that there is a ship right in front of me, neither does it discover several smaller vessels around her although they're all so near to me, as if they're only background, strangely the only ship which is identified is a fishing boat faraway...While in open sea a ship will be identified and reported several miles away once you can see it in periscope.

Is it because they're stationary so the sonar man cannot identify them by the sound of their propeller?

Yes. Use the hydrophone manually, and set their "Unknown" type to "Merchant".

If they're not moving, the soundman shouldn't be able to hear them at all, and neither should you. They should identify with the periscope, just the same as at sea.

I'm not sure why you're having this problem.

Hydrophone is an interesting thing. In the torpedo exam, the visual targeting of the target ships is very slow, takes 1-2 minutes (despite the clear weather), unless I hand-change them from Unknown to Merchant in the hydrophone.

Another interesting thing I've seen that occured with the LRT's no-collision mod: sunk ships kept on bouncing all around on the seafloor, and emitting "Merchant" "noises" (actually silence).