near Invincible ships in port
 

A lot depends on the port
If the port is shallow the ship will settle on the bottom without actually sinking
So you need more torps to reduce hitpoints till ship " sinks " thru damage not from slipping beneath the waves :up:

Another issue you may not be taking into account is a torpedo impacting on a seawall or dock from an angle oblique enough to mistake it for an impact on target. I have seen it in 3 and 4. Repositioning usually resolves this, but one cannot tell from 2500m that the impact is grazing a dock and masking a "miss".

Also, a ship that is fighting it's way through high waves and stormy winds in a storm will take in far more water when hit than ships in calm waters.

I have managed to sink the Royal Oak with a single torpedo in rough seas, as storms deals a slight portion of damage to the ships itself.

Earlier i always avoided engaging ships in storms, now i tend to like it as the high waves make it easier to kill, but harder to aim, so get reeeealy close and comfy to them ;)

Royal Oak: Built in 1916, 31,000 tons full load, recieved anti-torpedo bulges between the wars.

Nelson: Built in 1935, 41,000 tons full load, anti-torpedo bulges built in.

There was a very large difference between the two.