Much as I love S-boats, I thought that missions using a Narwhal class submarine would be challenging.
However, there is no Narwhal or Argonaut class boat in SH2 or Pacific Aces, so I simply modified the parameters for the P Class to make it function as a Narwhal class boat (back up your original files).
1. Dive time increased to 86 seconds (per war patrol reports), max depth achieved in combat was 340 feet (test was 300). The Narwhal was a very large boat (349 feet versus 307 for a Gato). Believe me, you'll really start sweating when you have contacts and want to submerge (run decks awash, that helps).
2. External appearance (jrf, etc). Nothing changed since I'm not a hex editor; the appearance stays the same until some experienced PA modder creates the Argonaut or Narwhal class boat (don't think it would be too difficult, only time consuming).
3. Mission changes to historical scenarios- I didn't change much from stock, just edited the files in Topp with new starting points, return to base, and some objectives to reflect historical missions. Otherwise I reach the stock objectives and just patrol areas mentioned in war patrol reports-
http://www.esryle.com/coblinks/links/PATROLREPORTS.html
4. Guns- Narwhal class used two 6"/53 caliber guns (load times mentioned were once every ten seconds according to the NavWeaps website). The P Class has a 5" gun, I didn't change any features otherwise.
5. Nautilus had a TDC, but being the hardcore type I use manual TDC.
Nautilus was fitted with SD radar in August 1942.
So, while you're technically still sailing a P Class (at least externally), the changes to the boat's handling are pretty noticeable.
Any modder from PA out there interested in creating a Narwhal class boat?
Yours, Mike