Brilliant! I read somewhere that because of my setup or game version (Steam with U-boat missions, 16GB computer), I didn't need the 4GB patch, but I researched it some more based on your suggestion.
The patch I found said "no such directory" when I tried to apply it, but then I found this:
http://www.subsowespac.org/the-patro...ry-boost.shtml
and it looks like everything works now! No white patches on the screen, no CTDs (I'm crossing my fingers.) I can captain a boat now!
I'm even running the RSRDC_RFB_Pre Pearl Harbor Career Start mod now, and no problems.
Thanks for your help!!!
Btw, I noticed with the "TMO" Porpoise mod we're both running it adds a captain's cabin room ("Interior Aft"), but everything's dark except for some dim light bulbs. I wish it showed up like it did in TMO itself with all the room detail. But at this point, I'll take it. :P