How close to Spezia were you?
You can just as SkvyWvr says if you are close enough. Say within 50km I think, tho might have to be closer. I don't think it works if you're out in the mid Atlantic but could be wrong.
You could have hit a mine, anther ship, or even a subnet if near your base. Using 1024TC near base or any other time is asking for what happened to your sub. You're missing important info that your computer can't keep up with. If using SHIII Commander set your TC to1 when near other shipping or enenmy planes detected. You'll be fine then and not have the problem again. Just go into SH3 Options in SHIII Commander. I even use TC of 1 when receiving Radio Reports so I don't miss out on plotting a Convoy intercept asap. To be safe make your changes while in base so SHIII doesn't CTD. Same goes for mods too.