Go slow and the sub will charge faster. Don't expect too much out of the old Sboats. You will appreciate the fleet boats more after spending valuable time in the SB. The weather works ok. Get yourself some historical convoy and military route maps and patrol along the routes between missions. You will get a lot of targets. Here is a source of maps that have worked for me:
http://www.valoratsea.com/cmbtcht.htm
-Pv-