Looking for answers to another topic I found this great bit of information from a book by Bud Gruner, who was not only captain of USS Skate but also technical advisor and supplier of information for SH1.
http://www.hnsa.org/doc/subsinpacific.htm
Did you know?
Only 1/3 of ships attacked actually sank.
US submarines were designed for a patrol lenght of 8 weeks, or 56 days. Three boats had patrols lasting 80 days or longer.
Average number of merchant sunk per war patrol: 0.8.
A lot of good stuff from someone who was there.