The Historic Naval Ships Association (HNSA) has provided a massive amount of documents and manuals all about naval ships. There are many that are pertain to WWII submarines.
These should provide hours of reading and discovery if you are interested in the how-tos of WWII fleet submarines. They might even help you in the creation of a SH mod.
They are constantly adding new things, so check out their Documents and Manuals page for updates.
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UPDATE: All American WWII submarine war patrol reports are now available online thanks to HNSA. You can also purchase these reports on disc individually at Submarine Memorabilia.
TONS OF WAR PATROL REPORTS ONLINE HERE!
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The Fleet Type Submarine, Navpers 16160, is the first in a series of submarine training manuals that was completed just after WW II. The series describes the peak of WW II US submarine technology.
Submarine Information and Instruction Manual, 1942 was used for training on board S-class (built 1918-1925) submarines.
Submarine Torpedo Fire Control Manual, 1950. This describes how to plan a submarine's approach and attack using the fire control systems on Fleet and Guppy submarines with straight running torpedoes.
Torpedo Data Computer Mark 3, Mark 3, Mods 5 to 12 inclusive, O.P. 1056, 1944 describes the most successful torpedo fire control computer of WW II. It is considered a masterpiece of mechanical computing design.
Submarine Attack Course Finder Mark I Model 3, O.D. 453, 1922. This describes the circular slide rule commonly know as an "is-was" used to calculate an approach and attack by an submarine.
Torpedo Angle Solver Mark VIII Operating Instructions, O.D. 3518, 1941. This describes the hand operated torpedo angle solver commonly called a "banjo". It is used to calculate the gyro angle when the Torpedo Data Computer is not available.
5-Inch Gun Mount Mark 40, OP 1029, 1944, describes the five inch, 25 caliber, wet mount gun used on U.S.N. submarines near the end of WW II.
20 MM Antiaircraft Gun, OP 911, 1943, is a Navy service manual for the anti-aircraft gun of choice during the early years of WW II.
20 MM Antiaircraft Gun Mount, OP 909, 1943, is a Navy service manual for the 20 mm gun mounts used aboard ships during during the early years of WW II.
40 MM Antiaircraft Gun, OP 820, 1943. The maintenance manual for the Bofors 40mm gun. This was the most widely used anti-aircraft gun of WW II.
A Brief History of U.S. Navy Torpedo Development, by E.W. Jolie, OP 353W or TD5436, 1978. This provides basic information on most US Navy torpedoes up to 1978 (Submarine, Surface and Air).
Torpedoes Mark 14 and 23 Types, OP 635, 1945, is the basic service manual for the principle U.S.N. steam torpedo of WW II.
U.S. Navy Torpedo Mark 18 (Electric), OP 946, 1943, is the basic service manual for the principle U.S.N. electric torpedo of WW II.
The Submarine Commander's Handbook, ("U.Kdt.Hdb."), 1943 describes the submarine U-boat tactics of Nazi Germany.
Standard Submarine Phraseology, describes a set of uniform voice procedures and phraseology for use throughout the U.S. WW II submarine fleet.
Unit Course in Marine Electricity, 1942. Museum ships stabilizing or restoring their electrical systems will appreciate many parts of the manual. Items such as the proper ways of working with armored cable, lacing, etc. are included.
Introduction to Radio, 1946. An introduction to electronics and naval radio. It covers basic tube electronics in the beginning, then explains the basic operating instructions for typical WW II radios.
Notes on Servicing Radio and Sound Equipment, 1942, is a training manual that introduces the art of tube radio and sound equipment repair. This included both receivers and transmitters, and the motor generators they use.
Transmitter Servicing Course 503/4, 1945, is a training manual that focuses on WW II naval transmitters TAJ, TBK, TBL, TDE and TBS.
Radar Operator's Manual, 1945. This is a WW II radar operator's manual. It covers the most typical radars of WW II.
Arma Gyro-compass Mark 7, Mod. 4, 1943, is a service manual for the preferred U.S. submarine gyro-compass of WW II. This was also used on small surface ships that had fire control, but only one gyro. It is very similar to the Arma Mk 8 Gyro-compass used on larger CA, BB, and CVs.
Catalogue of Electronic Equipment, NavShips 900,116, 1952, is a U.S. Navy catalog of electronics that includes almost all the WW II equipment and some of the early Cold War equipment.
Visual Call Sign, DNC 4(A), 1945, is a U.S. Navy list of visual (as compared to radio) call signs used during WW II.
SS-381 - USS Sand Lance - Booklet of General Plans, 1943, Balao Class
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There are other numerous great sources of information out there. PigBoats.com is a great resource on submarines from the Holland up to early fleet boats like the Tambor class.
And of course, if you want to actually SEE most of this stuff in person, make a trip to your nearest museum submarine. There are 6 Gato, 8 Balao, and 2 Tench class submarines here in the USA that you can tour. More on those can be found at SUBMARINEMUSEUMS.ORG.