Do any of you guys know how thick the W W 2 navel mine fields were. On the movies, they look like the mines are about 10 feet apart.
All of the submarines used in WWII were 20-30 feet wide (excluding the Japanese midgit suicide-subs and the British coastal defense ones). So setting mines at 19 feet apart would make it impassable for submarines.
But that would require an awful lot of mines to cover any significant area of the sea. It might be possible for a limited area, like a harbor approach. (But in places like that you need your own ships to be able to get through.)
Naval mines, back in the day, needed a certain amount of separation to prevent “unintentional countermining” or accidentally detonating when a neighboring mine detonated. When the extremely laborious task of clearing the WW1 North Sea Barrage series of minefields was being done, numerous instances occurred where minesweepers would detonate one mine, only to have a chain of mines go off up to a mile away from the original detonation. The book “The Northern Barrage” (Taking Up The Mines), published by the United States Office of Naval Records and Library, goes into this demining operation in some detail. You can find the full text at archive.org.
It is surprising difficult to find an easy cite for naval minefield laying procedures, even for historical naval minefields. In WW2, according to the book, Hellcats: The Epic Story of WW2’s Most Daring Submarine Raid, Japanese naval minefields, using the Type 93 contact mine, would be laid in two to three rows, each row 400 to 1000 yards apart. Each mine within the row would be 75 to 100 yards apart, to minimize the probability of the accidental demining mentioned above. A nice site that talks about Royal Navy minelaying operations in WW2 may be found here. It takes a lot of mines of deny a sea area to an adversary, even with modern, bottom-laying guided mines.
Mines were set based on the draft of ships that were desired to be excluded from the area—up to one hundred feet, according to the book—minus a small amount to account for ocean currents. (Currents would push on the mine, causing it to sink a small amount below that if it were able to float directly above its anchor.) Since submarines move in three dimensions, a minefield to exclude submarines must have mines moored at different depths.