This is why the Allies' secret DD tank failed on D-Day
If you've seen the blockbuster movies The Longest Day (currently on Netflix) or Saving Private Ryan, a big part of the story is how infantry fought through the obstacles on Omaha Beach (the wisdom of sending two divisions into that meat-grinder can be debated at another time).
But the lack of tank support wasn't part of the plan. In fact, it was one hell of an instance where that notorious and unwelcome Murphy's Law put in an appearance, costing the infantry some much-needed support. It would have been their secret weapon: the Dual-Drive, or DD, tank.
M4 Sherman modified into a DD tank. (British government photo)
The DD tank was a modified M4 Sherman that had a large canvas screen and propellers to enable it to swim in to shore from a distance. Tanks-Encyclopedia.com notes that the M4 had some good firepower for busting up fortifications — a 75mm gun with 90 rounds. At close range, that gun would more than do against the Nazi fortifications.
This is how the DD tank was supposed to work. Note the calm seas. On D-Day, they seas were rough. (YouTube screenshot)
There's just one problem: the DD tanks weren't tested in rough seas. Almost all of them ended up sinking when eight-foot-tall waves swamped them. And a tank on the bottom of the channel can't provide support for the grunts. In short, the grunts had to do the hard by themselves.
So, take a look at this History Channel video, and a piece of D-Day history some folks would like to forget.