Vince:

I believe I read this account it in Lowell Thomas's book "Raider's of the Deep". It has a number of colorful stories of WWI German U-boat exploits: A U-boat, pounding away with gunfire at near point blank range to sink a crippled British ship, hit some ammo that blew a heavy truck being shipped on deck up through the air and landing on the U-boat, sinking it. So, yes, this is a true story, but from WWI, not WWII.

Now, have I independently confirmed this? No. But it is not implausible. In WWI gunfire was a common way of dispatching merchant ships. Close range on a cripple - only a couple hundred yards or so - would be unusual but not unheard of for that era. And a heavy truck landing on a U-boat could swamp it, or crack open a pressure hull. Remember that WWI boats were a lot thinner skinned than the WWII boats, often with diving ratings of only a 50 to 75 meters.

I'll see if I can dig up the exact reference - I'm recalling this from memory.

Regards,

Kurt