Hi
Some clarification seems to be needed here:
I haven't seen any references to HMS Ultimatum and U-864.

However, if you check a bit around on the pages on Uboat.net you will find the following:


About HMS Ultimatum (and U-431 and U-73):

U 431:
Previously recorded fate (Last revised by FDS/NHB during November 1987).
Sunk 30 Oct, 1943 in the Mediterranean southeast of Toulon, France, in position 43.04N, 05.57E, by a torpedo from the British submarine HMS Ultimatum.

This attack was actually against U-73 inflicting no damage.


About U-864:
On 9 Feb, 1945 the British submarine HMS Venturer, commanded by James S. Launders, torpedoed and sank this boat. This is the only known incident in all of naval warfare in which one submarine sinks another while both are submerged.



- Both involved u-boats were submerged! One u-boat sinking another while both were surfaced happened quite a few times.


Cheers
Vidar