I always thought that a bold was a device making noise to attract the destroyers... And indeed, it seems to work like that in SH3. I never thought that it was creating a "cloud" of bubbles to stop/divert ASDIC sound...
In the film "The enemy below", the U-Boat releases a black "smoke" underwater which makes her (the u-boat) invisible to ASDIC. I don't know if it's historical.
Sometimes also in SH3, I encountered a "bug" : if a destroyer (or maybe it was a corvette) is on the point to run above you (to throw DC over your head), once you launch the bold, this destroyer (or corvette) stops immediatly and goes reverse (exactly the same way as if she wanted to avoid a collision)....
