Tides of War

The passive sonar on subs and surface ships should have a blind spot at the back of the ship like occurs on real ships and subs

Yes! I absolutely agree, this would give skilled players a good option for making convoy attacks.

The Asdic in game only "shines" in a cone out the front. Hydrophones on the otherhand can see EVERYWHERE.....

Right, so hydrophones should have a blind spot in the rear. I'd say about the same radius as the Asdic, 12 or 13 degrees, maybe a bit more. Enough to be significant, but small enough that you can compensate for it by slowly zigzagging.

Isn't the blind spot caused by the sounds of the props turning at the back of the ship? I think I read that in a book somewhere.

for Hydrophones, yes. However when a ship slows down or stops the engine, they will find you. The AI knows to try this if they miss you on a sweep (and they knew of your existence).

I don't recall seeing pictures that show the layout for asdic sensors of the period, but I do know that modern destroyers that have their sensor pack at a location both at the bow and below the keel. That implies that the blind spot is minimalized. The ping only gives you range to target. The pivot mounted hydrophones, if they can't inmediately get a bearing, it would mean that they have to turn around to get their bearing.

The twisting track of an overhead escort has nothing to do with listening, it's just a way to cover the widest possible area when dropping depth charges. It means that if you're close enough to see them twist you know that you are not going to be hit.

That's why I prefer to keep the escorts behind me. If you have them in front of you, you have to go through his drop pattern.