Ok, I'll throw my hat in the ring here too...

Did an entire patrol with Contact+Influence set and most of those puppies went right under the keel and kept going (had the depth right for a back-breaker, etc etc)

Will try flipping the switch to contact only and see how that goes on my current patrol.

I tested this too and it seemed to make no difference, both settings allowed magnetic detonation under the hull.

I did notice that you had to be pretty close to the bottom of the ship, a few feet or less for it to work. I think that has been changed since SH3.

I'm changing my mind on this, I think. I'm thinking that what appears to be a detonation under the hull is actually the hull's "hit box" being a bit larger than the 3D dimensions of the ship.

Also, does the magnetic detonator use the 3D model or the ships dimensions in the sim to determine depth below keel. If we use the Mogami as an example, the listed depth of its keel is 14.5 ft., but you have to run the torpedos at 25 ft. to clear the hull itself. If the magnetic detonator is using 14.5 ft., then that is over 10 ft. of separation, which might be deep enough to cause it the influence detonator to fail.

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Good point - will try a few on this patrol.