IIRC, the carrier's draft was ~25' or so. I had my fish set for 20'. All three hit amidships from just forward of the tower, to just aft of it. There was no flames or smoke following the hits, other than exhaust. The thing didn't even lose a knot's speed it seemed. He just went hard-aport and headed away from me. I noted there was not even the hint of a list either.

So they're not all that tough? Did I just find the one, Unobtanium-hulled carrier in the IJN?

Depends on mod, but most carriers you can sink with 3 torps, but the damage needs to be spread out some.. Funny, one time I hit a carrier in the bow with last two torps, it took on a forward list until the props came out of the water. It wouldn't sink, but the props were spinning fast out of the water, so it couldn't move either....