It's possible that the detonation of the torpedo that damaged the Montauk was collatoral and not directly intentional. This is most likely the case. I doubt that reporter Osbourne was in a position to observe the explosion of the charge that cracked the vessels hull. Other federal vessels that waded into the torpedos used by the Confederacy showed more substantial damage than sustained by the Montauk. The damage sustained is evidence of some distance from point of explosion. Perhaps shrapnel from fire, concussion from same, small arms fire, or even some large sea creature contacting the smaller mine in order to get clear of the larger Montauk. That Osbourne mentions the vessel being "slewed around" suggests the symmetry of the blast was off center of the vessel.

As stated previously by me numbers of high angle firing weapons would have been necessary to be certain of destruction of any attacking vessel. The ability to improvise same existed within the Confederacy.

Have you read some of the documents pertinent to the development of armored vessels in the 1840's? Plunging fire was very much on Porter's mind then and later with the modification of the U.S.S. Merrimac into the C.S.S. Virginia.