Henry,

Let's break this down. The chances of achieving a single full through shot with a mortar shell fired at a stationary monitor are slim. If such a hit occurs, then you can count on the boat trying to make its way back down river. Getting underway might take a few minutes, but the boat wouldn't be caught flat footed (since that sort of possibility always existed during an exchange.) The chances of mortars hitting it underway are an order of magnitude worse than the original hit. They would be lucky to get off one or two shots apiece. I'm not familiar with a moving vessel being hit by mortar fire at any time during the war, but I haven't been looking for it--perhaps something like that happened around Charleston. The only two mortar hits that come to mind are vs. the Passaic, and a reference to a work barge at Dutch gap. I believe both were stationary when struck.

I don't disagree that the Southern port defenses needed many more mortar tubes. They would have provided some deterrent, but they weren't going to drop consecutive rounds on a boat. The fire from the tubes is intrinsically far more scattered than that. Howitzers (seacoast, siege, and field) had lousy accuracy but were still an order of magnitude better than the best mortar fire of the time.

Whether or not the Montauk's torpedo damage was equivalent to a 10" hole I cannot say. It sounds similar--same order of magnitude anyway. It was certainly not the massive type of hole that would put a vessel on the bottom within a few minutes. One can rough out the initial leak rate assuming a head and orifice flow calcs--a quick glance at the Crane fluid flow tables is a useful cross check. That's how I came up with 3-4,000 gpm for a 10" hole.

The statements indicate that the Montauk reached the cover of the gunboats before deciding to beach (and presumably ended up doing so farther down stream.) The gunboats normally stood off out of effective range of the fort and provided harassing fire from a distance.

There is no reason to believe there was any rifle fire from the fort or from the other shore at this time. There were several folks exposing themselves on the deck at the time and no mention of fire by them. It appears that the action was essentially over.

The whole idea of firing a period rifle 800+ yards at an unseen, submerged cap is completely implausible to me. I know one thing, if someone had made such a shot, there would have been a whole lotta bragging going on. Such was not the case. The US Navy did some tests of water penetration and lethality near the end of WWII with .50 and .30 ammo. Straight on vertical the 50 caliber could only penetrate a 1" pine to a depth of 4-5 feet and the 30 caliber only 1 foot (did not penetrate at 2 feet.) At a 45-60 degree angle the 50 caliber lost another foot. Wonder if I can convince someone to let me borrow their rifled musket and pool for the afternoon?

I have some idea what type of critters are in the area and I'm a diver myself. Having dived with and touched such creatures I can assure you that they are not that clumsy and even a bump is rather gentle. I suspect there would have been a noteworthy quantity of blood visible had one inadvertently detonated a device.

As it is, Osbon's comment about passing over three torpedoes is intriguing. This may have been a supposition based only on observed cloth markers thought to be for torpedoes. Or it could suggest that the crew thought they heard caps snapping (as was mentioned in the running of the Mobile torpedo field.) This would be an indication of waterlogged torpedoes. In fact, if the torpedo took on water and sank I suspect it would probably end up lying closer toward the muddy bank (this was on a bend.) I don't have information on the particular type of torpedo used or how arranged. However, it wouldn't surprise me if the boat pinning the device against the bank actually provided the resistance needed to pop a cap that produced the explosion. (Any chance of a time delay based on orientation?) Powder behind some caps might have been wet, but popping several might find some sufficiently dry powder.

To me the simplest and most straightforward/plausible answer that matches witness statements is a torpedo with mostly wet powder or other defect.