Pilings/obstructions don't succeed without an accompanying battery. Pilings only delay, they don't prevent passage. Guns with their threat to wooden ships are what prevented vessels from steaming up and forcing a passage. The obstructions force ships to pass under the guns. It is not enough to be able to pass the obstructions once. One must be able to do so freely with supply vessels and transports. That's why Farragut had not won at Mobile until Fort Powell was abandoned--after that he could supply via Grant's Pass while the army systematically reduced Fort Morgan and Fort Gaines.

At McAllister it appears that the mortar battery was somewhat exposed to the fire of the ships. No, it was not direct, but it did have an effect. On Feb. 1, 1863 the mortar battery contended with the gunboats at long range, hitting none. Remember, unlike the monitors, the gunboats stayed out of the effective range of McAllister. Capt. Martin of the mortar battery said his men "were frequently covered with sand, and shell and fragments of shell frequently fell around us." This wasn't Fort Fisher with its long and relatively high earthen walls. Closer range fire by the monitors would likely have been far more problematic for the mortar crews, but they seem to have focused on the fort's guns instead. They proved a problem for the gun crews, even though the fort's gunners were firing directly at the monitor turret just before it fired.

Even Fort Macon gave the federal mortar battery trouble for a time during the siege bombardment, until the Parrot siege rifles knocked the enough of the fort's guns out of service and the mortars were able to improve their fire to keep the defenders from their pieces. Before that, damage to Flagler's mortar battery parapet had forced his gunners to lie flat and cease firing for a time. Later, while readjusting sighting stakes and serving as lookout one of the men of this battery was struck square in the chest by a 32 pounder shot--distance ~1 mile.

I made my Passaic soft area estimate based on a few drawings I found showing deck support beams center and aft--and also an image of the builder's half model for the class. Spacing of beams appeared to be about 7 units for every 22 total. I also noted that from above the deck projection was substantially greater than the hull area. One of the drawings showed some dimensions for the deck and hull width etc. I also subtracted the turret area--there was enough heavy stuff in the way there and below that I couldn't see a shot getting through to the hull through there. (A hit on the turret would likely knock out a gun, gun crew, and/or turret of course.) For the lower of my two estimates I also subtracted what I believed to be the sloped overhang of the armoured and backed sides above the hull itself. (For each I subtracted any overlaps so that I didn't double count my cutaway zones.) Not having full or easily legible drawings, there is a fair bit of estimation, but I'm not inclined to believe the effective numbers would move much one way or the other. The engine/boiler/steering estimate is far more approximate since I lack drawings of that (I compared to some other boat.)