Regarding the largest caliber gun used as an AA armament (in the USN, at least), there was the 6″ 47 cal. on the CL-144 Worcester class. Only two were built, and each mounted 6 twin 6″ turrets. The 6″ turrets rate of fire, and training and elevation, couldn’t cope with the coming of jet aircraft and anti-ship missiles, so they didn’t serve very long.

The AGS on DDG-1000 is not intended as an AA weapon.

Here you can learn more than you ever wanted about this gun: http://www.navweaps.com/Weapons/WNUS_6-47DP_mk16.htm

First off Sr D notes
” Close-in situations (or where one does not want to put a bird farm) can be addressed with Patrol Torpedo (PT) boats based on LCAC hulls.”

1) My hole snipe insights tell me that we already know what a PT boat is and does. You know, close in knife fights with bigger ships, slip up behind and stab in the back.

2) this does not help the boots on the beach with bad guys shooting at them. We still need EXPENDABLE hulls with 5″ or larger guns with which to shoot back. There I said it. we need to have enough hulls so that we can loose a few – or several and still deliver the goods.

So, why don’t we base the LCAC on that concept.

3) Personel andAdmin of the thing is a major problem, careerism is it’s name and it’s killing the navy. No one wants to be in a slot with no path to Admiral. That is the biggest problem with the current situation.

There are 300 years of data on hull design to do what you need. We just forgot that people have to have a way to survive and succeed in it.

Because the Legend class cutter isn’t a warship. Coast Guard cutters in general are not warships. They are coastal patrol and rescue ships with light armament for use in law-enforcement operations. I’d guess that converting a cutter, even a Legend cutter, to a true warship would entail a lot of internal and external modifications, and even then you’d probably wind up with a lot of compromises. If you want a warship, it’s better to build it from the start as a warship.

Which isn’t to say that the Navy couldn’t design a warship on the Legend class hull. 400 feet and 4300 tons is a useful hull size no matter what you want to do with it. OTOH, the Zumwalt is 50% longer and three times the displacement, so it’s debatable whether the Navy planners would accept a Legend hull as big enough.

Cobb’s idea for a PT/LCAC has a basis in early proposals for the basic LCAC design tossed around at JHU/APL in the early ’80s. I’ve read the book(s) several times and, while there are very interesting concepts, the story line was written against a semi-plausable hull configuration. Be that as it may, I think Cobb’s Sea Fighter is a better concept than his Cunningham (aka Zumwalt) class destroyer.

The Raytheon concept for a much larger hull/engine combination lost due to the sheer size and awkwardness of a helo capabable platform (not to mention the cost to engineer the skirt and fans).

There is a very fundamental argument regarding littoral warfare. The Navy must challenge the idea of larger hulls against smaller, faster, more maneuverable hulls. Littoral warfare is not only green water, it is brown water or even no water. The enemy is not battleships, cruisers or even destroyers. Rather, corvettes, frigates, cigarette boats and the odd rubber raft. The environment is restricted and reaction time measured in seconds. PT boats were, pound for pound, some of the most powerful hulls available to the Navy during WWII. Update the idea, improve armor and weapon systems to answer the need. Don’t spend billions on something that is not proven.

Oh, sorry for the sarcasm, the opening was too large to avoid. 8-}

Any gun of any caliber “can be used” in an AA role. The Japanese Navy developed antiaircraft shells for 16″ battleship main guns, for pete’s sake. How effective were they in that role? Not very, by the accounts I’ve read.

The US Navy put VT (radar proximity) fuses in projectiles of all sizes from 40mm to 5-inch, and they were murderously effective against conventional aircraft. OTOH, it seems from contemporary accounts that even with VT fuses, most AA kills were from crossfire, not by the targeted ship’s own guns. And most were also saturation kills, with so many ships firing multiple shells, and so many shells exploding around the target, that it was nearly inevitable one would eventually do lethal damage. Then there was the simple number of barrels firing: about twenty 5″ guns on a battleship or fleet carrier, 12-16 on a CLAA, 6-8 on a typical CA or CL, five or six on a destroyer, … and still, many kamikazes got through.

Then again, WW2 aircraft were largely empty space. Unless shrapnel hit a vital part (engine, pilot, control cables), the AA fire had to rip the plane apart to stop it. Cruise missiles, with their small, highly streamlined, stuff-filled airframe, may be more vulnerable.

As for the “wall of water” tactic, yes it will work — IF you have the range set just right AND the timing just right AND the inbound is low enough to hit the water spray in the handful of seconds before it falls again. And you’re likely to get only one shot per inbound. Sounds like a big gamble to me.

Could the Zumwalt’s main gun be used as an AA defense? Certainly. Should it be trusted as an effective AA defense against supersonic sea-skimmer missiles? I wouldn’t. Not until it demonstrated its ability to target and hit multiple inbounds from multiple directions with a high rate of success.