Originally posted by Reportage what would one look for in such a vessel?
When speaking of a naval ship, the meaning of flagship is precise and is not the same as the usage as the word when NOT talking about naval ships. The common useage is derived from the more precise meaning, and bears a resemblance to the more precise meaning, as by analogy. The analogy, specifically, is more in line with the historical outward appearance of a flagship in the 18th/19th centuries.
A flagship is associated with a fleet or squadron commander. Usually this is a "flag" officer. There is a flag associated with the officer/commander and the flag can be transferred to another ship at which point that ship becomes the flagship.
So, for example: "When Yorktown was struck at Midway... Fletcher transferred his flag to the cruiser Astoria and placed Spruance tactically in charge."
It is, in its nature a temporary designation. The duration may be relatively long, but it is temporary as the flag follows the officer.
I think the senior flag officer in the USN is Chief of Naval Operations. Here is his flag:
Admiral Gary Roughead, USN is the 29th and current Chief of Naval Operations. I do not think he has a designated flagship, but I suppose that under certain circumstances once could be designated.
What would one look for in such a vessel? Putting aside the difficulties raised above, generally the criteria for flagships have evolved.
In the age of sailing ships, the flagship was typically a first-rate... First Rate was the designation used by the Royal Navy for its largest ships of the line. While the size and establishment of guns and men altered over the 250 years that the Rating system held sway, from the early Georgian period the First Rate comprised those ships mounting 100 guns or more on three gundecks.[1] In the Nelsonic period, First Rate vessels carried over 800 crew and displaced in excess of 2,000 tons. (cf: Wikipedia)
In the 20th century, ships became large enough that most types could accommodate commander and staff, and during World War II admirals would often prefer a faster ship over the largest one. Some larger ships may have a separate flag bridge for use by the admiral and his staff while the captain commanded from the main navigation bridge.
Because its primary function is to coordinate a fleet, flagships are not necessarily more heavily armed or fortified than other ships. Increasing communications and computing requirements have resulted in the design of specialized command and control ships to serve as flagship. (cf: Wikipedia)
On this latter point, see this article:
USS Blue Ridge (LCC-19) - Wikipedia, the free encyclopedia
That smallish ship is the flag ship of the United States 7th Fleet. Not exactly what you would expect. Clearly there has been an evolution here...
woof