I started playing this game about a month ago. In reading the forums and the wiki, I've noticed that there is a lot of variation in ship classification. In doing a Google search to make sure I had a more or less realistic classification for my ships, I discovered there is a large variation in real-world ship classification, as well. There are a lot of nations that want to call their frigates destroyers (or vice versa in the case of some EU nations).
I decided to make a listing for my own reference, and have decided to share it with the community.
It can spot a 10,000-ton battleship [. . ]
-Aurora Wiki Active Sensor Design page
I included that quote from the wiki because it gave me a chuckle. This is like calling a chihuahua a horse.

In general, this is what I came up with through various Google searches on naval displacement, tonnages, and classifications. The most valuable source was the AMI International Definitions of Vessel Types (hxxp: www. amiinter. com/pagex. php?pg=vesseltypes).
<700 tons - Fast Attack / Patrol craft --
FACs are small (under 700 tons), fast (over 25 knots) vessels that are intended for quick, hit-and-run strike operations within 100 miles of the coast. In Aurora terms, any craft under 500 tons is treated as a fighter, so this class in Aurora is generally from 501 - 1000 tons.
700 - 2000 tons - Corvettes / gunships / coastal patrol craft --
Corvettes are generally the smallest platforms capable of accommodating the sensors, weapons, and combat systems needed to operate in a medium threat environment. In Aurora terms, this would be used for defense of a planet, moon, or listening post, but would not give chase to an enemy once driven off.
2000 - ~5000 tons - Frigates --
A frigate is generally the smallest surface combatant that can conduct extended blue-water missions in a high-threat environment.
~5000 - ~ 10,000 tons - Destroyers --
Generally, a destroyer is considered to be a ship that has all of the sensors (including a sophisticated phased-array radar), combat systems, and weapons needed to operate in a high-threat environment. This ship is the backbone of most navies as these ships are generally more cost effective than, and almost as capable as, cruisers.
~10,000 -
?? - Cruisers --
A cruiser generally displaces over 10,000 tons, and is fully capable of a wide-range of independent warfare operations in a multi-threat environment. These are the most familiar types of vessel in science-fiction. There does not appear to be an upper limit for cruisers. Even the old battleships could be classified as cruisers. Logically, though, the cruiser class shoufd probably end where the battleship class begins.
>~34,000 tons - Battleship/Dreadnaught --
The only real world equivalents are from WII, as no one has built a BB since ~1943. These ships are huge and expensive. Note, they are not necessarily slow! The Iowa class battleships of WWII could easily keep pace with the Essex class aircraft carriers they escorted and had a top speed of 31 knots. Respectable even for modern naval vessels. These ships carried the largest and most powerful armaments ever put on a naval vessel. The Yamato class battleships used 18" guns, the Iowas 16".
Some examples of past naval vessels and their sizes are below. (Taken from hxxp: www. rpsoft2000. com/shipsize. htm)
Passenger and Military Type Ships
approx. feet feet knots
date name owner weight (tons) decks people length width Speed
Passenger / Military
1912 Titanic White Star Line 46,328 9 3,000 882.5 92 21
1934 Queen Mary Cunard 81,237 12 3,131 1,019 28.5
1939 Bismark German War Ship 50,000 880 120 29.5
1962 Carrier Enterprise US Navy 89,600 5,830 1,101 133 30
1973 Carrier Nimitz US Navy 97,000 5,680 1,092 134 30
1991 Monarch of the Seas Royal Caribbean 73,941 11 2,350 880 106 19
2000 Explorer of the Seas Royal Caribbean 138,000 15 3,114 1,020 157.5 23.7
2001 Adventure of the Seas Royal Caribbean 138,000 15 3,114 1,020 157.5 23.7
2002 Brilliance of the Seas Royal Caribbean 90,090 13 2,501 962 105.6 25
2004 Queen Mary 2 Cunard 150,000 23 3,873 1,132 147.5 30
2006* Ultra-Voyager Royal Caribbean 160,000 4000+