Federation Starbase 23 - Essays & Rants

Federation Starfleet Ship Classifications


"Cruisers and Destroyers and Frigates, Oh My!"

By Scottish Andy


This article was inspired by the confusion in both fanon and licensed Star Trek works about the classifications of Starfleet's starships. This confusion is generated in part by the fact that Starfleet's vessels are not merely the warships that Navy vessels are in the present-day world, and neither are their crews only military people.

Or to put it another way, how often do you see scientist officers on a Daring or Ticonderoga? Uniformed diplomatic personnel?

As such, Starfleet's ships cannot be pidgeonholed into the narrow 18th-, 19th-, 20th- or even 21st-Century warship types – and yet they still are, as the creators of these designs do not take the extra step into the future that these ships inhabit to see that the responsibilities of a particular ship type could be expanded from their traditional roles, or not fit into any traditional roles at all.

And so comes the problem of how to – of even if we should – apply current warship designations to the ship designs we want to include in our Star Trek universe.

The confusion over these ship classifications is mainly concentrated around the smaller ship types – the eponymous frigates, destroyers, and cruisers – and just what their roles in Starfleet are outside of bare military designations. Or more specifically: how is each design of ship actually different from one another, both in terms of similar hull forms in different roles, and different hull forms in similar roles?

Factors in the Classification

  1. The Mass of the vessel: More mass = more volume = more capabilities.
  2. Engine power: How far and how fast can it go?
  3. Fire-power: How much deterence can it offer, what punishment can it dish out?
  4. Hull configuration: General layout is defined by the role, but is the role likewise defined by the layout?

Traditional Classifications


During the Age of Sail from the 14th to the 18th Centuries the term "cruising" referred to certain kinds of missions fulfilled by a sloop or later a frigate, such as independent scouting, raiding, or commerce protection. As such, these vessels were referred to as the "cruising warships" of a fleet. The term "cruiser" was meant as the purpose or mission of a ship rather than a category of vessel. However, the term was nonetheless used to mean a smaller, faster warship (as compared to a Ship of the Line) suitable for such a role.

In the 17th Century, a Ship of the Line was generally too large, inflexible, and expensive to operate to be dispatched on long-range missions (for instance, to the Americas), and too strategically important to be put at risk of fouling and foundering by continual patrol duties.

During the 18th Century the frigate became the pre-eminent type of cruiser. A frigate was a small, fast, long range, lightly-armed (possessing of only a single gun-deck) ship used for scouting, carrying dispatches, and disrupting enemy trade. The other principal type of cruiser was the sloop, but many other miscellaneous types of ship were used as well. In 1748 the informal useage of "cruiser" became codified by Lord Admiral George Anson of the British Royal Navy to be a classification which included all Fourth-, Fifth and Sixth-rate frigates and sloops, as being fit for patrol and convoy work.

The 19th Century saw the introduction of the steam-powered ironclad. The first ironclads of the 1860s were frigates in the sense of having one gun deck, but as the most powerful units in the fleet they were effectively Line of Battle ships. Smaller ironclads were developed for overseas cruising duties. These "station ironclads" were the beginning of the development of the "armoured cruisers", a type of ironclad made specifically for the traditional cruiser missions of fast independent raiding and patrol. These cruisers were split into two categories: "protected" and "armoured". Protected cruisers had only a single deck of iron covering the machinery spaces. Armoured cruisers added to this with side "belt" armour and upper deck armour.

Unarmoured cruising warships, built out of wood, iron, steel or a combination of those materials, remained popular until towards the end of the 19th Century. The ironclad's armour often meant that they were limited to short range under steam, and many ironclads were unsuited to long-range missions or for work in distant colonies. The unarmoured cruiser – often a screw sloop or screw frigate – could continue in this role. Even though mid- or late-19th Century cruisers typically carried up-to-date guns firing explosive shells, they were unable to face ironclads in combat.

Shortly after the turn of the 20th Century the growing size and power of the armoured cruiser resulted in the battlecruiser, with an armament and size similar to the revolutionary new dreadnought battleship. However, the Battle of Jutland showed the battlecruiser's vulnerability in fleet battles and the type was discontinued. The surviving battlecruisers were later refitted with increased armour, slowing them, and battleships became faster, finally resulting with the dispensing of the battlecruiser type altogether due to the advent of the Fast Battleship. In WWII, the fast, light capital ships were referred to as "battlecruisers", such as the legacy WWI British BCs, the German Scharnhorst-class, and the French Dunkerque-class ships.

At around the same time as the battlecruiser was developed, the distinction between the armoured, protected, and unarmoured cruisers finally disappeared. By the introduction of the British Town class, it was possible for a small, fast cruiser to carry both belt and deck armour, particularly when turbine engines were adopted. These "light armoured cruisers" began to occupy the traditional cruiser role once it became clear that the battlecruiser squadrons were required to operate with the battle fleet.

Some light cruisers were built specifically to act as the leaders of flotillas of destroyers.

After WWII the obsolescence of the battleship left the cruiser as the largest and most powerful surface combatant. The role of the cruiser varied according to ship and navy, often including air defence, commerce raiding, and shore bombardment. The U.S. Navy in the Cold War period built guided-missile cruisers primarily designed to provide air defence, while the navy of the USSR built battlecruisers with heavy anti-ship missiles designed to sink NATO carrier task forces. The Kirov-class' main roles are Fleet Command and Anti-Ship Warfare, though it packs substantial Anti-Air and Anti-Sub missile capabilities. Most other cruisers are Anti-Air protection for battlegroups and as such can be classed as "escort cruisers".


Originally conceived in the 1890s as large, swift, and powerfully-armed torpedo boats designed to destroy other torpedo boats, the "torpedo boat destroyer" first evolved in WWI to become light vessels with little endurance for unattended ocean operations; typically a number of destroyers and a single destroyer tender operated together, and often a number of destroyers would be led by a destroyer leader or light cruiser flotilla leader.

The initial purpose of the destroyer was:

  1. Screening the advance of a fleet when hostile craft were expected.
  2. Searching hostile coastlines for unseen enemy presence.
  3. Blockading a port to spy on comings and goings, harass enemy shipping, and prevent the return of ships to that port.
  4. Attacking an enemy fleet.

In WWI, capital ship engagements were scarce but destroyers saw constant raiding and patrol actions. Later, the evolution of massed submarine attacks led destroyers to become primarily Anti-Submarine Warfare (ASW) escorts designed to detect and sink submarines, with their Anti-Surface Ship Warfare (ASuW) role much reduced.

By WWII, with cruisers still performing in the Anti-Ship role, destroyers evolved again to become multi-purpose vessels with both Anti-Air Warfare (AAW) and Anti-Sub capability. Their brief also changed, to escort not only convoys but also battle groups. As such they increased in size to improve their seakeeping capabilities and endurance in open ocean, and in value as targets themselves.

After WWII, the advent of the guided missile allowed destroyers to take on the surface combatant roles previously filled by battleships and cruisers. This resulted in larger and more powerful destroyers capable of independent operation. By the 1980s they were up-sized to cruiser-weight hulls and became fast and manoeuvrable yet long-endurance warships intended to escort larger vessels in a fleet, convoy, or battle group and defend them against smaller, powerful, short-range attackers.

Currently, destroyers are the largest surface combatants used in most of the world's navies. Only the U.S. and Russia still field missile cruisers, with destroyers (now up-sized to cruiser-weight; ref: Daring, Arleigh Burke, Zumwalt classes) taking their role as defenders of the fleet.

In effect, they became light cruisers.


In the 18th century, the term "frigate" referred to ships which were usually as long as a Ship of the Line and were square-rigged on all three masts (full rigged), but were faster and with lighter armament and used for patrolling and escort. Very specifically, during the Age of Sail a frigate had a single gun-deck and in later years this was used as the primary definition of a frigate.

Frigates were perhaps the hardest-worked of warship types during the Age of Sail. While smaller than a Ship of the Line they were formidable opponents for the large numbers of sloops and gunboats, not to mention privateers or merchantmen. Frigates could also carry marines for boarding enemy ships or for operations on shore.

Able to carry six months' stores, they had very long range. Frigates scouted for the fleet, went on commerce-raiding missions and patrols, and conveyed messages and dignitaries. Usually frigates would fight in small numbers or singly against other frigates. They would avoid contact with Ships of the Line, but frigates were involved in fleet battles, often as "repeating frigates". In the smoke and confusion of battle, signals made by the fleet commander, whose flagship might be in the thick of the fighting, might be missed by the other ships of the fleet.

Unlike larger ships that were "laid up in ordinary" (placed in reserve, tied up at dock under officers and a skeleton crew), frigates were kept in service in peacetime as a cost-saving measure and to provide experience to frigate captains and officers which would be useful in wartime.

Frigates remained a crucial element of navies until the mid-19th Century. The first "ironclads" were classified as "frigates" because of their single gun deck. However, terminology changed as iron and steam became the norm, and the role of the frigate was assumed first by the protected cruiser and then by the light cruiser in WWI, and finally the destroyer in WWII.

Frigates are often the vessel of choice in historical naval novels due to their relative freedom compared to ships of the line (kept for fleet actions) and smaller vessels (generally assigned to a home port and less widely-ranging).

In modern navies, frigates are used to protect other warships and merchant-marine ships, especially as anti-submarine warfare (ASW) combatants.

Leading to even more confusion, throughout naval history ship classes dubbed "frigates" have also more closely resembled corvettes, destroyers, cruisers, and even battleships.

Modern frigates are related to earlier frigates only by name. The term "frigate" was readopted during World War II by the Royal Navy to describe an anti-submarine escort vessel that was larger than a corvette, smaller than a destroyer, and were about equal in size and capability with the American destroyer escort. Anti-submarine escorts had previously been classified as sloops by the Royal Navy, and the Black Swan-class sloops of 1939-45 were as large as the new types of frigate, and more heavily armed. 22 of these were reclassified as Frigates after the war in 1948, as were the remaining 24 smaller Castle class corvettes.

The frigate was introduced to remedy some of the shortcomings inherent in the corvette design: limited armament, a hull form not suited to open-ocean work, a single shaft which limited speed and manoeuvrability, and a lack of range. The frigate was designed and built to the same mercantile construction standards (scantlings) as the corvette, allowing manufacture by yards unused to warship construction. The first frigates of the River class (1941) were essentially two sets of corvette machinery in one larger hull, armed with the latest Hedgehog anti-submarine weapon.

The frigate possessed less offensive fire-power and speed than a destroyer, but such qualities were not required for anti-submarine warfare.

It was not until the Royal Navy's Bay class of 1944 that a British design classified as a "frigate" was produced for fleet use, although it still suffered from limited speed. These anti-aircraft frigates, built on incomplete Loch-class frigate hulls, were similar to the United States Navy's destroyer escorts (DE), although the latter had greater speed and offensive armament to better suit them to fleet deployments. The American-built destroyer escorts serving in the British Royal Navy were rated as Captain-class frigates.

The introduction of the surface-to-air missile after the Second World War made relatively small ships effective for anti-aircraft warfare: the "guided missile frigate".

From the 1950s to the 1970s, the United States Navy commissioned ships classed as guided missile frigates which were actually anti-aircraft warfare cruisers built on destroyer-style hulls. These "frigates" were roughly mid-way in size between cruisers and destroyers. This was similar to the use of the term "frigate" during the Age of Sail where it referred to a medium-sized warship, but it was inconsistent with conventions used by other contemporary navies which regarded frigates as being smaller than destroyers. During the 1975 U.S. Navy ship reclassification, the large American frigates were re-designated as cruisers or destroyers, while ocean escorts (the American classification for ships smaller than destroyers) were renamed as frigates.

One of the most successful post-1945 designs was the British Leander-class frigate, based on the previous Type 12 anti-submarine frigate but equipped for anti-aircraft use as well. Still later models had provisions for a light Anti-Ship role with the addition of Exocet container-launchers. This sets the precedent for role-specialised sub-variants in a single class.

Nearly all modern frigates are equipped with some form of offensive or defensive missiles, and as such are rated as guided-missile frigates (FFG). Improvements in surface-to-air missiles (e.g., the Eurosam Aster 15) allow modern guided-missile frigates to form the core of many modern navies and to be used as a fleet defence platform, without the need for specialised anti-air warfare frigates.

The Royal Navy Type 61 Salisbury class were "air direction" frigates equipped to track aircraft. To this end they had reduced armament compared to the Type 41 Leopard-class air-defence frigates built on the same hull. This sets the precedent for "scout frigates" with weapons reduced to allow increased sensor and data processing capability.

Multi-role frigates like MEKO 200, Anzac, and Halifax class frigates are designed for navies needing warships deployed in a variety of situations that a general frigate class would not be able to fulfil and not requiring the need for deploying Destroyers.

Many modern frigates are also specialised for the ASW role, usually in larger navies which have sufficient depth to allow for class specialisations. These ships carry auxiliary craft (helicopters) to extend their reach and increase their offensive punch, defensive depth, and general utility with helo transfers of everything that would normally have been done by lowering a boat.

However, to include all these capabilities once again modern frigates have been up-sized to match or exceed their predecessor destroyer classes and indeed have taken the role of their destroyer predecessors also (ref: German Type 125 Baden-Württemberg class).


A small, manoeuvrable, lightly-armed warship. Age of Sail corvettes were smaller than frigates and larger than sloops. Originally the British used the term "Sloop" where the French used the term "Corvette" to describe the same type of ship. Later, the Royal Navy adopted the term "Corvette" to refer to a ship larger than a Sloop.

Steam corvettes were used in less advanced regions of the world such as colonial holdings for battles with aboriginals.

WWII corvettes were an easily-built patrol and convoy escort vessel which could be quickly mass-produced, primarily or exclusively geared towards ASW operations. Originally designed for offshore patrol work, they were ill-suited for long-range escort duties and had almost no anti-air defences. With these limitations in mind, the new frigate design was created which then supplanted the corvettes in that escort role.

Modern corvettes are down-sized frigates which most industrialised nations can build in their commercial shipyards, though sophisticated tactical and surveillance systems often have to be imported. They mount a mix of Anti-Aircraft, Anti-Submarine, and Anti-Surface Warfare weapons in small numbers, and often a small helicopter. Many modern navies use ships smaller than frigates for coastal patrol but do not use the term "corvette"; "Offshore Patrol Vessel" or "OPV" is the an oft-used substitute term.

While originally smaller than a frigate, some current corvette classes have been up-sized to frigate-weight to allow for greater capability and versatility.


In the Age of Sail, sloops were originally unrated (i.e. mounted less than 20 guns) warships of any type.

In the 20th Century, inter-war steam-powered "screw sloops" were non-battlefleet warships intended for gunboat diplomacy deployments, surveying duties, and convoy escort. They could not make fleet speed, but were fast enough to escort their convoys.

The only class of sloops from WWII – the Royal Navy's Black Swan-class – were a specialised convoy escort vessel with sophisticated Anti-Air and Anti-Sub capabilities, but also used in the Minesweeping role. With their increased sophistication they were unable to be mass-produced quickly and easily, and so were dropped from production in favour of the simpler corvette and frigate types. These sloops had superior Anti-Air fire control to both frigates and destroyers. They also had heavier weaponry, were larger, longer-ranged, and faster than contemporary destroyer classes.

Transposition into the Star Trek Universe

What Era-Analogy of Naval History Does Star Trek Occupy?

This may seem like a bizarre question, but consider it more deeply.

Currently, all warship design is based around the threat of air power. Ships must be capable of defending themselves against air attack, or be protected by those that can. But in Star Trek we see no fighters until Deep Space Nine's Dominion War, though small armed craft do seem plentiful through The Next Generation's run. We certainly have never seen a fighter carrier in any canon Star Trek series. As such, defence against massed fighter attack is not of paramount concern to Star Trek's ship designers.

Despite their undoubted complexity, Star Trek's ships are therefore not directly comparable to the ships of our current navies. To have a more logical and realistic analogy, we have to wind back the clock until before the rise of naval air power; i.e. pre-Pacific Theatre in World War II.

Similarly, both cruise and ballistic missile analogies in Star Trek are unheard of so likewise defence against massive Beyond Visual Range (B.V.R.) missile attack is similarly not considered in Star Trek ship design. In canon Star Trek we do not even see the equivalent of Anti-Ship or Surface-to-Surface Missiles (ASMs or SSMs).

The 'Star Fleet Battles' tabletop game and its computer game adaption 'Starfleet Command' do have fighters, carriers, and drone-armed ships. These games being the wellspring of my entire writing venture they are very definitely included in Starbase 23's canon. However, these are tactical-range assets only.

With regards to missiles, if you have no lock-on to your target – i.e. they are not in your ship's sensor range – you cannot fire your drones at them. Further, drones are very much a secondary armament to all Starfleet Command species except the Mirak, and maximum salvoes are of twelve (12) drones.

Similarly, Starfleet Command's fighters are short range craft initially only capable of low warp; in the inverse of our real world, the carrier vessel is faster than the fighters. This means the equivalent of massive long-range alpha strikes against enemy targets hundreds of miles away could not happen.

Thus we also have to wind back the clock in this aspect to a time at the dawn of the guided missile era, or circa 1960-70.

The design of smaller ships of today's larger navies are geared towards the other primary threat they face: submarines. In Star Trek, the analogy for the submarine is the cloaked ship. In today's navies, submarines are prized for their very deniability and the deterrence offered to opponents merely at the thought of an enemy submarine being in the same area as their valuable naval task force. Being able to surveil or attack from beneath the waves, never revealing themslves, is the hallmark of today's submarines.

However, in Star Trek the cloaked vessels have to decloak to attack. They are less analgalous to our modern nuclear-powered attack subs than their honoured predecessors who preferred to surface the boat to attack by deck gun rather than torpedo from under the waves. Thus we similarly have to wind back the clock on these capabilities and counter-capabilities to the era of WWII again.

In conclusion, it is apparent to me that Starfleet of the 23rd Century at least, and quite likely through to a century later, is in a modified-Jutland Era of WWI as opposed to the Pacific Theatre of WWII or the current modern-day era. All the ships of the various navies are "Big Gun" warships, with the later current-world developments of air power, guided missiles, and permanently-submerged submarines only in their infant stages, and held there due to the nature of their enviroment.

And as such, the ship types utilised by Starfleet and the other "big navy" nations will reflect this state of affairs.


I originally intended here that Starfleet's battlecruiser would be a multi-threat-capable fleet command vessel like the Soviet/Russian Kirov-class; this would have made her the Excelsior-class as both Starfleet Command and Klingon Academy have it. On reconsidering, however, this Kirov-style role actually makes the Starfleet equivalent its own dreadnought type: fleet command capabilities with massive defensive and offensive fire-power. So, in light of this Starfleet's battlecruisers will actually be stripped-down heavy cruisers with increased weaponry.

In the absence of massed interstellar or interplanetary missile attacks, Starfleet's heavy cruisers will be "multi-role gun cruisers", capable of a myriad of duties of widely differing natures.

Starfleet's light cruisers will merely be smaller, less capable multi-role cruisers with less mission endurance, created as a cost-saving measure to free up heavy cruisers for their deep space role by replacing them on short-range missions.


In Star Trek the submarine analogy is cloaked ships, but these ships cannot fight cloaked; they must decloak to attack. Thus Starfleet destroyers maintain their primarily WWI roots as fleet screening elements and anti-ship combatants, with long-range convoy escort and space-lane patrol their primary missions in the absence of war. On tense borders they will act as quick responders, often in division or squadron strength depending on the strength of the incursive force.

With the advent of high-speed drones and fightercraft as first utilised against the Federation in the Federation-Mirak War of 2275, escort destroyers (not to be confused with destroyer escorts) are developed to once again protect their charges from these "new" threats.


As with destroyers above, there is no submarine threat to face or specialise for, and the smaller a Trek ship gets the more vulnerable to fighters and missiles it becomes. We have specialised and multi-purpose variants, with the specialised versions being smaller.


Offshore / Inshore Patrol Vessels in Star Trek terminology would be utilised by non-Supra-National Navy organisations; i.e. a Federation world's independent planetary defence fleet. The OPV could be called an "Interplanetary Patrol Vessel". Inshore Patrol Vessel would be called an "Orbital Patrol Vessel", inverting the current acronyms. To maintain the same acronyms, we could use instead "Outer-system Patrol Vessel" and "Inner-system Patrol Vessel".


In all cases, a ship type is:

  1. Introduced for a specific role in the fleet developed from a need or perceived need.
  2. Over time from its introduction asked to do more, often or usually taking on duties from the type above it.
  3. The type is gradually up-sized to be able to include the capabilities needed to perform the extra roles it takes on.
  4. In essence, the previously-smaller type becomes the type formerly above it in all but name, which it retains.
  5. A new type of ship is now needed to perform the duties left open by the ascension (re-roling and up-sizing) of the previous type.


Battle Cruisers (Hull Code BC; In Service: 1908-1918) were the upper split of the Armoured Cruiser classification. As with their precedessors, Battle Cruisers were designed to operate as a long-range, independent warship, capable of defeating any ship apart from a battleship and fast enough to outrun any battleship it encountered.
However, as more were built they began to operate in squadrons as part of the main fleet. The Battle of Jutland suddenly revealed them as being too fragile for fleet actions, and by the end of the war capital ship design had developed with battleships becoming faster and battlecruisers becoming more heavily armoured, blurring the distinction between a battlecruiser and a fast battleship. Also the Washington Naval Treaty, which limited capital ship construction from 1922 onwards, treated battleships and battlecruisers identically. All this led to them being discontinued as a type.

Armoured Cruisers (Hull Code CA; In Service: 1873-1911) were large cruisers with large-calibre main guns which encompassed the technological progression from sails to steam. They were designed like other types of cruisers to operate as a long-range, independent warship, capable of defeating any ship apart from a battleship and fast enough to outrun any battleship it encountered. They were the predecessors of the Battle Cruiser type and were supplanted by them.
The sole exception is the German panzerschiffe ("Armoured Ship") class of the late 1930s which the British famously nicknamed as "pocket battleships": the Deutchland-class, which was itself formally reclassified by the Germans in 1940 as a Heavy Cruiser. This class featured six battleship-cailbre main guns of 11" on a large cruiser hull.

Heavy Cruisers (Hull Code CH; In Service: 1922-1957) were more powerful Light Cruisers with either larger mid-calibre guns (8" or more) or far greater tonnage (more than 10,000 tons). With the rise of guided missiles in the late 1950s giving cruisers even longer-ranged firepower, large calibre guns were quickly phased out of new cruiser design even as cruiser tonnage increased, giving rise to the Guided Missile Cruiser classification.

Light Cruisers (Hull Code CL; In Service: 1908-present) were the lower split of the Armoured Cruiser classification from pre-WWI, used to classify smaller cruiser-role ships with smaller main guns but which also mounted an armoured belt.
With the demise of the Armoured Cruiser and the later introduction of the Heavy Cruiser classification in the London Naval Treaty of 1930, the distinction in cruiser classification now resided in the calibre of a cruiser's guns, as its hull mass was highly variable. Any cruiser mounting guns of 8" or more were Heavy Cruisers regardless of tonnage; similarly cruisers mounting guns of 4" to 6" were Light Cruisers. Some Light Cruiser hulls mounted Destroyer-calibre dual-purpose guns for use as Anti-Aircraft Cruisers.
The rise of the guided missile in the 1960s caused the cruiser to evolve again. No longer mounting multiple turrets of two or three big guns each, cruisers once again split but this time along national philosophies. The Soviets built cruisers mounting Anti-Ship Missiles (ASMs) to attack NATO surface groups, and NATO developed the Air Defence Cruiser designed to defend both themselves and their task force against massed missile attacks.
This new paradigm lasted for another 30 years until building such large, complex warships becme too expensive. At that point many of the roles that defined a cuiser were taken up by another smaller, purportedly cheaper, ship type: the Destroyer.

Destroyers (Hull Code DD) were created as attack ships and by WWI they were surface combatants designed to act as a screen for a fleet, seeking out and destroying opposing attack ships to prevent them attacking the larger ships. They were also formed into squadrons to be the attack ships that opposing destroyers would seek out and engage to protect their own ships. In WWII they then develop into purely escort ships, initially against the emergent sub threat and then again as anti-air escorts. Now destroyers are the multi-role combatants replacing more expensive cruisers with Anti-Air, Anti-Surface, and Land Attack capabilities, and often with a helo for Anti-Sub operations.

Frigates (Hull Code FR) were introduced in their current form in WWII to take over the ASW role from Destroyers, being smaller, cheaper, easier to build and faster to produce. Post-WWII frigates evolved with Missiles to offer modest (close-in, self-defence) Anti-Air capabilities, sufficient at the time for aircraft attacking with "dumb" iron bombs and small numbers of missiles, and Helos to extend their Anti-Sub reach. With Destroyers taking on the roles of cruisers, the Frigates had to include even more capabilities to take on what the Destroyers were discarding. To accomate all this, modern frigates have been upsized to destroyer-weight.

Corvettes (Hull Code CT) were originally coastal/littoral-only patrol ships built to commercial specs specifically for short-range ASW duties. They have since evolved into "multi-role light frigates" for nations that cannot afford even a modern frigate, with the addition of capabilities and the increase in size required to include them.

Summary of Analysis and Examples

Sloop (Hull Code: n/a)
In History: In the Age of Sail a small, unrated warship. In WWII a sophisticated frigate.
Star Trek Role: Since I am not using the Age of Sail definition of a Frigate I'll dispense with the Sloop type in my Starfleet and merely use Corvette.

Corvette (Hull Code: CT)
In History: In the Age of Sail a small, manoeuvrable, lightly-armed warship larger than a sloop but smaller than a frigate. In WWII a slow and short-range convoy escort ship with an anti-submarine specialisation which is small and quickly-built. In modern times a small coastal patrol vessel with limited multi-threat counter-capability that is built to commercial/civilian specifications. Larger classes are often used in the frigate role.
Star Trek Role: In-system patrol and interplanetary escort. Space lane patrol in quiet regions.
Perfect for my Class Two (Oberth-variant) specialised spacecraft. Black Swan, Asmodeus, Orca can all be classified as Corvettes with different armaments.

Frigate (Hull Code: FR)
In History: In Age of Sail, a cruiser. In WWII, an ASW convoy escort. In modern times, a respectable dual-threat (ASW & AAW) escort with limited ASuW capability. Has helos for extended reach in ASW & ASuW.
It is not a fleet combat unit but used for patrol and escort duties. Very effective against unsophisticated and lightly-armed pirates.
Star Trek Role: Interstellar patrol and escort. Space lane patrol and convoy escort in regions with known but light pirate activity. Anti-piracy patrol of unclaimed star systems within UFP space. Perform sweeps of star systems in regions with light pirate activity and no major belligerent interests. Perform preliminary surveys and investigations of all objects of interest discovered.
Just like I have my Okinanwa-class Falklands doing.

Scout Frigate (Hull Code: FRS)
In History: Post-WWII an air defence radar picket ship providing advance warning of air attack, with a reduced weapon suite to make room for more sensor and data processing equipment. Star Trek Role: ECM/ECCM support of task group operations. Picket duty for important convoys and small task groups. Scouting and charting less important sectors. SIGINT, COMINT, & ELINT gathering & processing duties.
A standard frigate which has heavy weaponry replaced by more powerful and sensitive sensor systems and processing power. Developed to increase the number of scout-capable hulls more quickly and introduce scout capability to barren regions where a full-size or "fleet" scout would be wasted.

Perimeter Action Ship (Hull Code: PA)
In History: No analogy in the current world or in history, these ships are a combination of the small Offshore Patrol Vessel and Battleship with all systems geared for offence.
In Star Trek: These ships are the opposite end of the spectrum from the Heavy Destroyer. They are, in essence, a Frigate outfitted for the traditional duties of a Destroyer. All the limited multi-role aspects of a Frigate are removed to make space for the tactical capabilities of a Destroyer, such as: increased weapon emplacements over a Frigate, greater torpedo storage, stronger (i.e. larger) shield generators, no diplomatic or cargo capability, drastically reduced science capability, much smaller landing / boarding party contingent, etc. Sometimes referred to as Light Destroyers, Small Destroyers, or even War Destroyers as these are the terms other nations use for ship types with these capabilities. This is a TacFleet ship type.
Star Trek Role: Perimeter / border space patrol. Early warning of impending incursion. First response to perimeter / border violations.

Destroyer (Hull Code: DD)
In History: Pre-WWI a defence unit to destroy small torpedo boats attacking capital ships. In WWI a picket unit and hunter-killer, screening a task force against enemy ships, seeking out their enemy counterparts, and attacking capital ships in packs.
In Star Trek: Hunter-Killer fleet unit. Medium-range but fast and well armed.
Star Trek Role: Rapid Deployment Emergency/Incursion Response. First response unit for ships in distress, and to stabilise the situation if need be. Often operates in packs or loose squadrons when responding to border incursions. Interstellar patrol and escort. Space lane patrol and convoy escort in regions with high pirate activity. Offensive escorts for high-value targets in a hostile region; their escort brief is to attack the attacking ships.
Just as I have Saladin-class Cortés doing.

"Fleet" Scout (Hull Code: ST)
In History: No analogy in the current world or in history.
In Star Trek: A scout built on a Destroyer hull, has heavy weaponry replaced by more powerful and sensitive sensor systems and processing power. Introduced as a "Fleet" or full-sized Scout to be deployed with a fleet for enhanced sensor range and capable of fleet speeds and long-range solo endurance. Intended mission profile is geared less for top end speed and more for extended running at maximum sustainable speed. As such, hull frame is heavily reinforced and power to the SIF is as high on Heavy Cruisers.
Star Trek Role: ECM/ECCM support of task group operations. Picket duty for large task groups and fleets. Scouting and charting unexplored space. SIGINT, COMINT, & ELINT gathering & processing duties in both military and scientific/exploratory capacities.

Super Scout (Hull Code: SS)
In History: No analogy in the current world or in history.
In Star Trek: A scout built on a Destroyer hull, has heavy weaponry replaced by more powerful and sensitive sensor systems and processing power. Has additional superstructure dedicated to active searching or passive listening sensor antennae, or a mix of both. Introduced as a means to increase the sensitivity of on-board sensors. Intended mission profile is geared for maximum top end speed to zoom the vessel out of danger and less for extended running at maximum sustainable speed. As such, hull frame is heavily reinforced and power to the SIF is as high on Heavy Cruisers.
Star Trek Role: Sensor "tripwire"; Distant Early Warming of border incursions. Monitoring of opposition fleet communications and ship movement. Scouting and charting unexplored space. SIGINT, COMINT, & ELINT gathering & processing duties in both military and scientific/exploratory capacities.

Escort Destroyer (Hull Code: DDE)
In History: In WWII a frigate-sized vessel with weapons loadout geared towards Anti-Submarine Warfare, with limited or no capability for attacking other ships.
In Star Trek: A standard destroyer which has all-defensive weaponry; that is, geared to block or destroy incoming fire.
Star Trek Role: Convoy and task force escort in high-threat regions and situations; their brief is to protect and to — if need be — physically shield their principal from incoming fire.
Just as I have Saladin-class Drake doing.

Command Destroyer / Destroyer Leader (Hull Code: DDC/DDL)
In History: In WWII a larger desroyer-type vessel which provided space, equipment and staff for the destroyer flotilla commodore (who typically held the rank of captain), including a wireless room, senior engineering and gunnery officers, and administrative staff to support the officers.
In Star Trek: A standard destroyer with more advanced and extensive C4I capabilities at the expense of other capabilities. In the Saladin-class, several science labs are replaced with a "flag bridge" or CIC. As such, they are TacFeet vessels; their scientific capability is reduced to that of a frigate.
Star Trek Role: Co-ordination and direction of anti-pirate activities across several star systems. Co-ordination of small-scale and/or escort-level joint exercises. Co-ordination of small-scale Sapientarian Assistance/Disaster Relief (SADR) activities. Command asset for anti-ship destroyer group in times of war.

Light Cruiser (Hull Code: CL)
In History: Pre-WWI a long-range armoured patrol vessel mounting mid-calibre guns for duty around the world patrolling distant colonies and outposts.
In Star Trek: The longer-legged work-horses of Starfleet. Whereas Frigates patrol closely-spaced star systems or quieter regions where time is less critical, Light Cruisers cover the further apart systems and more active regions. With greater range and increased capabilities over frigates both qualitively and quantitively these ships are in much demand for "milk-run" missions. Also able to carry a full squadron of fightercraft for greater interdiction capability if assigned to a mission with a profle that requires them.
Star Trek Role: High-priority cargo and personnel delivery in further apart systems and more active regions. Colony resupply and colonist checkups for star systems on the UFP frontiers. Anti-piracy patrol of unclaimed star systems on the UFP frontiers and regions with widespread pirate activity. Takedown of discovered pirate bases. Survey missions and initial scientific investigation of systems and phenomenon within their region of responsibility.

Scout Cruiser (Hull Code: CLS)
In History: Pre-WWI a cruiser which was smaller, faster, and mounting smaller-calibre guns and less armour than Protected Cruisers, used for fleet scouting duties and as destroyer flotilla leaders.
In Star Trek: Built on a Light Cruiser hull, has heavy weaponry replaced by more powerful and sensitive sensor systems and processing power. Can have additional superstructure dedicated to active searching or passive listening sensor antennae, or a mix of both, introduced as a means to increase the sensitivity of on-board sensors. Intended mission profile is geared less for top end speed and more for extended running at maximum sustainable speed and long-range solo endurance. As such, hull frame is heavily reinforced and power to the SIF is as high on Heavy Cruisers.
Star Trek Role: ECM/ECCM support of task group operations. Picket duty for large task groups and fleets. Scouting and charting unexplored space. SIGINT, COMINT, & ELINT gathering & processing duties in both military and scientific/exploratory capacities.

Heavy Frigate (Hull Code: FH)
In History: The Continental Navy's 44-gun "super frigate" is the only historical precedent, and these vessels were simply larger and more heavily armed frigates.
In Star Trek: A Light Cruiser outfitted exclusively for the patrol duties of a Frigate. Cargo, diplomatic, and scientific capabilities are drastically reduced and provision made for a moderate increase in auxiliary craft and landing / boarding parties, rendering it a TacFleet ship type. Able to carry a half-squadron of fightercraft for additional interdiction capability.
Star Trek Role: Neutral Zone/Border patrol.

Heavy Destroyer (Hull Code: DH)
In History: No analogy in the current world or in history.
In Star Trek: Can be either a up-sized Destroyer or a downsized Battlecruiser (or failed Battlecruiser design like the Menagha). In essence, a Light Cruiser or even a Cruiser hull outfitted for the traditional Destroyer role of attacking other ships or installations in a fleet combat situation or time of war. Instead of a multi-role cruiser, the removal of diplomatic and scientific facilities renders it a TacFleet ship type.
Star Trek Role: Offensive destroyer operations such as attacking larger or well-defended pirate bases, hunting the raiding destroyers of other nations, and screening battlegroups in times of war.

Escort Cruiser (Hull Code: ECL)
In History: In WWII, a Light Cruiser outfitted with dual-purpose high-angle destroyer-calibre main battery guns for dealing with both light surface vessels and aircraft.
In Star Trek: Built on a Light Cruiser hull, it has all-defensive weaponry, i.e. geared to block or destroy incoming fire. The Escort Cruiser is the flip side of the Heavy Destroyer coin; a Light Cruiser hull outfitted for defensive escort duties.
Star Trek Role: Protection and shielding of high-value targets such as large important singleton ships; vital convoys; and battlegroups containing troop ships, dreadnoughts, and fleet carriers.

Command Light Cruiser / Light Cruiser Leader (Hull Code: CLC)
In History: In WWI this larger ship effectively performed the same role as the smaller Destroyer Leader type.
In Star Trek: This type has more advanced and extensive C4I capabilities at the expense of other capabilities, such as cargo space, passenger quarters, or science failities being exchanged for a "flag bridge" or CIC.
Star Trek Role: Co-ordination and direction of wide-spread anti-pirate activities across a sector. Co-ordination of small-scale and/or escort-level joint exercises. Co-ordination of small-scale Sapienitarian Assistance/Disaster Relief (SA/DR) activities. Command asset for anti-ship destroyer group or light cruiser squadron in times of war.

Exloratory Cruiser (Hull Code: CE)
In History: During the Age of Sail, various Barques and Sloops of the British Royal Navy were outfitted as long-range exploratory ships. Prime examples are Lieutenant James Cook's H.M.S. Endeavour (1768) and Charles Darwin's H.M.S. Beagle (1831).
In Star Trek: Built on a Heavy Cruiser hull, it retains heavy weapons, main armaments and defences of its base class. However, it uses downgraded or second-line offensive equipment, mounts greatly reduced tactical equipment and computer systems, and embarks fewer tactical crew. In exchange for these it embarks greater scientific capability through an increased complement of science perrsonnel, expanded laboratory and research facilities, and a greater complement of specialist science auxiliary craft.
Star Trek Role: Exploration of deep space regions outside of the spheres of influence of the Federation's main adversary nations of the Klingons, Romulans, and Tholians. Long-duration exploratory cruises of still-unexplored Federation interior space.

Hull Classification Codes

After some supplemental work, I came up with the following definitive system. The ships will have a 2-character hull code denoting their fleet role as follows:

Class One Multi-Role
Class Two Specialised

Within these basic hull classifications and/or standard roles are more specialised variants leveraged for a specific mission profile. They are as follows:

Frigate Hulls:
Destroyer Hulls:
Light Cruiser Hulls:
Heavy Cruiser Hulls:
Carrier Hulls:

Within these Standard and Specialised hull types there are equipment and internal layout differences which further modify the role of the vessel. They have standardised code suffixes to denote what these roles are, as detailed below: