By Chuck Hill

Classification of surface warships as cruisers, destroyers, frigates, or corvettes, has become like pornography. There are no generally accepted definitions, but “I know it when I see it”–except that everyone sees it a little differently.

Since this is “Corvette Week” what are we really talking about?

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My Combat Fleets of the World, 16th Edition, which I have used here extensively for reference, defines Corvettes as, “Surface Combatants of less than 1,500 tons but more than 1,000 full load displacement–essentially, fourth rate surface combatants.”  But goes on to note that “…the designation as used here essentially refers to smaller frigates and does not correspond to the European concept of corvettes as any warship larger than a patrol craft but smaller than a frigate.” I feel to confine the definition within a 500 ton range is too restrictive. In fact it would have excluded the Castle class corvettes of WWII as too large, and other corvettes as too small.

Pre-WWII

During the age of sail, corvettes were originally warships typically smaller than a frigate, but larger than a sloop, usually with guns on a single deck. Some ships continued to be called corvettes as steam was introduced, but in the Royal Navy, in 1877, corvettes along with sloops and frigates were subsumed under the new designation “cruisers.” Corvettes, as a type, essentially disappeared from the English naval lexicon until 1939. The term was kept alive in some navies (including the French, German, and Italian) as a rank that translated corvette-captain, a rank generally equal to Lieutenant Commander.

World War II

Corvettes as a type reemerged just prior to WWII. As it became clear that U-boats would be a major threat, Britain saw the need for an escort vessel that could be built quickly and in large numbers, in yards that had not been considered capable of building warships. Just before WWII, they ordered the first of 267 “Flower Class” corvettes that were built in the UK and Canada. They modified the design for a whale catcher named Southern Pride, enlarging it to 205 feet overall and a displacement of 1245 to 1390 tons. They were terrible warships, weakly armed, cramped, uncomfortable, and slow. Single screw, reciprocating steam propulsion gave them a maximum speed of only 16.5 knots, a knot slower than a typical (Type VII) surfaced U-boat. They were originally intended only for coastal operations, but because of their long range, they were thrown into the Battle of the Atlantic, where they were by far the most numerous transatlantic convoy escorts for the critical early years, taking slow merchant convoys across the mid-Atlantic air gap, while the Home Fleet’s more capable, but shorter legged, fleet destroyers were generally held back to escort the battle fleet or met convoys only as they approached the British Isles.

Reportedly Winston Churchill had a hand it designating this new class “corvettes,” probably in an attempt to make them appear more glamorous than the term “patrol vessels” which had been applied to similar vessels previously. Two years after the re-introduction of the term “corvette,” the term “frigate” was also resurrected to describe another war emergency escort program, this one more complex and more capable but still using reciprocating steam propulsion. Larger commercial yards converted to making frigates (301 to 307 ft, 1920 to 2420 ton), but smaller yards continued to make corvettes of the improved Castle class (252 ft, 1590 to 1630 tons), while naval yards continued to produce small numbers of sloops like the Black Swan class that were the true premier ASW escorts of the Royal Navy.

Australia also built corvettes, 60 ships of the similar but even smaller, slower Bathurst Class (186 ft). Initially they were classified as minesweepers, but found more employment as escorts, so were more frequently referred to as corvettes.

Japan, Germany, and Italy all made similar escort ships, but only the numerous Italian  Gabbiano class (193 foot, 728 tons, with combined diesel or electric propulsion no less),  were actually referred to as corvettes.

All of the WWII corvettes were primarily ASW escorts, but their were a number of classes of vessels, many built prior to the war, that share DNA with today’s missile armed corvettes. These were small, fast, torpedo armed vessels that resembled destroyers, but most had a standard displacement of 1000 tons or less. Usually they were referred to as “torpedo boats.”  Japan built twelve, The Germans built 48 (the last 15 were large enough to have been considered destroyers in other navies). The French Navy completed twelve. The Italians completed 69 (some of which were closer to frigates or destroyer escorts). The Italian Spica class (269 ft, 885 to 1,030 ton, 34 knots) may serve as an example.

Generally, the war emergency programs had one thing in common. They were not the ships these navies would have chosen to build in peacetime. In wartime priorities change; planning horizons contract. Producibility may trump quality. They were all compromised in some fashion–in their speed, survivability, weapons, or economy of operation. Corvettes filled a need for large numbers of escorts, but after the war, most were quickly discarded.

The MCM Connection

The Flower Class Corvettes were originally also equipped to sweep mines. As noted the Australian Bathurst Class began life as minesweepers. While the US built no “corvettes” during the war, the minesweepers of the Raven (220 foot/1040 tons), Auk (221 foot/1,250 tons), and Admirable ((180 foot) classes frequently functioned in this role. In fact, with minor modification Admirable class ships were redesignated PCEs (Patrol Craft, Escort). All these minesweepers were built with sonar. By the end of the war, most were equipped with hedgehogs, depth charge projectors (K-guns) and dual depth charge racks, having enjoyed priority for ASW equipment second only to destroyer escorts.

Post WWII

Since the end of WWII corvettes have generally fallen into two categories, with some designs attempting to incorporate elements both types. They tend to be either:
—Small, fast, missile armed vessels optimized for ASuW, like Sweden’s Visby Class (40 knots, 239 ft, 650 tons) usually expected to operate in groups, either with others of their kind or acting as flagships for even smaller missile boats, or
—Smaller versions of frigates with moderate speed optimized for patrol and presence in peacetime and escort during wartime like the Damen designed SIGMAs or  India’s Kamorta Class (25 knots, 358 foot oa, 3100 tons).

Largest Operators of Corvettes

The largest operator of corvettes is Russia with approximately 53 (3 Buyan, 1 Buyan M, 7 Parchim II, 23 Grisha V, 4 Grisha III, 2 Dergach Project 1239, 13 Nanuchka) (80 if you count the 27 Tarantuls that fall slightly below the 500 ton threshold I have assumed).
India, China, South Korea, Indonesia, and Italy also maintain large numbers of corvettes.

Corvettes in the USN

While the US Navy has never built corvettes for its own use, the type is not without precedence in the US.

In the early days of WWII, when the US navy was desperately short of escorts, 18 Flower class corvettes were transferred to the USN. Eight of those were manned by USCG crews.

In the 50s the Navy was interested in experimenting with types that might be built hurriedly in an emergency. The result was the four ships of the Claude Jones class (DE-1033-1036) built by Avondale between 1956 and 1959. At 312 feet long and 2000 tons, they were essentially the same size as the preceding Dealey Class, but they were  simplified, diesel powered, slower, and more lightly armed. These ships were really a update of the corvette concept of a cheap simple escorts that lent itself to rapid construction. (Similarly about the same time the British were building 14 HMS Blackwood Class  (Type 14) that were “2nd Rate Frigates” of 1536 tons, powered by a single shaft steam turbine plant with no gun larger then 40mm.)

In the late 1960s the US built four corvettes, given US hull numbers PF-103 to PF-106, that were immediately turned over to the Iranian Navy. They became the Bayandor Class (275 feet long, 1,135 tons).

In the early ’70s, two additional PF-103 class ships (PF-107 and 108), built to a modified design, were delivered to Thailand’s Navy. These were the Tapi Class.

Between 1977 and 1983 Tacoma Boat built a class of four CODOG powered “PCG” for Saudi Arabia, the Badr class, 245 feet, 1,038 tons, 30 knots.

Between 1983 and 1987 Tacoma Boat built two diesel powered “PFMMs” for the Thai Navy Ratanakosin class 252 foot, 960 tons, 26 knots.

Between 1989 and 1995 Northrop Grumman Litton built three CODOG Corvettes for the Israeli Navy, the Sa’ar 5 class, (281 foot, 1,275 tons, 33 knots).

Between 2008 and 2013, VT Halter Marine has been building a class of four missile corvettes for the Egyptian Navy, the  Ambassador MkIII class (205 feet, 700 tons, 41 knots). The first has already been delivered.

While the Littoral Combat Ships are not normally considered corvettes, on June 10, 2013, Rear Admiral John F. Kirby, the Chief of Information for the Navy called them Corvettes. Without a mission module or aviation detachment, they are really more like OPVs. But when the Mine Warfare module is mounted they become MCM vessels. When an ASW or ASuW module is mounted, they start to look like corvettes.

The Claude Jones class ships were transferred to the Indonesian Navy and continued in service there until 2006. Of the six PF-103 class ships, two Iranian ships were lost in combat with Iraq, but the remaining four are still in service with the Iranian and Thai Navies and have been updated. The Badr class and the  Ratanakosin class are still in service with their respective navies, and the Sa’ar Vs are still the most advanced surface ships in the Israeli Navy. All but the two Thai Navy Ratanakosin class (PF-107 and 108) have been equipped to launch anti-ship cruise missiles.

The Coast Guard Connection

During WWII Coast Guard Cutters were frequently used as ASW escorts, some quite successfully, filling corvette and frigate roles. After the war, new construction frequently included provision for ASW systems either as built or as planned upgrades in the case of a major conflict.

The 16 Reliance class Medium Endurance Cutters (210 feet, 1,050 tons, 18 knots) delivered 1964 to 1969, were built with provision for adding sonar, hedge hogs, and torpedo tubes. They were originally to have been designated PCs. a designation shared with the sub chasers of WWII.

The 12 Hamilton Class High Endurance cutters (378 feet, 3,050 tons, 29 knots) completed 1967 to 1972, were built with ASW systems installed and their systems were upgraded and provision for harpoon installed 1989 to 1992. As built, they were not the equal of contemporary Destroyer Escorts with their AN/SQS-26 sonars, but were comparable to those built only a few years before. An argument can be made that these ships, as built and later modified, could be considered, if not frigates, at least corvettes.

The thirteen Bear class cutters (270 feet, 1,780 tons, 19.5 knots) completed 1983 to 1990, were built without ASW systems, but had provision for adding a towed array and supporting a LAMPS I helicopter. If these systems had been provided, then the ships might have also been considered corvettes.

The Coast Guard’s National Security Cutters, of the Bertholf class (eight ships planned, 418 ft/4,500 tons) have no installed ASW systems or ASCMs, but they do have excellent aviation support facilities and the ship has been marketed as the basis for a frigate program. Aside from Exocets carried by the French ships, they are in most respects more capable warships than the Floreal “light surveillance frigates” (307 ft/2950 tons) and similar to the French Lafayette Class frigates (410 ft/3,600 tons) which also currently have no sonar.

The Coast Guard is in the process of procuring a new class to replace its Medium Endurance Cutters. The resulting ship is likely to be similar to the Floreal class (90 to 100 meters in length and 2500 to 3500 tons) but faster and will share sensors and some weapons with the Bertholf class and the Littoral Combat Ships. Addition of ASW or ASCM systems would result in ships many would classify as light frigates or corvettes.

Bottom Line–What is a Corvette?

Corvettes slot under frigates but above patrol boats or missile boats as a classification of surface combatants. To me, this means that they are the smallest or perhaps least capable ocean-going warships. This is a bit of a stretch for Corvettes like the Visby, but in fact the Swedes have deployed even smaller warships to the Indian Ocean for counter piracy operations. That sets the low end of the the displacement range at about 500 tons, but when we look for an upper limit, it seems a moving target, with no similar performance based limit.

The US and Britain already build destroyers the size of WWII cruisers. Germany and in the near future Britain will build frigates over 6,000 tons full load. Japan’s Coast Guard has OPVs displacing 9,350 tons full load.  If we tripled the displacement of WWII corvettes as we have done with WWII Frigates and Destroyers, Corvettes could displace almost 5,000 tons, so I don’t think displacement is a reliable determinant.

Strict naval vessel construction standards don’t necessarily distinguish a corvette from an OPV either. They were not applied to the original “Flower” class, and they don’t apply to the Damen designed Sigma class, built or building for Indonesia, Morocco, and Vietnam, or to the French Lafayette class (also operated by Taiwan, Singapore, and Saudi Arabia) and Floreal class (also operated by the Moroccan Navy) which are rated as frigates but which it might be argued are actually corvettes.

The only metric that doesn’t seem to have changed much over the last 70 years is crew size. Corvettes generally have crews of 120 or less, frigates from 120 to perhaps a bit over 200, while destroyer crews begin slightly under 200 and go up to about 350, and cruiser crews are larger still. The DDG1000s will apparently have a frigate sized crew, but their final crew may be larger than currently planned. OPV crews tend to be corvette sized or smaller.

Just as the difference between Spruance Class Destroyers and Ticonderoga Class cruisers was mission and associated equipment, not displacement, the differentiation between the various types of warships and between Offshore Patrol Vessels (OPVs) and corvettes may simply comes down to their missions and equipment. OPVs include a wide range of ships, but the common thread, generally accepted, is that they have no ASW weapons, no heavy anti-ship cruise missiles, and only a self-defense AAW capability. Adding an ASW capability and/or cruise missiles would convert an OPV into a corvette. Perhaps they would not make very good warships, but then the original Corvettes weren’t very good warships either, but they served a vital role. Conversely an old frigate or corvette, stripped of all its weapons except a medium caliber gun and heavy machine guns would become an OPV, even if it nominally retained its frigate or corvette designation as in the case of Portugal’s Joao Coutinho and Baptista de Andrade class or some of Italy’s Minerva class.

If we had no history, and we could start ship designations on “a clean sheet of paper” we might define ships types based on their missions and equipment, saying destroyers are vessels designed with robust capacity to perform well in all three major surface combatant warfare areas, AAW, ASuW, and ASW. Frigates are designed to perform well in only two missions areas  (with possibly modest self defense capability in the third). Corvettes would be single mission specialists with only modest capability in the other two missions (if at all). OPVs would be vessels equipped for missions that did not require robust capabilities in any of these three mission areas. All four types might be called generically “cruisers” which would bring that designation back to its original meaning, a vessel smaller than a ship of the line that can operate independently.

The Future of Corvettes

WWII corvettes were small ships packed with crew and weapons.They were small because there was an urgent need for many ships that could not be met by the shipyards that normally built warships. They were a way of making the small commercial yards serve the war effort. If we are ever engaged in a prolonged conflict against a near peer adversary we may again resort to a similar expedience. If so, the resulting corvette is more likely to be based on a petroleum industry offshore support vessel rather than a whaling or fishing vessel.

But when ships are built in peace time, for a 20 to 40 year life, other factors beside construction cost start to dominate. In the West, crew costs weigh heavily, while increasing hull size appears less important, provided we do not load up the larger hull with additional systems which will in turn drive up crew costs. Larger hulls are more seaworthy, allow greater endurance, and may be made quieter. They may even be more economical to operate and maintain because of easier access.

Some European Countries that formally operated a number of Corvettes seem to have abandoned the type in favor of ships with more range and better seakeeping including The Netherlands, Denmark, and Norway. Denmark has instead produced frigates and a novel class of ships, the Absalon Class “support ships,” (450 ft/6,600 tons) that include a relatively large hull of modest speed, with a relatively small crew of about 100, and a large reconfigurable spaces–an open one topside midships where missile systems can be placed and a “garage” area under the flight deck that can accommodate vehicles and containerized loads. These ships are perhaps too large to be considered corvettes, but they are not nearly so well armed as the frigates of the similarly sized Iver Huitfeldt-class. They do have characteristics I would expect to see on future corvettes, a relatively commodious hull (because “steel is cheap and air is free”), a relatively small crew (because that is the most expensive component over the life-cycle of the ship), and reconfigurable spaces and weapon systems, that allow the ships to be adapted to different missions (because that is allow us to hedge our bets regarding what capabilities will be needed, while allowing that minimal crew over most of the life of the ship).

Because Corvettes are always compromised, they are likely to be controversial. Many will not agree with the compromises accepted. That is certainly true of the new American Corvette, the Littoral Combat ship.

In some respects the LCSs may be the prototype of the future corvette, in that it is not particularly small, but they were made cheap to operate with a minimal crew, and they are single mission ships, but with the advantage that the mission can be changed over time, although not as quickly as once advertised. Other aspects of the ship were perhaps not as well thought out, but they will serve a purpose, and perhaps the next generation LCS  or convertible corvette will better meet our needs.

Chuck retired from the Coast Guard after 22 years service. Assignments included four ships, Rescue Coordination Center New Orleans, CG HQ, Fleet Training Group San Diego, Naval War College, and Maritime Defense Zone Pacific/Pacific Area Ops/Readiness/Plans. Along the way he became the first Coast Guard officer to complete the Tactical Action Officer (TAO) course and also completed the Naval Control of Shipping course. He has had a life-long interest in naval ships and history. Chuck normally writes for his blog, Chuck Hill’s CG blog.  

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