Decades ago, I read a Royal Navy research paper that asked essentially, “What is the proper length for a corvette?” A balance of adequate seakeeping and minimum cost? The report had originated shortly after World War II when the British had operated hundreds of 205 foot (62.5 meter) long Flower class and 252 ft (77 m) Castle class corvettes. Their answer was 270 feet (82.3 meters, probably they were talking waterline length. In this post, I will be using length overall (loa) and full load displacement (fl)), but there was a caution, that while that was appropriate for the Atlantic, operation in the Pacific would require a larger ship because of the longer swells prevalent in the Pacific.

This introduced me to the idea that there might be a step function in effectiveness based on length, and that it might vary with the environment.

A very pleasant experience with the 327 foot (100 meter) cutter Duane also seemed to suggest there might be a sweet spot where ships become much more seaworthy.

Is there a minimum length for a vessel to function effectively as an Offshore Patrol Vessel? There is room for some scholarly research into the question.

Lacking a naval engineering background, I approach the question in a different way. On the assumption that given experience, OPV users move toward an optimum balance of capability and cost, let’s look at the experience of navies and/or coast guards of five nations that have created more than one class of OPV over the last 30 or so years, the US Coast Guard, French Navy, UK Royal Navy, Japan CG, and Indian Navy and Coast Guard. We will look at the ships created about 30 years ago and the ships they have chosen to build most recently.

FRANCE

France has the largest EEZ of any nation, 11,691,000 km², about 103% that of the US, most of it in the Pacific and Indian Oceans.

The Old

Floreal class. Six ships (plus two for Morocco) commissioned 1992-1994: 93.5 meter, 307 feet, 3000 ton, 20 knots. FS Ventose pictured. 

The New

European Patrol Corvette. First Delivery expected 2030: 110 meters. 361 feet. 3,000 tons, 24+ knots.

France has their open ocean coast guard functions performed by their navy. In 1992 the French Navy commissioned the first of six Floréal class “surveillance frigates” pictured above. These are expected to be replaced by a version of the 110 meter, approx. 3,000 ton, 24 knot European Patrol Corvette.

The Old

P400 class of ten vessels. Commissioned 1986-88, 54.8 m (179 ft 9 in). 480 tons fl. 24 knots. 

The New

Patrouilleurs Outre-mer (POM), or Félix Éboué class: Six ships to be commissioned 2023-25, 80 m (262’6″), 1300 tons fl.

Their latest acquisitions are six 80 meter Offshore Patrol Vessels (Patrouilleur d’Outre-Mer – POM).

They replace the P400 class. Like the Floreals, these ships and their replacements are of simple design intended to operate for long periods from austere bases in French overseas territories. Most of the P400s were operating in the Western Pacific in the same environment where we currently operate the smaller Webber class FRCs. Significantly, “Since late 2008, ships of the D’Estienne d’Orves class, with their heavy armament removed, were planned to replace the P400 in the high sea patrol role, a task for which the P400 class have proved to be underweight.” This was 20 years after the the vessels were commissioned but clearly the French thought they were “going to need a bigger boat.” The replacement POM class turned out to be very similar to the D’Estienne d’Orves class in length and displacement, 80 m and 1,300 tons.

La Fayette class Frigate Surcouf 

In 1996 France commissioned the first of five nominally general purpose 125 m (410 ft 1 in), 3,800 ton, 25 knot La Fayette class frigates, but as built, they had no ASW capabilities, so they were more high performance OPVs not unlike the USCG Hamilton class WHECs. France’s EEZ is widely distributed and varied. The French seem to have a wide array of solutions to their patrol requirements. They seem to produce designs for specific outposts. In addition to those discussed above they have built these relatively unusual OPVs: here and here.

Perhaps it is telling that, when French shipbuilder Naval Group built an OPV on speculation for a world market, the result was the 87 meter L’Adroit, which after service with the French Navy was sold to Argentina along with three new construction near sisters. Recently France contracted for ten Offshore Patrol vessels that have been reported to be 90 meters in length. I suspect these will be similar to L’Adroit

US Coast Guard:

The US EEZ, the second largest, is very nearly as large as that of France at 11,351,000  km².

The Old

Bear class Medium Endurance Cutters, class of 13, commissioned 1984-1991, 82 m (270′), 1800 tons, 19.5 knots.

The New

Offshore Patrol Cutter, projected class of 25, projected delivery 2025-2038, 110 m (360′), 4,500 tons, 22+ knots

Thirty years ago, the US Coast Guard had just completed the 82 meter, 270 foot, 1,800 ton, 19.5 knot Bear class, the first of which was commissioned in 1984, the newest, Mohawk in 1991. They replaced 143-, 205-, 213-, 311-, and 327-foot cutters built during and before the Second World War.

The US Coast Guard would not build any additional Offshore Patrol vessels until the 127 meter, 4,500 ton Bertholf class National Security Cutters (NSCs), the first of which was commissioned in 2008. Despite the age difference, the NSCs were not replacements for the WMEC270s, but rather the twelve 115 meter (378-foot) Hamilton class commissioned 1967 to 1972.

The direct replacement for the Bear class WMEC270s (as well as the 64 meter, 210 foot Reliance class) is the 110 meter Argus class Offshore Patrol Cutter, (OPC).

The relatively large size of both the NSC and the OPC, displacing at least 4,500 tons full load, reflect demanding requirements for launch and recovery of boats and helicopters in heavy weather.

These requirements seem to reflect experience on Alaska Patrol. While the Bear class seem to have served well in the Atlantic and the equatorial Eastern Pacific they were found to be inadequate for operations in Alaskan waters.

The 64 meter Reliance class were never really designed to be offshore patrol cutters in the modern sense. Designed before the United Nations Convention on the Law of the Sea, when the territorial sea was still three miles, and before the advent of the 200 mile Exclusive Economic Zone, they were only expected to go out 50 miles offshore.

Meanwhile, the relatively small 47 meter Webber class have proven remarkably capable but are operated with an eye on their endurance and weather limitations.

Looking at the large cutters that were in the fleet when I entered the service in the ’60s, we had three classes of ships that stood weather station patrol, demanding long open ocean patrols. Of these, the 100 meter Secretary class 327s were considered the most successful. The 95 meter Casco class “311s” were also successful and their huge fuel capacity allowed great operational flexibility. The 77.7 meter Owasco class “255s” operated at significant disadvantage, in that they were rough riding and their limited fuel capacity forced them to drift when being able to maintain a favorable heading would have been desirable.

United Kingdom’s Royal Navy: 

The UK’s EEZ is the fifth largest in the world at 6,805,586 km² (60% that of the US).

The Old

Castle Class, two ships, commissioned 1981 and 1982, 81 m (266′), 1,550 ton fl, 20 knot 

The New

River Class (Batch II), 5 ships, commissioned 2017 to 2021, 90.5 m (297′), 2,000 tons fl, 25 knots

The Royal Navy has never had a great many OPVs. They currently have eight in commission. They have tended to use frigates and MCM vessels to perform coast guard duties.

The Castle class were designed to replace the 61.1 m (200′), 1,280 ton, 16 knot Island class OPVs commissioned 1977 to 1979, that were criticized as unseaworthy, too slow, and lacking a flight deck. Originally it was to have been a class of six, but only two were completed and the Island class soldiered on into the 21st century. The two Castle class served the Royal Navy for 28 and 29 years before being sold to Bangladesh in 2010 where they still serve.

The Castle class was followed by three 79.5 m (261′), 1,700 ton, 20 knot River class (Batch I) and a forth modified, slightly longer River class the 81.5 m (267 ft 5 in) HMS Clyde that unlike the earlier ships included a flight deck. Initially the River class Batch 1 ships were leased, but the first three were subsequently purchased and continue to serve as fisheries protection vessels. HMS Clyde has been sold to Bahrain.

These were followed by five 90 m River class Batch II ships. Despite the similar nomenclature, these are an entirely different class.

Japan Coast Guard: 

Japan’s EEZ is 4,479,388  km² (39.5% that of the US) and is the eighth largest in the world. Their entire EEZ is in the North Pacific, which can be a challenging environment.

The Old

Ojika class, seven ships, commissioned 1991 to 2000, 91.5 m (300′), 2,006 tons fl, 20 knots

The New

Kunigami-class, 20 ships+1 building (+ 2 for the Philippine CG, commissioned 2012 to 2020, 96.6 m (317′}, 2260 tons, 25 knots

Unlike the US Coast Guard, the Japan Coast Guard produced “High Endurance Cutters” (PL and PLH) continuously over the last 30 years, 62 ship in 13 classes, including some very large cutters, some up to 150 meters and approaching 10,000 tons.

Thirty years ago, the Japan Coast Guard was building the Ojika class, seven ships commissioned 1991 to 2000. Six of the seven are still in service, they are 91.5 meters (300 feet) in length, with a full load displacement of 2,006 tons, a max speed of 20 knots, and a crew of 34.

The most recent, general-purpose “High Endurance Cutters” built by the Japan Coast Guard have been the 96.6 m (317 ft) Kunigami-class. There are currently 20 of these cutters in service with three more on the way. In addition, Japan has built two modified versions of this class for the Philippine Coast Guard. With no end in sight for their construction, these may become the most produced OPVs of the 21st century.

These are relatively simple ships, not much different from the Ojika class.

Frequently cutters of different classes have been built concurrently rather than consecutively.  Aside from three 79 meter Aso class, commissioned 2005/2006, designed as high-speed interceptors of North Korean spy ships, the smallest “high endurance cutters” built in the last 30 years are the nine ship, 89 meter Hateruma class pictured above, commissioned 2008 to 2010.

The JCG cutters discussed here, unlike their larger PLH cutters, have no hangar, only basic weapon and sensor suites. Their size is not determined by features incorporated in the design. It appears their size is a reflection of the environment where they operate, the North Pacific.

India:

India’s EEZ is 2,305,143 km², 20.3% that of the US, but still more than the entire EEZ under the Atlantic Area.

In 1986 the Indian Coast Guard had 80 officers and 400 men. They have come a long way. The Indian Coast Guard got a big boost after the 2008 Mumbai Terrorist Attack, which came by sea. The Indian Coast Guard operates 27 Offshore Patrol Vessels, all commissioned within the last 28 years, 18 since 2015.

The Old

Vikram-class offshore patrol vessels, nine ships commissioned 1983-92, 74 m (243′), 1,224 tons, 22 knots.

The New

Vikram-class (different Vikram class) offshore patrol vessels, Class of seven ships commissioned 2018-2021, 97 m (318’3″), 2140 tons, speed 26 knots.

And

Samarth-class, 11 ships, commissioned 2015 to 2022, 105 m (344.5′), 2450 tons fl, 23 knots

The first Vikram class was not considered entirely successful because they could not operate helicopters in heavy weather because of the ships’ roll characteristics. The class is no longer in service with the Indian Coast Guard, but two ships were transferred to Sri Lanka.

Since 1990 the Indian CG has not commissioned an OPV of less than 93 meters. Their largest are 105 meters. None of their smaller patrol craft (WPCs) exceed 51.1 meters so there is a clear distinction between OPVs and Inshore Patrol Vessels.

INDIAN NAVY

The Indian Navy operates 12 Offshore Patrol Vessels, six of each of the two class below.

The Old

Sukanya-class, seven ships, commissioned 1989 to ’93, 101 m (332′), 1,890 tons fl, 21 knots

The New

Saryu class, six ships, commissioned 2013-2018, 105 m (344′), 2,230 tons, 25 knots

OBSERVATIONS

Speed: Speed has typically increased by about four knots. The trend seems to be to a maximum speed of 24-26 knots. Increasing speed is made easier by increasing waterline length. It may not be an accident that a waterline length of 97.8 m (320.8′) provides a hull speed of 24 knots, typical max speed for a modern OPV, while 20 knots, the typical speed for an OPV 30 years ago, only required a waterline length of 67.9 m (222.8′).

There is now little or no difference between the speeds of OPVs and corvettes or light frigates. For warships, except those that have to be able to keep up with nuclear powered carriers, the value of absolute top speed has diminished while the value of high sustained cruising speed has increased. This is apparent if you compare the Oliver hazard class FFGs with the new Constellation Class. Top speed is down 3-4 knots to 26 knots, but range is up with a powerplant designed for economical cruising.

Aircraft: Flight decks are now ubiquitous. Hangars are common but are not included in every new design. Designs that support two embarked helicopters are still the exception. The French POM, which seems to be a minimalist approach, bucks the trend and includes aviation facilities for only unmanned air systems.

Length: Of the current generation OPVs, none of those discussed is less than 80 meters (262,4) and only the minimalist French POM was that small.

The Royal Navy study I mentioned seems to have gotten it right, that 270 feet (82.3 m) is a reasonable minimum and in some environments, length should be greater. Desire for greater capabilities (e.g., greater speed, better aviation facilities, more weapons and sensors} may to be driving designs of greater length than would be required simply for seakeeping, but the choices made by the Royal Navy and Japan and Indian Coast Guards tend to confirm that about 90 meters or more and at least 2,000 tons is the new norm.