In a well-argued piece in the Winter 2026 edition of Line of Defence, Rear Admiral Jim Gilmour (Ret) offers a comparison between the two options for the combat element of the Maritime Fleet Renewal program announced by New Zealand’s Minister for Defence earlier this year – the Japanese Mogami class and the British Type 31 frigates.

The Babcock Type 31 and upgraded Mogami are answers to different questions, writes Andy Watts. It depends on the mission.

Mogami has been selected by the Australian government to replace existing, soon to be life expired Anzac class frigates, and Type 31 is under construction for the British Royal Navy. Both options create opportunities for partnerships with the Royal Australian and Royal Navies in workforce development, supply chain sustainment, doctrine development, and technology insertion over time.

For a small navy seeking to manage these aspects of capability in an era in which societal, geo-political and technological stability is a thing of the past, such partnerships are very important in navigating complex organisational challenges.

I underwent my primary professional training and higher defence education in Australia, and served alongside ADF naval, air, and land forces throughout my career. I had less direct but still substantial contact with the Royal Navy – enough to develop a deep respect for our UK counterparts and enduring friendships with Royal Navy people.

Although my background pre-disposes me towards partnership with Australia (please note that I am currently employed as a consultant to an Australian company, Austal Ships Ltd), I recognise that the Royal Navy’s organisational depth and the historical goodwill that exists between our two navies would make such a partnership worthwhile.

Acquiring Mogami class frigates in partnership with the ADF or Type 31 frigates in partnership with the Royal Navy would provide a sound platform for the long-term management of the many complex organisational challenges facing the RNZN. However, in his article, Jim assets that Mogami is only marginally superior to Type 31 in capability. I must respectfully take issue with this point.

The first point to make is that Mogami and Type 31 are answers to different questions. Type 31 was conceived as a means of maintaining Royal Navy surface combatant fleet numbers in the face of escalating costs in advanced programs, particularly the Type 26 frigate. It was intended from the outset to perform “maritime security” missions, which are defined in the UK National Strategy for Maritime Security “… in terms of upholding laws, regulations, and norms to deliver a free, fair, and open maritime domain…”.

Accepting that this definition is open to wide interpretation, it uses language more commonly associated with what is termed the maintenance of good order at sea as opposed to the deterrence of state actor aggression. By adopting this definition as the basis for its requirement, the Royal Navy was able to specify a performance baseline that kept unit procurements costs to a very low GBP250M (not including a range of Government Furnished Equipment (GFE)), allowing sufficient numbers to be acquired to bring overall fleet numbers to a sustainable level.

In 2019, the UK MoD selected a design offered by Babcock Ltd based on the Danish Iver Huitfeld air defence frigate for its Type 31 requirement. From a professional naval perspective, there is much to like about these ships. For one thing, they are large – nearly 140m in length at a displacement of nearly 6,000 tonnes, against the RNZN Anzac class at 118m and 3,600 tonnes.

Size confers many advantages, including seakeeping ability, range, and maintainability (systems are easier to “get at” for maintenance and component replacement). Greater stability means that crewed and autonomous aircraft and surface and sub surface vehicles can be operated safely in higher sea states, and wear and tear on equipment and people is much reduced. Downstream modification for new capabilities is also much easier for a large ship than a small one.

However, the same size advantage applies to the upgraded Mogami (which is the version in contention for the NZ requirement), which is 142m in length at 6,200 tonnes displacement. In addition, Mogami has been designed from the keel up to accommodate a specific range of platform and mission systems without precluding future upgrades or system replacement. Design philosophy extends beyond the integration of these systems – they are designed as a single, coherent capability.

T31, on the other hand, is specifically designed to accommodate a range of systems according to customer requirements – systems fits across the T31 parent design and the Polish and Indonesian export variants vary widely. By definition, T31 cannot be as space and weight efficient or as ergonomically laid out as Mogami.

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I have advocated for a modular approach to capability in many articles in this publication and elsewhere, and I won’t repeat those arguments here. However, I respectfully take issue with Jim’s assertion that Type 31 is a modular design. It is a traditional design in which most of the mission systems are tightly coupled with the platform, but with sufficient design flexibility to allow space and weight for modular systems. At some point in the life of these ships, an expensive upgrade of those mission systems that are coupled to the platform will be required.

Type 31 obsolescence can’t be wholly managed by exchanging a new module for an obsolescent one. The cost and schedule risk associated with such upgrades was amply illustrated during the Anzac Frigate Systems Upgrade project.

Jim and others will rightly argue that the same limitation with regard to the upgrade of primary mission systems applies to the Mogami class. He also asserts, however, that Mogami has “…very limited ability to embark personnel beyond the crew, severely limited training capacity, limited space for uncrewed systems, disaster relief stores, mission bays and little growth margin allows for very limited capability breadth beyond its combat functions…”.

The Mogami class in its basic configuration requires a complement of 92, against 110 for the Type 31. A complement of 18 fewer sailors per ship (compared to Type 31) has great advantages in relation to workforce management. The total number of bunks available is 127, 35 more than are needed for the core complement, allowing a significant margin for embarked trainees and military forces.

Notwithstanding its traditional frigate configuration, Mogami is specifically designed as an autonomous system mother ship. Mogami is one of the first frigate types to embark both an Uncrewed Underwater Vehicle (UUV) and an Uncrewed Surface Vessel (USV) as standard elements of its mission system, not as ad-hoc additions.

The design incorporates a stern ramp and mission bay under the flight deck for launch and recovery of these craft, while a full-size flight deck and hangar support an H-60 series helicopter (such as the recently ordered MH-60R) and, potentially, unmanned air systems. The mission bay can also be used to accommodate other mission modules and equipment, including disaster relief stores.

Jim asserts that Mogami only offers a marginal increase in combat capability in comparison to Type 31. I disagree. Firstly, Type 31 in its Royal Navy configuration has no anti-submarine sensors and shipboard weapons. In my view, ant-submarine capability is central to frigate combat capability. Submarines are proliferating in the Asia-Pacific region and present a very real threat to the free movement of goods and people across the oceans and information flows through undersea cables.

Providing Type 31 with a towed active/passive sonar and adding the necessary code to the Combat Management System would create an expensive acquisition and integration problem for New Zealand, together with a training and maintenance burden that could not be shared with the Royal Navy.

In addition to its comprehensive anti-submarine capability, Mogami is fitted with the OPY-2 Active Electronically Scanned Array (AESA) radar using planar arrays with fixed 360-degree coverage. It is thus better able to defeat the saturation missile attacks that could easily become a feature of operations in our area of primary strategic interest than Type 31, which is fitted with a rotating Thales NS110 surveillance radar. The latter is no doubt an excellent system, but most authorities seem to agree that in installations where their weight and power demands can be accommodated, fixed arrays offer superior coverage and effectiveness.

There are other significant differences between the two ships. T31 features a 57mm main gun – very useful for self-defence, but ineffective for fire support to land forces. Mogami is fitted with the BAE Systems Mark 45 Mod 4 five inch (127mm) medium gun, which can fire extended range guided munitions. The latter enables greater compliance with the law of armed conflict principles of distinction and proportionality, in that targets can be engaged with much greater precision than with unguided rounds, allowing the necessary effect to be achieved at a much lower risk of collateral damage.

Adopting T31 in the Royal Navy configuration would mean the RNZN is no longer capable of providing fire support to land forces operating in the littoral. In the event of serious conflict, this could cost lives.

Another significant Mogami capability is its hull-mounted mine hunting sonar and OZZ-5 mine hunting UUV. Uniquely for a frigate, Mogami can hunt for and dispose of maritime mines – a capability hitherto limited to dedicated, highly expensive mine countermeasures vessels.

Mines were the only direct military threat faced by New Zealand during WW2; the minesweeper HMNZS Puriri was sunk by a mine with tragic loss of life while conducting sweeping operations off the Northland coast in 1941. Notwithstanding the superb efforts of RNZN operational diving, mine clearance, and hydrographic specialists over many years, our mine countermeasures capability is wholly unequal to the potential mine threat. Mogami could reduce this gap.

Finally, the Mogami class OYQ-1 Combat Management System (CMS) and Combat Information Centre (CIC) are amongst the most advanced in the world. A warship’s CMS is both its software-driven brain (with the ultimate cognitive element reserved for humans) and its central nervous system. It collects and synthesises information from the ship’s own sensors with that received externally via data link, presents that information to the ship’s operators, and controls the ship’s weapon systems. The CIC is the space in which humans interact with the CMS.

The Mogami CMS/CIC features a 360-degree augmented reality screen projection which provides operators and Commanding Officers with a level of situational awareness unmatched by any comparable system.

To return to my main point, Type 31 and Mogami are answers to different questions. Type 31 is a good ship and partnership with the Royal Navy will deliver a wide range of organisational benefits, but it is intended for maritime security functions and does not have the combat capability needed for New Zealand to add weight to the deterrent effect needed to avoid conflict in our region.

Mogami, however, is designed explicitly for combat, while retaining mission flexibility comparable to Type 31 through its basic design, autonomous vehicle, and mission bay capability. Partnership with the Royal Australian Navy would deliver benefits at least comparable to partnership with the Royal Navy, with the added advantage of geographic proximity.

In deciding whether Mogami or Type 31 is best suited to our maritime combat requirements, we first have to decide on the nature of the geostrategic environment in which we must live and operate, and what our Navy must therefore be able to do. Does it require the ability to contribute to good order at sea with maritime security level capability, or should it have the ability to contribute to the deterrent effect needed to avoid a tragic miscalculation leading to actual combat at sea? If the former, Type 31 is a good choice. If the latter, Mogami is a better choice.

As indicated above and in my other public writings since my engagement as an advisor by Austal, I cannot claim to be a disinterested party in this debate. However, I have tried to avoid partisanship as much as possible. There have been some excellent contributions to the discussion of late, including a paper by Major General John Howard (Retired) published by The New Zealand Initiative, and Line of Defence articles by Jim Gilmour, Graeme Doull, Marc Parsons, and others.

In my view all these contributions have added great value, notwithstanding any differences I have had with content. I very much hope that there is more to come.