Fairbanks Morse Marine Engines: OP Diesel and Naval Power

Fairbanks Morse Defense, based in Beloit, Wisconsin, is the principal domestic supplier of propulsion and power-generation diesel engines to the United States Navy. Its opposed-piston two-stroke diesel, the 38D 8-1/8, has been in continuous production since 1938, powered every class of US fleet submarine through World War II, and remains in active naval service today as emergency and auxiliary power on Los Angeles-, Seawolf-, and Ohio-class nuclear submarines. Since 1973 the company has also held a SEMT Pielstick license, building medium-speed four-stroke engines for surface combatants, landing ships, and fleet oilers.

The company’s history spans more than 130 years and three distinct eras: a broad industrial manufacturer (1893 to 1958), a division inside successive conglomerates (1958 to 2020), and a focused naval-defense supplier under private equity ownership (2020 to present). The marine and naval engine thread runs through all three eras without interruption. For the broader context of US large-bore engine builders, see the marine engine makers index. For the working principles behind the opposed-piston design, two-stroke marine diesel engine fundamentals and doxford opposed-piston engines give useful comparative depth.

Corporate origins: Beloit, 1893

Fairbanks Scales and its predecessors

The Fairbanks name in American manufacturing goes back to 1823, when Thaddeus Fairbanks founded a company in Saint Johnsbury, Vermont, making cast-iron plows and stoves. The platform scale, patented in 1832, became the company’s commercial foundation. By the 1880s E. & T. Fairbanks & Company was selling scales across the United States, Europe, South America, and China.

The Beloit chapter opened in 1889, when the Eclipse Wind Engine Works of Beloit, Wisconsin merged with the Fairbanks operation. Eclipse had been founded by Leonard Wheeler, a former missionary who designed durable windmills for water pumping after the Civil War. The Williams Steam Engine Company joined at the same time. The combined entity, reorganized as Fairbanks, Morse & Company in 1893, brought together weighing scales, windmills, steam engines, and water pumps under one roof in Beloit. Charles Hosmer Morse, who had led the firm’s Chicago expansion, gave the company its second name.

From Beloit, Fairbanks Morse added engine lines. The firm purchased the Charter line of oil and naphtha engines in the 1890s, making it one of the first US manufacturers to market a commercially successful gasoline engine, in 1893. Power plant fuel progressed by decade: kerosene in 1893, coal gas in 1905, semi-diesel in 1913, and the first full high-compression cold-start diesel in 1924. That 1924 engine, the 75-horsepower Type Y style VA, is preserved as ASME Landmark No. 208 and represents the transition from ignition-assisted semi-diesel to full compression-ignition diesel in American industrial practice.

Fairbanks Morse’s rural-electrification diesel sets and the “Z” engine line, introduced in July 1914 as a 1.5-horsepower headless model, found markets on farms, in sawmills, and at pumping stations across the US. Over half a million Z-series engines were produced in the thirty years after introduction. By 1910 the firm’s product catalog ran to more than 800 pages.

The opposed-piston turn, 1930s

The decisive move for Fairbanks Morse’s marine and naval future was its early-1930s acquisition of a license for the Junkers opposed-piston diesel design. Junkers had developed the concept for aircraft propulsion, most visibly in the Jumo 205 aero-diesel that powered German flying boats in the 1930s. The Junkers design had two crankshafts, one at each end of the cylinder, with opposing trunk-style pistons sharing a single combustion space. No cylinder head existed; ports in the cylinder wall handled both intake and exhaust, timed by the relative piston positions.

Fairbanks Morse ran its first opposed-piston prototypes in 1934, testing a six-cylinder unit with a 5 x 6-inch bore and stroke in railway service. The same year, an endurance test of a 1,000-horsepower eight-cylinder engine with an 8 x 10-inch bore and stroke attracted the attention of the US Navy. Submarine procurement officers saw in the design exactly what they needed: high power density in a compact envelope, no cylinder-head joints to fail under depth-charge shock, no mechanical valves to stick, and a running gear that could be maintained with shipboard tools.

In December 1934 the Navy placed the first production order: eight 38A8 engines, four each for USS Plunger (SS-179) and USS Pollack (SS-180) of the Porpoise class. Problems with the 38A8 led to a redesign, and the 38D8 became the standard submarine variant.

Opposed-piston thermodynamics and the scavenging cycle

Why the architecture works

The thermodynamic case for the opposed-piston engine rests on one fundamental fact: there is no cylinder head. In a conventional water-cooled diesel, the cylinder head is a fixed metal wall sitting at the hottest point of the combustion cycle. It absorbs heat from combustion gases and dumps that energy into the cooling water, regardless of what the designer wants. The head can’t extract work from that heat; it discards it.

An opposed-piston cylinder bounds the combustion space entirely with moving surfaces. Both pistons compress the charge from opposite ends and then retreat to extract work from the expanding gas. No fixed wall exists at the combustion space. This structural fact raises closed-cycle thermal efficiency through three compounding effects. First, the surface-area-to-volume ratio of the combustion chamber is lower than in a conventional cylinder of equal displacement, because two converging pistons create a more compact combustion geometry at top dead center than a flat head over a single piston crown. Less surface area at the hottest moment means less heat loss into metal and coolant. Second, the two-stroke cycle lets the engine fire once per crankshaft revolution rather than once every two, doubling the power events per unit of swept volume, which dilutes the energy density per combustion event and gives the charge less opportunity to heat the cylinder walls during the compression stroke. Third, the leaner air-fuel ratios that the two-stroke cycle permits at a given load shift the ratio of specific heats slightly, improving the thermodynamic work extraction from each unit of fuel burned. Achates Power’s technical analysis of this architecture quantifies the combined effect: opposed-piston two-stroke engines achieve an indicated thermal efficiency of 53 percent in simulation, against 50.1 percent for a conventional opposed-piston four-stroke and 47.5 percent for a standard four-stroke.

Uniflow scavenging in the 38D

Gas exchange in the 38D 8-1/8 is uniflow-scavenged. The upper crankshaft carries the pistons that control the intake ports, which are positioned near the center of the cylinder. The lower crankshaft carries the pistons that control the exhaust ports, which sit near the top of the cylinder. Fresh charge enters from one end of the gas column and exhaust exits from the opposite end, so the flow is always in one direction through the combustion space. This is the defining characteristic of uniflow scavenging and it’s why the design achieves better scavenging efficiency than loop-scavenged two-stroke layouts where fresh charge must reverse direction. See uniflow scavenging in two-stroke marine engines for the detailed comparison.

The phasing between the two crankshafts is not equal. In the 38D, the lower (exhaust) crankshaft leads the upper (intake) crankshaft by 12 degrees of crank angle. This asymmetric timing ensures the exhaust ports open before the intake ports on each cycle. Combustion gases start leaving the cylinder before fresh charge arrives, which drops the cylinder pressure ahead of the scavenging event. When the intake ports open, the fresh air entering from the blower is already moving into a lower-pressure space, and it carries the remaining exhaust gas out through the still-open exhaust ports ahead of it. The 12-degree lead also creates a brief period at the end of scavenging when both intake ports and exhaust ports are open simultaneously; a controlled “ram effect” during this overlap pushes additional fresh charge into the cylinder. This contributes to the engine’s thermal economy at the moderate scavenge pressures available in a submarine where turbocharging was constrained by air supply limitations.

The scavenging air produces a swirling motion as it enters the cylinder through the intake ports, which are angled tangentially to the cylinder bore. This swirl persists through the compression stroke, improving fuel-air mixing when the injectors fire. The combined uniflow direction and tangential swirl is the mechanism behind the relatively clean combustion the 38D achieves even at conservative scavenge-air blower pressures.

Thermal asymmetry and the exhaust-piston challenge

Uniflow scavenging creates an inherent thermal asymmetry: the exhaust piston operates in a hotter environment than the intake piston. The exhaust ports sit at the top of the cylinder, and the exhaust piston spends more of each cycle near the hotter end of the gas column. The intake piston, by contrast, is exposed to incoming cool scavenging air during the gas-exchange phase, which provides a degree of internal cooling. This asymmetry was manageable in the marine 38D, which runs at conservative 85 psi brake mean effective pressure with long overhaul intervals, but it became an operating problem in locomotive service where high-duty cycling heated the exhaust piston beyond what the design’s ring-and-crown geometry could reliably sustain. The locomotive program’s piston failures traced directly to this thermal loading in sustained mainline operation.

The 38D 8-1/8: engine design and operating principles

Configuration and geometry

The designation encodes the basic geometry. “38” identifies the cylinder height of 38 inches (970 mm). “8-1/8” gives the bore in inches: 206 mm. Each piston strokes 10 inches (254 mm), so the effective total stroke for compression purposes is 20 inches (508 mm) over the shared cylinder space. Displacement per cylinder is 17.0 liters. The engine runs at 900 rpm (60 Hz applications) or 1,000 rpm (50 Hz), giving a mean piston speed of 7.6 m/s at the lower speed.

Cylinder count ranges from 4 to 12 in a single inline bank. WWII fleet submarines used 9- or 10-cylinder versions as main propulsion engines, rated at 1,600 horsepower per engine. The 12-cylinder variant (12L) is the largest configuration currently offered for power generation, producing 3,165 kWe turbocharged at 1,000 rpm. Navy variants were rated at 85 psi brake mean effective pressure, a conservative figure that suited the durability requirements of submarine service.

The dry-block construction means the cylinder walls do not contact the cooling water directly; a separate sleeve arrangement keeps water away from the liner surface. Two separate crankshafts run at the top and bottom of the engine, geared together to turn a single output shaft. Counter-rotating crankshaft pairs inherently balance the primary forces, which suits submarine installation where vibration signature matters.

No cylinder head exists. Gas exchange happens entirely through ports: intake ports near the middle of the cylinder are controlled by the lower piston, exhaust ports near the top are controlled by the upper piston. Overlapping port timing creates the scavenging event. Because intake and exhaust ports are at different vertical positions in the cylinder, the design is uniflow-scavenged: fresh charge enters from one end and exhaust leaves from the other, rather than looping back on itself. This is the same scavenging advantage that the Doxford and Junkers families claimed, and it’s one reason the 38D ran cleanly at the relatively low scavenge pressures available on a submerged submarine where turbocharging airflow was constrained. The doxford opposed-piston engines article covers the parallel British tradition using the same principle.

The 38ND variant for nuclear submarines

When the US Navy began commissioning nuclear submarines in the late 1950s, the opposed-piston diesel found a new role. Nuclear submarines don’t rely on diesel propulsion, but they carry diesel generators for emergency power when the reactor is shut down. The 38ND 8-1/8 (the “N” denoting the nuclear-submarine variant) is a modified version rated for this role. It equips Los Angeles-class submarines (commissioned 1976 to 1996), Seawolf-class (commissioned 1997 to 2005), and Ohio-class ballistic-missile submarines (commissioned 1981 to 1997). The design is still in service on hulls that will remain in commission well into the 2030s and 2040s.

The endurance of this engine in service is a direct consequence of the design’s simplicity. No cylinder head, no mechanical valve train, and a conservatively rated 85 psi BMEP all reduce the failure modes that matter on a submarine. A crew can perform significant maintenance tasks on the 38D with the tooling already carried aboard, a requirement the Navy built into the original specification in 1934 and has never stopped requiring.

Power ratings across configurations

The current FM 38D 8-1/8 product data from Fairbanks Morse Defense shows the following output table for power generation applications:

Physical dimensions for the 12L configuration are 9,304 mm long, 2,662 mm wide, and 3,239 mm tall, with a dry weight of 38 metric tons. Operating conditions are rated at 32.2°C ambient and maximum 457 m altitude. The engine supports 110 percent overload for two hours per 24-hour period.

US Navy and Coast Guard vessel classes: the 38D in service

Fleet submarines in World War II: Gato, Balao, and Tench classes

Fairbanks Morse became the dominant diesel engine supplier for US fleet submarines in World War II. The standard WWII fleet submarine carried either four or eight main propulsion diesel engines depending on class, using them to charge the large battery banks that powered underwater propulsion. On the surface, the diesels drove both the propellers directly and the charging sets simultaneously; submerged, stored battery energy drove electric motors.

Three main fleet submarine classes of the war era carried Fairbanks Morse 38D engines: the Gato class (launched 1941 to 1943, 77 boats), the Balao class (launched 1942 to 1945, 122 boats), and the Tench class (launched 1944 to 1951, 31 boats in WWII service). Each class used four main propulsion 38D engines in 9- or 10-cylinder configuration, each producing 1,600 horsepower, plus two smaller auxiliary diesel generators. The four main engines gave each submarine a combined 6,400 horsepower on the surface.

The Beloit factory produced almost 2,300 engines during the war, maintaining a pace of one engine per day. That production rate, sustained against the demands of a wartime economy short of skilled labor and raw materials, was the practical measure of Fairbanks Morse’s wartime contribution. The company ranked 60th among all US corporations by value of WWII military production contracts.

Surviving WWII submarines in US maritime museums carry the 38D: USS Pampanito (SS-383, Balao class) at San Francisco carries four 10-cylinder 38D8-1/8 engines using air starting. USS Torsk (SS-423, Tench class) is at Baltimore, USS Ling (SS-297) at Hackensack, and USS Blueback (SS-581) at Portland. Blueback, the last diesel-electric submarine to serve in the US Navy (decommissioned 1990), used 38D engines throughout its career.

Postwar conventional submarines: Barbel class

After 1945, Fairbanks Morse continued supplying submarines under the diesel-electric regime. The Barbel class (SS-580, launched 1956 to 1959, three boats), the last diesel submarines designed for the US Navy, carried Fairbanks Morse 38D8-1/8 engines following the same propulsion arrangement as the wartime fleet boats.

The late 1940s saw an experiment with radically different submarine engines: the Tang class (SS-563) initially received “pancake” flat opposed-piston engines designed by General Motors for extreme compactness. These proved unreliable in service, and the Tang-class submarines were re-engined with conventional Fairbanks Morse 38D units, restoring the design standard that had worked through the entire war.

The 38D remained the submarine standard through the conventional diesel boats commissioned into the early 1960s, after which nuclear propulsion dominated new construction and the engine transitioned to the emergency/auxiliary role described above.

Nuclear submarine emergency power: Los Angeles, Seawolf, and Ohio classes

The 38ND variant serves as the sole emergency diesel generator on three nuclear submarine classes. The Los Angeles class (SSN 688 series, 62 boats, commissioned 1976 to 1996) carries one 38ND set per hull. The Seawolf class (SSN-21, SSN-22, SSN-23, commissioned 1997 to 2005) follows the same arrangement. The Ohio class (SSBN/SSGN 726 through 743, 18 boats, commissioned 1981 to 1997) also depends on the 38ND for emergency power during reactor shutdowns, missile-tube maintenance periods, and shipyard periods when shore power is unavailable.

This accumulates to more than 80 nuclear submarines in the active US fleet that carry the 38ND. The Virginia class that follows the Los Angeles class uses a different emergency power system, but the legacy Los Angeles, Seawolf, and Ohio boats will remain in commission into the late 2030s and beyond. Each hull requires periodic engine maintenance and parts support, which is why the 38D production line in Beloit remains economically sustainable despite the engine’s age.

Edsall-class destroyer escorts

The Edsall class of World War II destroyer escorts used four Fairbanks Morse 38D8-1/8 engines in a reduction-geared diesel arrangement, which is why the class carried the type designator “FMR” (Fairbanks Morse Reduction-geared). Each engine was rated at 1,500 horsepower, giving the ship a combined 6,000 shaft horsepower driving two screws. The FMR arrangement gave a maximum speed of 21 knots in trials. The 85 ships of the Edsall class, built 1942 to 1944, represent the most numerous single use of the 38D on any one surface-ship class. Cruising range was 10,800 nautical miles at 12 knots, a range figure that suited trans-Atlantic convoy escort operations.

In the early 1950s, 11 Edsall-class destroyer escorts transferred to the US Coast Guard as WDE (Destroyer Escort, Coast Guard) vessels, extending the 38D’s service into Coast Guard patrol work.

Wind-class icebreakers

The Wind-class icebreakers (seven ships, commissioned 1945 to 1955) used six Fairbanks Morse diesels each, rated at 2,000 brake horsepower per engine, for a combined 12,000 hp driving three electric propulsion motors in a diesel-electric arrangement. The diesel-electric drive suited polar operations: variable ice conditions demand precise low-speed maneuvering that electric motor control provides more smoothly than direct mechanical gearing. Wind-class ships served both the US Navy (as AGBs, icebreaking vessels) and the US Coast Guard (as WAGBs), with USCGC Westwind (WAGB-281) and USCGC Northwind (WAGB-282) among the better-documented Coast Guard units. Some Wind-class hulls transferred to allied nations under military assistance programs.

Hamilton-class cutters (WMEC-901 type)

Hamilton-class Coast Guard cutters (378-foot class, 12 ships, commissioned 1967 to 1972) incorporated the 38TD 8-1/8, a 12-cylinder turbocharged variant, in a CODOG (combined diesel or gas turbine) arrangement. The diesel provides economical cruise propulsion; the Pratt & Whitney FT4A gas turbine delivers maximum speed. The Hamilton class entered service from 1967 and remained in front-line patrol through the 1990s and 2000s, with some units decommissioning only after successor National Security Cutters arrived. The 38D family’s durability in open-ocean patrol work, including extended periods in the Pacific and in Caribbean counter-narcotics operations, confirmed the engine’s fitness for sustained offshore service without major-depot support.

Canadian Coast Guard icebreakers also received the 38D family. The engine’s ability to run on degraded or variable-quality fuel, and to restart quickly after shutdown, suited polar operations where resupply intervals could be long.

The SEMT Pielstick license, 1973 to present

Taking the license

In 1973 Fairbanks Morse signed a manufacturing license with SEMT (Société d’Etudes de Machines Thermiques), the French firm that owned the Pielstick engine designs. The Pielstick range covered medium-speed four-stroke engines in several families, from the PA6B for naval and commercial applications to the larger PC2 and PC4 series for high-power shipboard duty. The context of these engines is covered in the semt-pielstick-engines article; Fairbanks Morse’s role was to manufacture them domestically for US naval procurement.

The US Navy required domestic manufacture for major propulsion components on Navy ships. A foreign-built engine, however capable, created a supply-chain and technology-access problem that procurement rules would not accept. Fairbanks Morse’s Beloit factory and its existing Navy relationships made it the logical US licensee. MAN Energy Solutions, which later acquired SEMT and the Pielstick brand, marked the 50th anniversary of the Fairbanks Morse license relationship in October 2018.

The Colt-Pielstick designation

During the period when Fairbanks Morse was a division of Colt Industries (1964 to 1990), the Pielstick-derived products were marketed and delivered under the Colt-Pielstick name, sometimes written Colt/Pielstick. The “Colt” prefix simply reflected the corporate parent, not a design change. After Colt Industries became Coltec Industries in 1990, the products were known as Fairbanks Morse Pielstick or FM Colt-Pielstick depending on the program. Current Fairbanks Morse Defense documentation uses “FM | Colt-Pielstick” as the brand string on these engines.

The PC2/PC4 family: design principles and the medium-speed four-stroke cycle

The Pielstick PC (Pielstick Charge) series uses a conventional four-stroke cycle: intake, compression, combustion, and exhaust on alternating strokes of a single piston in each cylinder. Gustav Pielstick, the designer, optimized the architecture around a bore-cooled cylinder liner that extracts heat from the combustion chamber wall and feeds it to the cooling circuit rather than losing it to the exhaust. Combined with carefully matched turbocharging, this produced engines that could sustain high cylinder pressures over long continuous-duty periods.

The PC2 family uses a 400 mm bore. In the PC2.5 STC variant built by Fairbanks Morse for US Navy San Antonio-class ships, the bore is 400 mm and the stroke is 460 mm, giving a stroke-to-bore ratio of 1.15:1. The engine runs at 520 rpm, which places it firmly in the medium-speed category (typically defined as 300 to 900 rpm). At that speed, mean piston speed is 7.97 m/s, close to the 38D’s 7.6 m/s, which helps explain why both engine types can coexist in one company’s service infrastructure. Turbocharging on the PC2.5 uses a constant-pressure exhaust manifold arrangement, where exhaust pulses from multiple cylinders feed into a shared manifold rather than individual pulse pipes; this smooths the energy delivery to the turbine wheel and allows it to operate at a steadier, more efficient point. The STC suffix in “PC2.5 STC” denotes Suralimenté Turbo-Compresseur, the French designation for the turbocharged configuration.

The larger PC4 family uses a 570 mm bore. The PC4.2, the version Fairbanks Morse built for US Navy fleet oilers, produces up to 1,215 kW per cylinder in its B-rated configuration. The 10-cylinder 10 PC4.2 set used on T-AO 187 class oilers delivers 16,270 horsepower per shaft in a twin-engine arrangement, totaling 32,540 horsepower for a two-shaft ship. The PC4.2 is a physically large engine: the 570 mm bore is more than twice the 38D’s 206 mm bore. Shipping a PC4.2 from the Beloit factory to a shipyard requires heavy-lift transport and specialized handling.

PC4.2: the largest diesel built in the United States, 1981

In 1981 Fairbanks Morse shipped the first Colt-Pielstick PC4.2, which the company described at the time as the largest diesel engine manufactured in the United States. The PC4.2 is a four-stroke medium-speed engine with a 570 mm bore and 620 mm stroke, with a compression ratio of 11.79:1. The US Navy selected the PC4.2 to power the T-AO 187 class fleet oilers: each ship received two 10-cylinder PC4.2 engines rated at a combined 16,270 hp for propulsion. The T-AO 187 (Henry J. Kaiser class) totaled 18 hulls and the engines remain in service on the oilers in the Military Sealift Command fleet.

PC2.5 STC: San Antonio-class propulsion

The San Antonio class (LPD 17, amphibious transport dock) uses four 16-cylinder FM Colt-Pielstick PC2.5 STC engines as its main propulsion diesels, each rated at 7,755 kW at 520 rpm. Combined output totals 41,600 shaft horsepower, driving two shafts through reduction gears and controllable-pitch propellers. The class is capable of speeds exceeding 22 knots. Fairbanks Morse has supplied these engines for all San Antonio-class hulls, including LPD 29 (Richard M. McCool Jr., commissioned 2022) and LPD 31 (Santa Barbara, contracted 2018 as the first LPD Flight II hull). The engines are manufactured in Beloit and delivered to Huntington Ingalls Industries’ Pascagoula, Mississippi shipyard for installation.

The LPD-17 propulsion installation places two PC2.5 engines on each reduction gear, so both engines on a single shaft can run together or independently. At full power, all four engines drive the ship at maximum speed. For economical transit, the ship can operate on two engines, one per shaft, with the other two engines shut down and their shafts clutched out.

PA6B STC: Freedom-class LCS

The Freedom-variant Littoral Combat Ship (LCS) uses two FM Colt-Pielstick 16PA6B STC medium-speed diesels alongside two gas turbines in a CODOG arrangement. The 16PA6B STC is shock-qualified to MIL-S-901D standards. Freedom-class ships are in service with the US Navy, and the PA6B STC is certified for 17 navies and coast guards worldwide.

FM 28/33D STC and FM 32/44 CR

In addition to the Pielstick-licensed families, Fairbanks Morse Defense now offers the FM 28/33D STC (5,460 to 10,000 kWb) and FM 32/44 CR (3,600 to 12,000 kWb) in its current catalog. The 48/60 CR, derived from the MAN 48/60 family, covers 7,200 to 19,200 kWb and represents the upper range of the FMD medium-speed portfolio. These engines target large naval vessels, auxiliary ships, and shore-based power plants where domestic manufacture and military-grade qualification are required.

Locomotives: a parallel history, 1944 to 1963

The opposed-piston engine’s railway application is worth covering briefly because it ran in parallel with the marine program for nearly two decades and shares the same engineering base.

Fairbanks Morse entered locomotive production in 1944 after the Navy’s wartime engine orders had proven the opposed-piston design in demanding service. The H-10-44 switcher, rated at 1,000 hp, was the first product in 1944. The Erie-built cab unit, 2,000 hp, followed in 1945. The 1953 H-24-66 Train Master, at 2,400 hp, was the most powerful single-unit diesel locomotive available in the US when introduced.

Total production reached 1,460 locomotives. However, the engine’s lower exhaust piston ran close to the cylinder-wall exhaust ports, giving it limited cooling airflow and causing elevated piston temperatures in sustained mainline service. Piston failures and occasional crankcase fires undermined customer confidence. Fairbanks Morse built its last domestic locomotive in 1958 and its last export unit in 1963. The Soviet Union reverse-engineered the opposed-piston locomotive engine, producing the 2D100 that powered the TE3 diesel locomotive series (up to 7,600 units built); turbocharged 10D100 variants followed in the TE10 series, though reliability problems eventually led to re-engining programs at many Soviet railway depots.

The locomotive experience established the engineering lesson that the 38D’s marine and naval service had already demonstrated: the opposed-piston design performs reliably at conservative ratings and steady loads but is unforgiving in highly cyclic, thermally demanding service where the lower piston sees both combustion heat and limited cooling.

Corporate history: from industrial manufacturer to defense specialist

The conglomerate era, 1958 to 2020

Fairbanks, Morse & Company merged with Penn-Texas Corporation in 1958 to form Fairbanks Whitney Corporation. Penn-Texas also held Colt Manufacturing, the firearms maker, and when the merged group reorganized in 1964 it took the name Colt Industries, the name staying until 1990 when Colt sold its firearms business and became Coltec Industries. This is the period during which the Pielstick-licensed engines were sold under the Colt-Pielstick label.

In 2002, Goodrich spun off Coltec’s non-aerospace industrial businesses as a new public company, EnPro Industries. The Fairbanks Morse Engine operation became an EnPro division, headquartered and manufacturing in Beloit. EnPro retained Fairbanks Morse Engine until January 2020.

On 21 January 2020, Arcline Investment Management acquired Fairbanks Morse from EnPro for $450 million. Arcline renamed its new subsidiary Fairbanks Morse Defense, a deliberate signal of the company’s strategic focus going forward.

The Arcline era: building a naval systems integrator, 2020 to present

Under Arcline, Fairbanks Morse Defense has grown through acquisition into a broader naval systems supplier. Key additions since 2020:

• Ward Leonard (acquired January 2021): ship propulsion control systems, motor drives, and electrical equipment for naval applications.
• Hunt Valve Company (acquired September 2021): specialty manufacturer of submarine and naval valves and electromechanical actuators, critical to the same submarine platforms the 38ND equips.
• Welin Lambie (acquired December 2021): UK-based designer and manufacturer of marine davit and boat-handling systems.
• Federal Equipment Company (acquired January 2022): surplus industrial and military equipment services.
• Maxim Watermakers (acquired January 2022): reverse-osmosis watermakers for naval vessels, based in Shreveport, Louisiana.
• Rolls-Royce Naval Propulsors and Handling Business (announced September 2024, completed July 2025): propellers, waterjets, controllable-pitch propeller systems, and marine handling equipment.

The Rolls-Royce Naval Propulsors acquisition

The Rolls-Royce acquisition is the most consequential addition in Fairbanks Morse Defense’s history. The deal closed on 16 July 2025, celebrated at the Pascagoula, Mississippi foundry with Mississippi Governor Tate Reeves and FMD CEO Steve Pykett in attendance.

The acquisition includes two US facilities: the Pascagoula, Mississippi propeller foundry and the Walpole, Massachusetts operation, plus the Naval Handling business in Peterborough, Ontario, Canada. The Pascagoula campus is the only privately owned foundry in the United States qualified to cast propellers for the US Navy’s surface and submarine fleet, a status that makes it a designated national asset. It manufactures controllable-pitch propeller blades and hub body castings, large fixed-pitch propellers, and waterjets. Rolls-Royce divested the business as part of its own corporate transformation toward civil aerospace that began in 2023.

This acquisition means Fairbanks Morse Defense now covers both the engines that drive a naval vessel and the propulsors those engines turn. No other US domestic supplier has assembled that combination. FMD propulsion products link to the marine diesel propulsion systems context; for the specific engineering of controllable-pitch propellers, the medium-speed four-stroke marine engines article gives relevant context on how propeller pitch and engine torque curves are matched.

FMD currently operates an 800,000-square-foot manufacturing facility in Beloit employing over 550 people, with five service centers positioned across the US for parts and field service.

The ALCO engine product line, 1993

In 1993, during the Coltec Industries period, Fairbanks Morse acquired the ALCO 251 engine product line. ALCO (American Locomotive Company) had built medium-speed four-stroke diesels for marine, stationary, and rail applications before leaving the locomotive business. The FM ALCO 251F remains in the current FMD catalog, covering 764 to 3,060 kWb in marine and stationary service. It extends FMD’s product range below the opposed-piston and Pielstick families.

The Trident OP: a modern opposed-piston revival with Achates Power

The partnership

Fairbanks Morse signed a joint development and licensing agreement with Achates Power in October 2013. Achates Power had been developing modern opposed-piston two-stroke diesel concepts using contemporary computer-aided engineering tools, including multi-dimensional computational fluid dynamics to map the scavenging flow, injection spray, and combustion chemistry inside the cylinder. Fairbanks Morse contributed 80 years of opposed-piston manufacturing and operational knowledge to the partnership, along with its established Navy customer relationships and its knowledge of what the 38D’s 40,000-hour service life actually demanded from the design.

Development of the Trident OP was formally approved in February 2014. The partnership combined Achates Power’s thermal and combustion engineering with Fairbanks Morse’s production capability in a way that neither company could have replicated alone. Achates Power had demonstrated that modern materials, injection systems, and ring-coating technologies had solved the exhaust-piston overheating problem that undermined early opposed-piston locomotive engines. Piston thermal management was the key enabling technology: bore cooling, advanced thermal-barrier coatings on piston crowns, and oil-gallery cooling in the piston body collectively keep the exhaust piston crown within acceptable temperature limits even at high power density.

Technical claims and target applications

Fairbanks Morse introduced the Trident OP in December 2017. The 3.8 MW engine claims 50 percent brake thermal efficiency, which the company positioned as best-in-class against leading medium-speed four-stroke competitors. The Achates Power thermodynamic analysis underlying this claim computes three compounding contributions: the low combustion-chamber surface-area-to-volume ratio reducing heat transfer losses, the two-stroke cycle’s diluted energy density per combustion event reducing wall heating, and the leaner air-fuel ratios shifting the ratio of specific heats favorably. In real-world testing of Achates Power’s 10.6-liter opposed-piston engine announced in December 2023, peak brake thermal efficiency exceeded 49 percent, which is consistent with the Trident OP’s 50 percent claim at the larger bore and lower specific power of the stationary application.

The design uses counter-rotating crankshafts with counter-reciprocating piston assemblies for inherent mechanical balance, a heritage trait of the 38D family. Moving parts count is 30 percent lower than comparable medium-speed engines, partly because the absence of a cylinder head eliminates the head bolts, head gaskets, rocker gear, camshaft-driven valve train, and associated sealing hardware.

The 40,000-hour interval to first major overhaul was a central selling point. Competing medium-speed engines typically require a minor overhaul around 20,000 hours; eliminating that outage reduces operating cost in base-load applications where the engine runs 8,500 hours per year. The engine carries an EPA Tier 4F rating on the 60 Hz model, with selective catalytic reduction handling NOx. Target markets were base-load and prime power generation: utilities, nuclear plant backup power, municipal facilities, and remote installations. A PoweReliability-as-a-Service platform with embedded sensors and cloud-based condition monitoring accompanied the launch, offering fuel consumption and reliability guarantees backed by FMD’s service-center network.

The FM 175D and the DDG(X) program

The FM 175D high-speed diesel, with a 175 mm bore available in 12-, 16-, or 20-cylinder configurations, covers 1,740 to 4,400 kW at 1,800 to 2,000 rpm. FMD introduced the FM 175D into the US market in 2023 to meet demand for high-power-density generator sets in naval and commercial applications. The engine accepts DMA, DMZ, F-75, and F-76 naval fuels.

In July 2025, the US Navy awarded FMD a contract to supply an FM 175D engine for integration into the DDG(X) land-based propulsion system test site. DDG(X) is the Navy’s next-generation large surface combatant, planned to enter construction in 2032. The Fiscal Year 2025 National Defense Authorization Act mandates that all key propulsion technologies for DDG(X) be tested in land-based environments before ship construction begins. The land-based test site will validate the FM 175D’s load-following behavior, thermal efficiency, maintenance predictability, and generator output under variable conditions relevant to a next-generation destroyer’s mission systems, propulsion drives, and high-energy weapons. The DDG(X) program requires at least 40 megawatts of available reserve power, and the FM 175D’s power density makes it a candidate for the generator sets that supply that margin.

Engine families in the current catalog

The Fairbanks Morse Defense engine portfolio as of 2025 spans five active and two legacy-supported families:

Legacy families receiving parts and service support but no longer manufactured include the 38F, 38TDS, 38TDD, PC2.3, PC2.6, PC4, 32/40 DF, and 42/44. The PC4.2 that powered the T-AO 187 class falls into this supported-but-not-built category.

The opposed-piston design in context

The fundamental attraction of the opposed-piston layout is thermodynamic: eliminating the cylinder head removes the surface through which a conventional diesel loses a significant fraction of combustion heat. A cylinder head in a water-cooled engine sees the hottest gas in the cycle and dumps heat into the coolant whether you want it to or not. A cylinder with two opposing pistons has no fixed wall at the combustion space; the combustion chamber is bounded by two moving surfaces, both pistons, that extract work from the expanding gas rather than absorbing it as coolant loss. This is why opposed-piston diesels can approach or exceed 50 percent brake thermal efficiency, a figure that contemporary four-stroke medium-speed engines struggle to reach.

Against this is complexity. Two crankshafts, phased precisely, with a geared connection between them, add mechanical complexity relative to a conventional single-crankshaft engine. The two pistons must be sealed by their rings against the cylinder bore from both ends, and the port geometry at the center of the cylinder, where the two pistons pass close together on each stroke, demands careful timing. Thermal management of the lower (exhaust) piston is the persistent engineering challenge: it sits nearer the exhaust ports and sees more heat. The locomotive program’s piston failures came from this problem under high-duty cycling. Modern thermal-barrier coatings and bore-cooling techniques, applied in the Trident OP, have reduced but not entirely eliminated this asymmetry; the Achates Power research program produced detailed piston thermal management patents (US 9,464,592 and 10,174,713) specifically addressing this problem.

The semt-pielstick-engines article and the cooper-bessemer-marine-engines article each describe the competing medium-speed four-stroke design philosophy that Fairbanks Morse adopted for its licensed products alongside its own two-stroke heritage. The coexistence of both types in one company’s catalog is unusual and reflects the company’s position as a naval supplier that must match the engine to the ship specification rather than advocate for any single cycle.

For the engineering that underpins how any diesel converts fuel to shaft work, the marine diesel engine article and the medium-speed four-stroke marine engines article give the working detail. Brake mean effective pressure and brake thermal efficiency calculations relevant to comparing the 38D to Pielstick families are available through the BMEP calculator and the brake thermal efficiency from SFOC calculator.

Why the engine is still in production after 88 years

The 38D 8-1/8 has been in continuous production since 1938. No other marine diesel engine currently in production has an equally long unbroken run. The reason is specific: the US Navy needs it.

Every Los Angeles-class submarine in commission, every Seawolf-class hull, and every Ohio-class SSBN and SSGN carries a 38ND 8-1/8 as its emergency diesel. These submarines are not being replaced quickly. The Virginia class that follows uses a different emergency power arrangement, but the existing hulls will operate for decades. The Navy cannot switch emergency diesel suppliers without a major re-qualification program on every hull type, and qualification takes years and costs substantially. Fairbanks Morse Defense is the only US domestic manufacturer of the engine.

That captive demand is the commercial foundation of the 38D line. It also explains the company’s investment in the Trident OP and in the Achates Power partnership: Fairbanks Morse wants to apply its opposed-piston manufacturing knowledge to next-generation programs before the 38ND requirement eventually terminates.

The industrial logic runs both ways. The Navy’s dependence on a single domestic source for a critical submarine component creates an obligation to sustain that source. FMD’s manufacturing facility and workforce in Beloit are in effect a mobilization asset: if a future conflict required building emergency diesel generators for submarines in volume, Beloit is the only place in the United States equipped to do it. The Rolls-Royce naval propulsors acquisition extends that logic to propellers and waterjets, building FMD into a more complete naval machinery supplier that is harder to displace.

Limitations

This article covers the design history, technical characteristics, and naval applications of Fairbanks Morse engines from verifiable primary and official sources. Specific per-cylinder fuel consumption figures, detailed emissions test data, and complete hull lists for each ship class using FM engines are not reproduced here because they vary by rating, build year, and upgrade level and are properly read from the OEM’s technical documentation and naval records rather than summarized.

The corporate ownership chain from Penn-Texas (1958) through Fairbanks Whitney, Colt Industries, Coltec Industries, EnPro Industries, and Arcline/Fairbanks Morse Defense (2020) is documented but the precise dates of every internal division restructuring are not always publicly available; the main transition dates given here are confirmed from primary announcements.

Engine specifications for active naval programs, including exact power ratings per vessel class, are subject to export-control and operational-security considerations. The figures cited here come from FMD’s own public specification pages and from unclassified procurement and press reporting; they should not be treated as complete operational specifications.

The Trident OP performance claims (50 percent brake thermal efficiency, 40,000-hour overhaul interval) are drawn from the December 2017 introduction press release and from Achates Power’s published technical analysis. Independent third-party in-service operational results for the Trident OP in sustained service are not publicly available at the time of writing.

See also

• Marine engine makers: the index of builder-brand histories on this site.
• Two-stroke marine diesel engine fundamentals: the cycle and scavenging principles behind the 38D.
• Uniflow scavenging in two-stroke marine engines: the gas-exchange mechanism the 38D uses.
• Doxford opposed-piston engines: the parallel British opposed-piston tradition.
• SEMT Pielstick engines: the licensed engine families built by Fairbanks Morse since 1973.
• Cooper-Bessemer marine engines: another US large-bore engine builder with a parallel naval and industrial history.
• EMD marine engines: the Electro-Motive Division two-stroke diesel, the other major US Navy engine supplier in the WWII era.
• Medium-speed four-stroke marine engines: the broader class that includes the Pielstick families.
• Marine diesel engine: working principles common to all engine types discussed here.
• Marine auxiliary engines and generators: the generator-set role of naval emergency diesels.

Related calculators:

• BMEP calculator: brake mean effective pressure from power, displacement, and speed.
• Brake thermal efficiency from SFOC: efficiency from fuel consumption and heating value.
• Compression ratio calculator: geometric compression ratio from cylinder volumes.