NOHAB and Polar Marine Diesel Engines

What NOHAB and Polar were

Nydqvist & Holm AB, known by the acronym NOHAB, was a Swedish industrial firm in Trollhattan that built water turbines, steam and diesel locomotives, and marine diesel engines. The firm traces to an 1847 mechanical workshop on the Gota alv river. Its marine diesel line carried the brand name Polar, a name that began not at NOHAB but at a separate Stockholm firm, AB Diesels Motorer, before passing through AB Atlas Diesel and arriving at NOHAB in 1948.

The Polar name covers more than a century of marine practice. The earliest Polar engine ran in 1908 as one of the first reversible marine diesels built in series. The latest descendants ran under the Wartsila Nohab badge after Wartsila absorbed NOHAB’s engine business between 1978 & 1984. Between those dates the brand traveled across at least three Swedish owners and one British licensee in Glasgow.

This article traces the corporate chain and the engine designs. It keeps to dated, named events. Where a specific power or fuel figure cannot be tied to a primary or heritage record, the text describes the design qualitatively rather than quote a number. For the underlying engine principles, see two-stroke marine diesel engine fundamentals and four-stroke marine diesel engine fundamentals. For the wider field, see the marine diesel engine overview and the marine engine makers index.

Trollhattan origins, 1847

The workshop that became NOHAB opened in 1847 in Trollhattan, on the Gota alv where the river drops through a series of falls. The site mattered. The falls gave mechanical power before electricity, and later gave hydroelectric power once turbines could use the head. A mechanical works on that river had ready demand for water-wheel and turbine work.

Antenor Nydqvist was the engineering founder. The early business was hydraulic: water turbines, sluice gates, and the iron work that a canal-and-river town needed. The Gota canal system, which links the Baltic to the Kattegat through the Gota alv and the great lakes, ran traffic past Trollhattan, so the works sat on a transport artery as well as a power source.

The firm moved into railway work in the 1860s. It built its first steam locomotive in 1865. That single product line ran for more than a century and made Trollhattan one of the recognized European locomotive towns. By the early 1910s the works had delivered well over a thousand locomotives, a volume that put it among the larger continental builders outside the giants in Germany and Britain.

The name Nydqvist & Holm came from two of the principals, Antenor Nydqvist and Carl Olof Holm. The firm reorganized as a joint-stock company under the NOHAB acronym in 1916. By then it carried three product families that would define it: turbines, locomotives, and, soon after, diesel engines. The diesel work did not start in Trollhattan, though. It started in Stockholm, & it started with a different company.

The water-power that drew the works to Trollhattan also shaped its turbine business through the early twentieth century. The Trollhattan falls became the site of one of Sweden’s first large hydroelectric stations, commissioned in stages from 1910, and a turbine builder on the spot held an obvious advantage. NOHAB supplied water turbines to Swedish and export power schemes for decades. That hydraulic line, the firm’s oldest, sat alongside the engine and locomotive work and gave the company a third leg that smoothed demand when one of the other two slowed.

A locomotive and turbine works of this size carried heavy plant: a foundry for iron and steel castings, large boring and turning machines for cylinders and shafts, balancing rigs for rotating parts, and test floors for finished machines. None of that plant is specific to railways or to power stations. It is exactly the plant a marine-engine line needs. When NOHAB took on Polar diesel production after the Second World War, it did so on an existing industrial base, not from a standing start, which is part of why the transfer of the engine line into Trollhattan worked.

The Polar name and AB Diesels Motorer, 1898

Rudolf Diesel was granted his German engine patent in 1893 and ran his first working high-compression engine in the late 1890s. Swedish industry moved on the idea quickly. AB Diesels Motorer was formed in 1898, with backing from the Wallenberg financial group, to build Diesel-licensed engines in Sweden. The company set up at Sickla, on the southern edge of Stockholm.

AB Diesels Motorer is the true source of the Polar brand. The firm needed a trade name for its engines that buyers would remember, and it chose Polar. The choice fit a Nordic builder selling into Baltic and North Atlantic trades, where ships worked cold water and ice.

The technical figure at Sickla was Jonas Hesselman, who joined the firm in 1901 and led much of its engine development through the following years. Hesselman’s name attaches to two early contributions that shaped the Polar line: reversibility for marine service, and solid fuel injection without compressed air. Both are covered below under the engineering sections, because both outlived the company that first built them.

The marine problem in 1900 was not whether a diesel could turn a propeller. It was whether the engine could run astern. A steam reciprocating engine reverses by changing valve timing; the engine itself runs backward. An early diesel could not, because its fuel and valve events were timed for one direction of rotation. Without a reversing solution, a motor ship needed either a reversing gearbox, a controllable-pitch propeller, or an electric drive, none of which were mature in 1900.

The commercial stakes behind that problem were large. A coal-fired steam cargo ship of the period spent a heavy share of its deadweight and crew on the power plant: bunkers of coal, a fire-room gang to shovel it, boilers that needed cleaning and survey, and ash to dispose of. A diesel ship cut all of that. Liquid fuel takes less volume per unit of energy than coal and pumps itself; the engine needs no fireman; there is no ash. Owners on long trades, especially the Scandinavian tramp and timber operators who worked thin margins, watched those savings closely. The reversible marine diesel was the technical key that unlocked them, and a Swedish builder that reached it early sold into a market that wanted it.

The 1908 reversible Polar engine

In 1908 AB Diesels Motorer delivered a directly reversible marine Polar engine. It ranks among the first reversible marine diesels built anywhere, and it is generally treated as the first Swedish-designed marine diesel of consequence. The achievement was the reversing mechanism, not the combustion cycle.

Direct reversal means the crankshaft itself turns the other way, driving the propeller astern without any gearbox between engine and shaft. To do that, the fuel-injection and valve events have to be re-timed for the opposite direction of crank rotation. Early solutions used a camshaft that could be shifted axially so that a second set of cams, ground for reverse running, took over the fuel pumps and valves. The engine is stopped, the cam carrier is moved, and the engine is restarted in the opposite sense, often with a blast of starting air to get it turning the right way.

This is what made the diesel a credible main engine for a single-screw cargo ship. A reversible direct-drive diesel let an owner delete the steam plant, the boilers, the bunkers of coal, and the firemen, and connect the engine straight to the tailshaft. The economic case for the motor ship rested on that one mechanical capability, and the 1908 Polar was an early proof that it could be built and sold.

The reversing approach was not unique to Sweden. Builders across Europe were attacking the same problem in the same years, and several reached direct-reversing marine diesels in the 1905 to 1912 window. What the Polar engine shows is that a small Swedish licensee, not one of the original Diesel licensees in Germany, reached a working reversible marine engine early and turned it into a product line.

AB Atlas Diesel, 1917

In 1917 AB Diesels Motorer merged with Nya Atlas, a Stockholm engineering and railway-equipment firm, to form AB Atlas Diesel. The combined company kept the Sickla works and kept building Polar marine engines, now under the Atlas Diesel corporate name. This is the source of the Atlas-Polar designation that appears on engines and in records from the 1917 to 1948 period.

Atlas Diesel ran two broad businesses that would later split. One was the diesel engine line, marine and stationary, carrying the Polar name. The other was compressed-air and rock-drilling equipment, the business that Atlas-origin firms had built since the nineteenth century. Through the 1920s, 1930s, and 1940s the Sickla works turned out Polar marine engines for Scandinavian owners and for export. The detailed model-by-model record from this period is the subject of the companion article on Atlas-Polar marine engines.

The two halves of Atlas Diesel sat together until after the Second World War. The diesel half went to NOHAB in 1948. The compressor and rock-drill half kept the Atlas name and became Atlas Copco in 1956, a firm still active in compressed-air and mining equipment. The shared ancestry of a major compressor maker and a Polar marine engine is a detail that the corporate genealogy preserves even though the two products have nothing in common.

NOHAB acquires the Polar marine line, 1948

In 1948 NOHAB acquired Atlas Diesel’s marine diesel engine business and moved Polar production toward Trollhattan. This is the point where the Polar brand and the NOHAB name join. From here, engines carry the NOHAB-Polar designation, and the Sickla marine activity winds down as Trollhattan takes over.

The fit was logical. NOHAB already ran a heavy-engineering works with foundry, machine-shop, and assembly capacity built up over a century of turbine and locomotive work. Adding medium-bore diesel engines used the same skills: large castings, precision boring, crankshaft and bearing work, and test-bed running. A locomotive builder and a marine-engine builder share more practice than the two products suggest.

NOHAB’s product mix after 1948 had three legs. Hydraulic turbines for power stations were the oldest line. Locomotives were the largest and most visible. Polar marine and stationary diesels were the newest of the three to sit under NOHAB ownership. The marine engines served Swedish coasters, fishing vessels, tugs, and small to medium cargo ships, plus naval auxiliaries and the ice-capable ships that Baltic service needs.

The Baltic and the Bothnian Gulf freeze in winter. Swedish, Finnish, and Norwegian owners needed engines that would start cold, run hard against ice, and take the shock loading that ice contact puts through a propeller and shaftline. A medium-speed diesel with strong reserves suits that duty better than a lightly built high-speed engine. The Polar line’s reputation in northern waters rests on that match between a sturdy medium-bore design and a hard operating environment.

Ice navigation puts a particular load case on a propulsion plant. When a propeller blade strikes ice, the engine sees a sudden torque spike and the shaftline takes a shock that a steady open-water run never produces. An engine for ice service needs torsional margin, sound bearings, and the ability to hold low-speed torque without stalling when the ship works its way through broken ice. The Polar engines’ medium-bore, medium-speed layout, with substantial reciprocating and rotating parts relative to their rating, gave them that margin. The same quality that made them heavier than a high-speed engine of equal power made them suited to the duty.

Cold starting is the other northern demand. A diesel needs enough compression heat to ignite the charge, and a cold engine and cold intake air make that harder. Builders for the Nordic market paid attention to starting-air systems, to the warming of jacket water before a cold start, and to the margins that let an engine fire on the first attempt in winter. An engine that will not start on a frozen morning is no use to a coaster owner who has a tide to catch. The Polar engines earned their place in that trade by being dependable in exactly those conditions.

The Polar two-stroke engines

The Polar marine engines split into two-stroke and four-stroke families, and the two-stroke side carried the older lineage. A two-stroke engine fires every revolution, so for a given size and speed it makes more power than a four-stroke, at the cost of a scavenging system that has to clear exhaust and admit fresh air in the short window around bottom dead center.

The Polar two-strokes were port-scavenged medium-bore engines. Port scavenging uses ports cut in the cylinder liner, uncovered by the piston near bottom dead center, rather than poppet valves in the head, to handle the gas exchange. Early Polar two-strokes used cross-scavenging or loop-scavenging arrangements, where the incoming air is aimed to push the burnt gas out without short-circuiting straight to the exhaust. The principle is set out in two-stroke marine diesel engine fundamentals.

The key output metric for any such engine is brake mean effective pressure, the average pressure that, acting on the piston over one power stroke, would produce the measured brake work. It lets engines of different size and speed be compared on a single intensity figure.

For a two-stroke, the firing-stroke factor in that relation is one, because the engine fires every revolution; for a four-stroke it is two. That single difference is why a two-stroke of the same bore, stroke, and speed shows a different power and a different BMEP from its four-stroke counterpart. The Polar two-strokes used the factor-of-one path, which is part of why they suited heavy continuous propulsion duty.

Charging arrangements changed across the production life. The earliest port-scavenged Polar engines were naturally aspirated or used mechanical scavenge blowers driven off the engine. Later marine two-strokes across the industry moved to exhaust-gas turbocharging, first with pulse systems that use the exhaust pressure pulses from individual cylinders, then with constant-pressure systems that feed a common receiver. The Polar engines followed that general path as turbocharging matured through the mid-twentieth century.

Turbocharging is the single change that did most to raise marine-diesel output per cylinder. A turbocharger uses the energy in the exhaust to drive a compressor that forces more air into the cylinder; more air allows more fuel and so more power from the same swept volume. For a two-stroke, the scavenge air also has to clear the cylinder of exhaust, so the charging system serves two duties at once, gas exchange and boost. The progression from naturally aspirated to pulse to constant-pressure turbocharging across the Polar line mirrors the wider marine-diesel record, where the same boost technology lifted ratings two- and threefold over the decades without changing the basic cylinder dimensions.

The two-stroke layout also shaped maintenance. Port-scavenged liners wear at the port edges and need watching; the scavenge spaces and exhaust belts need cleaning to stay clear of carbon and oil residue; and the absence of cylinder-head intake and exhaust valves on a port-scavenged engine removes one set of components but puts more demand on the liner and piston rings. Engine-room crews on Polar two-strokes worked to those particulars, which differ from the four-stroke routine.

The Polar four-stroke engines

The four-stroke Polar engines covered the medium-speed end of the range. A four-stroke takes two crankshaft revolutions per power stroke and uses poppet valves in the cylinder head for intake and exhaust, which gives cleaner gas exchange than port scavenging and suits higher running speeds. The trade is one power stroke every other revolution instead of every revolution. The general arrangement is described in four-stroke marine diesel engine fundamentals.

These engines served propulsion in smaller and coastal vessels and served as generator prime movers across the fleet. A medium-speed four-stroke turning a few hundred to several hundred revolutions per minute pairs well with an alternator for ship’s electrical power, and pairs with a reduction gear for propulsion in vessels too small to justify a slow-speed direct-drive engine. The Polar four-strokes filled both roles.

Fuel economy is the figure owners watch over a ship’s life, and it is captured by specific fuel oil consumption, the mass of fuel burned per unit of brake energy delivered. Lower SFOC means less fuel for the same work. From SFOC and the fuel’s heating value, the engine’s brake thermal efficiency follows directly.

Reported SFOC depends on the reference air conditions at the test bed, because intake air temperature and density change the mass of air the engine breathes and therefore the combustion it can support. Standards bodies set reference conditions so that an engine measured in a warm shop and one measured in a cold shop can be compared fairly, and so that a buyer’s guarantee figure means the same thing in any climate.

For the Polar engines, no single guaranteed consumption figure transfers across the whole family, because bore, speed, charging, and vintage all moved over the decades of production. The point that holds is qualitative: a sturdy medium-speed four-stroke aimed at coastal and genset duty trades a little peak efficiency for reliability and tolerance of variable fuel and load, which is what northern coastal operators wanted.

Polar engines in service: ship types

The Polar marine range settled into a band of ship types defined by its medium-bore, medium-speed character. At the small end were coasters and the timber and general-cargo ships of the Baltic and North Sea trades, single-screw vessels in the low hundreds to low thousands of gross tons that a directly reversible diesel of a few hundred to a few thousand kilowatts suited well. These ships ran short hauls with frequent maneuvering, and a sturdy direct-drive engine was the simplest arrangement for them.

Fishing vessels were a second large market. A trawler or seiner works its engine hard at variable load: full power steaming to the grounds, reduced power while fishing, and heavy auxiliary demand for winches and processing. A medium-speed diesel that tolerates load swings and runs on the fuel available in a fishing port fits that profile. Polar and Atlas-Polar engines went into Scandinavian fishing fleets across the inter-war and post-war decades.

Tugs and harbor craft formed a third group, where bollard-pull duty asks for high torque at low speed and constant maneuvering asks for quick, reliable reversing. Naval auxiliaries and patrol craft, in Sweden and through the British Polar Engines line in the United Kingdom and the Commonwealth, made up a fourth. Ice-capable ships, from ice-strengthened cargo vessels to support craft for winter navigation, drew on the line’s cold-service strengths. Across all of these, the common thread is a working ship that needs reliability and reserve more than it needs the last point of efficiency or the lowest possible weight.

NOHAB-GM diesel-electric locomotives

NOHAB’s most widely recognized post-war product was not a marine engine. It was the NOHAB-GM diesel-electric locomotive. From the 1950s NOHAB held a license to build locomotives around the two-stroke diesel engines of General Motors’ Electro-Motive Division, the American builder whose engines had displaced steam across North American railroads.

The license let NOHAB combine American prime-mover and electrical technology with European body and bogie design. The resulting locomotives, with their rounded noses, ran on Swedish, Danish, Norwegian, and other railways from the late 1950s, and many survived into preservation. To railway enthusiasts across northern Europe, NOHAB means these machines first.

The locomotive business matters to the marine story for two reasons. It shows that NOHAB ran two large diesel programs at once, marine Polar engines and EMD-licensed locomotive engines, drawing on a shared works. And it sets up the firm’s end, because it was the locomotive side, not the marine side, that drove the late-1970s collapse. The marine engines are the focus here; the locomotives are the more famous cousin.

British Polar Engines of Glasgow

The Polar design crossed to Britain through a license. In 1928 the Polar marine engine line was licensed to a Glasgow firm that became British Polar Engines. Glasgow, on the Clyde, was one of the world’s shipbuilding and marine-engineering centers, and a Clyde-built engine carried weight with British and Commonwealth owners.

British Polar Engines built airless-injection two-stroke marine engines under the Polar license. The engines went into British merchant ships, into Royal Navy auxiliary and patrol craft, and into export markets. A licensee in Glasgow gave the Polar design a manufacturing base inside the British Empire’s trading system, which the Swedish parent could not easily serve from Trollhattan against tariffs and shipping costs.

The license arrangement was typical of marine-engine practice between the wars. A successful engine design was a property worth licensing, and a builder in one country could extend its reach by licensing a maker in another rather than trying to export finished engines against tariffs, freight, and local-content rules. The licensee built to the parent’s drawings, paid royalties, and sold into its home market and the territories it could reach. For Polar, the Glasgow license put a recognized Clyde name on engines built to a Swedish design, which carried more weight with a British owner than an import would have.

The Glasgow firm outlived its Swedish parent. After NOHAB’s troubles at the end of the 1970s and the Wartsila absorption that followed, British Polar Engines continued as an independent Glasgow company. It positioned itself as the long-term service source for Polar engines worldwide, supplying spare parts and overhauls for engines built in Sweden under all the earlier owners as well as for its own Glasgow-built units. For a defunct engine line, a surviving independent service house is the difference between an engine that can be kept running and one that becomes scrap when the first major part fails.

Engineering: airless solid injection

The two technical ideas that define the Polar engines both trace to Hesselman’s work at Sickla, and the more far-reaching of the two was airless solid injection. Early diesels injected fuel using a blast of compressed air to atomize it into the cylinder, which is where the engine’s name in some languages, the air-blast or compression-ignition engine, partly comes from. Air-blast injection needed a high-pressure air compressor on the engine, which added weight, cost, and a maintenance burden.

Solid injection, also called airless injection, does away with the air blast. A high-pressure fuel pump forces the fuel through an injector nozzle whose own geometry atomizes it. No compressed-air system is needed for injection. The fuel pump and injector do all the work. This is the system every modern diesel uses, in the sense that none use air-blast injection any more.

Reaching solid injection early gave the Polar engines a simpler, lighter top end than air-blast contemporaries. The benefit compounds in marine service, where weight, the number of separate systems to maintain, and the reliability of each system all bear directly on a ship’s operating cost. An engine with one fewer high-pressure auxiliary is an engine with one fewer thing to fail at sea.

Engineering: reversibility and direct drive

The other defining feature was direct reversibility, present from the 1908 engine onward. A directly reversible engine connects straight to the tailshaft and runs astern by reversing its own rotation, which removes the reversing gearbox from the drivetrain. For a single-screw cargo ship, that is the simplest possible propulsion arrangement: engine, shaft, propeller.

The mechanism, as noted earlier, shifts the cam timing so the fuel and valve events suit the opposite direction of crank rotation, then restarts the engine the other way on starting air. Maneuvering a directly reversible engine in harbor is a hands-on operation, a sequence of stop, change over, and restart commands worked from the engine room or from the bridge through a telegraph. Crews trained on these engines knew the drill, & the mechanical simplicity paid back across the rest of the voyage.

Direct reversibility suited the medium-bore single-screw ships that made up much of the Scandinavian merchant fleet: coasters, small cargo ships, and the timber and ore carriers of the Baltic trades. It is no accident that the Polar line, designed by a Nordic builder for Nordic owners, settled into exactly that market and held it for decades.

Decline and absorption, 1978 to 1984

NOHAB’s end came through the locomotive business and the wider contraction of Swedish heavy industry in the 1970s. Wartsila, the Finnish group then building a medium-speed marine engine business by acquisition across Europe, took a majority stake in NOHAB’s diesel operations in 1978, buying 51 percent from the parent group. The move gave Wartsila control of the marine and locomotive diesel work.

NOHAB went bankrupt in 1979. The failure traced mainly to the locomotive side after the last large domestic locomotive order, against a backdrop of falling demand and rising costs across Swedish manufacturing. The diesel engine operations, by then under Wartsila’s majority control, carried on.

Wartsila acquired the remaining 49 percent in 1984, taking full ownership, and the engine business ran on as Wartsila Nohab at Trollhattan. Production of medium-speed marine and stationary engines continued at the site under the Wartsila badge into the following decades. The Polar brand as an independent maker’s name ended with these transfers; the engines and the works lived on inside a larger group.

Wartsila’s strategy in these years was to assemble a medium-speed marine engine business by buying and licensing across Europe rather than by growing one design organically. The group took stakes in and absorbed several national builders through the late 1970s and 1980s, gathering their designs, their works, and their service networks. NOHAB’s Trollhattan works and the Polar line were one such acquisition. The logic was scale: a few large groups serving the world fleet, each carrying several inherited engine families, in place of a crowd of national builders each serving mainly its home market.

The pattern is the European medium-speed consolidation of the late twentieth century, in which national builders were absorbed into a few large groups. Sweden’s other engine builders sat in the same current, and the contrasts are worth drawing: Gotaverken Swedish marine engines built large slow-speed two-strokes on the west coast, Kockums shipyard and engines in Malmo combined shipbuilding with engine work, and the earlier Bolinder hot-bulb marine engines had defined the small-craft motor before the high-compression diesel matured. The Polar line is one strand in that Swedish marine-engineering record.

Heritage and the preserved record

NOHAB’s public memory rests mostly on its locomotives. NOHAB-GM machines survive in railway preservation across Sweden, Denmark, and Norway, some of them still operated on heritage lines and at enthusiast events, and the firm’s railway output is documented in detail. The turbine work is recorded too, tied to the Trollhattan hydroelectric story and the wider history of Swedish power engineering.

The marine engine record is held in heritage and archive collections rather than spread widely in print. Tekniska museet in Stockholm holds Swedish diesel-engine and industrial material that covers the era and the technology. The Sjohistoriska maritime museum holds records of the merchant ships these engines powered. The Innovatum center in Trollhattan carries the local industrial heritage of the works itself, and the Riksarkivet holds company and industrial records that document the corporate chain. For a researcher tracing a specific Polar engine, these collections are the route to a builder’s plate, a delivery date, and a specification.

A surviving Polar-engined ship is now a rare thing, because the merchant fleet that used these engines has been scrapped down over the decades. Where one survives, in a museum ship or a preserved working coaster, the engine itself is a heritage object as much as the hull. Keeping such an engine running depends on spares and on the institutional knowledge held by British Polar Engines in Glasgow, which is why an independent service house outliving the original maker matters to preservation as much as to commerce.

Identifying and modeling Polar engines

A surviving Polar engine, in a preserved ship or a working coaster, carries a type designation on its builder’s plate that locates it in the family by bore, configuration, and cycle. Cross-referencing that designation against builder records held in Swedish heritage collections is the practical route to dating an engine and finding its specification. For decoding maker and model strings across the wider field, the marine engine model decoder helps parse the conventions different builders used.

Where an engine’s swept volume, speed, and brake power can be read from a builder’s record or a NOx technical file, the brake mean effective pressure calculator returns the output intensity, and the SFOC-to-efficiency calculator converts a measured fuel consumption into a thermal efficiency. These are general engine tools; they apply to a Polar engine exactly as to any other reciprocating diesel, with the firing-stroke factor set for the engine’s cycle.

For ship-speed and fuel questions at the operating level rather than the engine-test level, the cube-law relation between speed and propulsive power gives a first estimate of how fuel demand changes with speed.

That relation is a propulsion-side approximation, not an engine-bench measurement, so it sits a level above the BMEP and SFOC figures: it tells an operator how much power the hull needs at a given speed, which the engine then has to supply. For a Polar-engined coaster, the same physics applies as for any displacement ship.

Limitations

This article is a corporate and technical history, not a maintenance manual or a specification sheet. Several caveats apply to how it should be read.

Specific power, speed, and fuel-consumption figures for individual Polar types are deliberately not quoted, because the family spans more than half a century of production under several owners, and a number that fits one type and vintage misstates another. The general engine relations shown in the formula cards apply to any reciprocating diesel; they are not Polar-specific guarantees. To specify a particular engine, consult the builder’s record for that type held in Swedish heritage and archive collections.

The corporate chain, AB Diesels Motorer to AB Atlas Diesel to NOHAB to Wartsila Nohab, with the British Polar Engines license running alongside from 1928, is the spine of the brand’s history. Dates for mergers, acquisitions, and the bankruptcy are given as they appear in heritage and archive records. Readers tracing a specific engine should confirm which owner built it, because the same Polar type could be made at Sickla, at Trollhattan, or at Glasgow depending on the period and market.

Heritage and museum sources document the surviving public record well for the locomotive and turbine sides of NOHAB, which are more visible in preservation than the marine engines. Detailed marine-engine production records exist but are held in company and archive collections rather than in widely published form, so some fine detail of the Polar marine range is harder to verify than the firm’s railway output. Where this article cannot tie a claim to a heritage or archive source, it states the point qualitatively.

See also

• Atlas-Polar marine engines: the 1917 to 1948 Atlas Diesel period of the same brand.
• Gotaverken Swedish marine engines: the west-coast slow-speed two-stroke builder.
• Kockums shipyard and engines: Malmo shipbuilding and engine work.
• Bolinder hot-bulb marine engines: the Swedish small-craft motor before the high-compression diesel.
• Two-stroke marine diesel engine fundamentals: the cycle and scavenging principles behind the Polar two-strokes.
• Four-stroke marine diesel engine fundamentals: the cycle behind the Polar four-strokes.
• Marine diesel engine: the field overview.
• Marine engine makers: the index of builders.
• Marine engine model decoder: parse builder and model designations.
• Brake mean effective pressure calculator: output intensity from power, volume, and speed.
• SFOC-to-efficiency calculator: thermal efficiency from fuel consumption.