The Deltic story began in 1944 when the British Admiralty commissioned Napier to design a diesel engine for Motor Torpedo Boats and other lightweight, high speed craft. Hitherto such boats had been driven by gasoline engines, but this fuel is obviously highly flammable and made them very vulnerable to hostile fire. Up to this point, diesel engines were characterised by their poor power-to-weight ratio and their slow speed, but the Admiralty were sure that Napier, with its well-known aero-engine expertise, would be able to conquer such difficulties.
Napier rose to the challenge and designed a unique engine. The two-stroke engine was shaped like an inverted triangle, with three crankshafts, one at each corner of the triangle. The sides of the triangle were cylinders; the Deltic engine was an opposed piston design, meaning that instead of each cylinder having a single piston and being closed at one end with a cylinder head, the elongated cylinder would contain two opposing pistons driving in opposite directions. This is both a powerful and more importantly a compact design. The Deltic design used three of these opposed-piston cylinders driving three crankshafts, all three being connected together with phasing gears to drive one output shaft. Various models of Deltic engine could be produced with varying numbers of such three-cylinder banks, though nine and eighteen cylinders were the most common models, being of three and six banks respectively.
The first Deltic unit was produced in 1950, and by January 1952 six engines were available, enough for full development and endurance trials. An ex-German E-Boat, powered by three Mercedes-Benz diesel engines, was selected for these trials, since its power units were of approximately equal power to the new 18-cylinder Deltic engines. Two of the three Mercedes-Benz engines were replaced with Napier Deltics, the compactness of the Deltic graphically shown here - they were half the size of the original engines.
Proving successful, the Deltic diesel engine became a commonly used powerplant in small, fast naval craft. The Royal Navy used them first in the Dark Class Fast Attack Craft, and subsequently in a number of other smaller attack and minesweeper classes. Napier Deltic engines are still in service in Hunt Class Mine Countermeasure Vessels.
In addition, Deltic diesels served in MTBs and PT Boats built for other navies; particularly notable being the Norwegian built Tjeld (Nasty) class, which were also sold to Germany, Greece, and even the United States Navy. PTF Nasty class boats served in the Vietnam War in largely a covert ops role.
While the Deltic engine was certainly successful and amazingly powerful for its size and weight, it was also a highly-strung beast, requiring a lot of care and maintenance. This was made much simpler by a policy of maintaining by unit replacement rather than repair in place - rather ahead of its time. Deltic engines were also easily removed should they break down, generally being sent back to the manufacturer for repair.
To hear, though, it was and is magnificent. One expects a diesel to growl and rumble, like a truck or a regular locomotive engine or suchlike. The Deltic engine throbs under power like a big aero-engine in a fighter plane; it's like nothing else. Hardly surprising, really, considering Napier was best known for its aero-engines, equipping such planes as the Hawker Typhoon.