Outputs exceed 30,000bhp ceiling
In 1963 the marine engineering industry was certainly obsessed with power and bore size.
The two big news stories in The Motor Ship, May 1963 concerned the ‘Most Powerful Marine Diesel Engine’ – a Fiat unit of 32,000bhp-plus – and the fact that British company Doxford has revealed details of its J-type engine, which was claimed to be capable of outputs in excess of 30,000bhp.
But before we look at these in more detail, another article in the same issue, from Götaverken of Sweden, highlighted the problems of engine ratings. Despite the efforts of bodies like CIMAC, IEC and ISO, there seemed to be many different ways of rating marine engines. The two main methods were the ‘Continental rating’ and the ‘Scandinavian rating’. The former was based on MCR, with 85% to 90% of MCR expected to be employed during normal continuous service, while the latter was based on the recommended continuous service rating. Obviously engines rated in accordance with the Continental system enjoyed a market advantage, so to counter this, companies using the Scandinavian system stated a ‘maximum’ output, some 10% higher than the continuous output. The problem was exacerbated by the facts that many countries offered shipping and shipbuilding subsidy schemes, making higher stated outputs more attractive, and that shipyards tending to quote higher outputs than necessary to ensure that high trial speeds could be achieved, even if it meant a temporary engine overload.
Fiat’s powerful engine was a 12-cylinder unit, of 900mm bore, which was normally rated at 2,100bhp/cyl, i.e. 25,200bhp at 122rpm, but in overload conditions on the test bed it achieved 32,511bhp at 126.5rpm – highlighting the ratings confusion mentioned above.
Doxford maintained its characteristic opposed-piston design for the new J-type, on account of the fact that it offered smoother running and less stress on the welded parts of the engine. Other claimed benefits include lower weight and more compact dimensions, and improved scavenging. Despite these apparently strong claims, the industry in general had been moving away from the opposed-piston engine. Interestingly, Doxford’s engine production and design had previously been mostly influenced by the fact that its own yard built relatively smaller vessels, for which engines up to 10,000bhp were adequate, but following a merger with the J.L. Thompson yard which could build tankers of 100,000dwt, there was an incentive for the company to join the power race.
Mindful of the fact that the weakness of previous Doxford designs had been considered to be the crankshaft, the company had, according to our predecessors, started from this point and designed the engine around a stronger and stiffer crankshaft. The chosen design owed much to the contemporary high speed engines from the likes of Pielstick and Maybach, and it allowed shorter cylinder centres while maintaining stiffness and large-diameter journal bearings. A bore size of 850mm had been considered, with a six-cylinder version giving some 18,000bhp, but in the end Doxford settled on 760mm bore, producing 20,000bhp in 9-cylinder form, but could be developed for 30,000bhp from 10 cylinders. Equivalent engines on the market had bore sizes of 840-900mm and needed 10 or 12 cylinders to achieve 20,000bhp.Although the J-type was of similar height and width to competing engines, it was considerably shorter and lighter at 17.8m and 580 tons – saving 200 tons or more over similar engines.
Of course, even with fewer cylinders, there is a larger number of pistons than with other types, and at 2,180mm the stroke is considerably longer (in line with present trends?) but Doxford pointed out that its design needed no cylinder heads, valves or valve operating gear, which meant it could equal any other type of engine with regard to reliability, accessibility and maintenance cost.
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