The P.24 Monarch was an enlarged development of the Prince, designed by chief engine designer Captain A.G. Forsyth. Similar in layout to the Napier Dagger the cylinders were arranged vertically in two separate blocks, driving contra-rotating propellers via separate shafts and gears. Like the smaller Prince engine each bank of cylinders could be shut down in flight to drive only one propeller, an idea that was reused much later in the Armstrong Siddeley Double Mamba turboprop.
The idea came from the desire to deliver high power in a reliable form for naval use. A twin engined aircraft could not be designed such that even when "folded" it came within the limits for aircraft carrier use; with two power blocks the failure of an accessory would not lead to failure of the engine as a whole.
The engine was considered for use in the Hawker Tornado, and K9370 was later shipped to Wright Field in the US, where testing (Project MX-229) was carried out in 1942 with a view to using the potentially 3,000 hp P.24 in the Republic P-47 Thunderbolt, a total of around 250 hours of test flying in the Battle being completed at Wright Field before the idea was abandoned. Following cancellation, the engine provided three trouble-free years of service in K9370.
2,240 hp (1,670 kW)