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Merlin 30 was basically the Merlin 20 series with low supercharger gear only (contrary to the Merlin 45/50, basically the Merlin 20s with high supercharger gear only). Installing a 'normal' 2 speed Merlin 20 series should give the historical performance with low gear clutched in, and much better performance above 12-15000 ft, where the high gear is clutched in. The Merlin 45/50 should give similar performance, above 12-15000 ft, as Merlin 20s indeed.
But not the Seafire, which is what Fairey should probably have been building/developing!
One of the major issues with the FAA is that they never really seemed to have enough aircraft in theatre initially, particularly in the Mediterranean. Granted, they were limited by the number of aircraft that could be carried aboard their carriers, but I would have thought that more Fulmars, rather than any modifications to the basic design, which would have delayed aircraft getting to the front line, would have been a better interim solution until a more effective aircraft came along.
How about dropping the rear position crewmember and all the equipment associated with that area? Would the weight savings be worth it?
It would improve the climb some but the change in speed would probably be under 5mph. That is assuming there is no center of gravity shift that would require ballast.
The long range fighter that still can make 310 mph might give the defender better chances for multiple interceptions, rather than a short range interceptor that can do 330-350 mph?