The Class 42 and 43 Warships were a follow on from the original 6 prototype machines (D600-D605). They had very little in common with the original heavy locomotives and had an all up weight of 78-79 tons. The locomotives were split into 2 distinct classes. The first class had 2 Maybach MD650 engines and werfe fitted with twin Mekhydro 4 speed transmissions via a torque convertor. The original 3 locomotives (D800-D802) were rated at 2 000 bhp at 1 400 rpm and had 6 drivers control notches. The maximum rpm was subsequntly in 1962 iincreased to 1 440 rpm which increased the maximum power output to 2 070 bhp. A further upgarde to 2 112 bhp (diagram DH/4101/1) at 1 400 rpm occurred at a later date. The remainder of the class was rated at 2 270 bhp at 1 530 rpm. The class 43 locomotives were constructed by the North British Locomotive Company in Glasgow Scotland. They were fitted with 2 MAN L12V 18/21 engines rated at 1 100 bhp at 1 530 rpm. Each engine powered a Voith LT 306r hydraulic transmission. with 7 control notches. A singular locomotive (D830) was fitted with 2 Paxman YJXL engines rated at 1 135 bhp at 1530 rpm with each being equipmed with a Mekhydro 4 speed transmission via a torque convertor. At a typical 75% load factor the sfc is 0.37. On a typical run from London Paddington to Plymouth approximately 1.1 tons of fuel (205 gallons) would be consumed.
Although the two classes of locomotive are similar in terms of nominal brake horsepower the actual performance differeential between them is quite substantial. The Class 42 is the better of the two with higher efficiency from the torque convertor and 4 speed gear box combination compared to the 3 stage torque convertor of the Class 43. The class 43 is further disadvantaged by rge MAN engine which proved to be very probalematic and unreliable. This resulted in many failures and a number of fires from leaking fuel and oil (similar to the class 21 and 22 Type 2 units). It was reported at the time that few drivers were prepared to operate in notch 7 for any period of time and it may even have been stapped out. The Maybach engine in the Class 42 was substantially better although it did show a not insignificant drop in performance between overhauls. It was further compromised by the lack of test bed facilities at Swindon until around 1966 with a suspicion that Swindown tended to set the output during the 1962-66 period to something less than 100% in an attempt to mitigate the thermal stress issues with the cylinder head.