Allen Hazen wrote:Pity the Class 21 and Class 22 differ inso MANY respects (though you suggest good reasons why they may have): it makes it harder to regard them as a "controlled experiment" to compare the two transmission types! (And thank you for the diagrams: the drawings in the Cooper book are exterior side views, not general arrangements diagrams!)I suspect that had NBL endeavoured to make the 21 and 22 closer in equipment layout, it would have involved non-trivial compromise. They might be about as close as one could get, at least for single-engined locomotives. With two engines, the differences between a homologous pair might be smaller, although that would depend on the specific drive arrangements chosen for the diesel-hydraulic case.
Allen Hazen wrote:Cooper does give a small bit of information about the Class 22 truck ("bogie" in British English) design:Yes, the class 22 truck was a two-axle version of that used for the class 41/D600, which in turn was a modified version of the LMS Ivatt/Fox design used under LMS diesel-electric prototypes 10000/1, and which design in turn was said to have been inspired by the EMD/Blomberg A1A design used under the EMD E-series passenger locomotives. NBL couldn’t work out how to drive all three axles – the 41/D600 was originally intended to have a C-C wheel arrangement - so settled for A1A-A1A.
"Mechanical construction was on similar lines to the D600 class [= BR Class 41], the bogies being a two-axle version of the Class D600 three-axle bogie."
Allen Hazen wrote: Given the remark on the Class 41,In fact that form of construction was really NBL’s choice. For the D600, it had followed LMS 10000/1, which had used a “strength” underframe construction rather than conventional truss-sideframe cab unit construction for extraneous reasons. All of BTC’s own-design, own-production first generation diesel-electric cab units were of truss-sideframe construction, as were most, if not all of those acquired from the industry. NBL had lobbied the BTC to try some diesel-hydraulic locomotives; it had agreed (or perhaps there was some politicking and it was told to agree) and had actually ordered some from NBL ahead of the pilot-plan diesel locomotive orders. BTC decided that the Western region should have the diesel-hydraulics – which it may have regarded as being of passing interest, probably not foreseeing that by doing this it would create a monster. The BTC was probably unfairly blamed for the way the D600 turned out. It has been argued elsewhere that had it [the D600] been entrusted to a competent builder, the outcome could have been quite different. If the BTC was guilty of anything, it was probably of not caring enough about this sidebar issue to better direct and control NBL, although that could have been mission impossible anyway.
"Mechanical construction of the locomotives was in the substantial style favoured by the BTC [= British Transport Commission, the gornment "parent" o the railways] and also in the tradition of a firm with a long and famous history of building steam locomotives,"
This doesn't inspire confidence!
Returning to diesel-electric vs diesel-hydraulic locomotive equipment layouts, the Alco DH643 may be compared with the Alco C855: Allowing that the DH643 is roughly three-quarters of a C855, there are some similarities, perhaps more than in the BR 21, BR22 case.