Love what you’ve come up with here. Good looking aircraft! Whilst I think what you propose is perfectly possible I do wonder about a couple of aspects (though of course this is massive speculation so I freely admit there are no right or wrong answers). With regard to the airframe I find everything you say to be highly likely though I do wonder about the wings.
Supermarine Type 224 fighter was intended as a Gauntlet replacement, it never entered service.
Given that Mitchell had utilised cantilever wings on the S.4 and would use them again on the Spitfire (and indeed the more relevant to the timeframe Type 224), I am slightly surprised that he wouldn’t have applied them to the S.7. Pressures of time perhaps? I think they’d be a definite on the S.8 though. I wonder if he might have cleaned up the strut arrangement for the floats as well? The which Mitchell began designing in 1931 had an inverted gull wing and trousered undercarriage, something along those lines could fit the bill for a floatplane.
Our proposed S7 from last week
However, to my mind the Schneider trophy was won or lost not by a seaplane’s airframe but by its engine. The Gloster VI was in some respects more aerodynamically advanced than the S.6, it certainly had a finer fuselage form, and there is, so far as I can tell, no reason to suppose that Gloster would not have won the Trophy in 1931 or set the world speed record, with their own floatplane had it been fitted with the Rolls-Royce R rather than the notably long-in-the-tooth Napier Lion.
With regard to the Rolls-Royce R, I am not entirely sure that it was actually at the limit of its development. For the S.6s the engine was not flown at full power, and operated on an overly rich fuel mixture which also sapped power but increased reliability. Further development work was discussed, though not undertaken, by the RAE in 1932 advocating testing four engines to destruction. Not only that but the R was still a very young engine, having first run in 1929. By contrast the Napier Lion first ran in 1917 and developed 450hp, twelve years later it was delivering 1200hp, and before it managed that output it had powered the S.5 to win the Schneider in Venice. If one looks at the later Rolls-Royce Merlin (which owed a lot to the R) the power increase from an initial 750hp to 2000hp was mostly delivered through a process of aggressive supercharging and much higher octane rating of the fuel.
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An often overlooked, though totally critical figure in the success of the Schneider Trophy racers was fuel expert Rod Banks of the Anglo-American oil company. Whilst seconded to the Schneider team From 1929 – 31 he developed the 30% benzole, 60% methanol, 10% acetone (yes, the Supermarine S.6 was partly fuelled by nail-polish remover) plus 4.2cc of tetra-ethyl lead per gallon fuel blend that powered the S.6b to its world record speed in 1931. Later Banks was drafted in by FIAT when the AS.6 of the Macchi MC.72 failed to work properly. The success of his work there being reflected by the 440mph record set by that aircraft in 1934 and later still his work was crucial in maintaining the dominance of British aircraft engines over their larger volume German counterparts. It is likely that spurred by the further development of the S.7, Banks would have furthered the development of exotic fuels for the R engine, readily able to accept a more potent supercharge without detonation.
Hooker and a Harrier. Sir Stanley Hooker. From left to right Fred Sutton, Chief Flight Development Engineer, John Farley, Chief Test Pilot. Sir Stanley Hooker, John Dale, Chief Pegasus Development Engineer, Bill Bedford, former Chief Test Pilot. Image credit: Rolls-Royce.
The suggestion that the R might be developed into a V-16 is the most fanciful aspect you present though. Rolls-Royce has never built a V-16. Indeed, historically Rolls-Royce was far more likely to bolt two V-12s or V-8s onto a common crankcase in an X formation. Rolls-Royce’s solitary 16 cylinder engine was the Eagle XVI, an X-16. Later when the war was on and the need for increasing the power of the Merlin was at its most urgent (and money was no object) Rolls-Royce still didn’t go down the extra cylinders route but improved supercharging (notably Stanley Hooker gained a 30% increase in power in one fell swoop by improving the internal streamlining of the supercharger) and ultimately running on 130 octane fuel, though this of course was not available in 1931. Talking of things that were not available in 1931, I am unsure if ethylene glycol was available as a coolant but if it was it would have improved the radiator situation, likewise Rolls-Royce were tinkering with evaporative (or steam) cooling during this period for high performance applications and it is possible they might have managed to shoehorn it onto the S.7. Mitchell was not averse to the idea as his (unsuccessful) Type 224 employed this novel cooling system.
So, I propose a cantilever winged version with an X-24 engine formed from two Rolls-Royce Rs on a common crankcase (call it the Rolls-Royce RR) with an indeterminate calling system, a VP airscrew and cleaned up fuselage and float struts (though I suspect these might have been under too great stress-loading to be viable, ah well…).