The main reason why I brought the change to all-gas turbine propulsion forward by a decade was because gas turbine ships have smaller crews than steam turbine ships and the RN's biggest problem since the end of World War II has been recruiting and retaining enough sailors. I thought that being able to put more ships to sea with the same number of men would outweigh the disadvantages by a considerable margin.
This is true and is a laudable aim, especially post-National Service. Probably less of a driver when the Counties were designed - they soon inflated with some cruiser features for Admiral Staffs etc., teak decks and all sorts of oddments.
With experience of course it was found that centralised engine control was possible, even from the bridge.
Tertiary reasons were that I thought that gas turbines might be cheaper to buy and maintain than steam turbines. In the real world the 8 Counties and 7 Tribals had a grand total of 39 G.6 gas turbines (32 in the Counties and 7 in the Tribals). In this version of history it would be 268 for the Royal Navy (64 in 8 ALT-Counties and 204 in 51 ALT-Type 81s) plus another 100 in the 25 ALT-Type 81s that were built for other navies at the same time (2 Chile, 10 RAN, 6 RNLN, 4 RNZN and 3 SAN). Economies of scale might come into play.
My understanding was that gas turbines were no more thirsty than steam turbines. However, if that isn't true I don't mind building the ALT-Counties and ALT-Type 81s with larger hulls to accommodate larger fuel tanks because it won't significantly increase the cost (steel is cheap & air is free) and that makes it easier to fit the extra weapons and sensors such as the second target indicator radar in the ALT-County. If push comes to shove the extra fuel costs can be paid for via the reduced manning costs.
Gas turbines cost about 10% more than equivalent steam plants, but were cheaper to operate so made back this initial capital cost.
Fuel economy improved with development and indeed did outstrip that of steam plants.
It was the experience aboard the Tribals that gave rise to 'repair by replacement' for repairs, it was found to be possible and easier. The later Olympus-powered ships had 6m2 uptakes and downtakes for sufficient airflow which allowed easy access to lift out the entire engine. I don't think this was possible (at least via the funnel) on the Tribals and Counties.
Its worth remembering that the fuel used enabled water-displacement for the bunkers, but did need very good filtration systems.
I'm not sure that complicated refuelling of carrier task groups would have been a significant problem. As far as I know that must have happened in the 1970s in the real world when gas turbine powered ships appeared in large numbers and the RN still had Ark Royal, Bulwark, Hermes, Tiger and Blake. As far as I know the RN and RFA coped with this complication in the 1970s in the real world and I see no reason why they could not have coped with the same problem in the 1960s.
I also thought that it might be a non-problem because as far as I know gas turbines and steam turbines used the same grade of fuel. That's one of the reasons why the RN liked gas turbine powered aircraft. The lower flash point of gas turbine fuel (I can't remember if it was AVCAT, AVTAG or AVTUR) allowed it to be stored in normal fuel bunkers while AVGAS needed to be stored in special tanks to minimise the risk of fuel explosions. Or it might have been the other way around, i.e. the boilers generating the steam for the turbines could burn AVCAT, AVTAG and/or AVTUR.
I have probably over-stated the problem, as you say that was overcome anyway in time, but the age of steam was in no way over (its surprising how many ships during the Falklands War were steam-powered for example).
The most powerful "normal" Tyne engine that I know of was the RTy.20 rated at 6,100ehp (5,665shp) and 5,665shp is 93% of 6,100shp. On that basis the Tynes in the 10,000ehp class that were envisaged would be producing in the region of 9,300shp.
Ironically the advice from the aviation industry was to ignore the Tyne due to its very mixed performance on aircraft. But the Marine Tyne actually proved to be far more reliable!
I wouldn't read too much into the aviation ratings, the Marine Tynes had a new free-turbine section and different materials to prevent corrosion and of course their output was geared differently.
Exmouth had the Proteus (another dubious engine in aircraft), for me the Spey was the real game changer in terms of power and economy.
Smaller crews are the main why I find a gas turbine powered version of CVA.01 attractive. I also thought that having six Olympus engines instead of the steam turbines would reduce the building and maintenance costs through standardisation with the Olympus engines on the frigates and destroyers built in the 1970s.
CVA-01 was planned for a fairly modern high-efficiency steam plant (1000psi, 482C superheat, 135,000shp output) with self-contained compartments and 'repair by replacement' principles too. I don't know what the manning requirements for the engineering spaces were, but this would have been a fairly advanced set-up.
In some ways its surprising Olympus wasn't fitted to boost the speed, instant acceleration for example when launching and minimising the impact of steam-loss with heavy catapult use. An all-gas ship would still need a donkey boiler of course. I think again, the design period is slightly too early for the conservative thinking of the time to sanction a radical idea like this - no navy had ever built any COSAG carrier then, let alone COGAG.
The Tribals and Counties provided a lot of experience, as did the conversion of HMS
Exmouth, into refining the uptake and downtake design and developing the strengths of gas turbine power. The Navy (and Treasury) was conservative when it came to an all-turbine ship. It must be remembered that the RN were advanced enough to outpace US and Soviet use of naval gas turbines, but I agree that some go-ahead thinking given the relatively few problems in the Tribals and Counties should have pushed things.
I think perhaps timing was an issue given the need to get experience, perhaps the Batch 2 Counties should have been all-gas, there was a fair gap between them and the T82 though, so an all-gas T82 would be later than the 'Kashin' despite the RN's initial lead.
Actually a better scenario for this thread might be a Tartar-equipped COSAG Batch 1, COGAG Batch 2 and a further improved Batch 3 instead of T82 (maybe Ikara replacing the 4.5in mounts for example, C-band NSR radar).