Which is why I kinda like the idea of a gasturbine or two in the conning tower, where if you want to operate like that you can, and if the engine destroys itself its servicable, but that brings its own issues.
I like the idea of gas turbines for this situation. The compact size and weight for a given power output, lower vibrations/mechanical noise (!!) and lower maintenance requirements. The high fuel consumption may not be such an issue because it isn't being run constantly but being run intermittently to charge the batteries and turbines are good for that surge requirement. Intake and exhaust trunking greatly simplified eliminating the need to route it to/from the machinery space, which in turn can be reduced in size.
On the other hand there are some down sides that kill the idea. The fin is used heavily for optical and sensor masts, and I don't know enough about operating gas turbines to know if it would be suitable for snorkeling when the intake is intermittently cut by waves but I know the crew wouldn't be happy with the amount of air a gas turbine could draw from the hull even for a few seconds.
The big killer though is the air intake and exhaust requirements. Gas turbines need a lot of air fed to them which needs to be exhausted again, so the snorkel would have to be much larger and much more detectable. The exhaust gases would also be very hot, and unless it can be cooled first it is going to light up an IR sensor.
As I said, I kinda like the idea though, which is why I hope the Future Frigates are Diesel/Gas - Electric. All the benefits listed above with less issues. By the time a decision has to be made in 2022-2024 the use of IEP in combat ships (DDGs/FFGs) will be mature and the RAN will have experience with the LSD/LHDs.
I especially like how you can be a bit more creative with machinery arrangements regarding crew/weapon spaces, machinery can be acoustically isolated, deck arrangements optimised for intakes/exhausts and battle damage redundancy through separated power generation.
The QE-class carriers have their propulsion gas turbines separated under the islands and the diesels deep in the hull.
I can't help but think that if the Future Frigate is commissioned in 2026 and it is based on the F100 hull and machinery there is going to be Anzac-type upgrade problems post-2040 when choices regarding sensors or weapons are constrained by the hull/machinery, particularly the power generating capability.
