Correct, either option must be selected before construction and the underpinnings will determine the structural arrangement of the after part (if not more) of the vessel. Sorry, the idea of fitting shafts and then going to pods does not match with reality.I take it your referring to this quote from Naval technology
http://www.naval-technology.com/projects/cvf/
"The MOD has decided not to use nuclear propulsion because of high cost, and several alternative configurations have been considered for the propulsion system, including a 25MW WR21 gas turbine, as used on the Type 45 frigate, and a podded propulsion system based on Integrated Full Electric Propulsion (IFEP).
A configuration currently being considered is based on two Rolls Royce Marine Trent 36MW MT30 gas turbine alternators driving two electric motors. The motors power fixed conventional propeller shafts.
CVF will have two bronze propellers, each 6.7m in diameter and weighing 33t. The anchors will be 3.1m in height and weigh 13t."
I personally believe that they'll use the conventional electric drive motors & shafts, powered by generators fed from the the WR21's.
With such a large & complex design, these decisions will have no doubt already been made, as the earlier such decisions are made, the easier it is to plan construction & order the parts.
The idea of putting in even short shaft lines from the electric motors to the propellers, only to have then ripped out in "X" years time, to be replaced by pods is farcical! No shipbuilder in their right mind would consider this as a viable option.
It's either, Or, not both. The structure that would be needed to be put in place on the deck plates of the outer hull to support pods, just doesn't make it practical to fit shaft lines as well.
The MT30 can operate as a combined cycle unti incorpoating a free powr turbine for the priamry alternator and and waste hea boiler for a secondary steam turbine alternator. This has a claimed effeicney of 50%. The problem is the system runs best at it desing load. The ships hotel and service laod will be variable but where speed is not an issue these variations can be absored by the propulsion system. for a warship where constant speed is critical the variation in load is most likley to be addresse with the deisel alternators.These lessons will already have been learned from the huge commercial cruise ships that have been plying the global oceans for the last 10 - 15 years, since pods where introduced.
The use of the WR21's also would, yet again, be a sound, practical choice. With Type 45 already using them & getting good results, it would be foolish to switch to another semi-unproven system, purely for the continuity / spares factor, never mind the maintenance & operational differences.
Also, while gas turbines have been used elsewhere in the RN fleet, to great effect, they tend to be fuel hungry & have the additional drawback of only being able to run on cleaner, refined fuel oils, unlike most commercial diesels, which have been updated / adapted to run on everything from rough crude oil, to LPG !
Systems Adict
By the way for the power required the only HFO option you have is stroke slow speed desiels. The size and mass of these and their operating characteristics (slow acceleration. slow decelerations and not very forgiving if 'throw around') make them completley impractical for this type of vessel (or many passenger ships for that matter). The Gas turbine - electric drive option permist flexiblity in desing becasue the location of generation system is not driven by the positionof the propulsion drive line.
http://www.rolls-royce.com/marine/downloads/article/id6_fuelling_future.pdf
While the MT30 is a purpose built marine turbine the WR21 is a modified aero trubine but still incorporates the combine cycle (regenerative) technology and has an advantage of the order of 27% over simple cycle turbines. Added to this is the the fact the Trent, on which it is based, is a modern core and is more efficnet in that the older generation aero turbines in any case.
http://marine.rolls-royce.com/WR-21-marine-gas-turbine-engines/
Simply cycle is the arrangement used ships such as the FFG7 where the power of the turbine is harness by a free power turbine only.
For an electic ship with space an weight issues both are a good option.