I am not certain a modification like this would actually work. Lenghtening the ship by about 40% without modifying the beam, hull depth and main cross section likely leads to an ship that breaks in half in the first storm. You also increase drag significantly, so all new propellers, propeller shafts, bearings and stern shape might be required. (or you'd drop speed by over 40%, as the new engines could not be anymore powerful then what was in the ship already) The larger size would also require a new rudder. You already fitted an modified bow...... all in all, building a new ship would be both faster, easier and cheaper..........
Hi. I said before the width would be altered with what I'd call a "revolutionary concept": the special 8cm thick armour against torpedos would not be welded straight against the hull's wall, but would be outward shaped, concave, like in the U-boats and would be welded in compartments and, obviously, waterproof. This way you gain a protection but also an increase in width (in total by maybe 1,5 meter). And I said also in the description made that this addition of shaped armour would have inside several walls which by themselves would add a significant amount of strength to the vessel, becauswe all would be welded to the outside shaped steel wall and on the inside against the original hull. And one could use them as ballast tanks as well, when needed. The main point is the hydrodynamics involved. It would make the old hull like a huge U-boat hull... it would add very little drag, since very streamlined fore and aft and improve stability and hull strength.
Regarding the costs: These improvements in my AU would have been made in the original Spanish shipyard right after the local civil war had ended. Spain was then more then eager to get work to do, jobs for their working population. Germany, as I said also, had its shipyards then at full capacity... The main reason for giving this work to the Spanish yard (with German technicians supervising it). And why would adding an external concave watertight shell make the total cost so much higher than building a new vessel? Compare the labor costs there with Germany's... already back then...
Another question: I don't understand the point where you say:
You also increase drag significantly, so all new propellers, propeller shafts, bearings and stern shape might be required. (or you'd drop speed by over 40%, as the new engines could not be anymore powerful then what was in the ship already)
Did the U-boats have that amount of drag as you say? No. And yes, new engines, shafts, bearings, thrust blocks and screws would be necessary. Just like in a new project. And why couldn't they be more powerful than the original ones? Final comment: This is AU. I made this new project with discussion in mind. So please keep comments coming, I think we all enjoy the sharing of ideas and comparing points-of-view. Even weird ones, like mine here...
The finished ship, "DKM Prinz Alfons" escorting medium sized aircraft carrier:
And here the scheme of the propulsion on each shaft: two double acting 2-stroke Diesel engines coupled on each shaft, allowing the ship to sail with only half the power or do so with full power. Each engine has an output of 11.450 shp and drives massive four bladed screws. Total power is therefore 45.800 shp.
What do the experts say? Doable or not?
PS: This propulsion scheme I did does not reflect the real Diesel engines used, the ones of this project presented here are nine cylinder ones, not the massive V-12 shown in the diagram.