Because you just cant split a ship in half to increase its length or its beam.
Main turrets are designed then installed and the complexity of that for displacement and seaworthiness etc. are taken into account at time of design. Now the upgrading prior to installation can be done, but even that has its limits.
Additional Armour can be upgraded but again it has its design limitations. More armour equals slower speeds.
Engines can be upgraded but then you have to specialty design them for the existing compartments.
Main Armament can be increased by reboring the gun tube. A lengthy and costly process and this has it limitations due to length and especially diameter of the rebore so it can handle the new shell and subsequent pressure from charge etc. And that doesnt take into account the shell amd powder handling systems from magazine to turret. Which has to be upgraded if it can be.
The main parts that can be upgraded and are cost effective to lengthen a ships effectiveness are Secondary Guns, Anti-Aircraft, and Electronics. These are a lengthy process in and of themselves and require a ship to be pulled from combat status and drydocked.
It is much easier to just design a new class with all the improvements you want. Even that is limited by shipyard ability to build the actual ship. Take for example the US plans for Montana and Iowa Class BB's. Even those had to be redesigned due to 1 of the shipyards capable of even building them being behind a bridge so height became a factor in the design or it couldnt be sailed to sea. Also the Panama Canal was a factor for US ships prior to the Montana Class design.
Now I am no Naval Architect, but even a cusory examination of the fundemental aspects of Naval Design will bring these problems to the light.
Main turrets are designed then installed and the complexity of that for displacement and seaworthiness etc. are taken into account at time of design. Now the upgrading prior to installation can be done, but even that has its limits.
Additional Armour can be upgraded but again it has its design limitations. More armour equals slower speeds.
Engines can be upgraded but then you have to specialty design them for the existing compartments.
Main Armament can be increased by reboring the gun tube. A lengthy and costly process and this has it limitations due to length and especially diameter of the rebore so it can handle the new shell and subsequent pressure from charge etc. And that doesnt take into account the shell amd powder handling systems from magazine to turret. Which has to be upgraded if it can be.
The main parts that can be upgraded and are cost effective to lengthen a ships effectiveness are Secondary Guns, Anti-Aircraft, and Electronics. These are a lengthy process in and of themselves and require a ship to be pulled from combat status and drydocked.
It is much easier to just design a new class with all the improvements you want. Even that is limited by shipyard ability to build the actual ship. Take for example the US plans for Montana and Iowa Class BB's. Even those had to be redesigned due to 1 of the shipyards capable of even building them being behind a bridge so height became a factor in the design or it couldnt be sailed to sea. Also the Panama Canal was a factor for US ships prior to the Montana Class design.
Now I am no Naval Architect, but even a cusory examination of the fundemental aspects of Naval Design will bring these problems to the light.
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