If I understood all the comments up to this point well, efficiency was actually obtained and production time could be reduced, but at the same time that production efficiency was gained, there were efficiency losses every time new technologies were added to the project. So it makes sense that if there is no change in a battleship project, there would be gains in production efficiency that would reduce construction time to some extent. The gains are very limited and the larger the ship and the more complex the project, the less gains are obtained.
TL : DR: "there were very minor gains in efficiency, but if we're looking for a historically plausible prop for minor nations, this isn't the place to look - the gains were only realised by major nations that already had advanced shipbuilding capabilities, and they were so small they're not really worth worrying about. But if you're not worried about historical plausibility, then it's a perfectly reasonable game mechanic suggestion
".
Long version: I'm a bit tired now, so this may be a bit all of the shop, but as best I can manage (and as best I understand...):
There was - as well as this, for things like the liberty ships, and then some elements of at least later British escort ships (and possibly US as well, but my memory's super-hazy on that right now), as well as the Type XXI, there was some application of modular construction techniques - far less elaborate than those used today, but still a step forward in construction times, and eventually efficiency (although, as per the Liberty Ship example below, not always, and perhaps not at all in the HoI4 time period) - although my 2 cents would be to have this as a modifier improving construction speed for escorts, subs and merchants, rather than efficiency gains between models (and, particularly for the Type XXI subs, there were substantial teething troubles implementing this approach).
Note it's not just technologies, but also design - even using existing technologies for a vessel with a different purpose (for example, the Tribal class destroyers, as opposed to the 'standard type' being built by the British, didn't use a lot of new technology (iirc....), but as it was a different design, the existing technology was used differently (and different elements of the technology were emphasized).
Even with very similar designs, outside of a very small number of 'war emergency' situations, there isn't a huge amount of evidence for large "efficiency" improvements (and, as noted above, these were likely the opposite of efficient, but rather fast times achieved by intensive uses of resources
inefficiently - even the most efficient Liberty ship times were about 29 per cent more expensive than similar (and slightly more capable) designs built using normal production techniques and intensity*). Thus, even though British yards had been building destroyers that were incrementally similar for 12 years, the build times weren't much faster later than they were earlier. For example, the first British A-class destroyer,
Acasta, was laid down by John Brown on 13 Aug 1928 and commissioned on 11 Feb 1930, about 18 months. John Brown's last of the pre-war 'standard' types,
Ilex, was laid down on 16 Mar 1936 and commissioned on 7 July 1937 (16 months) - and John Brown built eight vessels of this broadly similar design (in four lots of two) over this period.
For a battleship project, the size and complexity of the build, combined with the time it takes, and the changes in technology over the period, would likely mean any benefit from efficiency is more than offset by the second battleship (produced perhaps 10 per cent quicker?) being a substantially less able ship that it could have been were a more optimised design used. I'm not aware of an example of battleships being built in serial to
speed up construction times in the WW2 period, but rather to avoid the long periods of time involved in designing new vessels (ie, the US decided to build more Iowas than it originally planned not because it expected later Iowas to be cheaper (they were still being built in parallel, more or less, rather than one-after-the-other), but because they'd received financing to build more battleships, and waiting until another design had been developed would significantly delay the time at which the new battleship could be operational.
For most minor nations (the only example I can think of that would be in with a shot would be Sweden and
perhaps the Netherlands - although as per the above, my brain's pretty hazy right now), the idea that they could build a battleship at all is pretty 'adventurous' - and any such vessel would likely take many years to complete. Taking destroyer build times in the HoI4 time period for minor nations, and noting that battleships are exponentially harder to build, would suggest that if a minor tried to build battleships in serial, they wouldn't be able to finish their second before 1948 rolled around. For examples of build times of minor nation destroyers (times are laid down to commissioned):
- Netherlands, Van Galen class, Van Ness, 15 Aug 1928 to 12 Mar 1931 (31 months)
- Brazil, Acre class, Araguaya, 20 Jul 1940 to 3 Sept 1949 (110 months - the time was extended due to interruptions in the availability of technical assistance from the UK, and changes to the design to incorporate technical assistance from the US instead - but that only underlines the difficulties Brazil had in building these ships).
- Romania, Regele Ferdinand class, Regele Ferdinand, June 1927 to 7 Sep 1930 (39 months)
- Sweden, Oland class, Oland, laid down at some point in 1943, commissioned on 15 Dec 1947 (minimum 48 months - and its sister was also laid down in 1943 and not commissioned until 31 Jan 1949)
Compare this with the 18 and 16 months for the British destroyers above, and it's clear that even for substantially smaller and less complicated vessels, minors found it took far longer to build them (and often required foreign assistance just to achieve that). It's not really plausible that a minor nation would be able to build back-to-back battleships during the period, and if they did build them to the same design, there's a fair chance the second ship would be pushing obsolescence when it was completed.
* This is a comparison between the 350,000 person-hours to build an "Empire" type freighter by the UK, compared with 450,000 at the 'most efficient' end of the Liberty-ship person-hours curve - the first 'round on the ways' for a Liberty Ship took 1,250,000 person-hours so, while still quicker than an Empire freighter, took over three times as much labour). Liberty ship figures from Ships for Victory, Empire freighter figures from Buxton's Warship Building and Repair in the Second World War.
