@WILD:
I have been thinking of something similar, and I think 2 turns is a good place to start for capital ships. Was even thinking 3 turns for capital ships (BB carrier), 2 turns for cruiser, and same turn for dd, sub, tpt (same as OOB for smaller ships). I’m not sure about being able to fully attack a ship being built, or allowing it to def (no guns in the early stages) but maybe allow SBR on a ship being built up to 6 ipcs? As the owner of the ship you must repair damage before paying for the next build level.
For purchasing say a BB in 3 turns you have to put down half (10 IPCs), and the next turn you can advance it to the next level for min half the balance (5 IPCs). Then on the 3rd turn you pay it out for the remaining 5 IPCs. If is takes damage from SBR you have to buy out damage before you can pay to advance to the next build level.
On half-built warships, my focus was on board game playability.
It is easier to put a piece on its side to figure the incomplete built. As such, it is similar to damaged Carrier (no flight ops, 1 hit) and BB (1 hit remaining) without movement allowance. Cruiser on its side still works as damaged Carrier and BB (full defense roll @2 /@4), so I give full defense @3 to half-built Cruiser too.
TheTactical bombing raids on units (instead of bases) seems an unnecessary complification, even if from a certain POV can be seen as more accurate.
I would considered the lower cost of two or three turn built.
A three turns built, bring another issue more drastically vs DDs and Subs. Operational warships working for two turns before Capitals ships going to war is like a waste of IPCs usefulness.