Hmm, now there is an idea, Modular ship construction.
An ability to combine 2 or more slipways within an individual shipyard to build an oversize ship might be a good idea. Not as a starting capability though. I think it should be a researched tech that is then applied to individual yards. To start with it should only add the ability to build oversize ship, later tech advances would be needed for improved ship construction rates. Maybe even a veriant of experience for yards that influences construction rates.
Grub for ponder?n