If I knew the answer to that question, I would probably be much richer—which means I might not be here right now if I knew the answer.
One obvious way of increasing productivity would be to build more ships domestically so that the shipyards become more efficient. Of course, if you start by building one ship every now and then, the knowledge acquisition has to not start from scratch, but it has to be regained. The experienced workers might not be updating and upgrading their skills as often.
That's one easy answer on how to increase efficiency and productivity of shipyards. Beyond that, I'm not sufficiently well versed in naval construction to be able to provide more solutions than that.