I noticed that quite a few of my provinces take twice as long to build ships, and wanted to check what the rule was for this - as far as I can tell if they are Cores, different culture/religion, lack of revolt risk, population size and so on don't matter. The only possibility I can see that was consistent was provinces with a continuous land connection to the capital, but this seems it would be a bit harsh for starting in places like Rhodes. The manual suggests you can only build ships on core provinces, but this seems to have incorrect (my fastest place is non-core, because it has cloth as well as wood).