There is a way to get free ships with rush buy. There are currently 4 way (that I know of) to rush buy ships. 2 of them give you free ships.
1. Rush buy from "Quick Build" (main map). On the main map with a shipyard selected, there should be a quick build button. If you click it, you will be able select a ship to build. You could either click a button to add a ship to production, or you could rush buy it. The rush buy option will not only give you a ship next turn, but you would have paid nothing for it.
2. Rush buy from available ships (ship management). In the ship management screen, you can rush buy a ship from the list of available ships. Select a ship from the list. If you do so, you can rush buy it and have it added to the end of the que. It will be built next turn, and you would not have paid anything for it.
----
To avoid confusion, I'm going to list other ways to rush buy ships. These do not give free ships.
3. Rush buy using the button found next to the build progress (main map). This button looks like a stack of coins with an arrow pointing up. That button will rush buy the ship. The ship will be built next turn.
4. Rush buy from manufacturing que (shipyard management). On the shipyard management screen, you can find list of all ships in build que. If you select a ship from that que, you can rush buy it. *
*Note: for number 4, it might not display the correct amount the money for rush buy. The problem is, the number shown does not take into account work already done. It will charge you the correct amount despite that.