Are there any VORs placed in the oceans for navigation. Is it true that VORs or TACANs are placed on ships to help oceanic flights, for example flights across the Atlantic?
There are no ship-based VOR's or TACAN's specifically dedicated to transatlantic flights. Transatlantic flights navigate using the inertial navigation system and GPS.
TACAN's are used on board naval ships for military aircraft to find their way to the ships, but this is typically not for flights across the ocean.