For large ships on ocean voyages the solution is to use hydrogen, ammonia or other forms of carbon free fuel produced using renewable energy.
The discussion is ongoing to find the best one.In the Future Marine Fuel thread many different alternatives has been presented:
Sail, solar, kites and batteries are only as additional power to reduce fuel consumption and thus GHG emission when using conventional fuel.
LNG or LPG is also just a way to reduce GHG emission, not to eliminate it, thus not meeting the 2050 target of carbon neutral shipping.
Wind power MAY be able to reach that goal in some trades and where speed and regularity is not at an essence, but most likely in combination with other sources of power.
Fuel cells with large enough capacity to power large ships are under development and being tested.