Ariel Winter: This Ass is Unattached!

Maybe Ariel Winter needs to buy a billboard -- or maybe pics like this will get the job done -- but she's desperately trying to tell the world ... SHE'S SINGLE!!!

The 18-year-old "Modern Family" star finally came right out and made the announcement this weekend ... with a little help from Kim K.

The next day she posted this pic on a boat to show off her ... nautical skills.

Also ... apparently she's single?