There are many synonyms that can be used for the phrase "plain sailing". These include smooth sailing, easy street, a piece of cake, a breeze, straightforward, no trouble, trouble-free and effortless. Each of these phrases implies that a task or situation is easy to handle or manage without any hindrances. The phrase "plain sailing" usually refers to a smooth journey, without any rough waters or obstacles along the way. It can be used to describe sailing on a boat, but it is often used metaphorically to indicate that a situation or task is progressing without any significant difficulties.