Sail through

Why do we say that someone will "sail through" their exam (or other difficult situation) meaning that they will do it easily, when we all know that sailing can be tricky and often hazardous?

I don't know for sure, but I imagine it is in part because despite the fact that sailing is difficult it can look easy to the observer - a yacht seems to be moving easily and gracefully from a distance, even though everyone is working away like mad onboard.

Also, in the days when sail was the normal method of powering ships, sailing would be the method used when conditions were right, and although not as simple as it looked, it was a lot easier than rowing, which would have been the alternative.

DFG