Sierra is made of a small blue square centered on a white background, and stands for "S" in the signal flag alphabet. Its appearance leads absolutely no one but me to call it "The Inverse Papa". It's probably the flag maker's favorite, since all you have to do is sew a small square onto a blank square. This is probably the one they start the apprentices off with.
You'd fly this flag if you were trying to slow your vessel down or go in reverse. Basically, if you're applying power away from the front of the boat, you fly this flag. It helps people around you know what to expect in terms of your maneuvering. Think of Sierra as the much less annoying version of the "BEEP BEEP BEEP" from the moving truck going into reverse at 7 am on a Saturday.
When displayed with numbers, Sierra is used to indicate a ship's speed in knots.
Check out the rest of our Signal Flag explanations!