Mike is a white cross on a blue background, aka a square Scottish flag, and is stands for "M" in the signal flag alphabet. I'm guessing there's no copyright for stuff like this, otherwise Scotland would be suing boats EVERYWHERE.
Mike means that your ship is stopped and making no way through the water. It's important to note the distinction "making no way through the water" - if you use a single anchor or mooring point, there's a big radius in which your boat could potentially spin. If you're flying Mike, you should be sure that you are well secured on multiple points.
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