It's an unusual method. There is now a standard method for this, which is printed on the SOLAS (Safety Of Lifes At Sea) cards we sailors are required to carry,

It's an unusual method. There is now a standard method for this, which is printed on the SOLAS (Safety Of Lifes At Sea) cards we sailors are required to carry, although the direction ones are not on all versions of the card I've seen. Here's one that has them (section "Air to Surface Direction Signals): http://www.marinelite.gr/images/detailed/1/221518a.jpg http://www.marinelite.gr/images/detailed/1/221518a.jpg