They Went Down to the Sea in Ships and Protected Their Dead with... CROSSES!

I was in a Barnes and Noble bookshop the other day and found some images in a pair of books showing the use of crosses for uses other than execution.

Source: National Geographic History [1]

The above door is from a tomb in Vergina, Greece, now confirmed to be the tomb of Philip II of Macedon, father of Alexander the Great. Notice something peculiar with the pattern of the strips and bosses on the doors?

Exactly. The two embossed strips at the middle of the door, one from top to bottom and the other from left to right, forms the schematic of a Latin cross -- the kind seen in Christian churches. Here this cross is "guarding" Philip II's final resting place. So the ancient Greeks had an apotropaic use for the cross. 

Source: Smithsonian History: from the Dawn of Civilization to the Present Day [2]

This is a scene from Homer's Odysseus, wherein the protagonist Odyssseus has his sailors lash him to the mast, or ἴκρια (pole, mast, spar) and stuff their own ears with cotton to get past the Sirens and avoid crashing on the rocks by being attracted by the aural beauty of their song.  Note the yard and the mast form a sort-of "cross" (nota bene: it's really a t-pole) -- the technical term is κατάπτερος (katapteros), "wing" or "winged thing".

Artemidorus (2nd Cent. CE) wrote in his Oneirokritikon (2.53) about sailors considering "crucifixion" being auspicious for their voyage at sea:

Being crucified is a good thing for all sailors. For a cross is made from posts and nails like a ship, and its mast is like a cross.

Σταυροῦσθαι πᾶσι μέν τοῖς ναυτιλλομένοις ἀγατόν καί γάρ ἐκ ξύλον καί ἤλων γέγονεν  ό σταυρός ὡς καί τό πλοῖον, καί ἥ κατάπτιος αὐτοῦ ἐστί σταυρῷ.

Nota bene: "is like a cross", not "is a cross".

Now being crucified being a "good thing" makes zero sense! Clearly, then, some other action is implied by Artemidorus's selection of the verb σταυρόω (staurow). Justin Martyr (100-165 CE) and Minucius Felix (?-ca. 250 CE) give us some clues.

From Justin Martyr's I Apology 55: "For the sea is not traversed except that trophy (τροπαῖον) which is called a sail abide safe in the ship." So the tropaion or victory cross comes into play here.

And indeed in the same chapter Justin also refers to the Romans' victory and funerary crosses or tropaea:

And the power of this form is shown by your own symbols on what are called vexilla [banners] and tropaea [trophies], with which all your state possessions are made, using these as the insignia of your power and government.... And with this form you consecrate the images of your emperors when they die, and you name them gods by inscriptions.

And now Minucius Felix weighs in. From his Octavius 29:

You, indeed, who consecrate gods of wood, adore wooden crosses perhaps as parts of your gods. For your very standards, as well as your banners; and flags of your camp, what else are they but crosses glided and adorned? Your victorious trophies not only imitate the appearance of a simple cross, but also that of a man affixed to it. We assuredly see the sign of a cross, naturally, in the ship when it is carried along with swelling sails, when it glides forward with expanded oars....

Indeed, Minucius says the same thing as Justin Martyr and Artemidorus: the "cross" in the ship is a victory cross, or a tropaion. And those crosses when decorated after a military battle or in memoriam of a deified Caesar did look like they had a man affixed to it. Like this one:

Caesar memorial cross

And this one:

Victory cross

So Artemidorus' remark about "Being crucified" (Σταυροῦσθαι) should really be translated as "Being crossed" (or "staurow'ed") instead... because the sailors recognized in the ship's "cross" (σταυρός [stauros]) an apotropaic item that was intended to turn away harm or evil influences -- such as a tropaion.

Notes:

[1] National Geographic History. National Geographic Society, Washington, DC, Dec 2015 / Jan 2016, p. 5.
[2] Rob Colson, Camilla Hallinan, David John, Kieran Macdonald, eds. Smithsonian History: from the Dawn of Civilization to the Present Day. New York, DK Publishing, 2015, p. 102.