undergirding the ship - that is, passing four or five turns of a cable-laid rope round the hull or frame of the ship, to enable her to resist the violence of the seas, an operation rarely resorted to in modern seamanship.
fearing lest they should fall into the quicksands - “be cast ashore” or “stranded upon the Syrtis,” the Syrtis Major, a gulf on the African coast, southwest of Crete, the dread of mariners, owing to its dangerous shoals.
they strake - “struck”
sail - This cannot be the meaning, for to strike sail would have driven them directly towards the Syrtis. The meaning must be, “lowered the gear” (appurtenances of every kind); here, perhaps, referring to the lowering of the heavy mainyard with the sail attached to it [Smith].