What does the phrase "The riven masts had gone by the board" mean?

We encounter this phrase early on in the first chapter. The ship the family is sailing on had been battered by a “raging storm” for seven full days. After a pummeling of such violence, the ship is unsurprisingly the worse for wear, and we see the phrase in question as the damage on the ship is being described. Riven is an adjective that means to be split apart; so the masts of the ship are split and broken, rendering them utterly useless. The phrase go by the board has become a general idiom in English to mean be abandoned or become lost or wasted; however, its early usage was nautical, meaning to fall overboard—that is, fall beyond the side of the ship, called the board.

So the phrase “the riven masts had gone by the board” means, quite simply, that the masts had broken and fallen overboard. This, when added to the myriad leaks in the ship and the amount of water on board, contributes to a very dire picture indeed for the Robinson family.