According to James Clavell's Shogun, "[a] rutter was a small black book [used for ocean navigation] containing the detailed observation of a pilot who had been there before."
Such a book was also known as a "portolano," which MW3 says is "from Medieval Latin 'portulanus,' harbor official." In Dutch, it's apparently known as a "scheepsjournaal." I love Dutch words; they make me grin. Scheepsjournaal! That's clearly an ovine diary. And now the Supersheep Overture is stuck in my head.
Found "rutter" and the Clavell quote (though not the sheep stuff) in Charlie Finlay's article The Artful Infodump.
Apparently a "rutter" can also be (in addition to "something that ruts") a horseman or trooper, from Dutch "ruiter," or "rider." There's that Dutch again.