A: "This is a taffrail log, and it is/was used for calculating nautical speed.

As you may be aware, for centuries sailors would measure their speed by casting a rope over the side, and measuring how fast the line would spool out. They would tie a knot in the rope at fixed intervals, and if they’re going fast enough that a dozen of the knots would be reeled out in a standard amount of time (e.g. ten seconds), then that’s a speed of twelve knots."