What is a Speed Log?

• Speed Log in an instrument to measure the speed and distance traveled by a ship.
• Only Doppler Log measures speed over the ground. All other logs measure speeds through water.

HISTORY

In the days of sails, a specially shaped woocd chip was attached to a length of line wound up on a hand reel. This line was marked with a knot at 47’ 3” from the chip, with 2 knots at 94’ 6” and so on. The wood chip was thrown overboard, a 28 sec sand glass measured the time the line ran. The number of knots which ran out in 28 sec., indicated ships speed in knots.

The   Dutch used a different technique. A piece of wood (log) was thrown into the water from the forward part of a ship. In the after part, sailor with a sand glass noted the time taken for the ship to pass the object floating log.  From the time and the known distance between the two points on the ship, ships speed was calculated.

The concept of speed through water & speed over the ground

Suppose there is a Southerly current of 3 knots. (From North to South as Current always flows towards)

It means that the water is flowing from North to South at the speed of 3 knots.

If you throw a piece of wood, it will move southwards at 3 kn.