A log has two closely related meanings aboard a boat. First, it is the instrument that measures speed through the water and, by adding up that speed over time, the distance run. Second, it is the ship’s log or logbook, the written record of the voyage in which those distances, courses, weather and events are entered. A beginner needs to know that the speed on the log is speed through the water, not speed over the ground, and that the two can differ by several knots in a tidal stream. The experienced navigator adds that the log is the one instrument that still works when the satellites do not, and that a calibrated log, a compass and a clock are all you need to run a dead-reckoning plot across a channel.
From chip log to paddlewheel
The word comes from the original instrument, which really was a log – a weighted wooden board on a knotted line. A seaman threw the chip log over the stern, let the line pay out for the length of a sandglass, and counted the knots passing through his fingers; the spacing was chosen so the count gave nautical miles per hour, which is why speed at sea is measured in knots to this day. The taffrail or patent log of the nineteenth century towed a small rotor on a long line and drove a counter on the stern rail, and some of those brass Walker logs still serve on traditional yachts.
Modern logs are through-hull sensors. The paddlewheel is the most common: a tiny impeller spins in the flow, magnets in its blades pulse a sensor and the instrument converts pulses to speed. It is cheap and reliable but fouls with weed and growth and must be pulled and cleaned. Electromagnetic logs have no moving parts and read the voltage induced by water moving past a sensor. Ultrasonic logs measure sound through the water and are the choice for serious racing boats, because they read accurately at very low speeds and are less affected by the boundary layer along the hull.
Speed through the water and speed over the ground
The log measures how fast the boat moves relative to the water around it; GPS measures speed over the ground, relative to the seabed. The difference is the current. Sail across a three-knot tidal stream and the log may read six knots while GPS shows anything from three to nine, depending on direction. Comparing log distance with GPS distance over a run gives the set and drift of the tide. For dead reckoning you plot the log distance, then apply the tidal vector from the almanac to reach an estimated position.
Why the log matters to sail trim
The sailmaker cares about the log because boatspeed through the water is the measure of how well the sails are working. Polar diagrams and target speeds are expressed as speed through the water at a given true wind speed and angle, and a helmsman driving to a target is watching the log, not the GPS. That only works if the log is accurate. Calibrate it by running a measured distance both ways in slack water, or by comparing it with GPS over reciprocal runs, and enter the correction into the instruments. A log that reads ten percent slow will convince you that a new sail is a failure when it is doing everything the designer promised, and a tenth of a knot gained by a trim change is invisible to the eye but obvious on a well-calibrated log.
The ship’s log
The logbook is the second meaning, and it grew from the practice of copying the chip log readings into a book at the end of each watch. A cruising log records, every hour or at each change of course, the time, log reading, course steered, wind, barometer, position and remarks about sail changes, sightings or engine hours. It is a legal record if there is an incident, the navigator’s backup if the electronics fail and, years later, a pleasure to read. Racing navigators keep a terser version – log and GPS at each mark, sail changes and the wind at each – so the debrief can match sail choices to conditions.
Frequently Asked Questions about the Log
Why is boat speed measured in knots?
Because the earliest speed instrument was a knotted line paid out behind the boat over a timed interval, and the count of knots gave the speed. One knot is one nautical mile per hour.
Should I trust the log or the GPS for speed?
Both, for different purposes. The log gives speed through the water, which is what sails and polars are about; GPS gives speed over the ground, which is what gets you to the destination. The difference is the tide.
How often should I clean the log impeller?
Whenever the reading becomes erratic or starts under-reading. In warm, fouling-prone waters that can mean every few weeks, and many sailors swap in the blanking plug whenever the boat is left on a mooring.
The log is where sailing performance becomes a number. Keep the sensor clean and calibrated, understand that it measures the water and not the ground, and write down what it tells you, so that every trim change, sail choice and passage plan is built on evidence rather than impression.