How to Use a Chip Log or Trailing Log for Distance Measurement at Sea

A chip log is one of the oldest and most self-reliant tools for measuring a vessel’s speed through water. It consists of a weighted wooden board, a line with knots tied at regular intervals, and a sandglass. A trailing log, often called a patent log, uses a towed rotator or spinner to record distance run on a dial. Both tools feed the dead reckoning plot, the foundation of traditional navigation. This article explains how they work, how to use them correctly, and where they fit in modern seamanship.

Main Explanation

What it is

In the context of an independent sailing reference and tools platform, the chip log and trailing log are treated as practical, low-cost seamanship tools rather than products of any single manufacturer. A chip log is a wooden quadrant or triangular board weighted so it floats upright and resists being pulled through the water. A bridle connects the board to a log-line. One leg of the bridle is fitted with a wooden pin that releases when the line is tugged, allowing the board to lie flat and be retrieved easily. The log-line has knots tied at precise intervals. A sandglass, typically 28 or 30 seconds, times the measurement. A trailing log is a mechanical instrument streamed astern; a rotator spins as it is towed, and the revolutions drive a dial that records nautical miles run.

Why it matters

Modern sailors are surrounded by GPS, chartplotters, and speed sensors, but these systems can fail, lose power, or be affected by signal denial. A chip log or trailing log provides a non-electronic backup that measures speed through water, which is exactly what dead reckoning requires. Speed over ground from GPS includes current and leeway, but dead reckoning traditionally starts with speed through water and then applies set and drift separately. Learning to use a chip log also builds a deeper understanding of what a knot means and how distance, speed, and time relate. In an era of information overload and conflicting online advice, returning to a simple, verifiable method helps sailors distinguish reliable seamanship from marketing claims.

How it works

The chip log works by converting a known length of line paid out over a known time into speed. The line is marked with knots at intervals that correspond to the distance a vessel moving at one knot covers during the sandglass interval. Traditional sources give 47 feet 3 inches for a 30-second glass and 42 feet for a 28-second glass, but the exact spacing must match the glass and should be calibrated. To use it, the board is thrown over the stern, and the line is allowed to run freely. When the first marker passes the hand, the glass is turned. The sailor counts the number of knots that pass through the hand before the glass empties. That number is the speed in knots. For example, if five knots pass in 30 seconds, the vessel is making five knots. A trailing log is streamed astern and the dial is read at the start and end of a period; the difference is the distance run through the water. Some trailing logs have a small spinner or rotator that must be kept clear of weed and line.

How to do it

For a coastal passage, prepare the chip log before departure. Check the log-line for wear, confirm the knot spacing against the sandglass duration, and test the sandglass against a stopwatch. Assign one crew member to handle the line and another to time. In calm water and clear of traffic, throw the chip board well clear of the stern and let the line run. When the first knot after the stray line passes, start the glass. Count each knot as it passes. When the glass empties, note the count. That is speed through water in knots. Record the time and speed in the logbook. Repeat every hour or when speed changes. To find distance run, multiply speed by time. For example, at 5 knots for 2 hours, distance is 10 nautical miles. Plot this on the chart along with course steered to advance the dead reckoning position. For a trailing log, stream it after clearing harbour, note the dial reading, and record the reading at regular intervals. The difference is distance run. Always retrieve the log before entering confined waters or reversing.

When not to do it

Do not rely solely on a chip log or trailing log in confined waters, heavy traffic, poor visibility, or when precise position fixing is required. These tools measure speed through water, not speed over ground, and they do not account for current, leeway, or steering error. A chip log is also impractical in very light winds, when the board may not hold steady, or in rough seas, when counting knots is difficult. A trailing log can foul the propeller, be damaged by floating debris, or give false readings if the rotator is not spinning freely. For complex passages, storm avoidance, or hull and rig inspections, professional training, a human weather router, or a certified marine surveyor is more appropriate. Use chip logs and trailing logs as backups and teaching tools, not as primary navigation in high-risk situations.

Visual

1. Prepare and inspect. Check the log-line for chafe, confirm knot spacing matches the sandglass duration, and test the glass against a stopwatch.
2. Set the chip. Throw the weighted board well clear of the stern, allowing the line to run freely without tangling.
3. Start the glass. When the first knot after the stray line passes your hand, turn the sandglass.
4. Count knots. Count each knot that passes through your hand before the glass empties.
5. Calculate speed. The number of knots counted is the vessel’s speed in knots through the water.
6. Record and plot. Note the time and speed in the logbook, multiply by elapsed time for distance, and advance the dead reckoning position on the chart.

Common Mistakes

Using the wrong knot spacing for the sandglass duration

If the knot spacing does not match the sandglass interval, the speed reading will be systematically wrong. Consequence: every dead reckoning position will be off. Correct approach: verify the spacing against the glass duration and calibrate over a known distance before relying on the log.

Counting knots from the wrong point

Including the first marker or the stray line in the count introduces an error of one knot or more. Consequence: speed is overestimated or underestimated. Correct approach: start the glass only when the first proper knot after the stray line passes, and count only the knots that follow.

Ignoring current and leeway when converting speed to distance over ground

A chip log measures speed through water, not speed over ground. Consequence: the dead reckoning position drifts with current and wind. Correct approach: use speed through water for the DR plot, then apply set and drift separately from tide tables or observations.

Relying on a chip log as the only navigation tool in confined or high-traffic waters

A chip log cannot provide the precision or real-time position fixes needed near hazards or shipping lanes. Consequence: increased risk of grounding or collision. Correct approach: use GPS, radar, visual fixes, and paper charts in confined waters, and treat the chip log as a backup or training aid.

Failing to calibrate the log or sandglass

A sandglass that runs long or short, or a log-line that has stretched or been re-knotted incorrectly, produces a consistent error. Consequence: all speed and distance calculations are wrong. Correct approach: check the glass against a stopwatch, measure the knot spacing, and calibrate over a measured mile or known distance.

Letting the line foul the propeller or rudder

If the log-line is streamed too close to the stern or retrieved carelessly, it can wrap around the propeller or rudder. Consequence: damage to the log, loss of the line, or even a fouled propeller. Correct approach: stream the line from the quarter, keep it clear of the prop, and retrieve it slowly while the vessel is moving slowly or stopped.

Safety Note

A chip log or trailing log is a useful backup and teaching tool, but it must fit into a broader safety framework. Digital tools complement, never replace, proper training, physical checks, and human judgment. Always cross-check critical data such as weather, tides, and navigation with official sources, including national weather services and coast guard broadcasts. Never use a tablet or phone as your only navigation tool; keep paper charts and a handbearing compass as backups. Take a certified safety and seamanship course, and follow the International Regulations for Preventing Collisions at Sea (COLREGS) and local maritime regulations. When using a chip log, ensure the line is clear of the propeller, wear gloves to prevent rope burn, and do not deploy it in heavy traffic, poor visibility, or rough conditions where handling the line becomes hazardous. A trailing log should be retrieved before entering confined waters or reversing, and its rotator should be inspected regularly for damage or fouling.