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Dutchman's log – measuring speed without a log

The log has failed or is clearly reading nonsense? With a float, the known length of the boat and a stopwatch you can still find your speed through the water – just as seafarers have done for centuries. In NauticTools2 this tool is called Rail Log.

• Length in m
• Time in s
• Speed in kn

How does it work?

An object drifting in the water stays practically still relative to the water while the boat sails past it. Drop it at the bow and time how long it takes to reach the stern: you then know how long the boat needed to cover its own length – and that gives the speed through the water.

When do you need it?

• Log failure: Fouling on the transducer, a jammed paddle wheel or an electrical fault – without speed through the water a key input for dead reckoning is missing.
• Checking the log: Is the log over- or under-reading? A few runs with the Dutchman's log give an independent comparison.
• Small boats without a log: Dinghies and tenders often have none at all.

Formula and working

• L – measured distance in metres, usually the length of the boat or a marked distance along the rail
• t – timed seconds

1. Convert the distance

   Distance in units of 0.5144 m (the distance covered in one second at 1 knot): L / 0.5144 m

2. Speed

   Divide by the time: speed = (L / 0.5144) / t

Why 0.5144? At 1 knot a vessel covers a 3600th of a nautical mile per second: 1852 m / 3600 = 0.5144 m, or about 1.688 ft. Rule of thumb: speed ≈ 2 × L (m) / t – about 3 % high, but fine for a quick check. With feet: speed ≈ 0.6 × L (ft) / t.

Worked example

Things to watch out for

• Speed through the water: The method measures speed relative to the water, not over the ground. Tidal streams are not included.
• Longer distance, several runs: On a short boat at speed a run lasts only a few seconds – a tenth of a second of reaction time is already several per cent. Better use two people at bow and stern and average several runs.
• Don't throw anything overboard that doesn't belong there: A piece of wood or a float on a thin line that you pull back in is ideal.
• Avoid light objects: Something blown along the surface by the wind spoils the result. It should sit as deep in the water as possible.

How it works in NauticTools2

1. Enter the length

   The measured distance in metres.

2. Enter the time

   The timed seconds, decimals welcome.

3. Calculate

   NauticTools2 shows the speed in knots and the intermediate step. Measurements can be saved and printed.

Frequently asked questions

Where does the word "knot" come from?

Speed used to be measured with a chip log: a wooden board on a line with knots at regular intervals. The crew counted how many knots ran out over the stern in a fixed time – hence the unit.

How accurate is it?

With careful timing and several runs, to about 5 to 10 per cent – good enough for dead reckoning and for checking the log.

What if I have no stopwatch?

Every phone has one. At a pinch counting ("one thousand and one …") works too, though much less accurately.

Related tools

• Distance, Speed and Time – use the measured speed for distance and ETA
• Current Triangle – from speed through the water to speed over the ground