[1]
Background
The compass, sextant, chronometer, and radar are tools that
fix the ships position on a chart, meaning the position is a known place “on
the ground”.
The speed of a ship is another parameter important to
navigation and historically has been achieved by measuring the passage of an
object alongside the ship. The principle is that an object thrown overboard stays
stationery as the ship moves past it. If we can measure the time it takes for
the ship to pass the object, we can measure the speed of the ship. There is an
important provision here. It measures distance travelled through the water not
“over the ground”. Therefore the effects of water mass movement, tide and wind
on the ship are not taken account of.
The structure of the log is simple. A quadrant-shaped piece of wood is weighted with lead to hold the quadrant vertical in the water. A bridle is attached to each corner of the quadrant and then to a log line that is wound on a spool. A release line is attached to one corner of the quadrant and a wooden plug secures it to the log line. A sharp tug on the log line will release the quadrant from its upright position in the water for retrieval.
Another version of the log uses a canvas drogue or mini sea anchor with the same construction.[2]
Attached to the logline is a series of
knots depicting how much line is released. The knots are placed a known distance apart, eight fathoms generally. So a double knot is the second knot on the line and signifies 16 fathoms of line are out. To mark the deployment of the log, the line is released until the first line marker is in the water. This is
usually a piece of leather or cloth.
[3]The element of time in the use of the log is supplied by a sandglass built especially for use with a log. The time to
empty the sand from the upper glass is usually 28-30 seconds.[4] So if 16 fathoms of line is run out in 30 seconds, the distance covered is 11520 feet in one hour.
[5]However the unit of distance at sea is the nautical mile defined as the distance of an arc of one minute measured on a meridian and is 6080 feet. The vessel’s speed in this example is 11520/6080 which is 1.9 knots.
Approximately 2 knots for the two knots on the logline.
[6]There
were variations in distance marking of logline and the time to empty the
sandglass but the principle for each ship was that the number of knots on the
logline should approximate the vessels speed in knots.
[7]Thomas Walker was a clockmaker who turned his attention to measuring distance at sea. After much experimentation the “patent Walker log” was produced in the 1880’s. A mechanical counter (1) was connected to a governor(2) and a braided logline(3). A rotating propeller(4) completed the assembly. The log was attached to the taffrail at the stern of the ship and was deployed on leaving port. The counter recorded distance and was read each watch and the result recorded in the ship’s logbook.
Reading of the log was usually done by a
junior officer and in pouring rain and sometimes heavy seas I would struggle
aft to read the log and report my findings to the officer of the watch. A
miserable job!
Later models had an electrical connection to a bridge repeater, much easier.
Deploying the log after leaving port was not that simple as
I found out. Thinking the best way to set it in the water was to lower the
propellor into the water and pay out the logline until everything was set out
resulted a set of knurled rope full of twists and knots.
You stupid boy, said the Scottish 2nd. Mate. You
pay out the line from the log first and then finally drop the rotator in the
water. So much to learn “on the job”.
Nevertheless, the patent Walker log stayed an essential tool
to the navigator for many decades until replaced by a rotator built into t the
hull of the ship.
References
‘Chip Log’. In Wikipedia, 28 September 2024. https://en.wikipedia.org/w/index.php?title=Chip_log&oldid=1248278445.
in, You are not logged in-Log. ‘Logg’.
Accessed 27 May 2025. https://digitaltmuseum.no/011024192632/logg.
———. ‘Logg’. Accessed 27 May 2025.
https://digitaltmuseum.no/011024192629/logg.
———. ‘Logg-Glass’. Accessed 27 May 2025.
https://digitaltmuseum.no/011024193987/logg-glass.
‘Nautical Mile’. In Wikipedia, 20
February 2025.
https://en.wikipedia.org/w/index.php?title=Nautical_mile&oldid=1276676565.
‘Thomas Walker & Son’. In Wikipedia,
5 May 2025.
https://en.wikipedia.org/w/index.php?title=Thomas_Walker_%26_Son&oldid=1289002093.
Walkers Patent Log. n.d. Norwegian Maritime Museum.
Acknowledgements
Bengt Malm, Ancient Mariner, and volunteer at the Norwegian
Maritime Museum
Camilla Nordeng, conservator for artifacts at the Norwegian
Maritime Museum
