South Seas Companion
|In the eighteenth century, the log was the name given to a device used to measure the speed at which a ship travelled, enabling mariners to determine how far they had sailed.|
The log was usually constructed from wood and a thin sheet of lead, and attached to a line marked into lengths of fifty feet (15.24 metres), with a special mark at 12 or 15 fathoms (about 22 to 27.5 metres).|
One seaman held a reel, around which the log line was wound, while another held a sand-filled glass, usually one designed to measure half a minute.
The officer or mate in charge the watch threw the log over the stern of the vessel, and ensured the line was let out so the log floated freely away, and no strain was placed on the line.
When the special mark placed at either 12 or 15 fathoms of line passed over the stern of the ship, the timing glass was turned to start measuring a half-minute.
As soon as all the sand had run through the glass, the line was stopped, and the log was drawn aboard.
The log was also designed to ensure that it floated away from the vessel in a straight line, and could then be pulled back to the ship through the water so as encounter a minimum amount of resistance.
The speed of the ship was then determined by noting the number of marks set at intervals of fifty feet that had passed over the stern in half a minute.
A good illustration of a typical log appears in plate V of the South Seas on-line edition of William Falconers Dictionary of the Marine.
On Royal Naval vessels, the log was generally used to measure the ships speed every hour.
Interpreting the results gained by using the log required a good of skill. It required the officer or the mate in charge of the operation to take into account the prevailing winds, currents and the wake of the ship. If any significant change in the wind occurred over the course of a watch, the officer then in charge on deck had to take into account how the readings from the log were likely to be affected.
Complicating matters further, changes in climate could make the timing class run slower or faster. Care also had to be taken to measure the log line regularly, because immersion in water and the strain of retrieval could cause it to stretch or shrink.
The time and results of each use of the log were recorded on a large sheet of paper together with information about wind direction, the ships course, its latitude as determined by celestial observation, and any other matters deemed to be of significance by officers in charge of the watch.
This sheet was called the log-board. At noon, the entries to the log-board made during the previous twenty-four hours were copied into the ships logbook. Officers in charge of each watch on naval vessels then checked the information derived from their watch and signed to verify its accuracy.
Published by South Seas, 1 February 2004
Comments, questions, corrections and additions: Paul.Turnbull@jcu.edu.au
Prepared by: Paul Turnbull
Updated: 28 June 2004
To cite this page use: http://nla.gov.au/nla.cs-ss-biogs-P000053