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Nautical Measured Mile Markers

Measured mile markers, Talland Bay

Visitors to Talland Bay can see two towers situated up on the hillside on the eastern side of the bay ( the Looe side). If you walk the coastal path to Looe, you will find two more towers on the hillside above Hannafore. These are “measured mile markers” used by ships to calculate their speed.

The ship is worked up to full speed and then approaches on a steady course of either 086(T) or 266(T). As the markers are in transit (in line with each other) the time is noted, and then again as the next markers are in transit. The ship then turns round, works back up to full speed on the reciprocal course and the process is repeated. The average of the two runs is taken, which makes allowance for any effect of wind and tide.

The markers are set one nautical mile apart. In years gone by, this was the only way to measure a ships speed and despite modern navigational aids, this method is still sometimes used by ships coming out of refit at Devonport. At night, the marker towers are still illuminated so that ships can see them.

A nautical mile is 6080 feet in length, as opposed to the land based statute mile which is 5280 feet in length.

A ships speed is given in knots and NOT knots per hour. A knot is one nautical mile per hour. The term “Knot” comes from the old sailing ship days and the way used to work out the ships speed. A line with a wooden “plate” on the end, was calibrate, and each length marked with a knot. The line was trailed astern and a sand glass (like an egg timer) used to measure a set time, the number of knots going over the stern in the given time was counted, giving the speed in “knots”.

Why is there a difference between a nautical mile and a statute mile ?? Why not ! its all part of the mystique of being a “ruffty tuffty sailor man” !! The statute mile is an arbitrary measure of distance and has been legally fixed at 5280 feet. The nautical mile is defined as “ the arc of the earth's circumference subtended by an angle of one minute at the centre of curvature”. Due to the fact that the earth is not a true sphere, but an oblate spheroid, i.e. somebody stood on it and flattened it at the poles, the length of one minute of arc varies from 6108 feet at the poles to 6046 feet at the equator. A mean length of 6080 feet is universally adopted in navigation, and a tenth of a sea mile is a cable.

Stay ashore it's easier!!

 

Tony White, Polperro, March 2003


Page dated 21 March 2003. www.polperro.org © 2003

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