Tin Tin's Sailing Calendar

Wednesday, 5 April 2017

Testing the Log

Thanks to information from George Taylor, Mark decided to make a log to test the accuracy of our speed instruments and hence whether the adverse current is real or not.

We could manage 35 metres of line, with a half filled water container instead of a weighted log of wood. Traditionally the log line ran out until a sand timer finished at which point the number of knots that had unreeled were read off. A knot was placed every 8 fathoms, and I calculate that the sand timer must have run for 30 seconds. There are 1000 fathoms in a nautical mile, which is amazingly convenient if you consider that a fathom is 6 feet, or basically the length of rope a man stretches between two outstretched hands. A nautical mile is defined as one second of arc on the north-south latitude scale defined as the circumference of the globe divided into 360 degrees and hence 60 times more into minutes of arc. So how does the span of a man's outstretched arms happen to fit exactly(ish) into a minute of arc, or did someone decide to divide the compass into 360 degrees for some related reason?

Suffice it say that on 5 tests at 5knots and 5 tests at 7 knots the speed measurement was consistent(ish) with the instruments.

Equally at different speeds the current direction and speed remained a consiste 0.7 knot heading 220 degrees across our path

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