WHAT IS A KNOT?

Auckland Star, Volume LXV, Issue 146, 22 June 1934, Page 13

WHAT IS A KNOT?

TERM MISAPPLIED. REFERS TO SPEED ONLY. The term " a knot " is often understood by landsmen to be a measure of distance. As a matter of fact, it is not a "nautical mile," as is commonly supposed, but a word used for the measuring of speed only. A knot is a speed of one sea mile per hour. A ship is correctly reported to " steam 20 knots." or " to be steaming at 20 knots," but ii is absolutely incorrect to write "20 knots an hour.'' A knot is not a length except on the log line. A statute, or land mile, is oliSOl't (SO chains), and it is used for all land measurements. A nautical mile is 6080 ft (equal to one minute of longitinle at the equator), and it is used in all navigation calculations. It is incorrect to say, " it is 10 knots to Awaroa Buy." It is Hi nautical miles, but it should be unnecessary to use the word " nautical " as all distances by water are measured in terms of nautical miles. Origin of the Log. The earliest and most primitive method of measuring the speed of a ship was to tie a line to a log of wood, drop the log overboard, and see bow fast the' line ran out. Later, the log gave place to a canvas bucket attached to a long line, which had its iirst mark at 1001't from the log ship, ami counting did not begin until this stray line had run out. Beyond this the line was marked at prescribed intervals "by knots and pieces of coloured hunting. At least once in every watch, two. men were detailed to cast the log, to determine the speed of the ship, one standing by to give the time, while the second bad the log. The shape of this canvas bucket kept it at rest in the water, acting as a sea anchor, and as tlie ship moved away from ii Hie line paid out. As the rope passed through the hands of the man in charge of that part of the proceedings, the number of knots which ran out were noted, the different pieces of coloured bunting aiding him to note how many of them bad passed. The number of knots which ran nut in 28 socnmls, measured by a sand glass, was the ship's speed in knots. If ten of these pieces of bunting had run through (he seamen's/ fingers in the 28 seconds. Ilien the ship's speed was 10 knots. Owing to the system on -which the line was marked and the relation of the period of observation to the full hour, the speed

of the vessel In nautical miles per hour was equal to the number of knots run out on the log during that period. The name "log" is still applied to all machines for measuring speed. Nowadays, the line is on a reel fitted with a brake, and the sand glass is replaced by a watch, and though patent logs have been invented almost in dozens, one principle is the same in all, although their details vary. .Some sort of float is towed astern, having on it something like the blades of a propeller. As the ship proceeds, these revolve, and the number of revolutions measures the distance run on a dial. Old Sailors Watched tho Log. In tho old days of the "fifties," when the tea clippers raced home from China with tho first of the season's crop, or in the "eighties" when the wool ships from Australia and this Dominion "carried on" to be first Home, one can easily imagine tho keenness with which the officers and men scanned the log at the end of each watch to note the distance run. Anxious eyes wero also cast round the horizon to see if any of their rivals were in sight, in the hope of overtaking tho leaders, or in dread of being overtaken. Speed is an essential thiug in ocean commerce to-day, and the perfecting of instruments to truthfully interpret it is a necessary adjunct to the safe control of any ship. The old methods of determining this speed by tho hand log may be practised perhaps occasionally during the run Home on some of the ships that now engage in the annual grain race from South Australia to Falmouth, but owing to the more modern methods available it is doubtful if much reliance would be placed upon the results. Nevertheless, the hand log is still a necessary item in a ship's equipment, and no surveyor will allow a vessel to proceed to sea unless one is included, and its markings are in their proper places. The British Board of Trade and our Navigation Act consider it expedient, and an added precaution against marine disaster, to retain some of the older methods of ensuring safe navigation, and many seafaring men would regret tho severing of a link with the past if they were dispensed with. Converting Nautical Miles to Statute Miles. The following shows the difference between the nautical mile, COBO feet, and the land or statute mile, 5280 feet. Nautical miles given first:—1 —1.151, 2—2.303, 3—3 454, 4—4.006, 5—5.757, 0—6.909, i — B.OGO, 8—9.212, 10—11.515, 11—12.666, 12—13.818, 14—16.121, 15—17.272, 16—18.424, 17—19.575 18—20.727, 19—21.875, 20—23.030, 21—24.181, 22—25.333, 23—20.454, 24—27.636, 25—28.787, 26—29.038, 27—31.090, 28—32.242, 29—33.392, 30—34.544, 35—40.302, 40—40.060, 45—51.818, 50—57.574, 00—69.088, 70—80.604, 50—02.120, 90—103.630, 100—115.148.