The derivation of a 'knot' - Ships Nostalgia
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The derivation of a 'knot'

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  #1  
Old 17th November 2011, 16:31
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The derivation of a 'knot'

I found this information in my files from the past and thought it should be posted into the Bridge section of SN. It will be of interest to some to read all ablut a knot.
Hand logs consisted of 150 fathoms of line,stretched and marked while wet,made up on a reel.On the end of the line a piece of wood shaped into a sector or a circle was weighted at the arc to enable it to swim upright with apex and one corner bridled to the logline,the other corner was secured by a peg which would release when the line was jerked,thus allowing the log to be hauled in without resistance.
Speed was determined by streaming the log astern and allowing ninety feet of 'stray' line to run out,the end of which was prominently marked by a piece of white bunting. As this bunting sped over the taffrail a 28 second sandglass was reversed. At intervals of 47 feet 3 inches the log line was marked by knots. 28 seconds is the same proportion to 1 hour as 47'3" is to 6,000 feet (one nautical mile). When the sandglass was emptied the logline was tripped.
If 189 feet of line had run out ( 4 Knots) then the ship was making a speed of 4 knots (nautical miles per hour) If only one knot 47'3" had run out the speed was 1 knot per hour,hence the meaning of the term ' knot ' and ' log ' as a record of a ships progress at sea.
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Old 18th November 2011, 14:57
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A great post!

In my sailing days (i.e. small boats) we used to call the knotmeter in the cockpit the "Taffrail Log" even though it was really a paddle wheel mechanism with magnet used to count revolutions and then display the "count" on a lighted dial. No lines and no hour glasses needed!

If memory serves, in days gone by I believe they had a Taffrail log that counted revolutions somehow by means of a mechanical meter that trailed a small propeller/fin behind the vessel that caused the line to rotate at a known/calculated RPM. And this total revolutions count was referenced each hour of the watch to determine speed through the water during the previous hours.

My Dad has an antique brass mechanism that appears to be set up to do just this. And he later found the brass propeller/water screw that was fitted to the line to stream it in the ship's wake.

A nice coffee table exhibit, that.
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Old 18th November 2011, 15:12
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Greetings,

The wood at the end of the log line was triangular with a hole in it for the logline pass through. An entry in the logbook used a triangle with a dot in it to indicate a log reading thus remembering the old logline.
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Old 18th November 2011, 15:28
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Quote:
Originally Posted by YankeeAirPirate View Post
A great post!

In my sailing days (i.e. small boats) we used to call the knotmeter in the cockpit the "Taffrail Log" even though it was really a paddle wheel mechanism with magnet used to count revolutions and then display the "count" on a lighted dial. No lines and no hour glasses needed!

If memory serves, in days gone by I believe they had a Taffrail log that counted revolutions somehow by means of a mechanical meter that trailed a small propeller/fin behind the vessel that caused the line to rotate at a known/calculated RPM. And this total revolutions count was referenced each hour of the watch to determine speed through the water during the previous hours.

My Dad has an antique brass mechanism that appears to be set up to do just this. And he later found the brass propeller/water screw that was fitted to the line to stream it in the ship's wake.

A nice coffee table exhibit, that.
YankeeAirPirate,

From the description you have given, you are referring to a Walker's Log similar to this one.
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Old 18th November 2011, 16:45
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Quote:
Originally Posted by Pat Thompson View Post
Greetings,

The wood at the end of the log line was triangular with a hole in it for the logline pass through. An entry in the logbook used a triangle with a dot in it to indicate a log reading thus remembering the old logline.
and a triangle with a dot is/was used to show estimated position on a chart. This allowed for set & Drift as well as course and speed.

and, according to Wiki, the Log Book was originally used to record the speeds.
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Old 18th November 2011, 17:57
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Taffrail Logs and the Like...

Gulpers,

Thanks so much. That is exactly what it is that Dad has had all these years.

The link to the nautical antique site was interesting. I am not ready to pay 500 Euros for a Russian Sextant; but some of the other artifacts are quite tempting. Must plan an trip to France for some shopping!

I have always wanted a ship's telegraph or full size binnacle for my den, living room or (eventual) Man Cave.

Has anyone out here done up their Man Cave as a sea cabin with wood paneling, port holes and helm and all that? Appreciate any and all pictures posted to give me some ideas on how to make it authentic.

Cheers all.
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Old 18th November 2011, 18:34
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I well remember, as apprentice, being tasked to haul in the walkers log at the end of a passage. Sixty fathoms of log line with the rotator attached with the ship still making fifteen knots took a considerable effort. To add insult to injury we then passed the inboard end out on the port side to allow the logline to untwist prior to final retrieval and stowing away.

The length of the log line was down to some formula using ships speed and freeboard, but Second Mates would stream a second log line on the other side, typically between The Brothers and Daedelus Reef in the Red Sea, a 100 mile run with virtually no currents.

Mahseer, loaded and making 13 knots needed, I think, a 42 fathom log line.
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Old 20th November 2011, 18:25
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That used to be the norm, once a day. Haul the log, stream the line, haul the line, stream the log. Turns all out, check the frapping on the clip end, job done.
Once you had the knack of going around the poop house, with a bit of practise you could do it with the warping winch quite smartish.
Was on a couple of ships where they had a 'midships streamed log. That was a cow to haul and untwist. In weather you could sometimes even see the rotator bounce out of the water!
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Old 20th November 2011, 18:33
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Sorry Folks, brain cell missing. Should have read: once a trip, not day.
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Old 23rd December 2011, 11:28
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There were many component parts of "Walker" version of the log; "governor" and "fish" I can remember.

Any more?

BW

J
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Old 23rd December 2011, 13:27
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A couple of illustrations: http://www.shipsnostalgia.com/galler...he-log/cat/501

http://www.shipsnostalgia.com/galler...he-log/cat/501
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Old 23rd December 2011, 14:05
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Quote:
Originally Posted by R798780 View Post
I well remember, as apprentice, being tasked to haul in the walkers log at the end of a passage. Sixty fathoms of log line with the rotator attached with the ship still making fifteen knots took a considerable effort. To add insult to injury we then passed the inboard end out on the port side to allow the logline to untwist prior to final retrieval and stowing away.

The length of the log line was down to some formula using ships speed and freeboard, but Second Mates would stream a second log line on the other side, typically between The Brothers and Daedelus Reef in the Red Sea, a 100 mile run with virtually no currents.

Mahseer, loaded and making 13 knots needed, I think, a 42 fathom log line.
in the thistle boats we had a harness which fitted over the shoulders and had an inglefield swivel clip (all the same flag clips)on a short length of log line. the log line was also fitted with two of these clips, without swivels, so that it could be broke just abaft the governor wheel. when broken the appropriate clip was quickly attached to the clip on the harness and the wearer took off up the deck with the log still rotating merrily until the rotator was clear of the water, and hey presto, no kinks, no paying out again.
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Old 31st December 2011, 17:43
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I often wondered what was meant by "two horizontal green lights over the stern at midnight". Then someone explained to me "Ships cat reading the log"
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Old 18th January 2012, 21:26
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Quote:
Originally Posted by jmcg View Post
There were many component parts of "Walker" version of the log; "governor" and "fish" I can remember.

Any more?

BW

J
Frog and rotator, not forgettng the actual recorder, which was zeroed at noon after taking the days run.
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