Knot

[JohnnyTarr]

Ok now I know that a know that a knot was measured by tossing a plank of wood tied to a rope overboard. The rope would have knots tied into it and over time would produce your speed. Now I have been looking as to how this was all done with more specifics. This is what I came up with and this is open for discustion and not ment to be used as fact as I am not shure of the facts.

I have found a 3 min sand glass that might have been used to measure speed. Then I found this: 1 knot is roughly equivalent to 101.268591 foot/minute. So if you had a vessle that would go 5knots you would need 500 ft of twine to let out in one min to measure your speed. Now I find this hard to believe but that doesn't make it true.

Anyone else out there that can help?

[Dorian Lasseter]

http://www.twogreens.com/wakeup/lifeatsea/logline.htm

Here ya go m'boy-o....

[jessie k.]

Very cool--that's the first time that ever made sense to me.

[blackjohn]

Link to previous posts on these topic...

https://pyracy.com/forums/index.php?showtopic=5812

[MorganTyre]

Here's a semi-related question - What knot was typically tied in log lines? I know this will vary wildly and GAoP era is probably impossible to know but I am curious what sort of knots were on the log lines you guys have seen.

[Fox]

All the ones I've seen have half hitches - they do the job and they use the least line. I recall seeing one (though damned if I know where or when) that had a figure-of-eight as every third knot to make counting easier.

[Fox]

Slightly post-GAOP, but Falconer says:

LOG, a machine used to measure the ship's head-way, or the rate of her velocity as she advances through the sea. It is composed of a reel and line, to which is fixed a small piece of wood, forming the quadrant of a circle. The term log however is more particularly applied to the latter.

The log, fig. 14, plate V. is generally about a quarter of an inch thick, and five or six inches from the angular point a to the circumference b. It is balanced by a thin plate of lead, nailed upon the arch, so as to swim perpendicularly in the water, with about 2/3 impressed under the surface. The line is fastened to the log by means of two legs a and b, fig. 15, one of which passes thro' a hole a at the corner, and is knotted on the opposite side; whilst the other leg is attached to the arch by a pin b, fixed in another hole, so as to draw out occasionally. By these legs the log is hung in equilibrio, and the line, which is united to it, is divided into certain spaces, which are in proportion to an equal number of geographical miles, as a half minute or quarter minute is to an hour of time.

Plate V

This instrument is employed to measure the ship's course in the following manner: The reel, fig. 16, about which the log-line is wound, being held by one man, and the half-minute glass by another, the mate of the watch at the same time fixes the pin, and throws the log over the stern, which, swimming perpendicularly in the sea, feels an immediate resistance as the ship advances. To prevent the pin from being drawn by the effort of this resistance, the person who heaves the log continually slackens the line over the stern, or quarter, so that it becomes almost streight on the water, and the log continues nearly in the same place where it first alighted, and is considered as fixed therein. The knots are measured from a mark fastened at the distance of 12 or 15 fathoms from the log; the glass is therefore turned at the instant when this mark passes over the stern, and as soon as the glass runs out, the line is accordingly stopped; when the water, acting forcibly on the surface of the log, immediately dislodges the pin, so that the log, no longer resisting the effort of the water, is easily drawn aboard. The degree of the ship's velocity is then readily determined, by examining the number of knots nearest to that part of the line, where it was stopped at the expiration of the glass, as the knots increase in their natural order from the mark above-mentioned. The space comprehended between that mark and the log is used to let the latter be far enough astern, to be out of the eddy of the ship's wake when the glass is turned.

If the glass runs thirty seconds, the distance between the knots should be fifty seet. When it runs more or less, it should therefore be corrected by the following analogy: As 30 is to 50, so is the number of seconds of the glass to the distance between the knots upon the line. As the heat or moisture of the weather has often a considerable effect on the glass, so as to make it run slower or faster, it should be frequently tried by the vibrations of a pendulum. The line, being also liable to relax or shrink from the same cause, ought likewise to be measured, as occasion requires.

It is usual to heave the log once every hour in ships of war and East Indiamen; and in all other vessels, once in two hours; and if at any time of the watch, the wind has increased or abated in the intervals, so as to affect the ship's velocity, the officer generally makes a suitable allowance for it, at the close of the watch.

[jim hawkins]

That plate is hot! Its my new wallpaper

Blackjohn and I were just discussing where/how to purchase/make a chip log. Very handy presumably cheap prop to explain to the public the whole 'Knot' thing.

[kass]

You have a friend with a wide format printer that can print on a range of media, you know? Fully colour. Sepia tone. Anything... :)

[jim hawkins]

yeah, its me! I do work in a map production joint remember.

[kass]

Ah! Yeah, I forgot you only pludered on the weekends!

[blackjohn]

Don't let him fool ya, he used to work in a map production joint!

However, I'm sure Alex would love to have you back!

mwahahahahaaa

[blackjohn]

That plate is hot! Its my new wallpaper

Blackjohn and I were just discussing where/how to purchase/make a chip log. Very handy presumably cheap prop to explain to the public the whole 'Knot' thing.

Got linkage to a better size for wallpaper???

I've had the dreaded book on how to build nav instruments for years now. It's just a matter of making/finding/getting the spare time to do it.

[Capt. Sterling]

Blackjohn and I were just discussing where/how to purchase/make a chip log.  Very handy presumably cheap prop to explain to the public the whole 'Knot' thing.

There are instructions in Chapter 15, Latitude Hooks and Azimuth Rings...if that's the book Blackjohn is referring to...

Looks pretty simple, need directions... I could snail mail you a set Jim, since Blackjohn seems pretty busy...?

[Capn_Enigma]

As the heat or moisture of the weather has often a considerable effect on the glass, so as to make it run slower or faster, it should be frequently tried by the vibrations of a pendulum.

The moisture of the weather has no effect whatsoever on the hourglasses' accuracy - and how would it? The glass is sealed hermetically, after all. If air moisture could influence the running time, no sauna hourglass would run correctly.

It is the air moisture trapped within the glass before sealing that can render it inaccurate, depending on the ambient temperature. Of supreme importance is whether or not the sand (or eggshells, or whatever is used for running medium) is hygroscopic.

[Fox]

Actually a lot of early sand glasses were made in two halves which were bound together so were far from hermetically sealed.

[Capn_Enigma]

"Bound together"?? You are kidding, right?

GAOP hourglass halves were joined with sealing wax, or by a glassblower or something like that. The knot had a merely decorative function, as it covered up the often unsightly joint and the diaphragm. GAOP hourglass halves were not "bound together" and they were as hermetically sealed as possible at that time.

[Fox]

"Bound together"?? You are kidding, right? 

Y'know, that's what I love about your contributions to these discussions... always bringing a certain level of mature politeness which seems to be lacking.

GAOP hourglass halves were joined with sealing wax, or by a glassblower or something like that. The knot had a merely decorative function, as it covered up the often unsightly joint and the diaphragm. GAOP hourglass halves were not "bound together"...

The binding was not "merely decorative", it prevented the two halves of the glass from slipping apart sideways, in the same way that the frame held it together lengthwise (amongst other things). Sealed with wax is not hermetically sealed.

...and they were as hermetically sealed as possible at that time.

Which is not the same as saying hermetically sealed per se, which in turn suggests that there was really no need to scoff at Falconer.

[Capn_Enigma]

Ed, you missed the whole point here: GAOP hourglasses were sealed hermetically "enough" so that ambient air moisture does not influence them, much in the same way as it does not influence the contents in a bottle of wine. What is really important is the level of air moisture trapped within the hourglass when it was made, especially when the sand is very fine and/ or hygroscopic.

[Fox]

I'm assuming you've tested a selection of GAoP period sand glasses for their reliability then?

In either case this is a point of science rather than practicality. Whatever the cause of the irregularity, it was a problem that GAoP era navigators had to face.

[Capn_Enigma]

I have studied period writings on the making of hourglasses and I have encountered two or three examples in museums and collections; I have even built two glasses (sealing them with sealing wax, and tying a decorative knot around the joint). There'd be no point in testing the acuracy of original GAOP glasses, as the grinding action of the sand has widened the diaphragm holes to a point where a former 30 minutes glass I came across a few years ago ran through in a matter of 24 minutes. Also, I guess the owners would have - rightfully - objected to the degree of maltreatment I would have subjected the glasses to.

But I did conduct a series of experiments with two different 30- minutes hourglasses from my collection: I clocked their running times in the freezer, in a normal 18°C surrounding and in the baking sunlight (42°C in the shades). Although they were as hermetically sealed as it gets (having closed glassblown bulbs), they showed different degrees of variations in their respective running time, due to the air moisture trapped within them.