fathoms

[Tea-Rex]

the royal navy defined 1 fathom = 6 feet
other countries use corresponding units defined as 1/1000 nautical mile, roughly an inch more.

i guess if there wouldn't have been the need to have the nautical mile 90x60 (5400)x distance pole to aequator, the royal navy would have defined the nautical mile as 6000 feet.

[gsheath]

We need to remember how each of these measurements are usually applied. The 'fathom' is typically used for depth measurement (unless depth in metres are used), but most charts, irrespective of the depth measure, use the Nautical Mile as the distance measure, of which a Cable is 1/10th of the Nautical Mile as previously discussed.

The only situation I have ever used 'Cable' is measuring distances between ships at sea while stationkeeping, or when reporting the distance to run in a ship to a defined anchorage position or to a calculated position for ' wheel over' when manoevring a ship, having regard to the ship's 'advance and 'transfer' when turning.

(ex Royal Australian Navy)

[Tea-Rex]

got a question here: how long will the traditional sea units survive?
the metre replaces more and more the fathom in seacharts, and with GPS there is no need to measure distances in a unit that converts to minutes of angles either.
the french proposed to do away with seamiles and the derived units during their revolution, but i never saw, or even heard of a sextant with a gon scale.
now with celestial navigation getting pushed aside it's just as easy to measure distances in metre and kilometre, and propably prefered by the international unit comitee.

so, what do you people think?

[Omatako]

Just a comment to Garyp:
1/60th of a degree is a minute not a second.

[gsheath]

Originally Posted by Tea-Rex

got a question here: how long will the traditional sea units survive?

Good question; not sure any of us can answer that. I hope 'forever' because it works; there is no need to change.

What Tea-Rex says regarding the use of GPS is quite true and is valid if your GPS is working. What would you do if the GPS wasn't working? If your modern electronic aids failed on your boat then the only answer is to resort to the traditional arts. I use a GPS too. However, I would not do an ocean passage without having a sextant based back-up, nor a coastal passage without a handheld compass at the ready. I always use paper charts.

Challenges to the validity of the traditional ways from time to time is healthy to keep reminding us of how important they are. I think there should be more questions about how we can ensure these skills are retained.

[camaraderie]

One more trivia bit...60 n. miles to the degree means that the circumference of the earth is 60x360 or 21,600 nautical miles!

[Tea-Rex]

actually, one nautical mile represents the distance of 1 arc minute based on a circumfence of 40003 km. in fact the earth's circumfence along a meridian over the poles is about 40040 km, along the aequator about 40088 km.

when the distance was first defined it was thought that the earth would be a perfect sphere, so the nautical mile represents 1 arc minute along a meridian at the latitude of greenwich, where the measurements were taken.

in fact, 1 arc minute varies from about 1848 to 1855 metres.

that might not sound like much, but if you'd to use the actual distance to the subpoint, the multiplied error would make celestial navigation next to useless.

[ebs001]

The factor for metres to feet is 3.289 A simple way for me is to multiply by 3 and add 10 percent Eg 15m is 3x15=45, 45=4.5 =49.5. 15x3.289=49.33 so it's within an inch. The smaller the number of metres the smaller the difference and since it only really matters at shallower depths this method works very well.
A nautical mile is one minute (not 1 second) of arc on the latitude scale which is 1/60th of a degree.
The original metre was derived from 1/10,000,000th of the distance from the equator to the north pole.

[Tea-Rex]

well, since i grew up in metric units, i aproach it the other way ...
1'=304.8mm=0.3048 m (btw, 1/0.3048=3.28084)
devide metres through 3, multiply with 10 (really just moving the decimal point) and cut an inch for every 5 feet ... for more precission add 1/2 inch for every 40 feet (or actually, 1/8" per 10 feet)

15m /3=5
5*10 = 50
50'-10"=49'2"[+1/2"]

indeed, 15m/0.3048=49.2126'=49'2.55"

[ebs001]

While I didn't grow up in metric many measurements in Canada are given in metric and because of the work I have done, I have had to convert hundreds of times from one system to the other both for distance and for temperature. I think I'll stick with my method - too much to remember using T-Rex's method and awkward using both feet and inches. If I need to be very accurate which in this case you don't, I check it on my measuring tape which has both metric and imperial.