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Old 17-11-2017, 05:17   #1
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distance between two fix

Hello,

On ocean crossings, skippers try to know the distance sailed in 24h. With the intention of writing an article on the various techniques (and tricks) used by sailors to determine this distance, I would like to know from our fellows blue water sailors how they do it. (Or if they do this daily routine).

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Old 17-11-2017, 06:04   #2
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Re: distance between two fix

I look at the chart plotter log lol

If you're talking about dead reckoning, it's looking at the log of speed recorded by the watch person and calculating that over 24 hours. The more frequent the log entries if there is a significant variation in speed over time, the more accurate the estimate. Current and leeway are also factored in if known.
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Old 17-11-2017, 06:07   #3
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Re: distance between two fix

or if you note your GPS position and enter it in your log book you can simply take the difference in the positions for any given 24 hour period
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Old 17-11-2017, 07:00   #4
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Re: distance between two fix

If you calculate the distance between two sets of coordinates, note that the distance of a degree of latitude remains constant, but depending upon latitude, the distance of a degree of longitude changes.

Once you've dealt with that, for short distances, a good approximation can be done with simple Pythagorean math. For longer distances, spherical math is more accurate.
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Old 17-11-2017, 07:51   #5
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Re: distance between two fix

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Originally Posted by henris View Post
With the intention of writing an article on the various techniques (and tricks) used by sailors to determine this distance, I would like to know from our fellows blue water sailors how they do it. (Or if they do this daily routine).
Each midday, I write in the logbook the GPS total log distance and the water log total distance. Then days run is simply that minus yesterdays figures. And hopefully the GPS days run is more than the log wheel as you've some nice current with you
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Old 17-11-2017, 11:09   #6
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Re: distance between two fix

It gets complicated when sailing west to east (or the other way?), eg when crossing from Oz to NZ - what time is chosen as "noon"? I believe the clipper ships running down the southern ocean would record 23 hour days as they could cover the distance between time zones in those high latitudes in a single day.
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Old 17-11-2017, 11:15   #7
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Re: distance between two fix

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I believe the clipper ships running down the southern ocean would record 23 hour days as they could cover the distance between time zones in those high latitudes in a single day.
Yet to have that problem on a daily basis...
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Old 17-11-2017, 11:41   #8
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Re: distance between two fix

I read my Walker Log at noon. Then there is my electronic knotmeter/log. Could also use use the plotted difference between noon sights but not completely accurate as NevisDog says. Last but not least is the good old GPS. I'm more interested in how the boat is sailing so speed through the water is what I'm most interested in.
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Old 17-11-2017, 13:38   #9
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Re: distance between two fix

Maybe the distance run in 24 hours should be the distance gained over 24 hours in the direction of the end point or goal. Could be negative.
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Old 17-11-2017, 13:45   #10
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Re: distance between two fix

Hello, Henri,

You can take the log readings for a 24 hr. period. That will give you distance through the water.

We record the positions, and work it out for over the bottom, which is often different, due to ocean currents.

Ann
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Old 17-11-2017, 13:58   #11
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Re: distance between two fix

How is this so complicated? Almost all of us have chart plotters, chart plotters have a log and/or trip function, and the 24 hour run distance is simply the difference between todayís figure and yesterdayís figure. You can use UTC to track 24 hour periods, which in boat hours might be 23 or 25 or even -1 or +49 (crossing the date line).

Sheesh, thereís even a set of records for 24 hour runs that use the same rules. Though they donít care about the traditional noon to noon. The current record is from 2009 and is over 900 miles! https://en.m.wikipedia.org/wiki/Speed_sailing_record

If your chart plotter doesnít have a log function, you can instead measure the distances between hourly or two hourly fixes and adding them up - that will be a little more accurate than measuring the distance between 24 hour fixes. But for sailing across an ocean, isnít that close enough?
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Old 17-11-2017, 14:00   #12
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Re: distance between two fix

Speed from a recently calibrated knot meter compared to GPS speed will give the effective current but not leeway. There are ways to calculate leeway (also called rhumb line sag) from DR positions but I have long forgotten the technique. Best I look it up again.

For some reason, latitude, adjusted by the little scale on paper charts, is taught in navigation schools to do time and distance problems rather that longitude, which in theory, should require no adjustment, ie, a degree of longitude at the North Pole should be the same distance as anyplace else on earth. However, the earth is pear-shaped, not a perfect sphere, so maybe this is why they teach latitude. Less effected by the pear shape when adjusted to the scale maybe?

Make all log book entries and chart notations in GMT (UST). Who cares about time zones?
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Old 17-11-2017, 14:24   #13
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Re: distance between two fix

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For some reason, latitude, adjusted by the little scale on paper charts, is taught in navigation schools to do time and distance problems rather that longitude, which in theory, should require no adjustment, ie, a degree of longitude at the North Pole should be the same distance as anyplace else on earth.
I think you need to take a look at a globe

A degree of Longitude is ZERO at the poles and increases in length as you move towards the equator.
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Old 17-11-2017, 15:20   #14
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Re: distance between two fix

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I think you need to take a look at a globe

A degree of Longitude is ZERO at the poles and increases in length as you move towards the equator.
Yes, it should be multiplied by the cosine of medium Latitude.
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Old 17-11-2017, 15:48   #15
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Re: distance between two fix

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...Make all log book entries and chart notations in GMT (UST). Who cares about time zones?
Well, I do. Anyone using a sextant will work noon to noon. The hell with plotters. (Be careful of using that longitude scale for distance - that will seriously increase your sea miles run.)
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