Personal Miles Logged

[stephenh]

miles logged

Is that 43 per days afloat or over entire period including days not on board ?

That's days afloat, not moored or at anchor, there's a good few 1/2 days in there.
I thought my average was extremely low and rather embarrassing....

 

[snooks]

what do others do?

I gave up counting in 2001

I passed the minimum requirement for the Yachtmaster, (and still haven't got around to taking it) then gave up counting.

Now, I don't really care how many miles I've done, and if I don't care about my milage, who else would?

There will always be people who have sailed more miles than me, there will always be people who have sailed less than me

I just keep learning

 

[bilbobaggins]

OK, 'AntarcticPilot', I'll bite....

If you wish to compute rhumb line distance, you are into very nasty mathematics indeed, and frankly, I wouldn't bother!

I had a peer into my elderly copy of AP 1234A Air Navigation, Vol 1, (Theory and Practice of Air Navigation ), Chap 3, Sect 3, Annex paras 10-13 'Mercator Sailing' ( that Chichester guy again.... )

10. Mercator sailing is the name given to the calculation of rhumb line tracks and distances using the plane triangle formed by the rhumb line track and the ( N to S ) ch lat and ( E to W ) ch long ( Δ ABC in fig 5 ). Usually the smaller angle between the meridian of departure and the track is solved. The problem then is to find the track angle 'α' and the distance in nautical miles between A and B.

[sorry, DesperateDan hasn't given us easy access to symbols and Greek letters ]

11. To do this correctly the scales used to measure the sides must be constant and equal, or at least, when considering the two sides in isolation, their scales must expand uniformly.

Track Angle
12. One constant scale unit which can be used to determine tracck angle is the meridional part....as in Fig 5@

tan α = BC/AC = ch long (mins)/ DMP ( diff. of meridional parts )

For the most accurate result DMP should be obtained from spheroidal tables.

13. ( an example.... )

Rhumb line distance
14. The distance along the rhumb line track joning A and B ( Fig 5 ) is most easily found from:
AB = AC sec α

Distance = ch lat (mins) sec α (nm)

Since the scale in nautical miles along AB can be assumed to match that along AC. If α has been found using MPs for the spheroid, a further refinement is possible by adding F% of AB, from Table 2 ( Corrections to Calculated Distances - Journal of Institute of Navigation, Vol 3, pg. 133 and Vol 9, pg. 371 )
i.e. Corrected Spheroidal Distance = AB = F% of AB (NM), etc.....

This is also well treated in such as Reed's Ocean Navigator and, of course, Bowditch. Appropriate tabular values can be found in Burton's and, I believe, Norie's Tables. Anyone remember them?

The point of including this is to remind ourselves that the calculation of rhumb line tracks and distances were routinely done by deep-sea navigators as part of their 'day job', using precomputed and tabulated values for convenience.

So also may we, if gaining half-a-mile in the Fastnet, or a day's run in the North Pacific, is worth the effort. And sometimes it is......

I don't imagine this is of much help to the OP, 'BigNickers', but as thread drift goes, it's up there......

Another small point, while we're here. While peering into that old AP1234A, I noticed an entry under 'Errors in Automatic Instruments - Altitude Errors'. This discussed the 'RoTh' correction to be applied when, at height, the length of the nautical mile, one minute of arc at the centroid, is not the standard surface 6080 feet, and the instrument over-reads.

"It is suggested that a subtraction of 1/2nm per 100 from the indicated distance (when) above 60,000 feet will suffice."

I find myself wondering just what aircraft the Royal Air Force were operating in 1967, when this was published, that were capable of flight at or above 60,000 feet....

And I do know one of these was seen above Marion Island in 1979. Now where could it have come from?

 

[sailorman]

OK, 'AntarcticPilot', I'll bite....



I had a peer into my elderly copy of AP 1234A Air Navigation, Vol 1, (Theory and Practice of Air Navigation ), Chap 3, Sect 3, Annex paras 10-13 'Mercator Sailing' ( that Chichester guy again.... )



This is also well treated in such as Reed's Ocean Navigator and, of course, Bowditch. Appropriate tabular values can be found in Burton's and, I believe, Norie's Tables. Anyone remember them?

The point of including this is to remind ourselves that the calculation of rhumb line tracks and distances were routinely done by deep-sea navigators as part of their 'day job', using precomputed and tabulated values for convenience.

So also may we, if gaining half-a-mile in the Fastnet, or a day's run in the North Pacific, is worth the effort. And sometimes it is......

I don't imagine this is of much help to the OP, 'BigNickers', but as thread drift goes, it's up there......

Another small point, while we're here. While peering into that old AP1234A, I noticed an entry under 'Errors in Automatic Instruments - Altitude Errors'. This discussed the 'RoTh' correction to be applied when, at height, the length of the nautical mile, one minute of arc at the centroid, is not the standard surface 6080 feet, and the instrument over-reads.



I find myself wondering just what aircraft the Royal Air Force were operating in 1967, when this was published, that were capable of flight at or above 60,000 feet....

And I do know one of these was seen above Marion Island in 1979. Now where could it have come from?

Alconbury Cambs

 

[BlueSkyNick]

.... I was commenting on the OP's remark that he found it difficult to follow a rhumb line. ....

Correction, I didnt actually remark that I find it difficult to follow a rhumb line. I was trying to point out that it is not always relevent. The rhumb line from The Needles to Cherbourg is a straight line just about 60miles long. The quickest way to cross the channel is to sail a constant heading through the water, allowing for the tide ebbing and flooding and hence the distance OG is more than 60, usually about 70. Similarly, if the wind is on the nose it is impossible to sail the rhumb line.

Putting aside all the science about different lines across the curves in the planet, I think Snowleopard was closest to the issue. The rhumb line is only relevant to the RYA to show that a passage was AT LEAST that distasnce.

 

[bilbobaggins]

Ah, yes, Nick. You're copmpletlely right. ( I've left my first attempt to remind myself of the effects on the fingers of Hardy's 'Parcel' Shiraz '05 )

Our dear friend Paul has confused the unwary in referring to 'the OP'. He meant, of course, 'the Ould Poughur', which would be my own good self - and not your own good self. But it really doesn't matter. None of this does....

 

[graham]

Correction, I didnt actually remark that I find it difficult to follow a rhumb line. I was trying to point out that it is not always relevent. The rhumb line from The Needles to Cherbourg is a straight line just about 60miles long. The quickest way to cross the channel is to sail a constant heading through the water, allowing for the tide ebbing and flooding and hence the distance OG is more than 60, usually about 70. Similarly, if the wind is on the nose it is impossible to sail the rhumb line.

Putting aside all the science about different lines across the curves in the planet, I think Snowleopard was closest to the issue. The rhumb line is only relevant to the RYA to show that a passage was AT LEAST that distasnce.

Nick I wouldnt worry about it,You must have done way over the minimum anyway.The examiner isnt going to quibble over a few miles here and there .I didnt have a cruising logbook when I took mine so just scribbled down roughly what I had done on a scrap of paper with approximate distances.Unless its a zero to hero type candidate with bare minimum experience I cant see anyone arguing over exact distances.

 

[photodog]

You old duffers really otta get with the times...

No one bothers with all that "Maths" stuff now.... scroll the plotter to were you wanna go.... hit the waypoint button..... stab the big red button on autopilot... adjust sails accordingly... open Gin.

Job Done.

 

[bilbobaggins]

Alconbury Cambs

No.

Not without a couple of stops. And very few places, in 1979, had the resources to 'do the needful' for one of those. Unlike the recce Canberras, which it was intended to replace in the SR role, it wasn't simply a case of 'top up the oil and water, light the tyres and kick the fires', and Offski.....

The U2 driver's Environmental Protection Suite needed a lot of kit 'n caboodle. And people. Specialists. Lots of 'em - and Hercules plural coming and going. Very noticeable.

Ascension - Marion - Ascension was barely possible in 1979, in 'still air planning'. The air isn't still. And that would also mean directly overflying South Africa, who's strategic nuclear research was the object of the operation. No-where in/on Africa was possible, then, as an operating base. Not likely.....

 

[AntarcticPilot]

OK, 'AntarcticPilot', I'll bite....



I had a peer into my elderly copy of AP 1234A Air Navigation, Vol 1, (Theory and Practice of Air Navigation ), Chap 3, Sect 3, Annex paras 10-13 'Mercator Sailing' ( that Chichester guy again.... )

Well, if you want to cheat by using precomputed tables....

Underlying those precomputed tables is the aforesaid very nasty mathematics (note what it says about computation of DMP; that's where it is hiding some of the more horrible bits of maths). Also, these methods will fail under some circumstances, where you encounter singularities in the trigonometric functions (tan and sec go to infinity for large/small angles). That's rare, but it can happen, especially as you're differencing angles. You will, of course, respond that a navigator will be aware of the special cases and will use alternative computational techniques (the tracks concerned are close to due North/South), but software isn't clever - it does what you tell it, so you have to build in all the possible failure modes!

I once had a tussle with someone on a computer related discussion board, with someone who couldn't understand why it wasn't "easy" to implement trigonometric functions using the standard schoolroom definitions and identities. Several of us had to point out that the schoolroom formulae don't work in some circumstances, and that you need a numerical analyst to work out algorithms that are bullet-proof!

 

[bilbobaggins]

Hee! Hee!

I'm emphatically reminded that I have other things to do today, involving masking tape, moving furniture, mixing paint, painting paint on bits that need it, removing paint from bits that don't.... so, before I concede on points in this particular 'pissing up the wall' contest,

Perhaps I might remark that I have 30 years experience of implementing software to deal with map projections, and work in a specialized mapping unit

I will just remark, as a parting shot, that I have 40 years 'experience of implementing software to deal with map projections' - and that software is located between my ears, not down in the basement of a server farm. There were, and are, plenty others far better than I was at this, but HM Queen used to pay me regularly for my modest contribution to the sum of human knowledge.

Further, I also spent years of my young life 'working in a specialized mapping unit', and that unit had two Avon 109 jet engines, a big wing, and a whole lot of mapping cameras. You can see a whole lot of infinity, at night, from 8 miles up....

 

[TargaLout]

Dear Bilbo, Apart from the inconvenience of continually needing to take a bearing to maintain a course on a rhumb line say Needles to Cherbourg West entrance. Would I arrive in Cherbourg before "my twin" if he preferred the modified great circle approach?

 

[tcm]

43 miles a day is erm 14000 miles a year. I am er er er about 30 miles day, or just over 1 miles an hour 24hours a day.

 

[bilbobaggins]

Would I arrive in Cherbourg before "my twin" if he preferred the modified great circle approach?

Nah! There's not enuff trans-meridional progression innit, and anyway it's too short. Better off, IMHO, using a Rectified Little Circle routing, with vertices projected on an Admiralty Pattern Gnomonic, and not going quite as far west as the Needles Fairway Buoy.

Besides, the 'modified great circle departure' is accepted as a damn sight more important to success than the 'm.g.c. approach'.