Personal Miles Logged

[BlueSkyNick]

I've just created a small Excel workbook to tally up all my miles, year by year.

I have nearly always recorded the GPS track, but note that the RYA say it should be the rhumbline. The miles over the ground seems more relevant to me in that it is the actual miles sailed. Taking a channel crossing as a good example, it is better to sail say 70 miles than to sail a rhumb line of 60.


Also, some of my logs for day sails are a bit vague, eg "Cowes and back - 20 miles". I have counted most of them but have omitted others, which are effectively "went out for a sail and came back in 2 hours later". ie rhumbline is zero!

what do others do?

 

[Sailfree]

Miles Logged

I use the log reading which means its always a lot lower that actual miles sailed over the ground as I usually manage to sail with the tide

 

[Doghousekeeper]

I have nearly always recorded the GPS track, but note that the RYA say it should be the rhumbline.

For personal interest and accuracy, "Miles Sailed" = miles sailed. It's the only way to build up an accurate total. I think the RYA requirement only applies if you're establishing your experience for something like a Yachtmaster exam: you have to show that you've made a certain number of passages of a minimum length, and that length is measured along the rhumbline (even if that's not the track you followed). I wouldn't get too hung up about it - I don't think too many RYA Examiners are quite that anal about the figures.

 

[snowleopard]

The difference between log reading, point-to-point difference and GPS track are only significant if you are just on the borderline of the mileage requirements for a qualification. Yachtmaster candidates present with an average of 4x the minimum mileage.

 

[stephenh]

miles logged

I have been doing this for years on an Excel spreadsheet, sometimes, like Moodynick, it's an educated guess , sometimes the rhumb line lifted off a chart or distances from the log or from the GPS.
A bit of everything...so I reckon it averages out.
I also keep a record of the number of days afloat.

Because of the ease with which Excel manipulates figures I calculated my 'miles per day' and to my great surprise found the figure of 43 m.p.d. !

Forty three miles per day ? This must be the most expensive and slowest means of travel ever invented.

What distances do others average ?

 

[Salty John]

Mine works out at 21 miles per day.

 

[cumbrian]

The miles over the ground seems more relevant to me in that it is the actual miles sailed.

rhumb line vs. COG - does it matter?

isn't the "type" of time spent or miles done more important than either, e.g 1 hour of challenging sailing = 20 hours of drifting on a gentle breeze?

or in fact, is it worth counting at all....?

 

[jhr]

Forty three miles per day ? This must be the most expensive and slowest means of travel ever invented.

I may have been dreaming this, but i.s.t.r. someone on here saying that they'd added up their mileages and costs over the past few years and concluded that it would have cost less per mile to travel by Concorde.....

 

[BlueSkyNick]

rhumb line vs. COG - does it matter?

isn't the "type" of time spent or miles done more important than either, e.g 1 hour of challenging sailing = 20 hours of drifting on a gentle breeze?

or in fact, is it worth counting at all....?

I suppose the difference between D(istance)OG and rhumbline could be seen as a measure of good navigation. eg if DOG is twice as long, was it because the nav was unsatisfactory, or because a lot of tacking was required.

Agree with the "type" of sailing but its impossible to measure in practice. Just crossing the atlantic and back can realise 7000 miles, without any more sailing, but it's only two landfalls!

 

[bilbobaggins]

I've always found it discouragingly hard to sail along a rhumb line, or loxodrome. Instead of all that frustration, I've reverted to 'modified great circle' routing, which some authorities, like Pedro Nunes would argue is/can be both shorter and possibly faster.

Knowing that there is a difference, and on which side of a Mercator course to 'err' when racing over, say, 100 miles or so ( the 2 legs of The Fastnet Race between the Scilly Isles and the Rock? ) may make a half-mile or so of difference.

One's GPS kit usually calculates the Great Circle Distance between two waypoints, and re-calculates it sequentially as one progresses, so following that - as most probably do these days - gives a fairly sound approximation to the Great Circle and the shortest distance.

So measuring and following 'rhumb lines' would give you greater distances to sail, and to put into the RYA log book, than going straight there.

But don't tell the RYA......

 

[BlueSkyNick]

Yachtmaster candidates present with an average of 4x the minimum mileage.

That's an interesting number - where did it come from?

 

[AntarcticPilot]

I've always found it discouragingly hard to sail along a rhumb line, or loxodrome. Instead of all that frustration, I've reverted to 'modified great circle' routing, which some authorities, like Pedro Nunes would argue is/can be both shorter and possibly faster.

Knowing that there is a difference, and on which side of a Mercator course to 'err' when racing over, say, 100 miles or so ( the 2 legs of The Fastnet Race between the Scilly Isles and the Rock? ) may make a half-mile or so of difference.

One's GPS kit usually calculates the Great Circle Distance between two waypoints, and re-calculates it sequentially as one progresses, so following that - as most probably do these days - gives a fairly sound approximation to the Great Circle and the shortest distance.

So measuring and following 'rhumb lines' would give you greater distances to sail, and to put into the RYA log book, than going straight there.

But don't tell the RYA......

A Loxodrome or rhumb line is a straight line drawn on a Mercator chart; that is why Mercator charts are used for navigation. It is also a line of constant bearing, so to steer a rhumb line, you just measure the bearing of your destination from your chart and follow that bearing. Following a rhumb line is what happens when you're steering on a constant bearing.

If you're heading for a far northern or southern destination, the rhumb line can become VERY long! It spirals towards the pole, and technically never reaches it - you end up disappearing up your own cockpit drain

There is software available for the technically inclined that will compute waypoints on a great circle very accurately - see http://trac.osgeo.org/proj/, and get the geod program. However, it is command line and not for the faint-hearted!

However, while great circles are technically the best route, for short distances the difference between the rhumb line and the great circle are insignificant, and in tidal waters you won't be following either in any case! And on longer passages, a sailing vessel will make a shorter passage by taking note of the prevailing winds and optimizing their use of them. The sailing route across the Atlantic is most definitely NOT a Great Circle! Great Circle routes are really for motor vessels, and are really of theoretical interest only for sailing vessels.

 

[bilbobaggins]

Ah, Paul Cooper, 'The Sailings' of yore....

If only it were that simple. I have two ( or three ) things to say; one is 'orthodromic' and the other is 'conformality'.

And...

'Mercators' Projection' is an Orthomorphic Cylindrical Projection.

Summary of Properties:
30a. Scale is correct only along the equator. Elswhere it increases as the secant of the latitude.
b. The secant of 90 degrees in infinity. Consequently the Poles cannot be shown on the Mercator Projection.
c. A straight line represents a rhumb line.
d. A Great Circle is represented by a curved line convex to the nearer pole.
e. The projection is conformal. It is not 'equal area'; shapes and areas are highly exaggerated in high latitudes.

From AP1234A Sect 3 Chap 3 para 30 ( largely written by Sir Francis Chichester )

One can calculate rhumb line distance, and compare it with great circle distance, so as to consider if the time-distance difference is worth pursuing.

Mr. Cooper is right about one thing - such stuff is of theoretic interest only to a few. I know of a 31' carbon racing trimaran which won its class in the 2003 Fastnet race by 4min 55sec. Half-a-mile less sailed here or there made a difference.....

 

[yelbis]

I have one thing to say. Mr Baggins. Anorak

 

[bilbobaggins]

I have one thing to say. Mr Baggins. Anorak

I have one thing to say, Mr Yelbis....

Knickers.

It's what The Queen paid me for.

 

[Refueler]

I have been doing this for years on an Excel spreadsheet, sometimes, like Moodynick, it's an educated guess , sometimes the rhumb line lifted off a chart or distances from the log or from the GPS.
A bit of everything...so I reckon it averages out.
I also keep a record of the number of days afloat.

Because of the ease with which Excel manipulates figures I calculated my 'miles per day' and to my great surprise found the figure of 43 m.p.d. !

Forty three miles per day ? This must be the most expensive and slowest means of travel ever invented.

What distances do others average ?

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

 

[AntarcticPilot]

Ah, Paul Cooper, 'The Sailings' of yore....

If only it were that simple. I have two ( or three ) things to say; one is 'orthodromic' and the other is 'conformality'.

And...



One can calculate rhumb line distance, and compare it with great circle distance, so as to consider if the time-distance difference is worth pursuing.

Mr. Cooper is right about one thing - such stuff is of theoretic interest only to a few. I know of a 31' carbon racing trimaran which won its class in the 2003 Fastnet race by 4min 55sec. Half-a-mile less sailed here or there made a difference.....

Yes, all you say is quite correct. However, I was speaking only of bearings; what I said is correct. I was commenting on the OP's remark that he found it difficult to follow a rhumb line. I said nothing about scale, and in fact I avoid Mercator projections in my day job precisely because of the scaling problems you mention. The fact that you can't represent the Pole is also an inconvenience - but all that we have said about normal aspect Mercator is also true of Transverse Mercator, which is also used for charts, and which can be used to represent the Poles. If you wish to compute rhumb line distance, you are into very nasty mathematics indeed, and frankly, I wouldn't bother!

It is true that NO projected Map (i.e. a 2D representation of the 3d surface of the Earth) can preserve scale except in rather special and limited cases.

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? For those who are really interested, a very useful publication for common map projections is USGS Professional paper 1395, "Map Projections - A working manual" by John P Snyder. This contains the (extremely nasty) mathematics required to do accurate computations on the real earth (taking into account the non-spherical form of the earth). But it is VERY hard going. Publications by D H Maling are also interesting, though less mathematically rigorous.

You should also note that implementations of Mercator's projection in it's geodetically accurate form using an ellipsoidal earth are approximate, and only valid within limits set by the desired accuracy. The relationships you note are only approximately correct (though good enough for maritime purposes), and of course cannot be used for a track that spans a range of latitudes.

All this is of theoretical interest only, and only important to people like me who are working in situations where we have geodetic information accurate to millimetres! But if it weren't for the overwhelming advantage of lines of constant bearing being straight lines, we'd never have used Mercator's projection as it's other properties are awful!

BTW, if you want a projection on which Great Circles are straight lines, the Gnomonic projection is your friend.

 

[bilbobaggins]

geodetic information accurate to millimetres!

'Pencils used by cartographer'

'Pencil used by navigator'

 

[AntarcticPilot]

'Pencils used by cartographer'

'Pencil used by navigator'

You mean you still use PENCILS??? This is what I use:

http://h10010.www1.hp.com/wwpc/uk/e...-3328061-3328080-3328080-3338748-3338852.html

I admit that it isn't exactly marine gear!

 

[puddock]

Yes, all you say is quite correct. However, I was speaking only of bearings; what I said is correct. I was commenting on the OP's remark that he found it difficult to follow a rhumb line. I said nothing about scale, and in fact I avoid Mercator projections in my day job precisely because of the scaling problems you mention. The fact that you can't represent the Pole is also an inconvenience - but all that we have said about normal aspect Mercator is also true of Transverse Mercator, which is also used for charts, and which can be used to represent the Poles. If you wish to compute rhumb line distance, you are into very nasty mathematics indeed, and frankly, I wouldn't bother!

It is true that NO projected Map (i.e. a 2D representation of the 3d surface of the Earth) can preserve scale except in rather special and limited cases.

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? For those who are really interested, a very useful publication for common map projections is USGS Professional paper 1395, "Map Projections - A working manual" by John P Snyder. This contains the (extremely nasty) mathematics required to do accurate computations on the real earth (taking into account the non-spherical form of the earth). But it is VERY hard going. Publications by D H Maling are also interesting, though less mathematically rigorous.

You should also note that implementations of Mercator's projection in it's geodetically accurate form using an ellipsoidal earth are approximate, and only valid within limits set by the desired accuracy. The relationships you note are only approximately correct (though good enough for maritime purposes), and of course cannot be used for a track that spans a range of latitudes.

All this is of theoretical interest only, and only important to people like me who are working in situations where we have geodetic information accurate to millimetres! But if it weren't for the overwhelming advantage of lines of constant bearing being straight lines, we'd never have used Mercator's projection as it's other properties are awful!

BTW, if you want a projection on which Great Circles are straight lines, the Gnomonic projection is your friend.

Oh Bugger, you took the words right out of my mouth .......
Only kidding but VERY impressive. My hat comes off to you