Brain hurts - Working out course to steer

[wingssail]

Originally Posted by Andy Todd

say I do all my nav on a tab

Mistake #1. doing all your nav on a tab. That is not the sign of a serious or competent seaman.

[Dockhead]

Originally Posted by cyan

Well, no. I'm advocating a bit more study for people who write things like "the shortest distance between two points is the rhumb line". Those of us who cross oceans would be more concerned than those who cross channels.
That's cool, though. I think sailing around in a spiral path is clever. It's just not the "shortest".

I don't think anyone missed that. Great circle issues are simply not relevant to passages of 60 or 100 miles, or even 250 miles, and it is correct to assume that the shortest distance between two points OVER GROUND is the rhumb line, when dealing with passages of this length.



Throwing this irrelevancy in is potentially confusing to people who are struggling with understanding CTS -- struggling to understand moving water. The roundness of the earth is an entirely different problem, from dealing with moving water.



Passage between Faroes and Iceland, from 62 N 21 6 W 52 to 64 N 56 13 W 38 is 237.6 miles is:


238.565 miles on constant 310.7 heading, by the rhumb line


237.518 miles by ideal great circle, for a gain of 1.5 miles or 0.6%. But no one steers an ideal great circle -- traditionally, ships sailing a long great circle passage changed course once a day at noon.


For 60 or 100 miles, it's simply irrelevant.

[cyan]

Originally Posted by Dockhead

I don't think anyone missed that. Great circle issues are simply not relevant to passages of 60 or 100 miles, or even 250 miles, and it is correct to assume that the shortest distance between two points OVER GROUND is the rhumb line, when dealing with passages of this length.



Throwing this irrelevancy in is potentially confusing to people who are struggling with understanding CTS -- struggling to understand moving water. The roundness of the earth is an entirely different problem, from dealing with moving water.



Passage between Faroes and Iceland, from 62 N 21 6 W 52 to 64 N 56 13 W 38 is 237.6 miles is:


238.565 miles on constant 310.7 heading, by the rhumb line


237.518 miles by ideal great circle, for a gain of 1.5 miles or 0.6%. But no one steers an ideal great circle -- traditionally, ships sailing a long great circle passage changed course once a day at noon.


For 60 or 100 miles, it's simply irrelevant.

It would be incorrect to discount 60 or 100 miles as irrelevant without first considering latitude and heading. It would be a mistake to label this common misconception an “irrelevancy”. We could all just believe the false rhumb line “shortest distance” statements on this forum, and still get to our destinations.

I prefer the lower latitudes, thus have no dog in this fight. However, a CTS discussion is not better when incorrect tales are repeated about rhumb lines. Simply tell the student of navigation that this rhumb line shortest distance tale is often close enough, but not the mathematical fact. Why is that confusing?

[Dockhead]

Originally Posted by cyan

It would be incorrect to discount 60 or 100 miles as irrelevant without first considering latitude and heading. It would be a mistake to label this common misconception an “irrelevancy”. We could all just believe the false rhumb line “shortest distance” statements on this forum, and still get to our destinations.

I prefer the lower latitudes, thus have no dog in this fight. However, a CTS discussion is not better when incorrect tales are repeated about rhumb lines. Simply tell the student of navigation that this rhumb line shortest distance tale is often close enough, but not the mathematical fact. Why is that confusing?

Can you show me any 100 mile passage on any heading and any latitude, where great circle versus rhumb line makes even a couple cables of difference? So I disagree -- not only is it not incorrect to discount this as irrelevant, it is absolutely incorrect to do anything other than eliminate this harmful overcomplication from the calculation. You can't even plan a 100 mile passage to that level of precision -- you can't even set your pilot to tenths of a degree, to implement a great circle route over 100 miles, and even if you could, you couldn't hold a course accurately enough, to realize the slight difference between the great circle route and the rhumb line route over that amount of distance.




I purposely chose a passage where great circle navigation gives potentially the greatest benefit, and purposely took a longer passage than you do tidal passage planning for, to show that it doesn't make any difference.


It is absolutely incorrect to overlay the geometric complexity of great circle navigation, onto a CTS problem. A straight rhumb line is an abstraction, which does not work for a 1000 mile passage, but which works very well for a 100 mile passage, as it is an extremely close approximation of reality at that distance.



It is just exactly like regarding the earth as a sphere -- that is another abstraction, which works for all of our navigation problems, but which is not true -- the earth is not a perfect sphere.


It would be like if I came into a discussion about great circle navigation, and said -- "You guys are doing it wrong -- your great circle assumes the earth is a sphere, but it is not. You are ignorant."



But they ARE doing it right -- using a useful abstraction which is plenty close enough and avoiding harmful unnecessary complexity, which increases risk of error for absolutely no purpose.



Same here with CTS problems.

[Uricanejack]

Ha ha ha, just what the heck does a great circle have to do with the price of eggs?
Have you any idea how much shorter a GC across the N Atlantic from Cape Race to the Lizard is than a rumb line? And how much further north it takes you.

Which do you think would be shorter in a sailboat.

[Uricanejack]

Once upon a time used to use something referred to as a “days work”

It’s a simple table using simple trig to calculate D lat and Departure.

From your start position. Lat long. N + S - E + W -

course speed distance diff latitude departure


For each course you steer apply the distance calculate the D lat and the Dep
Enter in table.
Apply current or set and drift just as any course and time. Calculate DLat and departure

Add all the d lats and departure

Give you a total d lat and a total departure. For the resultant vector.

For a short distance just plot it. Technicaly you should convert departure to Diff longitude. For short distances it doesn’t matter.

Bingo.

For longer distances up to 600 miles a Mid latitude sailing is accurate enough.
Just figure out the middle latitude by dividing the D lat x 2 apply and use mid latitude to convert departure to D long and plot the position.

For the purposes of thread drift into irrelevance.
Used to use this technique to to calculate a DR when doing a great circle just added expected set and drift as a course and distance.

[cyan]

Originally Posted by Dockhead

It would be like if I came into a discussion about great circle navigation, and said -- "You guys are doing it wrong -- your great circle assumes the earth is a sphere, but it is not. You are ignorant."

I think you make a good point regarding 100 mile CTS work, and nobody here was calling people names that I know of.

I am simply suggesting that navigation students might be best served if they are told the full truth about the rhumb line-- being a good estimate most of the time. There were incorrect rhumb line assumptions stated in this CTS discussion, and pointing that out now is not meant to be insulting whatsoever.

For the record, those crazy souls who sail from Cape Horn to Cape Town would be adding 200 nm to their cold weather adventure if they believed a rhumb line represented the shortest distance. I wouldn't wish another 200 miles of such a journey on anyone.

[Dockhead]

Originally Posted by cyan

I think you make a good point regarding 100 mile CTS work, and nobody here was calling people names that I know of.

I am simply suggesting that navigation students might be best served if they are told the full truth about the rhumb line-- being a good estimate most of the time. There were incorrect rhumb line assumptions stated in this CTS discussion, and pointing that out now is not meant to be insulting whatsoever.

For the record, those crazy souls who sail from Cape Horn to Cape Town would be adding 200 nm to their cold weather adventure if they believed a rhumb line represented the shortest distance. I wouldn't wish another 200 miles of such a journey on anyone.

OK, fair enough.


But I do think you underestimate your colleagues here -- I am pretty sure everyone here knows that the earth is round, and has heard about great circle navigation. I doubt anyone here would try to sail between the Capes on the rhumb line.

[Uricanejack]

I certainly wouldnt put a GC between those capes high on my to do list.
A composit maybe, A rhumb line much more likley.

The point here. The shortest distance and time between two points may not be the distance over the ground. Rather the distance through the water,
I would just add considerations about what the wind and weather will be and how it will affect your route are equally important

[cyan]

Originally Posted by Dockhead

OK, fair enough.


But I do think you underestimate your colleagues here -- I am pretty sure everyone here knows that the earth is round, and has heard about great circle navigation. I doubt anyone here would try to sail between the Capes on the rhumb line.

You might be right.
However, in my first "navigation" class, the instructor passed out maps with some sort of hyperelliptical projection, then proceeded to explain to the students that the 35th parallel was actually closer (in real life) to the North pole where it crosses the US East coast vs where it crosses the US West coast. (part of ASA103)
I laughed out loud.
He was serious.
Six students walked away believing the "instructor" that day.
It's a colorful world.

[SV_Harbinger]

Concerning the great circle debate, my (admittedly limited) experience with off shore sailing is that while they are indeed the shortest route, on my longest trip Fairhaven MA to Bermuda I was unable to take advantage of the benefits offered by a great circle, as the Gulf Stream was to be crossed first, before turning South. While this was a couple years ago, as I recall a rhumb line was to the tune of 750 miles, a great circle would have been about 700, and the resultant wiggly dog leg was closer to 1100... those pesky currents and prevailing winds... jerks the both of em. Now if that was more of a tradewinds type crossing a great circle would have been far more practical. But before I muddy the waters of this thread any more I'll shut up now lol.

[Dockhead]

Originally Posted by SV_Harbinger

Concerning the great circle debate, my (admittedly limited) experience with off shore sailing is that while they are indeed the shortest route, on my longest trip Fairhaven MA to Bermuda I was unable to take advantage of the benefits offered by a great circle, as the Gulf Stream was to be crossed first, before turning South. While this was a couple years ago, as I recall a rhumb line was to the tune of 750 miles, a great circle would have been about 700, and the resultant wiggly dog leg was closer to 1100... those pesky currents and prevailing winds... jerks the both of em. Now if that was more of a tradewinds type crossing a great circle would have been far more practical. But before I muddy the waters of this thread any more I'll shut up now lol.

Well, of course great circle navigation works -- the earth is round, after all, and not flat. No one doubts that. The great circle route between any two points on the earth's surface is always the shortest -- even two miles across the bay.



The issue is how long does a passage need to be, for it to be worth bothering with. Two miles across the bay, the difference between the rhumb line and GC distance will be measured in millimeters (if that), and would require to steer to thousandths of a degree. So a great circle route for such a trip exists only in theory.

Likewise for passages of 60 or 100 miles -- even at high latitudes. The gain might be a few cables, but you cannot hold a course closely enough to realize such a route, so in practice a great circle route doesn't even exist -- not one you can practically implement, anyway.




The passage you talk about -- from approximately the West cardinal buoy at the end of Cuttyhunk Island, to Bermuda -- is 605.2 miles on the rhumb line, on course 151.1 true. By great circle, it's 604.5 miles using an initial course of 149.3 true. A grand total of 7 cables of benefit, and requiring you to steer tenths of degrees to realize -- something you can't do even without the Gulf Stream.


So this is also a passage which is too short for great circle navigation -- that is, not only not worth it but you can't even do it -- and would be too short even if you didn't have to deal with the Gulf Stream. So I believe your memory is deceiving you here.








If anyone is interested in doing quick great circle calculations, this is an excellent utility:


https://www.nauticalalmanac.it/en/pd...humb-line.html


Another absolutely excellent tool for great circle nav is OpenCPN (not a surprise since O is generally the best tool for any given nav task). The "measure" tool will automatically switch from rhumb line to great circle nav when the difference is more than 2 nm. So when you're going passage planning with O, you know you don't even need to think about it, if the "measure" tool isn't showing you a GC route.



See: https://opencpn.org/wiki/dokuwiki/do...circle_sailing

[parkstone bay]

This thread was originally started by a question about using a tablet to resolve CTS questions (i.e. vector diagrams). Some contributors have got fixated with the differences between rhumb lines and great circle navigation (which do indeed exist, even on across the bay routes (unless your departure and destination lie on the same meridian of longitude, in which case they are the same)). But, in practice, there are loads of other things which often have greater impact on a passage plan. These include: Tidal current effect, (the original question, guys!); weather here and now/forecast; tidal gates; other shipping; state of your boat and crew; bits of land sticking out of the water, (we pedants call them "islands"); deep/shallow water; other shipping; whether a route takes you unduly North, or South; what town/island you want to visit, (or to avoid); blah, blah, blah. So the GC navigators aren't wrong, but for passages of less than about 600 miles (10 degrees of arc), often the impact of a GC is *relatively* unimportant, by comparison with everything else.

[El Pinguino]

Did I miss the bit where someone mentioned ' lee bowing' the tide?

Mind you I have no idea how you would do that on a 'smartphone' or a 'tablet' whatever they may be....

[El Pinguino]

Originally Posted by Uricanejack

Once upon a time used to use something referred to as a “days work”

It’s a simple table using simple trig to calculate D lat and Departure.

From your start position. Lat long. N + S - E + W -

course speed distance diff latitude departure


For each course you steer apply the distance calculate the D lat and the Dep
Enter in table.
Apply current or set and drift just as any course and time. Calculate DLat and departure

Add all the d lats and departure

Give you a total d lat and a total departure. For the resultant vector.

For a short distance just plot it. Technicaly you should convert departure to Diff longitude. For short distances it doesn’t matter.

Bingo.

For longer distances up to 600 miles a Mid latitude sailing is accurate enough.
Just figure out the middle latitude by dividing the D lat x 2 apply and use mid latitude to convert departure to D long and plot the position.

For the purposes of thread drift into irrelevance.
Used to use this technique to to calculate a DR when doing a great circle just added expected set and drift as a course and distance.

Exactly.... under 600 miles use the Traverse Tables.... no adding or subtraction required..... Mercator Sailing for longer distances in the low and middle latitudes....

Great circle? Rarely used in its pure form in high lats....... composite GC is more common with a limiting latitude... use the ABC tables to work out a fresh 'initial course' each day.... if you are realy keen just use the Haversine Formula for distance .... just replace the GP of the body with the Lat/Long of your destination... but you will still use the ABC tables for initial course.

Can somebody remind me how this has anything to do with crossing the Channel and lee bowing the tide?