31 10 2009




7 responses

23 11 2009

Is there an “inverse” circle route?
Winds on my last 2 Nov transat runs kept me very close to the rhumbline.
I get that the flattened sphere puts the GCR in a northerly arc from the rhumbline. Is there a southerly arc “Pretty Good Circle Route” that would be shorter than the rhumbline (but longer than great circle).
Maybe major and minor GCR would be better terms.

24 11 2009

Nope… The easiest way to visualise GC routes is probably to do the old globe-and-a-string trick, but the route you are talking about is longer for sure. Given a shortest route (which is a straight line, albeit on a curved surface) anything other than that is longer. There is a “more curved” route that is the same as the “less curved” route, but the cartesian straight-line path and anything south of that are the long way ’round…

Since nobody has globes these days, Google Earth is a good approximation. lay in a path point-to-point and observe the shape. Experiment with the options too, it should be easier to see what is going on.

2 12 2009

Thanks 76T,

I had a basic misconception about what a GC was.
I was visualizing the rhumbline course on SOL as what i now understand a great circle actually is. I thought a GC was an arc above that course and that it deviated north due to flattening of the sphere, and there must be a slightly longer inverse.

The first picture on the wikipedia page made it all clear…

Now, what else in life do I have completely wrong?


1 12 2009

Your articles are very informative. Trying to absorb this sailing common sense makes my brain feel like a pumpkin.

Lets say I’m in the middle of the Ocean going somewhere & the rest of the fleet has sailed over the horizon, but there is this one boat who I have been criss-crossing tracks with and I decide “well dammit” I want to beat that boat to the finish if its the last thing I do.

Could you please expound on the correct way to calculate “relative position” between just two boats. I know that I can place my cursor on that boat and check to see how the angle is either increasing or decreasing. Its just that there are times when I know I have a little more wind pressure but the other boat has a slightly better TWA. Lets say, to keep the example even simpler, My rival has been sailing a direct line and I also am doing the same but our tracks are not parallel. Should I draw a line perpendicular to my track, his track or the true wind direction to assess the situation before I decide whether I can continue or alter course ? Is it possible with the information provided to gauge the speed of my opponent, since I am moving what do I use as my point of reference ? I hope you can see my quandary.

Please end my confusion and show me the right way to think about such matters. I have so far to go before I sleep.

1 12 2009

Haha… complicated question, and we haven’t covered anywhere near enough techniques to figure it out by hand, unfortunately.

The range-and-bearing trick will let you know who is faster at the moment, which is often enough if the tracks are straight and the weather is constant. A good example is two boats converging on the finish, with one behind but with better angle – you should be able to tell who has the advantage.

Once you get into a solution that requires route-finding decisions, the best that I can offer is to attempt to calculate the time-to-finish for each boat independently and compare. Easier said than done, and since the focus of this blog is NON-software, it’s a bit of a cop-out to say “just chuck both boats in the router and see”…

But, since we brought it up, a cool feature of many more sophisticated routing packages is the ability to display “reverse isochrones” which are time-contours worked backward from a boat’s expected finish time… Using those, you basically get a line at or near your boat’s position. Anyone in front of the line is ahead, while those behind it are trailing you (if all boats sail “perfectly”, no major changes in wind, accurate data etc…).

1 12 2009

Thank you for your attention in this matter. I look forward to when you lay the groundwork for this type of discussion. I do not have a nautical background nor do I use routing software, for me its just a game that happens to befuddle me. Maybe that is why I enjoy it so much. I am only interested in what the correct way to approach the problem is using only such information as that can be gleaned from within the browser’s display. This will exclude me from the top grouping, of course, but I would like to sail more purposefully.

21 01 2010

miss you 76. sorry if i did not see the big picture… Arrrrrr

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