Great Circle or Rhumb Line for long cruises??? - SailNet Community

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  #1  
Old 12-02-2007
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Great Circle or Rhumb Line for long cruises???

Ok, I understand completely the differences between the two courses, but for those who don't wiki does a pretty good job. They can be checked out at:

http://en.wikipedia.org/wiki/Great_circle
&
http://en.wikipedia.org/wiki/Rhumb_line

So given the great circle is the shortest distance between two points on the earth, is that the best course for a cruising sailor to take? Obviously, on a day sail in the bay it doesn't matter, but what about the crossing from Panama to Tahiti? Now, the true great circle can never be achieved by a helmsman because it requires constant course changes at the helm that I don't think my raymarine autopilot is ready for just yet. But you can approximate it with multiple waypoints on a regularly scheduled, say two day interval, course change.

I am a pretty fair hand with Mercator navigation, but I've not come across anything that talks about plotting courses using the great circle routes. Does anyone use these or is it almost strictly for airline pilots? If you do, please give me some guidance on how to determine the great circle route and how great a distance savings it makes in comparison to the rhumb line in say, for example, the Bahamas to Bermuda run or Panama to Tahiti. Thanks, and look forward to hearing from you true navigators.

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  #2  
Old 12-02-2007
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Quote:
Originally Posted by LWinters View Post
So given the great circle is the shortest distance between two points on the earth, is that the best course for a cruising sailor to take? Obviously, on a day sail in the bay it doesn't matter, but what about the crossing from Panama to Tahiti? Now, the true great circle can never be achieved by a helmsman because it requires constant course changes at the helm that I don't think my raymarine autopilot is ready for just yet. But you can approximate it with multiple waypoints on a regularly scheduled, say two day interval, course change.

ILW
Your raymarine autopilot, linked to GPS, does sail a great circle route with a constantly changing heading if you ask it to steer to a destination waypoint. Even if its just a day sail across the bay.

Astronav courses talk about great circle routes, but unless you plan high latitude sailing its pretty much theoretical. On most passages, such as the two you give, the gain by sailing a great circle route is tiny in relation to other considerations. In practice yachts generally route themselves to take advantage of favourable winds and currents.

One well-sailed route which is far enough north to make a difference is from Newfoundland to England. The distance from Cape Race Light to Bishop Rock is 1,842 miles by the great circle route, compared with 1,871 for the rhumb line route. At one point the former route is 300 miles north of the latter.

You need a gnomic projection chart to lay off a great circle route. Because of the difficulty of the changing compass bearing, it used to be customary to steer a 'segmented rhumb line course', which is exactly what you describe. These days though, you can just follow your GPS to your destination waypoint to achieve a great circle route.

PS Having nothing better to do this evening I checked your Panama to Tahiti route. Say from Mala Head at 7°24'N, 79°56'W to Venus Point, 17°26'S, 149°32'W. The great circle route is 4387 nm (±2nm depending on your assumptions about the earth's shape). The rhumb line route is 3 miles longer.

Last edited by LynW; 12-03-2007 at 03:25 AM.
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Old 12-02-2007
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Your GPS will derive a GC route for you as stated. It is not at all a bad idea to plot GC waypoints on a chart or plotting sheet for the voyage contemplated. A navigational calculator will do that for you or you can use H.O. 229 and hand calculate them. Customarily, the GC course is actually sailed as a segmented rhumb lines with each one laid down for a traverse of 5 degrees of longitude. On a transoceanic passage, depending on latitude, this will mean a course alteration of somewhere in the neighborhood of 5 degrees heading for each five degrees longitude traversed.

The obvious advantage to following any electronically generated course device is that should you get set off your DR track, the device is already calculating a new DR track for you and a GC at that. To the extent that these devices may take you through the middle of islands, reefs, or areas of strong current they should not be relied upon solely. Laying out and updating a DR track on available charts and plotting sheets, as well as utilizing other sources of position finding, are always essential policies.

It should also be remembered, especially on board a sailing vessel, that the GC course, while the shortest, may not be the fastest due to vagaries of wind and current. It is not at all uncommon to use a modified GC course any more than it might be to sail north or south to take advantage of prevailing winds.

You can use a gnomonic projection, with the loss of some accuracy, to plot a GC route and then transfer the Lats & Longs derived to your Mercator projection charts and plotting sheets. The gnomonic chart's straight lines are GC routes and that's about all the chart is useful for in terms of navigation. Most choose to compute the GC course mathematically, either by hand and HO 229, or by calculator, and then transfer the points to the appropriate charts.

GC routes are used all of the time depending on departure and arrival points. The differences between rhumb line and GC will vary depending on those two factors. Any meridian is a GC route and so the closer a rhumb line is to north or south the closer it will be to a GC route. Bowditch explains the matter nicely, among other things, and I'd recommend it's being on board for any offshore passage as well as the planning of same.
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Old 12-02-2007
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It depends entirely on how much beer you can carry. As there is no greater peace attainable than that of being out on the seas with naught but nature and potable alcohol, when you're underway you'll want to take the longest and least direct route possible between any two points.

That would be the Rum Line (that's the CORRECT spelling of the term, the 'h' - and occasionally a 'b' - was added by the Fundamentalists in an attempt to make the idea of sailing less attractive to the easily-led).

The term "Rum Line" came into existence in the early sixteenth century, as sailors on the ship commanded by the quasi-notable Portugee Amerigo Vespucci were forced to follow half empty bottles of Captain Morgan that had been thrown over the side. It was (wrongly) thought that there was better stuff on board, and it was indeed lucky that the error was discovered before the castaway libations had disappeared from view.

As the floating bottles were tossed about by the winds and the seas, and as the Portugee mariners were hard-pressed to steer a straight course at the best of times, the Rum Line was anything but a straight one. It zigged, it zagged, it doubled back and folded under itself ad nauseaum as each and every bottle of Dark, Light and Amber were retrieved and consumed.

In order to avoid embarassment, ridicule and/or disciplinary action upon their return to port, the Captain and crew altered the ship's logs and redrew the charts into a close approximation of the graceful swoop we know today.

Hence, if you are blessed with copious amounts of stowage on board, you'll want to follow the Rum Line as it leads you through many joyous days of beer-sodden bliss. However, if you can't put much down below, or lack refrigeration, you'll want to follow a speedier course, indeed, you might even give some thought to flying...

Last edited by Sailormann; 12-03-2007 at 12:04 AM.
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sailaway21 is just really nice sailaway21 is just really nice sailaway21 is just really nice sailaway21 is just really nice
Sailormann's telling of a wholly incorrect tale does not fail to entertain regardless of it's lack of facts.(g) Rhumb line is derived from the Greek word, rombos, which describes a magic wheel or whirling motion which is what one scribes across the globe when sailing one. The OED lists "rum" as a possible slang usage of rhumb or rhombus.
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For all practical purposes you can ignore it unless you were in a highly competitive long distance race where even then the best course would be analysed by computer and special programs taking into account polars and expected winds and currents as well as the distance. The other factors would dominate.
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Old 12-03-2007
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Quote:
Originally Posted by sailaway21 View Post
To the extent that these devices may take you through the middle of islands, reefs, or areas of strong current they should not be relied upon solely. Laying out and updating a DR track on available charts and plotting sheets, as well as utilizing other sources of position finding, are always essential policies.
Sailaway's comment above reminds me of the J-Boat skipper who was heading transatlantic from NE US coast to UK a few years back. He plotted a perfect GCR on his big ocean chart, programmed waypoints into the GPS, and set off. One night the crew on watch was suddenly taken off-guard by breakers dead ahead. The boat got caught in the surf, rolled in to the shallows, and they waded ashore on an unknown (to them) sandy beach. Most of us saw the photos that circulated on the internet of the boat quickly getting swallowed in the shifting sands of Sable Island, a feature that was not depicted on the NOAA hurricane tracking chart of the North Atlantic, on which the skipper had plotted his GCR.

Sailormann, thanks for the chuckle.

Last edited by JohnRPollard; 12-03-2007 at 12:37 PM. Reason: typos
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Old 12-03-2007
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OK boys and girls.. The long and short of GC vs rhumb line in sailing is unless you are in one of the around the world races (i.e. Volvo Race) or your boat is capable of breaking ice through the poles, Rhumb (Rum) Line wins. We international airline pilots use great circle routes but also have waypoints at least every 330 miles or at each Longitude line. We also, like sailors, take full advantage of winds and will vary the tracks for crossing the Atlantic greatly from 48N to 66 N of latitude. Bottom line, wind and current always win! Always!
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Old 12-03-2007
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Often, as I sit on the stern sipping (okay, gulping) my Gossling and Coke Zero (GCZ) and dangling my toes off the steps as fish bait I notice that my wake kind of curves like a snake, is that a Rum line to which you refer? Should I use my Raymarine ST6000 to smooth out the curve or just let the boat steer itself like normal?

Having never navigated beyond 'point the bow at those trees' level I still believe I understand the Great Circle Route - that's the one where you keep turning to the right until you are back to where you started?

(just kidding, I certified for open ocean navigation while stationed on a USNS ship)
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Old 12-03-2007
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Even racers will give up the great circle for better conditions. The book I threw into the mix on that behemoth thread of recommended reading tells of Carleton Mitchell taking his boat way out of the way on a much longer course to get stronger westerly winds.
I don't like rum much, is there such a thing as a tequila line?
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