Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 15-10-2007, 13:58   #1
Registered User
 
Triton318's Avatar

Join Date: Aug 2006
Location: Hayes, Virginia
Boat: 1962 28' Pearson Triton
Posts: 289
Great Circle Routes

I'm planning a cruise to Bermuda (from Virginia) next June. This will be my first offshore passage, and I have a few questions regarding great circle routes. I understand the concept of a great circle route and the difference between that and a rhumb line course.

1. Is there a minimum distance for using a great circle route? There must be some distance where it doesn't really matter if you steer between waypoints on a great circle route or a rhumb line course. It's 623 NM to Bermuda--would this distance be best traveled using a great circle route?

2. What is the best way for determining compass courses to steer along a great circle route? I understand you can use a gnomonic chart--from the description I read, it's a pretty straightforward process. Are these charts available for the Atlantic Ocean? I'm not going to attempt a second method--spherical trigonometry. I have enough trouble with algebra! I also am not too keen on purchasing software. I visited a couple of websites for computing great circle routes. One only gave distance, the other gave distance and a heading, but I thought there would be several headings, and you have to periodically change course to remain on the great circle.

Any help would be appreciated. Thanks! (Even to the point of providing me with a series of courses/waypoints from the Chesapeake Bay Bridge Tunnel to Bermuda!)
__________________

__________________
Jay White
S/V Dove
1962 Pearson Triton, #318
Triton318 is offline   Reply With Quote
Old 15-10-2007, 14:16   #2
Moderator Emeritus
 
GordMay's Avatar

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 31,592
Images: 240
In practical terms, a Great Circle STEERING Route consists of a number of Rhumb Line WAYPOINTS (of varying distances apart).

Javascript Great Circle Calculator
Great Circle Mapper
Great Circle Distances
Marine Great Circle Navigation Calculator
__________________

__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"



GordMay is offline   Reply With Quote
Old 15-10-2007, 14:26   #3
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,458
Images: 69
Unless you are sailing very long distances non stop, and predominantly east-west or west-east, (i.e. trans-atlantic or pacific) the differences in distance aren't really significant. What's more the great circle route always curves towards higher lattitudes (numerically) than the rhumb line, which may expose you to more severe weather. Finally, the course you will actually be able to sail is more likely to be determined by the wind and weather anyway.
__________________
44'cruisingcat is offline   Reply With Quote
Old 15-10-2007, 14:38   #4
Senior Cruiser
 
Vasco's Avatar

Cruisers Forum Supporter

Join Date: Apr 2004
Location: Toronto
Boat: CS36Merlin, "La Belle Aurore" Ben393 "Breathless"
Posts: 7,140
For great circle sailing,the easiest way is to draw a straight line on a gnomonic chart and take waypoints off it and determine the compass course between these points. On a trip to Bermuda a rhumb line course will do. You will want to get across the gulf stream as fast as possible. We only used to use great circle sailing for Pacific crossings and then we'd have to decide how far north we'd go. The great circle route would take you too far north and into some nasty weather. We'd go up to a certain latitude and then run along it until we got to the great circle again. As for spherical trig. ugh!! Of course this was in the olden days when you needed sextants. Nowadays I'm sure all the info can be found on the net.
__________________
Rick I
Toronto in summer, Bahamas in winter.
http://groups.yahoo.com/group/beneteau393/
Vasco is offline   Reply With Quote
Old 15-10-2007, 17:43   #5
Registered User
 
Viking Sailor's Avatar

Join Date: Nov 2006
Location: San Francisco Bay
Boat: Fantasia 35 - s/v Feeling Good
Posts: 1,074
The way I learned to navigate is the same for RL and GC. Each time a fix is logged you manually recalculate and log the new RL or GC course, distance, ETA to the next waypoint or destination. This corrects for set, drift, and other steering errors regardless of the type of course you are sailing.

As stated above, RL course calculations can be (are) used for short passages (less then a few hours between fixes), passages with a constant latitude or a latitude limit, and north/south passages. Also, with the exception of north, south and equatorial passages, long passage distances need to be computed using CG distance calculations.

AFAIK, all manual calculations are based upon a spherical earth and are of limited accuracy. Good enough for planning or sextant based navigation. But, not good enough in today's world of super accurate electronic navigation.
__________________
Viking Sailor is online now   Reply With Quote
Old 15-10-2007, 18:32   #6
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
Jay,

To answer your question more directly, and practically: it makes little difference whether you travel a GC route or a RL route for relatively short distances, e.g., to Bermuda from the Chesapeake Bay.

Here are the figures from Chesapeake Bay entrance (37N 76W) to Bermuda (33N 65W), using round number starting and ending points for convenience:

Great Circle: 591.0 nautical miles Initial course: 110.7 True

Rhumb Line: 591.3 nautical miles Course: 113.9 True

If you were crossing the Atlantic to the Azores or the Med, then GC might be of interest, though as others have noted that's just one of the factors to take into account. For a sailing boat, it's more likely that anticipated weather, currents, etc. would be of greater interest.

Bill
__________________
btrayfors is offline   Reply With Quote
Old 16-10-2007, 02:30   #7
Moderator Emeritus
 
GordMay's Avatar

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 31,592
Images: 240
Bill:
How often was the GC course corrected* to calculate a Great Circle distance (from Chesapeake Bay entrance 37N 76W to Bermuda 33N 65W) of 591.0 nautical miles?
(ie: every 1, 5, other degrees?)

* Or how many Rhumb Line legs, comprise the GC?
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"



GordMay is offline   Reply With Quote
Old 16-10-2007, 06:21   #8
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
Gord,

"How often was the GC course corrected...?"

An infinite number of times. Note that the GC course is give as the "Initial Course", i.e., the initial true heading from the point of departure.

In practice, of course, you'd choose to correct the heading at convenient waypoints, which could be at any desired interval.

GC initial heading or course computation is often used by amateur radio DX operators to compute the direction to point their directional antennas to optimize long-distance contacts, thus the concept is well known outside the sailing world :-))

Bill
__________________
btrayfors is offline   Reply With Quote
Old 16-10-2007, 06:44   #9
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,637
As Bill and Gord have alluded, in days past no-one actually followed a GC. The path taken from the gnomonic planning chart would be transcribed to mercator charts, thus you would follow a succession of short RLs which would approximate the path of the GC; changing course by a few degrees at convenient intervals, like every 5 degrees of longitude. I would think with modern navigation electronics, the path between waypoints could be set as GC, and the exact heading would be transmitted to the auto-pilot or displayed to the helmsman, as it changed.

Kevin
__________________
Lodesman is offline   Reply With Quote
Old 16-10-2007, 09:02   #10
Registered User
 
Triton318's Avatar

Join Date: Aug 2006
Location: Hayes, Virginia
Boat: 1962 28' Pearson Triton
Posts: 289
Thanks for the responses! The information was very helpful.
__________________

__________________
Jay White
S/V Dove
1962 Pearson Triton, #318
Triton318 is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Routes To Caribbean SydneyTim Atlantic & the Caribbean 2 06-09-2007 18:32
Pacific cruising routes beau Powered Boats 10 31-01-2007 15:22
Bahamas (Abacos) Routes LaLeLu Other 7 05-10-2006 16:27
Atlantic Circle ~ by John Kretschmer GordMay Atlantic & the Caribbean 0 05-10-2005 05:55



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 09:34.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.