Correction for Leeway - Coastal Navigation Question - Page 2 - SailNet Community
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Old 07-30-2011
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Quote:
Originally Posted by kwindancer View Post
Hello All,

Let's say my point of departure is at point A and destination is to point B.

The track (Course to Make Good) from A to B is 145 degrees magnetic.

The winds are west at 22 knots when I depart point A at 1530. Let's say the leeway is 4 degrees and the boat speed is 6 knots. What course, corrected for leeway, should I steer from point A to point B?

Thanks so much.
The 'solution' lies within your question.
A—B = 145°M
leeway = 4°
therefore steer 145° + 4° = 149° to arrive at B

rigorous solution:
SIN 4° = 0.06975
1 mile = 6080 ft.
1 mile X SIN 4° = 6080 X 0.06975 = 424 FEET
therefore aim 424 FEET (per nautical mile per total distance travelled .... or add 4°) to the 'right' (windward side) of your course and you will be 'ded-on' at your arrival: 145 + 4 = 149°M
If the wind would be from the east and causing the same value of leeway, then subtract 4°; 145° - 4° = 141°M
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Old 07-30-2011
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Quote:
Originally Posted by MarkSF View Post
"If it's 2.9 miles and the drift is 4 degrees per mile..then 2.9 x 4 = 11.6 degrees of correction add east ..11.6 + 14 = 25.6 or 26 degrees."

I've never heard leeway being done that way. It's a constant error.

Get lost much?
Well, Mark....the 1st post...has the problem...the 5th post has the supposed answer......it wasn't my problem, and it wasn't my original answer..115 is not my solution.

I've been trying obtain additional information from the OP....and trying to figure out how he /they could come up with an answer of 115.

And no, I don't get lost..

BTW...Winds are always stated in direction " From" . a West Wind ...comes from the West, no clarification is usually necessary at least not.. on the east coast..

Last edited by tempest; 07-30-2011 at 07:00 PM.
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Old 07-30-2011
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Quote:
Originally Posted by RichH View Post
The 'solution' lies within your question.
A—B = 145°M
leeway = 4°
therefore steer 145° + 4° = 149° to arrive at B

rigorous solution:
SIN 4° = 0.06975
1 mile = 6080 ft.
1 mile X SIN 4° = 6080 X 0.06975 = 424 FEET
therefore aim 424 FEET (per nautical mile per total distance travelled .... or add 4°) to the 'right' (windward side) of your course and you will be 'ded-on' at your arrival: 145 + 4 = 149°M
If the wind would be from the east and causing the same value of leeway, then subtract 4°; 145° - 4° = 141°M
Yes! Agreed..I subtracted at first....it is an add.. 149 deg. steer into the wind..
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Old 07-30-2011

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Gotta love the wankers who make up these tests. If the wind is from the west and you want to go about SE, the wind will be from behind the beam and leeway will be negligible. In any case, if you steer 115 degrees on the compass you aren't going to get there.
Old 07-30-2011
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Quote:
Originally Posted by billyruffn View Post
Enough with the math! Vectors, smectors!

Put the coordinates of Point B in the GPS, note the bearing thereto, use that heading to start and after a few minutes note the COG on the GPS. Adjust your heading so that the COG equals the bearing to Point B.

And dont’ tell me about losing power and GPS failure and all that. If that happens you’re just SOL.

PS -- it helps to plot a line from A to B on a real chart so you can be sure there are no hard things lurking along the rhumb line.

But seriously, in small boats it’s hard enough to steer within 10 degrees of the desired heading. So pre-planning a few degrees of windage is an interesting exercise, but it shouldn’t replace knowing where you are and where you’re going in real time. That’s what makes GPS so great!
I'm with Billyruffin here! I cetainly use GPS to take the sigmoid curve out of any crossing of a currect vector or adjustment for the effects of leeway. I did not respond up in the second post with the thought that this was an exercise from a workbook or course. The answer I gave was the practical application I used in the 70's for crossing the Gulfstream with RDF and dead reckoning before GPS. My practical application of leeway undersail was always to adjust according to my RDF fix along with the intuitive development that all sailors have knowing that, especially when close hauled, you are not headed as the bow points. I favor the means of calulating this adjustment for leeway and respect the course work, but it's not a function in today's practical application. Take care and joy, Aythya crew
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Old 07-30-2011
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149 degrees!
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Old 07-30-2011
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I like this simplistic answer from acunningham to just counter the four degrees of leeway, but I'm curious. In a real world scenario who gives you the original information that your leeway is 4 degrees unless you calculated it from an earlier point and then a fix on your new position?
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Old 07-30-2011
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Puzzling problem...never started a journey knowing what my leeway would be (in degrees!) before I started out. Don't have a leeway instrument. A good substitute is bearing to next waypoint feature of gps as billyruffin described above.

In the real world this problem's solution can be estimated prior to getting underway, and course to steer can be adjusted to approximately offset the effects of set and drift due to wind and current by observation, experience and history, but after you are underway, it is worked out by calculating set and drift, which can occur due to current or wind.

By getting a fix and comparing it to your dead reckoned position, using your observed speed through the water and course for the time since your last fix, you establish a vector between the DR and the new fix. The vector's length is the distance traveled in the time since last fix (distance divided by time is speed, or set) and the direction of the vector is the drift, the direction of motion caused by current and wind. If you draw a vector from your original fix to the new fix, that is the actual course and speed you have made good.

The difference in bearing of your DR position from your last fix, and the bearing of your new fix from your last fix, is the amount you have to adjust your course on the opposite side from your steered course to offset the drift.

If you adjust your course in the direction opposite you are simply forcing your ship to make the same amount of speed to the right (or left) of your desired course that the set is pushing you to the left (or right).

Or if the destination is 2.9nm away, or 5800 yards, and a physical feature, look at it through the binoculars and steer toward it. Just as in the case steering bearing to waypoint using GPS, you will be steering a curved track rather than a rhumb line course, but my guess is after a few minutes you can estimate how much you have to crab to offset the wind and current. Another method when you can physically see the destination is to check the true bearing to it every few minutes. If it draws left, you are steering too much to the right; if it draws right, you are steering too much to the left.
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Old 07-31-2011
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I think the point is that once you have calculated leeway a few times you will then have a feeling for your boat's leeway and how it relates to wind conditions.
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Old 07-31-2011
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The OP's problem sounds like it's right out of the USCG's questions for a Master's license. Lot's of irrelevant data and lack of real (modern) world information, and an incorrect answer just to drive you nuts!

John
SV Laurie Anne

1988 Brewer 40 Pilothouse

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