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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > Correction for Leeway - Coastal Navigation Question
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Thread: Correction for Leeway - Coastal Navigation Question Reply to Thread
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Topic Review (Newest First)
08-03-2011 02:06 AM
kwindancer Thank you everyone for taking the time to reply. It all make sense to me now.
07-31-2011 07:02 PM
Tempest Let me add more information, if this helps. From the Current Tables, current velocity is 3.48 Ebb, and Ebb direction is 266 degrees TRUE.

Does this help? Thanks.[/QUOTE]


Well...yeah, Information regarding a 3.5 kn current would have been useful.

Any more rabbits in the hat?

So this is a compound problem... Set and Drift... and Leeway.

You need to compute the course to steer using the set and drift.... This is where your, time, distance and speed now become a factor....

Then apply the leeway... after.... you figure out the CST from the set and drift problem

Good luck...

Don't forget to use TVMDC..to go from true to mag..or visa versa..

Normally when I see these course books...they give you a deviation table to work with too...keep looking..I'll bet you it's there somewhere.

here's a you tube video for ya ... he could have done this a little better..but you get the idea.

07-31-2011 05:15 PM
kwindancer Hi All,

Thanks so much for your help. This question is actually from a home study workbook from a sailing school in the West Coast (SF Bay Area) for a U.S. Sailing Coastal Navigation certification. The problem is written by the school.

The problem states that "the winds are west at 22 knots when you depart point A at 1530 assuming a leeway of 4 degrees and a boat speed of 6 knots.

The variation on the chart is 14 degrees East. There is no mention of deviation.
The track from A to B is 145 degrees magnetic.

My assumption (correct me if I'm wrong) is the wind is coming from the WEST as opposed to from the EAST.

Let me add more information, if this helps. From the Current Tables, current velocity is 3.48 Ebb, and Ebb direction is 266 degrees TRUE.

Does this help? Thanks.
07-31-2011 05:06 PM
tomandchris I thinkthat JRD is probably correct and the OP is trying for a captains license or taking Coastal Nav from ASA which is I think more difficult. It is all fine to suggest that he just look at it or use his GPS, but that does not answer his question.

Leeway and current set can be calculated. I suggest Pyzel's Coastal Navigation book which is used for ASA. I took the course but lent the book to someone and have never gotten it back. I admit that I do just look at it and use my GPS, but also keep paper charts and plot for the practice. I just don't worry about Leeway or set as usually my next mark is not more than 50 miles away.
OP, know that there are some incorrect answers in both Pyzel and the USCG books, as well as many of the ASA books.
07-31-2011 03:03 PM
jrd22 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!
07-31-2011 01:07 AM
MarkSF 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.
07-30-2011 11:15 PM
fryewe 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.
07-30-2011 10:52 PM
CaptainForce 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?
07-30-2011 09:58 PM
acunningham 149 degrees!
07-30-2011 09:31 PM
CaptainForce
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 dontí tell me about losing power and GPS failure and all that. If that happens youíre 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 itís 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 shouldnít replace knowing where you are and where youíre going in real time. Thatís 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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