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post #1 of 18 Old 04-10-2009 Thread Starter
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Help needed with a plotting question

When told to take a bearing on the light of a lighted buoy do you use the open circle at the base of the buoy or the open circle that (I think) represents the light on the top of the buoy?
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post #2 of 18 Old 04-10-2009
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When taking a bearing on a buoy, you would put the line through the center of the open circle, which indicates the buoy's actual location. However, be aware that the actual, real world, position of the buoy can be significantly different from that on the chart as it is on a chain and has a swinging circle, much like an anchored boat, and the length of the chain and the depth will determine how large that swinging circle is. Also, buoys can be dragged out of position by ice, boats, storms, etc... so, you should generally take the position of a buoy as a general, rather than a specific point on a map.

A daybeacon or daymarker, which is fixed to the bottom, will be a more accurate way of locating your position, as they are not subject to the inaccuracies of a swinging circle.




I'd highly recommend you get Richard K. Hubbards's book, Boater's Bowditch, which is an abridged version of Bowditch specifically re-written for the small craft navigator. Or you can download Bowditch for free from HERE.

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post #3 of 18 Old 04-10-2009 Thread Starter
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Sailingdog, this buoy is the shape of an inverted cone with the top cut off and two open circles, one at the base and one on the top. In the question I am answering it asks me to take a bearing on the light of the specific buoy. I don't know which open circle to use.
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post #4 of 18 Old 04-10-2009
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well....

How accurate a reading can you get? Can you get less than 2 degrees error?
At a range of 1nm (6000') your 2 degrees error is going yield an error of about 200 feet. I highly doubt you're going to see a buoy move that much from it's charted position.

Less than a mile why are you even taking a sight? Just steer the boat.

GENERALLY baseball navigation works. (you're in the ballpark)
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post #5 of 18 Old 04-10-2009
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Here is a link to the updated 2002 version of the American Practical Navigator.
Maritime Safety Information
Check out the "Menu Options" dropdown for add'l info you may find useful.

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post #6 of 18 Old 04-10-2009
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What kind of chart are you using. I've never seen symbols like the one you're describing. Can you post a photo of the chart in question??? The one at the bottom of the inverted cone is probably the correct one, since the base of the buoy should be its nominal position.



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Sailingdog, this buoy is the shape of an inverted cone with the top cut off and two open circles, one at the base and one on the top. In the question I am answering it asks me to take a bearing on the light of the specific buoy. I don't know which open circle to use.

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post #7 of 18 Old 04-10-2009
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You really don't want to use a buoy for bearings. Buoys are like a dog chained to the middle of your yard and moves around the plotted position of its anchor. Also I have seen buoys dragged off of its position so many times I have lost count. Nope won't use a buoy for plotting. But that is Me.
Day marks are on piles and don't move. Use the daymarks for bearings.

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post #8 of 18 Old 04-10-2009
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dog, Bosun, et. al...

I think you missed his question. From it's phrasing, and from his response, I think he's taking some sort of course or exam.

Castoff,

3 assumptions:

1. you're using U.S. or Canadian charts;
2. you're asking about a question on an exam;
3. you still have your Valiant 37 and got it OK to Lake Ontario :-)

Most buoy locations are marked by a dot...like a thick period...at their base. A few, like private marker buoys, have a small circle at the base of a diamond to show their location. They are also generally in the shape of a diamond, not a triangle. The charted location of the buoy is indicated by the dot. Lighted buoys are indicated by circles or teardrops in color, generally purple/red. The colored circles are centered on the dots.

I know of no chart symbol with two circles. Sometimes, two items may be very close and if the scale of your chart isn't right -- particularly on electronic charts -- it may appear that one buoy has two circles.

When plotting a course directly to a buoy, you'd use the dot.

dog, Bosun, et. al are correct about not depending too heavily on buoy positions in the real world. They can move on their chains in deep water. They can be blown off course. They can be mis-plotted. They can have been moved, and your chart just doesn't show their new location yet.

Still, despite the caveats, in relatively shallow water I've found that buoy positions of U.S. buoys along the East Coast are generally dead smack balls-on accurate (an industry term). The Coast Guard takes great pains to ensure that they are, and they do a damned good job of it.

However, as ever, your mileage may vary :-)

Bill

Last edited by btrayfors; 04-10-2009 at 05:45 PM.
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post #9 of 18 Old 04-10-2009
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The only thing I can think of that the OP may be trying to describe is a sector light.


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post #10 of 18 Old 04-10-2009
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Could you mean the buoy on the right? (I think it's a North Cardinal Buoy - there's a different one in Canada)

If that's the case, the coordinates of the buoy should be taken at the open circle in the middle of the bottom horizontal line.




The marker on the right is an isolated danger marker. We do use the same one on Canadian Charts - I've just didn't remember it from my courses and I've never seen one.

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Last edited by flyingwelshman; 04-10-2009 at 06:33 PM. Reason: Found out what the marker is.
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