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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 02-28-2010
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Last I checked, wouldn't that be 0˚T not 360˚T...
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  #22  
Old 02-28-2010
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Quote:
Originally Posted by TheBishop View Post
If the wind is reported as 360T, it's coming from the North. If the current (set) is reported as 360T, it's going to the North. Who decided this, and was it done just to torture me?
I also have the same question and just suppose that it's just convention. Much like a circuit diagram in EE where electrical current is shown going from the positive to the negative post whereas in reality the electrons flow from negative to postive. Not sure how all this "conventional wisdom" got started.
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  #23  
Old 02-28-2010
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The winds are named from where they are coming from because when you face into the wind you are looking in that direction. Thus the North wind is from the North because that is the direction you are facing.
About everything else is the direction they are going; Currents, Your Boat and so forth.
This is not that hard to understand... Not hard at all.

BTW: My post #13 here has not been answered yet... It is about figuring distance with a known chart scale.
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Last edited by Boasun; 02-28-2010 at 11:03 AM.
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  #24  
Old 02-28-2010
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Quote:
Originally Posted by Boasun View Post
One other:
the scale of a chart is 1:80,000. What distance at sea does a distance of 3.5 inches on the chart represent? What distance on the chart represents an actual distance of 14.3 NM?
3.5 in at 1:80,000 = 3.84 nm
13.3 nm at 1:80,000 = 13.03 in

Quote:
Remember that a NM is 6076.11549 ft thought we use 6000 ft for practical purposes.
Okay,
3.5 in at 1:80,000 = 3.89 nm
14.3 nm at 1:80,000 = 12.87 in
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  #25  
Old 02-28-2010
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Quote:
Originally Posted by sailingdog View Post
Last I checked, wouldn't that be 0˚T not 360˚T...

You are closer than me.

Vol 5 of Canadian Tides and Currents uses 000 for currents flowing to True North.

I will have to fix that.
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  #26  
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Boasun and others

Re: Chart scales.

I use Canadian Chart 3462 in my navigation class; it is 1:80,000 chart. However, the 1:80,000 is measured at 49˚ 30'N which is not on the chart. The top of the chart is 48˚ 54'N. As this is a Mercator projection, it seems to me that converting distances measured using the scale would not be exact.

In you view, would this be a correct interpretation?
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  #27  
Old 02-28-2010
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If the chart is 1:80,000 that means that you have 80,000 square inches representing land & water area squeezed into 1 square inch of the chart.

But when using the latitude scale on the chart for measuring distances, I have found that there can be a slight difference from near the bottom of the chart compared to the top of the chart.
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  #28  
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How far will a vessel traveling at 16.5 knots travel in 34 minutes? In 2h 24m?

How long will it take a ship traveling at 14 knots to go 0.8 miles? To go 69 miles?
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  #29  
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Answers in bold
Quote:
Originally Posted by Boasun View Post
How far will a vessel traveling at 16.5 knots travel in 34 minutes? 9.35nm In 2h 24m? 39.6nm

How long will it take a ship traveling at 14 knots to go 0.8 miles? 2.98 minutes To go 69 miles? 4.29 hours.
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  #30  
Old 02-28-2010
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Quote:
Originally Posted by Boasun View Post
If the chart is 1:80,000 that means that you have 80,000 square inches representing land & water area squeezed into 1 square inch of the chart.

But when using the latitude scale on the chart for measuring distances, I have found that there can be a slight difference from near the bottom of the chart compared to the top of the chart.
Jacdale's post was spot on.
Boasun, you probably teach others this:
Chart is a flat paper representing a sphare. It is impossible to make it an accurate presentation.
Your statement is only true around one (reference) latitude. at any other latitude it is either less or more - but wrong.
Nautical charts are pretty accurate in representing angles, but not so good in representing surface areas. That is why we read distances at the closest left of right edge of the chart at about the same latitude as the distance was taken.
If we would only measure that distance and multiply it we could not get a very accurate result.
The error is neglectable for charts representing small areas, but can be significant in a general charts like Ca4700 or US13003.
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