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Old 07-08-2009, 23:37   #1
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USNA Short Course: Chapter Two - Radian Rule

This is a Bowditch classic. I won't go into the deep math of things, (the broccoli) I'll just give you the end rule (dessert). This is a game of approximation. All things are rounded off so you can figure things out in your head as you're passing time on the mid-watch.

Some givens...
Round of a Nautical Mile to 2000 yds.
D = Distance
S = Lateral Separation
A = Angular Difference in degrees

D/60 = S/A
DA/60 = S
etc... you can form it to fit your purposes.

Situation 1. You're 6 miles from your next port sailing due North. The wind shifts and you have to sail 010. How far off course will that make you.

S is what we're figuring out. So (6 x 10)/60 = 1 nm. Use whatever units for distance are handy, just keep it consistent.

What's really handy about that is that when you're doing some "what if" work when trying to reach a destination, just figure out the distance for 1 degree. So the distance divided by 60. 6 miles = 12000 yds. 12000/60 = 200 yds. Then as you choose your course you can multiply by the degrees off course. If you can only make 015, you have 15 x 200 yds = 3000yds - 1.5 NM.

Situation 2. Turn that around and make is something you want to avoid. We're looking at the chart and see that the lighthouse on the starboard bow at 030, 3 miles away, marks a point with hazardous waters. Lets say we want to give it wide berth and stay 1000yds (half mile) from the thing. You can get the plotter going or just do some quick head math.

3 Miles = 6000 yds. Divide by 60 = 100yds separation for each degree off of 030. 1000yds desired separation divided by 100 yards per degree = 10 degrees. So we need to steer 020 or left of that to stay clear .

Sure it's not all 6 and 12 mile figures, but with sailing there's usually time for a little long division in your head. Also this gets less accurate as the angle gets larger because in reality each degree would be slightly larger separation. But under 45 degrees angle the difference is negligible and we're just trying to get in the ballpark.

Any of this making sense? Not sure if this is news to anyone, but please post comments.
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Old 23-10-2017, 13:17   #2
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Re: USNA Short Course: Chapter Two - Radian Rule

Such a good post and nobody has replied? What am i missing here?
jon
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Old 23-10-2017, 14:48   #3
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Re: USNA Short Course: Chapter Two - Radian Rule

These are simple triangles used for solving wind, set/drift, current etc. They are on USCG license exams.
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Old 23-10-2017, 17:40   #4
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Re: USNA Short Course: Chapter Two - Radian Rule

This is indeed a pretty good approximation. Assuming that the Lateral Separation is measured at the closest point of approach to the point at the end of "Distance" , the error looks like this:

5°: 4.4% short
10°: 4.0% short
15°: 3.4% short
20°: 2.5% short
25°: 1.4% short
30°: exact
35°: 1.7% long
40°: 3.7% long
45°: 6.1% long

And as mentioned, beyond 45° the error gets much worse. But for angles under that the precision is certainly useful.
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Old 24-10-2017, 04:00   #5
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Re: USNA Short Course: Chapter Two - Radian Rule

Quote:
Originally Posted by David M View Post
These are simple triangles used for solving wind, set/drift, current etc. They are on USCG license exams.
But, i assume that most of us will never take or see or study for the Coast Guard exam. That is why this is so important to be posted here. It can teach someone like me!
blessings
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