Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 08-08-2009, 00:37   #1
Registered User
 
Down2TheC's Avatar

Join Date: Aug 2009
Location: Lake MI
Boat: Tartan 37
Posts: 83
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.
__________________

__________________
... and all I ask is a tall ship and a star to steer her by...
Down2TheC is offline   Reply With Quote
Old 23-10-2017, 14:17   #2
Registered User
 
longjonsilver's Avatar

Join Date: Nov 2014
Location: montreal
Boat: foilcat 14
Posts: 135
Re: USNA Short Course: Chapter Two - Radian Rule

Such a good post and nobody has replied? What am i missing here?
jon
__________________

__________________
Canadian Basic with Honours VE0XYZ
longjonsilver is offline   Reply With Quote
Old 23-10-2017, 15:48   #3
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,143
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.
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 23-10-2017, 18:40   #4
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,803
Images: 4
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.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 24-10-2017, 05:00   #5
Registered User
 
longjonsilver's Avatar

Join Date: Nov 2014
Location: montreal
Boat: foilcat 14
Posts: 135
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
jon
__________________
Canadian Basic with Honours VE0XYZ
longjonsilver is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
USNA Short Course: Chapter One Down2TheC Seamanship & Boat Handling 20 17-08-2009 13:23
USNA Short Course: Chapter Three - 6 Mile Rule Down2TheC Seamanship & Boat Handling 11 08-08-2009 19:38
New chapter in our lives bayoubouy General Sailing Forum 3 18-04-2008 06:26
brand new and about to start the next chapter peter d christensen Meets & Greets 7 12-03-2007 04:14
Another Sad Chapter for the Stuemer Family Knottygirlz Cruising News & Events 5 16-08-2006 09:07



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 01:36.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.