Cruisers Forum

Join CruisersForum Today

Thread Tools Rate Thread Display Modes
Old 02-07-2006, 06:45   #31
Moderator Emeritus
GordMay's Avatar

Cruisers Forum Supporter

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 29,326
Images: 240
Yes, you measure (/w dividers, etc) in any direction, then scale it to nm from min. of Latitude .

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

GordMay is offline   Reply With Quote
Old 02-07-2006, 11:23   #32
Senior Cruiser
SkiprJohn's Avatar

Cruisers Forum Supporter

Join Date: May 2006
Location: Kea'au, Big Island, Hawaii
Boat: Cascade, Sloop, 42 - "Casual"
Posts: 14,169
"a mile a minute."

Always use the latitude scale on the right or left on your chart for your DR position because it remains constant. Never use the top or bottom longitude scale because it represents distance changes the further north or south you go.


SkiprJohn is offline   Reply With Quote
Old 03-07-2006, 07:14   #33
Registered User
markpj23's Avatar

Join Date: Nov 2005
Location: Black Hills, SD
Boat: Now Boatless
Posts: 1,148
Images: 47
Originally Posted by S/V Elusive
if you use the latitude scale in the longitudal plane, don't you get an accurate measurement of nautical miles for longitude?
Yes - assuming you have a Mercatur projection chart. Be careful with this for other chart types.

markpj23 is offline   Reply With Quote
Old 03-07-2006, 09:09   #34
Senior Cruiser
Talbot's Avatar

Cruisers Forum Supporter

Join Date: Jun 2004
Location: Brighton, UK
Boat: Privilege 37
Posts: 3,578
Images: 32
Further detail on the IALA buoyage standards.

UKHO charts all include an arrow in the charts showing the IALA direction, as unless you understand the direction or origin of the system, you wont know which one goes where.

Personally I dont worry about it greatly. I orient myself to where I am on the chart, and where the deeper water is, then worry about what buoys I might meet purely so I dont run into them.

Dont forget a buoy is not a statement of location, but merely a guide. there have been lots of cases of buoys being in the wrong place, and if your charts are not exactly to the latest notice, they may have moved it deliberately.
"Be wary of strong drink. It can make you shoot at tax collectors - and miss."
Robert A Heinlein
Talbot is offline   Reply With Quote
Old 04-07-2006, 09:59   #35
Registered User
Sunspot Baby's Avatar

Join Date: May 2003
Location: New Bern, NC
Boat: Prout Manta 38' Catamaran - Sunspot Baby
Posts: 1,521
Images: 14
Right On!
Left Off!

A handy way to remember that when turning left, the compass reading gets larger, and the reverse is true turning left.

Sunspot Baby
She took my address and my name
Put my credit to shame
Sunspot Baby, sure had a real good time
Bob Seger
Sunspot Baby is offline   Reply With Quote
Old 13-07-2006, 12:06   #36
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,333
Originally Posted by markpj23
Angle on the Bow - Estimating CPA

Quick trick for computing closest point of approach (CPA) for another vessel. You can do this in your head easily and with good accuracy.

Kown officially as the Radian Rule, I also learned it as "Target Angle" and "Angle on the Bow." Surface Navy types use Target Angle, submarine service uses Angle on the Bow - since EVERYTHING is a "target" to submariners

I use Angle on the Bow, so here we go. Observe the other vessel and estimate how many degrees off of his bow YOU are - assuming that you are standing on the other vessel's bow. Another way to say this is "how many degrees of the other vessel's bow can you see?" The answer will be either Port or Stbd, in a range from 0 to 90 degrees.

For a vessel dead ahead on a reciprocal course to yours, the Angle on the Bow will be zero. A vessel dead ahead running perpendicular will be either port or stbd 90 angle on the bow.

So how do we compute distance at CPA? Take the Angle on the Bow as a percentage X the present distance = Distance at CPA.

So if you are 10,000 yards apart with a Port 20 deg Angle on the Bow, the distance at CPA will be 10,000 * .20 = 2,000 yards. This is a good rough estimate you can do in your head very quickly.

For those exacting types, you actually need to divide the angle on the bow by 90 degrees to get the exact multiplier. For example, a 45 degree angle on the bow means that you will close to exactly half the distance at CPA.
45 deg AoB / 90 degrees = .5
Distance at CPA = Current Distance * .5
Distance at CPA = 10,000 * .5 = 5,000 yards

This will NOT work if the other vessel is dead in the water - you will quickly notice the dreaded Constant Bearing, Decreasing Range scenario
Sorry Mark, your math is all wonky. If this is what the USN teaches its watchkeepers, it sure explains a lot

While I agree that determination of CPA is dependent upon the angle you're on the other vessel's bow, you must also take into account the relative bearing of the other vessel from you. Take your example of a vessel 45 degrees AOB at 5 NM. You have the CPA at 2.5 NM. If that vessel is 45 degrees on your bow, on a converging course at the same speed as you, the CPA will be zero. (picture a vessel northeast of you heading due west at 5 knots, with you heading due north at 5 knots).

By the same right, the vessel at Port 20, if on a similar or reciprocal course will have a CPA equal to about one-third the distance (sin 20) or 3300 yards (rounded down for safety).

Radian rule, and the lesser known Sine rule, can be readily used on stationary and near-stationary objects. Radian rule subscribes that 6 degrees will subtend one/tenth the distance. That is if you have an object 6 degrees off your ship's head, you will pass at a CPA equal to one/tenth of the current distance. Example, buoy is bearing Red 6, range 5 NM - the CPA is 5 cables or 1/2 NM on your port beam. You can interpolate or extrapolate for other relative bearing amounts - ie. 12 degrees is 2x 1/10 the distance, 3 degrees is one/twentieth the distance, etc.

Sines are much more accurate - the sine being the opposite over the hypotenuse, where the hypotenuse is the distance to the other ship/object, the opposite will be the CPA. Sin 45 is approximately .7 - range is 10,000 yards 45 degrees on your bow, then the CPA will be .7 x 10,000 = 7000 yards. Radian rule gets progressively inaccurate as the bearing increases - 6 into 45 = 7.5 ; 7.5 x 10,000/10 = 7500 yards CPA.

Your 3 min rule, I know as the 631 rule. At speed X, you travel X cables in 6 min, X00 yards in 3 min, X00 feet in 1 min. (a cable is 200 yards or 1/10 NM).

As you can probably see, it is quicker and easier to determine CPA of a stationary object, as with a moving vessel, you must first calculate CPA for the vessel relative to you (assuming it is stationary), then calculate CPA for you relative to the other vessel (assuming you are stationary), then average them weighting for the faster vessel. Really it is only a guesstimate; an accurate determination can only be made by systematic plotting. If you don't have an ARPA, you can use plotting tables, or plot directly on your chart.

The final caveat is that this doesn't account for set - assuming other boats underway will be set as well, you can make a reasonably accurate assessment of CPA based on angle from your ship's head, but for stationary objects, you would be better off using bearings relative to your Course Made Good (or course over ground).

IIRC, Bowditch explains these tricks. Now does anyone want to learn about 'doubling your angle on the bow'?


Lodesman is offline   Reply With Quote

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
Yanmar Tips GordMay Engines and Propulsion Systems 18 29-07-2012 05:04
Prout 37 tips and tricks? exposure Multihull Sailboats 1 21-06-2004 19:52

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 00:16.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.