Calculating CPA

[Dockhead]

In another thread, some disagreement arose about how to calculate CPA in a crossing situation. I was surprised - this is a pretty basic, uncontroversial navigational technique. Here's what was written:


"The sailboat starts out, with bow perpendicular to, and 180 ft away from the port stern corner of the lead ship.

"This places the sailboat 1.25 nm forward of the following ship.

At 90 degrees from the ships course, the sailboat has to travel 320 ft, to cross the original 180 ft to the port course line, the 100 ft beam of the ship, and clear the starboard course line with it's 40 ft length, in advance of the following ship. "


Anyone care to discuss what is CPA, and how do we calculate it?

[Dockhead]

Here is a very good reference on how to do it by plotting:

http://msi.nga.mil/MSISiteContent/St...RNM/310ch3.pdf

the way many of us learned it in basic radar or navigation courses.

[UNCIVILIZED]

I'm not understanding the specific CPA problem that you posted, but CPA's are easy enough to work out with pencil & paper on a maneuvering board. Or for that matter, on a radar screen using a popsicle stick & a grease pencil.


If you Google or Bing "maneuvering board exercises", or "maneuvering board practice problems", you should get plenty of hits, as well as links to websites with instructionals on how to calculate same.