Plotting an intercept course

[ldomustang]

Does OpenCPN have a feature that will allow me to plot an intercept course to another vessel?

[SeanPatrick]

If it doesn't, and if you're interested, I can post instructions on how to do it on paper. It's simple and doesn't take long.

[nohal]

No, there is not such a feature. Just out of curiosity - what is "another vessel"? An AIS target? Something else?

[C420sailor]

Time to dust off a maneuvering board!

Somewhere, an old surface warfare officer’s ears are burning.

[SailRN]

Originally Posted by SeanPatrick

If it doesn't, and if you're interested, I can post instructions on how to do it on paper. It's simple and doesn't take long.

I'd love to see how to do this!


Thank you.

[requiem]

Until Sean posts, here's one I happened to run across just before noticing this thread:

https://cdn.fs.teachablecdn.com/1k2BDPlQ3KyPY9XPPRDZ

It should pair well with this Youtube playlist which I found helpful, and I now see they're from the same person.

[goboatingnow]

Originally Posted by requiem

Until Sean posts, here's one I happened to run across just before noticing this thread:

https://cdn.fs.teachablecdn.com/1k2BDPlQ3KyPY9XPPRDZ

It should pair well with this Youtube playlist which I found helpful, and I now see they're from the same person.

That’s an app for that !!

[SeanPatrick]

Originally Posted by SailRN

I'd love to see how to do this!


Thank you.

[The first time I posted this, I found some errors. Here's attempt #2.]


In the attached image, I've used a maneuvering board. But you can use a plotting sheet, graph paper or even blank paper. You just need a way to measure angles, such as a protractor or compass rose, and a way to measure distances, such as a ruler or chart scale and dividers.

Let's say there's another vessel at a bearing of 140° at a distance of 3 NM. We've determined* that its true course and speed is 280° @ 5 kts., and we'd like to intercept it. We've found that we can also make 5 kts. in a southerly direction, so we'll use that speed†.

First, we plot the other ship's position relative to us (the red square in the image). Then, we decide what relative motion we'd like the other ship to make. Since we want to intercept it, we want it to move on the reciprocal of its bearing from us, 320°, so that it will appear to move straight toward us.

Next, we plot the other vessel's true speed and course as a line extending from the center of the plot (the "top" of the triangle in the image). This is a relative motion plot and is completely separate from the plot of the ship's position. Any convenient scale can be used for both plots and they don't have to be the same.

Then, we draw a circle centered on the plot and with a radius equal to our desired speed, 5 kts. We then draw a line in the direction of the desired relative motion, 320°, which ends at the end of the other ship's true motion vector (the "left side" of the triangle in the image). We draw this line long enough that it also intersects the speed circle we drew earlier.

Finally, from the point where the relative motion line intersects the speed circle, we draw a line back to the center of the plot (the "right side" of the triangle in the image). This is our true course to make good to intercept the other vessel at a speed of 5 kts., about 189° in this case.

If we measure the length of the relative motion line, we see that the other ship will appear to close on us at 7.1 kts. At a distance of 3 NM, this means that our CPA (closest point of approach) will occur in about 25 minutes and - if we do nothing else - will be a collision.

As others have written: there are instructions on how to find this and many other solutions online. I recommend the Radar Navigation and Maneuvering Board Manual, Chapter 6. Also as stated: there are apps that can do all of these calculations for you. But, as I wrote, it can quickly and easily be done on a sheet of paper, too.


* There are ways to determine true course and speed of another vessel using this method. See manual linked above.


† One can also find the course to make good and speed to use for the fastest intercept time if speed is not limited. However, speed limitations can often prevent a direct intercept. In that case the distance, bearing and time of the closest possible approach can still be determined.

[SeanPatrick]

Ugh ... third time's a charm:


Course to make good: 180°
Relative Speed: 7.6 kts.
Time to CPA: 24 min.


If I had done this on paper instead of trying to do it on my computer, I probably wouldn't be making so many mistakes.

[ldomustang]

What do I mean by "another vessel"? From a maritime law enforcement perspective, another vessel could be an AIS position OR a radar target that is not broadcasting its AIS as required by UNCLOS. I do a lot of training in Africa to teach Navy and Coast Guards how to conduct interdictions of suspected illicit traffickers at sea.

I'm a prior US Navy guy and understand Moboards very well.

[ldomustang]

SeanPatrick. You referred to some Apps that can do this? Can you elaborate?

[SeanPatrick]

Originally Posted by ldomustang

SeanPatrick. You referred to some Apps that can do this? Can you elaborate?

The images I posted are from just such an app. It's called MoBo (short for maneuvering board) and the only place I know of to get it is the Subsim Radio Room Forums. It has a feature that will do the vector calculations for you. (I should have taken advantage of that.)


The Radar Plotting app for Android will calculate CPAs and simple maneuvers. I'm sure there are others.

[C420sailor]

https://msi.nga.mil/api/publications....pdf&type=view

US Navy Maneuvering Board Manual

Page 66 (of the manual, not the PDF) is where maneuvering boards start.

[Dsanduril]

And if you want a crude method of doing this, and the vessel is using AIS, then turn on "show CPA" option and turn your own boat until it says you're going to crash. It's crude, but your brain can probably give you a reasonable first order estimate and then you go from there.

[ChrisJHC]

Originally Posted by Dsanduril

And if you want a crude method of doing this, and the vessel is using AIS, then turn on "show CPA" option and turn your own boat until it says you're going to crash. It's crude, but your brain can probably give you a reasonable first order estimate and then you go from there.

Assuming you want to “intercept” them for friendly reasons, make sure the other vessel is aware of what you’re doing if you use this method.
Otherwise their AIS alarms will be going mad and they will be panicking!