Nowadays most ships are sailing with an Arpa ( Automatic Radar Plotting Aid). This is a tool which helps the navigational officer of the watch to determine if there is a possible danger of collision with another ship.
On LinkedIn there was an discussion in March 2015 about a case mentioned in MARS 201504. The discussion was about regulation 7 of the Colreg. At LinkedIn I could not upload the picture of my handmade plot, so I use it in this small blog.
How to Plot 2 Ships to Determine a Collision Course:
- Put your own ship head-up ( in the video the ships course is 99 degrees and 11.5 knots)
- Bearing of the other ship is 60 degrees ( this means at 99-60 = 39 degrees relative from your port bow)
- Draw the line and at 10 mile (is 20 cm in the video) the other ships course will be drawn
- Other ship course is 202 degrees ( 202 – 99 = 103 degrees relative)
- Speed will be drawn in a distance of 6 minutes
- Six minutes is 1/10 * speed and therefor an easy time frame for calculations
- Own vessel is 11.5 knots is 23 mm (same scale as 10 mile is 200 mm: 200 mm / 10 mile * 11.5 mile/hour * 6 minutes / 60 minutes = 23 mm )
- Other ship is 13 knots is 26 mm ( 200 mm / 10 mile * 1.3 mile = 26 mm )
- In the video you can see that the other ship will cross your bow at approx 0.5 mile
In the text of MARS 201405 you can read that the other ship was changing course at 2.5 mile.
Information given by the plot:
The plot does give enough information to determine if there is a possible danger of collision between the two ships. It also give you information about the bearing from one ship to the other. So despite the information given in the MARS 201405 is not much, still a lot is given by a manual plot.
As there is nothing known from the second ship, one can not say if the distance of 2.5 mile is a lot or not. Maybe the course change was started at 3 mile but only noticed at 2.5 mile. From colreg point of view, the conclusion is that a bow crossing of 0.5 mile is not seen as safe practice. In some coastal waters 2 miles crossing can be considered as close, but sometimes unavoidable. At high seas where there is enough space to manoeuvre the distance should be much higher, probably up to 5 miles.