Good P&I Bulletin on Bridge Watchkeeping and Collision Avoidance

Bridge Watchkeeping and Collision Avoidance Japan P&I Club

This bulletin’s description of watch officer behavior when evaluating traffic for collision avoidance matches what I’ve observed watching less experienced watch officers.

A navigator using an ARPA is tempted to make an alteration of course which will simply bring about a prediction of the safe passing of the other ship by achieving the required mathematical result on the computer. That is to say, he will make an alteration of course which results in the predicted CPA distance, as calculated by the ARPA, being equal to the minimum required CPA distance. This may, or equally may not, result in a change in the aspect of the masts/ cranes or navigation lights, or as a change in target trail on a radar screen which can be seen by the navigator of the other ship. If the COLREGs are adhered to, then the alteration should be large enough to be observed visually or by radar. It is important to remember that an alteration of course of less than 10° is unlikely to satisfy the requirement.

It boils down to making a course change large enough to be readily apparent to other ships. Mates like to watch the calculated CPA on the ARPA and tweak the course to get the required minimum.

How did we ever manage before radar?

They most not have survived before GPS, either LOL

Here’s another interesting section:

In the days before ARPA, in almost all clear weather encounters with shipping traffic, navigators relied upon visual bearings of an approaching ship to determine whether or not there was a collision risk. In the appraisals made in those days, the value for CPA was assumed to be either “zero” or “sufficient” depending upon the rate of change of observed relative bearings. One of the advantages of such a visual assessment is that the visual check requires the navigator to become physically involved with the dynamics of the situation

To become physically involved with the dynamics of the situation, I’ve mentioned this a couple times before.


The karate chop method for visual bearings or wind direction.