NavList:

A few thoughts on my previous query re CPA.
Viewing the display of a radar in the relative motion mode own vessel appears as if fixed and if the target vessel has motion it is shown moving across the display in the direction DRM(Direction of Relative Motion). If this DRM is posing a threat by aiming at own vessel you need to alter course so that the targets new DRM will pass clear. For a vessel not fitted with ARPA (automatic radar plotting aid) the practice is to construct a ‘plot’ using the relative velocity triangle and from this determine change of course required to achieve the desired DRM to avoid the threat. In waters with restricted sea room to manoeuvre and no time to complete a ‘plot’ a guesstimate of the amount of alteration is made. If own speed and targets speed are the same the course alteration is twice the difference of the initial DRM and the desired DRM. I wasn’t sure if this was a unique situation or other multipliers could be used as a ‘rule of thumb’ when own vessels speed is greater than the targets.
If the targets true course is constant then the angle between this and the desired DRM should be proportional for different values of change of course for a consistent value of u(own vessel speed)/v(target vessel speed) ?
I have attempted to find multipliers for difference of DRM’S (initial and desired) and some values of u/v. (attachment) which appear to give a ‘rule of thumb’ result for a target vessel in the own vessel’s forward starboard hand quadrant irrespective of determining the targets course.
Difference DRM’s x 1.25 when u=3v; x 1.5 when u=2v ; x 1.7 when u=1.5 v; x 2 when u=v.

Regards,

Kevin

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