Thought it might be nice to share some shortcuts related to navigation
and seamanship. Looking forward to hearing what tricks others have learned over the years...
Angle on the Bow
- Estimating CPA
Quick trick for computing closest point of approach (CPA) for another vessel. You can do this in your head
easily and with good accuracy.
Kown officially as the Radian Rule
, I also learned it as "Target Angle" and "Angle on the Bow." Surface Navy
types use Target Angle, submarine service
uses Angle on the Bow - since EVERYTHING is a "target" to submariners
I use Angle on the Bow, so here we go. Observe the other vessel and estimate how many degrees off of his bow YOU are - assuming that you are standing on the other vessel's bow. Another way to say this is "how many degrees of the other vessel's bow can you see?" The answer will be either Port or Stbd, in a range from 0 to 90 degrees.
For a vessel dead ahead on a reciprocal course to yours, the Angle on the Bow will be zero. A vessel dead ahead running perpendicular will be either port or stbd 90 angle on the bow.
So how do we compute distance at CPA? Take the Angle on the Bow as a percentage X the present distance = Distance at CPA.
So if you are 10,000 yards apart with a Port 20 deg Angle on the Bow, the distance at CPA will be 10,000 * .20 = 2,000 yards. This is a good rough estimate you can do in your head
For those exacting types, you actually need to divide the angle on the bow by 90 degrees to get the exact multiplier. For example, a 45 degree angle on the bow means that you will close to exactly half the distance at CPA.
45 deg AoB / 90 degrees = .5
Distance at CPA = Current
Distance * .5
Distance at CPA = 10,000 * .5 = 5,000 yards
This will NOT work if the other vessel is dead in the water
- you will quickly notice the dreaded Constant Bearing, Decreasing Range scenario