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On 6/19/2017 9:43 PM, Frank Reed wrote:
> Yes. I suppose so. If the container ship was on course 070° (which the
> AIS indicates at the suggested collision time of about 1629 UT) and
> which corresponds nicely with that track south of Oshima Island, then if
> the impact angle was about 45° from starboard aft (sorry, I'm not
> "salty" enough to describe this right)....

Perhaps from the Department of Redundancy Department for most, but it
strikes me that the Traditional Navigation discussion group is comprised
of many non-sailors. For example, discussions of Lewis and Clark/surveying.

For those members who do not have a copy of "The International
Regulations for Preventing Collisions at Sea" (COLREGS) handy a few
definitions might shed some light on what is being discussed in regards
to the orientation of the ships.

A ship is considered to be "overtaking" if she is 22.5 degrees abaft of
abeam, and is required to keep clear.

The beam is the waist or widest part of the ship, generally about 1/2
way between the bow and stern.

Abeam is 90 degrees to the keel (a line fore and aft dividing the left
and right sides of the vessel). Mostly.

Abaft is towards the rear/stern of the vessel.

A crossing situation is exactly what the name implies; not overtaking
and subject to roughly the same rules as a 4-way stop sign in the US.

Given the above I have questions regarding 22.5 degrees abaft of abeam
rule that some of our Navy, professional mariners or sea lawyers can
address.

Reducing the argument to the absurd consider overtaking in the case of
an aircraft carrier and a 42' power vessel on a possible collision
course. The smaller craft's lubber line is at a constant angle of 113
degrees to the carrier's lubber line. At one stage she could be pointed
at the carriers bow, hence not 22.5 abaft of abeam. In the next phase
she is pointed amidships so is overtaking. In the finally phase she is
pointed just forward of the carriers stern, so overtaking.

Common sense would suggest the change of relative bearing of a given
point on the carrier would have the 42' craft pass safely behind given
enough distance between them (or get beat up in the carriers prop wash).

Now let's apply this to two large ships like the Fitzgerald and ACX
Crystal. Which part of the crystal would have to be oriented 22.5 abaft
of abeam to be deemed overtaking? The bow, the beam, the stern or the
lubber line?

   

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