Overtaking vs. Crossing -- Navigational Thoughts From An Engineer
As we learn more about the collision between the USS Philadelphia and M/V Yaso Aysen, I've been wondering how the "blame-apportionment" will turn out now that the Navy has relieved Captain Oxholm. Based on open source information I trust, it appears that the Yaso Aysen's port side hit the Philly's starboard side while the Philly CO was asleep, about 0200 local. The thing that confuses me is that most reports say the merchant was outbound from Bahrain for the UAE, but the Philly was inbound; this wouldn't support an "overtaking" situation, unless the merchant decided to turn around. I'm wondering if, instead of an overtaking situation, it turns out that it was closer to a "crossing" situation -- the merchant had steamed north out of port, then turned towards the southeast when she got to "clear" (no navigation hazards) water in that direction. In this case, the Yaso Aysen might have been approaching from Philly's quarter, in such a way that both vessels thought they were the "stand-on" vessel. This probably isn't what happened, but hear me out as I think aloud:
The one story that gives Yaso Aysen's port of origin was wrong about everything else, but it could be right about this. It says it got underway from "Asry, Bahrain", which could very well be the Arab Shipbuilding and Repair Yard, located south of the airport at the entrance to the harbor. (You'll need to click twice on that last link to get a map of the shipyard's location.) If, for some reason, they headed north after clearing the harbor, and the sub was coming in from the northeast, the merchant would have had to turn across the sub's path at some point to head towards the southern Gulf. If, in fact, the Yaso Aysen did get underway from Bahrain, it would make a classic "overtaking" situation unlikely in this case.
According to the "Rules of the Road", a vessel is overtaking another if they are coming up on the other vessel at greater than "two points abaft the beam", or within 67.5º of the stern (Rule 13). However, if the vessels aren't in a head-on situation, and one vessel is not running up the other guy's stern, it's a "crossing" situation (Rule 15). This rule states:
"When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel."
If it turns out that this was a crossing situation, the Philly would have been the "give-way vessel" (since the merchant was to her right) and would have been required to maneuver to avoid a collision.
Now, the Philly would have clearly known the exact bearing to the merchant (looking through the scope, as well as on radar if she was using it, as I suspect she was). The crew of the Philly also knew their exact course, and by doing a little mental gym, could have easily determined whether it was a crossing or overtaking scenario. Let's assume for a moment that this was the case, and they determined that it was an overtaking situation, by a few degrees. The CDO makes a determination that the sub is the stand-on vessel, and the boat continues on the same course and speed.
At this point we need to interject some information on "sidelights". All big ships have sidelights, which are visible from directly ahead to "two points abaft the beam"; the Rules of the Road say (Rule 21(b)):
"Sidelights" means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of horizon of 112.5 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on the respective side."
Likewise, a "stern light" is on the stern (duh!) and covers the area not covered by the sidelights. In theory, the only time you'll see the sternlight and a sidelight is if you are at a relative bearing of exactly 112.5º on either side of the bow from the ship you're observing. (Likewise, the only time you'll see both sidelights is if you're looking directly down the target's bow.)
Back to the real world. A submarine's sidelights are actually visible for more than the specified 112.5º; how much more depends on the specific class. (It's the same with all ships, but subs normally have more of a "sidelight/sternlight overlap" in my experience.) Assume the driver of the merchant sees a green light off his bow. He looks at the radar, and it's a tiny little blip... hardly worth his notice. He could plot the course of the contact, but why bother? He's over 50,000 tons -- the "Law of Gross Tonnage" applies, in the mind of the merchantman. He figures the ship's so small it doesn't even have a sternlight. "He'll get out of the way", the merchant sailor thinks as he goes back to reading his magazine -- probably that really cheesy porn you find in Bahrain. He sees the green light, so to him, "green means go". (That's the way sidelights are designed -- if you see the other guy's green light and no sternlight, you're the stand-on vessel; if you see the red light, you're the give-way vessel.) By the time both ships figure out the other isn't changing course, it's too late.
Bottom line: I could imagine this being a situation where both vessels thought that they were the stand-on vessel. The merchant in this case would be guilty of violation of both Rules 13(c) and 17(b), but it could be enough to convince an Admiralty Court to blame both vessels.
Staying at PD...