Keeping the blogosphere posted on the goings on of the world of submarines since late 2004... and mocking and belittling general foolishness wherever it may be found. Idaho's first and foremost submarine blog. (If you don't like something on this blog, please E-mail me; don't call me at home.)

Friday, September 23, 2005

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...


Blogger Skippy-san said...

Per your recent request, fresh babes are posted. I even posted an extra one , just for you.

9/23/2005 1:45 AM

Blogger Bubblehead said...

Sweet... you're too kind, skippy.

9/23/2005 5:21 AM

Blogger PigBoatSailor said...

This comment has been removed by a blog administrator.

9/23/2005 7:50 AM

Blogger Vigilis said...

A very thoughtful, fact-filled exposition.

Has the thought gone through your mind of a possible absence from the bridge? (CO charges would have been other than UCMJ 110, [92, 98]right)?

9/23/2005 11:42 AM

Blogger PigBoatSailor said...


I don't think the CO not being on the bridge would have changed the charges. CO's are not expected to be on watch at all times. What other violations do you think would possibly apply here?
Art 92(3) "derelict in the performance of his duties"?
Art 134 "all disorders and neglects to the prejudice of good order and discipline ... all conduct of a nature to bring discredit"?
I really don't think either of those really apply, and those are the only other ones I see that could really be brought to bear.

But I digress...

This article states that the Merch was heading into the UAE. I put together this rough map (sorry it is not better, not enough time to find a better one on lunch break). It seems plausible that the Merch was heading somewhat north and east to get around Qatar to head into the UAE. Where the Philly was coming from is not stated, but let's make a guess:
If she was heading in from the north, she would have to try and cut south past Al Hidd to pull into Manama (where I assume she was heading). If the Merch cut too far north the crossing scenario you describe is possible, although for slightly different reasons. However, it is also possible that the sub tried to take evasive action (too little too late, obviously), and that is what caused the strike on the stbd side, so in that case, she could have been coming from any direction, really, south or east, or even just loitering outside of everyone's territorial waters until daylight - waiting until sun up to station the manuevering watch and head into port. In this case she might have just been in a night steaming box, and could very well have been overtaken - midwatch, in a box, people's attention might wander and not notice the BIGA$$ ship coming up behind them.

I guess what I am saying is, while your scenario is definitely possible, I don't think we have enough information to pin down exactly what the scenario was.

9/23/2005 2:14 PM

Blogger Vigilis said...

The CO was sleeping, PBS. In the back off my mind, this could not happen if the OOD were on the bridge (short of the sad tale I posted Sept 5th at UQnm). Now, see what I'm saying?

9/23/2005 2:39 PM

Blogger PigBoatSailor said...

Vig, what are you talking about? The CO can sleep if the ship is surfaced and the OOD is stationed in the bridge. Try making a 30-hr surface transit without him sleeping. He is not only allowed to do so, he is expected to. It is why we have a CDO, so he can stand in the COs place and the man can stay rested, as he is required to.

9/23/2005 3:35 PM

Blogger Vigilis said...

PBS, let me review, in my day the guy standing in for the CO had to be on the bridge at all times. Was this guy on Philadelphia's bridge the whole time? I do not see how he could have been. Yes, the CO was asleep thinking watchful eyes were in his place.

9/23/2005 3:41 PM

Blogger PigBoatSailor said...

Not true anymore.
Our CDO regularly stayed belowdecks, so he could keep an eye on Radar, VMS, SONAR, etc. The only time the CDO or CO was required in the bridge was in restricted waters, ie. Piloting Party or Maneuvering Watch.
As neither of these parties were likely stationed 30nm away from land, there was no negligence by not having the CO or CDO in the bridge.

9/23/2005 7:49 PM

Blogger Bubblehead said...

PBS is right; in my experience, the CDO normally came up to the bridge every couple of hours for a few minutes, then spent most of his time in Control.

9/23/2005 8:37 PM

Anonymous Bernie said...

In a nutshell, the blame for this collision could go either way.
Whomever can afford the best Lawer is the one who can get off the hook.

9/23/2005 9:57 PM

Blogger Vigilis said...

PBS, Bubblehead, thank you. Now the collision appears possible. Before you explained that CDOs could absent from the bridge, it just made no sense. In my opinion that particular relaxation of the watch is not well-considered, and must be news to more than a few older subvets.

Bernie, interesting you would say that, too.

9/24/2005 12:04 AM

Blogger PigBoatSailor said...

Heh, Vig, I'll debate with ya wether that is a good watchstanding decision some other time. Until then, I'll just say some of my best memories on the boat are those of just me and a lookout up in the bridge =)

9/24/2005 7:34 AM

Anonymous questionable.dood said...

Rumor has it that the the Phil hit an anchored ship? Some on the safety center have put that out in training.......Remember, the FW planes had damage on the FRONT.
Just a question posted.....

9/24/2005 10:15 AM

Blogger PigBoatSailor said...


Only rumor. The Merchies owner would be screaming of that were true. Plus, the towed array fairing and the rudder and the screw were all damaged - lends credibility to the "getting run over" account.
Not that a sub running into an anchored ship is implausible. I am pretty sure it has happenned. I cannot remember who it was - Probably the Jax. She was the biggest bad luck boat around for a while there....

9/24/2005 10:44 AM

Blogger Bubblehead said...

I think that rumor got started when some of the initial reports said that the merchant "raised anchor and returned to port". After the collision, the ships remained locked together for over an hour while damage was assessed (this is SOP for a collision at sea) and the merchant dropped anchor during this time to keep the two boats from drifting.

9/24/2005 8:24 PM

Blogger loddfafnir said...

Wasn’t the J'ville. 82 was a merchant on the surface, 84 was a barge in tow on the surface and 96 was a merchant on the surface. Although I can only personally verify the last one, I believe all the other vessels were underway at the time.
I completely agree with you though about those times in the bridge, some great memories there.

9/25/2005 1:32 AM

Anonymous submarine.dood said...

I'll second that, the Naval Safety Center is putting out she hit a anchored ship. She had damage to the forward side of her fairwater planes. This one is pretty interestting.

9/25/2005 8:23 AM


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