Wednesday, January 10, 2007

Venturi Effect To Blame In Newport News Collision?

Cross-posted from The Stupid Shall Be Punished:

The Navy has apparently come up with an explanation for how the USS Newport News (SSN 750) hit the Japanese tanker M/V Mogamigawa that actually makes some sense and may spare the CO his career. From The Virginian Pilot:

The submarine Newport News was submerged and leaving the Persian Gulf when a mammoth Japanese oil tanker passed overhead at a high speed, creating a sucking effect that made the sub rise and hit the ship, the Navy said Tuesday.
That is the preliminary finding of Monday's collision between the Norfolk-based submarine and the Mogamigawa, a 1,100-foot-long merchant ship displacing 300,000 tons.
Both were southbound, crossing the busy and narrow Strait of Hormuz while heading into the Arabian Sea.
"As the ship passed over the sub, it ended up sucking the submarine into it," said Lt. Cmdr. Chris Loundermon, a spokesman for Submarine Force in Norfolk.
"It is a principle called the venturi effect," he said.
I'm sure they'll have to do some calculations to make sure this is plausible, but if the ship reported they weren't trying to come to PD, and assuming the ship control party didn't just randomly lose depth control at the worst possible time, this probably sounds as reasonable as anything else. There are lots of forces involved in operating a submerged submarine, and an upward force from the venturi effect is one submariners don't practice a lot in the dive trainer. (My old boat once popped to the surface because we hit a patch of colder water due to hitting the boundary where a river was discharging into open ocean, but that's another story...)

Assuming this explanation is true, the question now becomes: will the CO and crew be exonerated? The Sub Force has already shown that uncharted seamounts aren't a justification, and they do always warn you about the dangers of being "zoofed" -- submarine slang for having a surface ship pass directly over your position. The reason I always learned was that you didn't want to have someone above you in case you had to emergency blow, but it could be that there's a warning about the Venturi Effect buried in some tech manual. If there is, the CO is probably sunk. If this truly is a "first time we've thought of it" thing, though, the CO and crew might be fine -- unless, of course, all the "helpful" squadron, group, and force types who pour onboard a boat after an incident find anything that shows the Newport News wasn't operating completely in accordance with approved procedures...

In any event, the sub is apparently heading back to Bahrain for an inspection. If the boat really was sucked up into the tanker's stern, and if the tanker's propellers got ahold of the sonar sphere, we could see some interesting pictures.

Staying at PD...

Update 2014 10 January: Skimmer Dave over at The Galloping Beaver has a great explanation of the Venturi Effect near the bottom of this post. An excerpt:
I've handled a VLCC tanker. They leave a huge hole in the water, particularly when loaded to the marks and up to a typical service speed of 17 knots. That hole gets filled with water rushing into the cavity created at the stern of the ship. In simple terms the water filling the cavity rushes down from three sides and creates a force which moves in the same direction of the ship and operates like a swirling vortex, sucking everything from both sides of the ship down, once it reaches the stern and up and towards from the water column below. At the risk of over-simplifying a description, it's very much like effect of a vacuum cleaner nozzle.
A submarine too close to that vortex, with little warning, would be sucked into the filling cavity and propelled in the direction of the stern of the surface ship. The thing about it is, I've actually seen it happen...
You'll have to head over there yourself to read about what he saw.


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