A tugboat’s hull tears open against a tanker’s spinning propeller in a Texas ship channel.
Five crewmembers feel the deck lurch as water works into two compartments.
The captain turns for a bank outside the channel rather than risk sinking in deep water.
Nobody aboard is hurt, but the tug is taking on water fast.
Investigators would later trace the whole mishap back to one turn made only moments earlier.
So what actually pulls a spinning tug toward a ship more than ten times its own length?
What draws a turning tug toward a moving ship’s stern
As a large ship pushes through a narrow channel, water accelerating toward its propeller leaves a band of low pressure suction along the hull’s quarters. That suction is strongest near the inlet side of the propeller, and the forces it generates climb steeply as the ship’s speed rises. A small boat that drifts inside that band is pulled sideways toward the bigger hull rather than pushed away from it.
A tug’s defense against that pull is reserve power, the thrust it still has left after holding station. A boat already running close to its rated top speed has almost nothing in hand, so the Z-drives can be turned and the throttles opened without the hull actually moving away.
Which is close to what happened in the Corpus Christi Ship Channel.
A five boat escort and a turn at speed
The tanker Nisalah was moving inbound through the channel at close to 10 knots with five escort tugboats dispatched for the harbor assist toward Ingleside. The tug Mark E Kuebler held the starboard quarter assignment and met the ship near buoys 19 and 20. One tug was already made up to the tanker’s stern, while the others transited on either side without lines passed.
The Kuebler’s hawser and winch sat on its bow, and the chock it would use was set where the tanker’s hull curved in toward the stern. Its mate used the Z-drives to spin the tug clockwise through 180 degrees, planning to transit astern into position. Spinning a tug that size near a moving ship is routine on paper, but it puts the boat’s vulnerable end close to a very large propeller.
That is where the margin for error disappeared.
The hardest number in the investigation
According to the National Transportation Safety Board’s report on the January 2023 collision, the Kuebler fell back toward the tanker’s stern during the spin, and the mate increased power to regain the starboard quarter position. Running astern about 50 to 60 feet off the tanker, the tug reached 11.6 knots, just 1.4 knots below the 13 knots it was capable of at full speed, as laid out in the investigators’ report.
With little reserve left, the tug’s thrust could not overcome the forces drawing its stern inward, and the hulls met. The tanker’s propeller cut multiple gashes in the Kuebler’s stern, puncturing the aft peak tank and the Z-drive machinery room, and both spaces filled to the waterline. The propeller itself was damaged in the contact.
All six vessels had been moving through the channel at about 10 knots when the hulls touched.
Why the captain chose a bank over open water
The captain took the helm and at first believed the damage was confined to the tug’s mast and stacks. Then came a bilge high-water alarm, and a check of the machinery spaces found serious flooding in the Z-drive compartment. Facing a real risk of sinking, he intentionally grounded the tug on the bank outside the main channel.
None of the five crewmembers aboard was injured, and neither were the 27 people on the tanker, which carried on to its terminal and moored. A small hydraulic oil sheen, roughly 3 by 5 feet, appeared near the grounded tug and was picked up with absorbent pads.
The bill was not small: damage to the tug was estimated at $3 million and damage to the tanker at $3.9 million, close to $7 million between them.
What the finding means for busy harbor channels
The safety board’s probable cause was the maneuvering of the tug near the tanker’s starboard quarter, which left it drawn in by hydrodynamic forces it had insufficient reserve power to counteract at the speed both vessels were making. It advised owners and operators of Z-drive tugs to set speed limits for advanced maneuvers such as stern-first approaches, and to pass those limits to the masters and pilots they assist. The tug’s operating company afterward capped stern-first landings at 7 knots or less.
The episode rhymes with other close calls in tight shipping lanes, including an aircraft carrier in a crowded chokepoint far from Texas. The common thread is simpler than any one maneuver, that a few extra knots can erase the margin a crew is counting on.
Harbor authorities elsewhere lean on shore-based expertise, the way some harbor pilots now advise ships from a control room rather than the bridge. For the Corpus Christi Ship Channel, the fix is less dramatic than new technology, just a slower turn next time a tug has to spin in a tanker’s shadow.
Read the whole thing?
Get the week's signal, not the noise
Our sharpest reporting on energy, climate and nature — free, once a week.
