NTSB: Over-reliance on autopilot caused tugboat grounding on rocks

Just minutes into a nighttime voyage to meet an incoming tow, the tugboat Justice was sharply off course near the Cape Cod Canal.

Although he was concerned about its position, the mate helming the vessel never manually changed course, instead expecting the autopilot to make the necessary corrections.

Moments later, he grabbed the controls to steer the 5,400-hp tug back into the channel. By then it was already too late, and the vessel grounded on rocky ledge.

National Transportation Safety Board investigators said the accident was likely caused by “the mate’s ineffective use of the vessel’s autopilot to maintain a course within the navigable channel and his delay in taking manual control as the vessel approached charted hazards.”

The accident occurred at 0001 on March 21, 2013, near the Hog Island channel just west of the Cape Cod Canal. Justice left its pier at Massachusetts Maritime Academy with a five-person crew at about 2340 the night before to meet a vessel it was to escort through the canal.

Four minutes after leaving the pier, the mate used the tugboat’s autopilot to set the port azimuthing stern drive (ASD) to 8.5 knots and a 216-degree course, the report said. The mate had traveled the same route many times in the seven months leading up to the incident and used a similar heading each time.

Five or six minutes later, the vessel was hugging the western edge of the 500-foot-wide channel. The tug came within a few feet of hitting a lighted buoy marking the channel’s outer edge, the report said.

The mate told investigators he was constantly checking the vessel’s radar and chart plotter and watching his surroundings. However, an AIS readout of the vessel’s position showed the vessel left the channel for several hundred yards before grounding on a rocky ledge present on navigational charts.

“The mate said he did not immediately take manual control of the steering because he anticipated that his current autopilot command (requested course) would cause the ASD to ‘kick in’ and adjust the vessel’s heading to the minor course corrections he had entered,” the report said.

Investigators determined the autopilot equipment was functioning properly. The mate had been inputting incremental changes during the voyage to hold the vessel’s course. He told investigators that weather and currents were not a factor.

Justice is outfitted with a Simrad AP50 autopilot system. The vessel has two autopilot settings, “work” and “auto,” that do not account for set and drift. The report said the “work” setting holds a tighter course and provides a quicker response than the “auto” setting.

Nobody was hurt in the grounding, but part of the starboard stern drive was sheared off and more than 230 gallons of gear oil spilled from the vessel into Buzzards Bay. Damage to the vessel was estimated at more than $1.2 million.

Justice is owned by Reinauer Transportation Co. of Staten Island, N.Y., and operated by Boston Towing and Transportation. It was built in Tacoma, Wash., in 2009 by J.M. Martinac Shipbuilding Corp. and designed by Robert Allan Ltd.

Attempts to reach Boston Towing and Transportation for comment on the accident were not successful.

After the accident, the master awoke to a rumbling sound. He directed the crew to check the vessel for flooding and took control of the vessel from the mate. The master then steered the listing tug back to its pier at Mass Maritime.     

By Professional Mariner Staff