NTSB: Loss of Tow-Line Shackle Pin Led to Sinking of U.S.-Flagged Tug in Pacific

Mangilao (on left) and Chamorro (on right) under way prior to the accident voyage. (Source: Cabras Marine Corporation)

The loss of a towline shackle pin resulted in a towing failure that led to the sinking of a U.S-flagged tug during a developing storm in the Pacific Ocean last year, the National Transportation Safety Board has determined.

The NTSB released a Marine Accident Brief on Thursday detailing its investigation of the sinking of the towing vessel Mangilao on August 5, 2019, about 800 miles northwest of Guam.

No pollution or injuries were reported as a result of the sinking. The sunken vessel, however, has not been recovered.

The US-flagged Mangilao, a 114-foot-long tugboat, was being towed from Guam to a drydock in Subic Bay, Philippines, by the US-flagged Chamorro, a 97-foot-long tugboat, when the accident occurred.

Both vessels were owned and operated by Cabras Marine Corporation. No one was aboard the Mangilao during the tow.

According to the NTSB, the Mangilao was being towed behind Chamorro on a 2-inch wire rope with approximately 1,000 feet extended; a 14-inch-diameter-by-60-foot PolyDac plaited 8-strand hawser; a 1.25-inch chain terminal; and two anchor-type shackles connecting each part—one 35-ton shackle closest to the Chamorro and one 50-ton shackle to the 1.25-inch chain closest to the Mangilao.

From the NSTB’s Marine Accident Brief:

The Chamorro’s crew checked the Mangilao and the towing gear several times a day during the voyage and was recorded as being “in good order.” On Aug. 4 the Chamorro and Mangilao were beyond the halfway point of the planned 1,517-mile journey as a tropical cyclone developed over their location. The Chamorro’s captain explained to investigators that even if the tow turned back toward Guam, it would still have to endure significant weather, so he did not want to turn back.

A safety briefing was held the morning of Aug. 4, at which time the captain had the crew extend the tow wire to about 1,400 feet. Throughout the remainder of the day the Mangilao rolled and pitched in 39- to 46-knot winds and 12- to 13-foot seas.

About 3:45 a.m., Aug. 5, the chief mate completed a final visual check of the tow just prior to being relieved by the second mate for the 4 a.m. to 8 a.m. watch. The chief mate stated he could see the navigation lights of the Mangilao. The Chamorro’s speed was about 2 knots. As the second mate assumed the watch heavy rain and storm conditions prevented him from being able to see the Mangilao or its navigation lights. The rain passed in about 20 minutes, but the second mate still could not see the Mangilao’s navigation lights. About 10 minutes later the second mate awakened the chief mate, asking him to help look for the Mangilao. The Chamorro’s speed was 5.5 knots, and the chief mate said at that point he knew they had lost the tow, and he notified the captain.

The captain ordered the tow wire be retrieved and secured. The wire and hawser came aboard, but the shackle closest to the Mangilao was missing its pin. Due to the heavy seas it took approximately 50 minutes for the crew to secure the tow wire and equipment.

The Mangilao was located after about a 2-hour search and was found with a 15-degree list to port and with the port quarter of the vessel submerged. Sea conditions and the indications of flooding aboard the Mangilao made it too dangerous for the Chamorro’s crew to board the Mangilao and attempt to stop the flooding. The Mangilao sank at 7:42 a.m.

During the investigation, the captain of the Chamorro told investigators he believed the chain, the one from the bitt on the foredeck of the Mangilao, was not long enough to clear the bow fendering and the shackle pin likely worked loose from repeated contact with the fender.

According to the NTSB, the recovered shackle was bent, indicating that the load was uneven for a time, and, as the pin was working itself loose from the shackle, it may have fallen or snapped off, resulting in the Chamorro immediately disconnecting from the Mangilao.

A U.S. Coast Guard marine inspector completed a dead ship movement inspection before the Chamorro’s departure, and according to the marine inspector, a survey was conducted of the primary and emergency towing arrangements and verification that the exterior structure of the vessel was watertight, the NTSB said.

In its analysis the NTSB indicated that had the chain from the Mangilao’s bow been longer and the shackle extended out beyond the bow fender, the chain, rather than the shackle, would have contacted to bow. This likely would have prevented the shackle pin securing mechanism (cotter pin) from failing, and the tow would have remained connected, the NTSB said.

The NTSB determined the probable cause of the sinking of the Mangilao was the failure of the Chamorro’s towing arrangement due to the loss of a towline shackle pin, which left the Mangilao adrift and resulted in the ingress of water from boarding seas in a developing typhoon.

The NTSB’s Marine Accident Brief can be found here: MAB-20-33