(WASHINGTON) — The National Transportation Safety Board (NTSB) has issued Marine Accident Brief 20-33, detailing its investigation of the sinking of the towing vessel Mangilao on Aug. 5, 2019, in the Pacific Ocean about 800 miles northwest of Guam.
No pollution or injuries were reported in connection with the sinking. Mangilao’s value was estimated at $437,227. The vessel was not recovered.
The U.S.-flagged Mangilao, a 114-foot tugboat, was being towed to a dry dock in Subic Bay, Philippines, by the U.S.-flagged Chamorro, a 97-foot tugboat, when the accident happened. Both vessels were owned and operated by Cabras Marine Corp. No one was aboard Mangilao and there were 10 crewmembers aboard Chamorro.
Mangilao was towed astern of 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 Chamorro and one 50-ton shackle to the 1.25-inch chain closest to Mangilao. The vessels departed Apra Harbor, Gua, on July 29, 2019.
Chamorro’s crew checked Mangilao and the towing gear several times a day during the voyage and it was recorded as being “in good order.” On Aug. 4, Chamorro and Mangilao were beyond the halfway point of the planned 1,517-mile journey as a tropical cyclone developed over their location. 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 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 Mangilao. 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 Mangilao or its navigation lights. The rain passed in about 20 minutes, but the second mate still could not see Mangilao’s navigation lights. About 10 minutes later the second mate awakened the chief mate, asking him to help look for Mangilao. 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 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.
Mangilao was located after about a two-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 Mangilao made it too dangerous for Chamorro’s crew to board Mangilao and attempt to stop the flooding. Mangilao sank at 7:42 a.m.
The captain told investigators he believed the chain (from the bitt on the foredeck of Mangilao) was not of sufficient length to clear the bow fendering and the shackle pin likely worked loose from repeated contact with the fender. 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 Chamorro immediately disconnecting from Mangilao.
A U.S. Coast Guard marine inspector completed a dead-ship movement inspection before 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.
In its analysis, the NTSB indicated that had the chain from Mangilao’s bow been longer and the shackle extended out beyond the bow fender, the chain, rather than the shackle, would have contacted the bow. This likely would have prevented the shackle-pin securing mechanism (cotter pin) from failing, and the tow would have remained connected.
The NTSB determined the probable cause of the sinking of Mangilao was the failure of Chamorro’s towing arrangement due to the loss of a towline shackle pin, which left Mangilao adrift and resulted in the ingress of water from boarding seas in a developing typhoon.
– National Transportation Safety Board