On a sunny July afternoon, the owner of the 79.9-foot yacht Pegasus and an employee refinished wood surfaces with teak oil at Peninsula Yacht Basin in Gig Harbor, Wash. The task involved applying the oil with a brush then wiping away and excess with a cloth.
They wrapped up for the day at about 16:30 and left the oily rags in a bag under a wooden table on the vessel’s aft deck. Some nine hours later, smoke and flames were visible on Pegasus’ aft end. The fire continued to burn until the stern sank into the water.
National Transportation Safety Board (NTSB) investigators traced the fire at 02:04 on July 15, 2022, to “spontaneous combustion due to the self-heating of used oil-soaked rags that had been improperly disposed of on the aft deck of the vessel.”
Pegasus was built in 2012 in Oundle, England with a fiberglass-reinforced plastic hull. Nobody was on board at the time and there were no injuries, but the $1.5 million vessel was a total loss.
Neither the vessel owner nor the employee was identified in the NTSB report, which highlighted the risk of self-heating and combustion from rags soaked in certain kinds of oil-based finishes and improperly stored.
The owner and employee used a name-brand teak oil that contained a raw linseed oil on Pegasus’ wooden surfaces. A warning label on the container indicated rags soaked in teak oil can spontaneously catch fire if not stored correctly.
The label suggested placing used oily rags in a sealed container filled with water. In this case, used rags were wrapped in clean rags and placed within the bag, raising the risk of self-heating and combustion.
“Self-heating is a process in which heat is created within a material through a biological or chemical process and without the application of an external heat source,” the NTSB report said, citing a bulletin from the National Fire Protection Association. “If self-heating increases the temperature of the material above its ignition point with sufficient oxygen present, self-ignition can occur.”
That’s what NTSB investigators believe happened. Surveillance footage from the Peninsula Yacht Basin began at 01:00 on the morning of the fire. The flames grew for an hour before a bystander called 911 at about 03:00.
“Firefighters knocked down the flames, but the fire was persistent and continued to flare up,” the report said. “At 04:14, the stern of Pegasus sank with the bow rising out of the water.”
Investigators considered other possible causes of the fire, including arson and the owner’s suggestion that an errant firework might have landed on the aft deck. Surveillance cameras showed no evidence of “undetected arson or accidental human interference such as a lit cigarette or fireworks.”
“Further,” the NTSB found, “there was no evidence to suggest the fire was caused by an electrical system failure, and damage to engine room components indicated that the fire originated above the space and was not the result of a mechanical issue.”
Based on those factors, local fire investigators and the NTSB determined ignition of the oily rags due to self-heating was the most likely cause.