Acetone aboard: what the Ultra Dash fire teaches

What happened

On the afternoon of Wednesday, August 18, 2021, two catamarans caught fire at a marina at 399 Lewis Lane in Dania Beach, Florida, just south of Fort Lauderdale. One was Ultra Dash, a Leopard 50 that John and Ellie of the YouTube channel Barefoot Doctors Sailing had bought about six months earlier. The other was a Leopard 45 tied alongside it. Both boats were afloat at the dock, not hauled out, and repair work was going on. Neighbors heard a loud boom, then saw smoke and flames, Local 10 News reported.

Two workers were burned. Broward Sheriff Fire Rescue told CBS Miami that one man had significant burns to his lower legs and the other had minor burns, and that both were expected to recover. Both catamarans were destroyed. The couple, who were in Australia at the time, say Ultra Dash burned and sank, blocked the canal for a few days and wrecked the pontoon, and that a nearby superyacht was moved clear before the fire could reach it. They also say the boat was unlivable within about five minutes and had sunk within roughly ten to fifteen.

The cause, as the owners tell it

More than two years later, once the insurance claims and a mediation involving at least 16 parties had finished, the couple posted a video explaining the cause as they understood it from that process. In their account:

Two workers, both sent by the contractor the couple had hired, were aboard at the same time. Neither knew the other was there; each thought he was working alone.
One was doing clean-up work with acetone in the stern of the port hull. The vapor, heavier than air, sank into the bilge and spread forward along it.
The other was doing electrical work in the bow. When the vapor reached him, his electrical gear set it off.
The flame ran back along the bilge to the stern, where an open can of acetone at the first worker's feet went up and burned his legs. The electrician's burns were to the backs of his hands. The couple say both were treated and sent home the same day.
They ruled out arson, the boat's wiring, lithium batteries (they had none) and the Leopard build, and called it human error.

What we could and couldn't confirm

This cause is the owners' account, not an official finding. News reports at the time said the U.S. Coast Guard was investigating because the fire started on the water. We could not find any published Coast Guard, fire marshal or NTSB report on this fire.
Fire rescue told CBS Miami the workers were resurfacing or reglazing the boat. The owners describe the acetone job as clean-up work. Neither source says painting or fiberglass work was under way.
Which tool or switch made the spark has never been published.

Why acetone is so unforgiving below decks

About 2Vapor density of acetone (air = 1). It sinks into bilges and low lockers instead of drifting away.

−20 °C / −4 °FClosed-cup flash point. It gives off vapor that can ignite at any temperature you would work in.

2.5–12.8%Flammable range in air. The lower limit, 2.5%, is 25,000 ppm.

A correction about rags

In the same video the couple say acetone rags can catch fire on their own. Clean acetone doesn't do that: it simply evaporates, and without a spark or flame it won't ignite below about 465 °C (869 °F). The danger from an acetone rag is the flammable vapor coming off it. Self-heating comes from a chemical reaction in the waste. Rags carrying drying oils such as linseed oil can heat up and ignite in a pile. Sawdust, rags and other combustible absorbents soaked with MEKP, the catalyst for polyester and vinyl ester resin, can ignite on their own. Mixed epoxy left to cure in a pot can get hot enough to set nearby material alight, and epoxy hardener spilled on sawdust can combust. Fiberglass clean-up rags often carry exactly these, so a covered metal can, off the boat, is still the right place for them.

Acetone aboard: what not to do

Don't let two jobs run aboard without one person in charge of who is on the boat and what they are doing. On Ultra Dash, by the owners' account, one worker made the fuel and the other the spark, at opposite ends of the same bilge, and neither knew the other was there.
Don't mix solvent work with electrical work, grinding, heat guns or any other hot work. OSHA's shipyard rule for flammable solvents bans open flames, arcs and spark-making equipment in the area. Do the solvent job, clear the vapor, then start the electrical or hot work.
Don't leave the boat's power on while solvent is open. Switch off the battery switches, unplug shore power and chargers, and turn off automatic bilge pumps and blowers. Anything that switches itself on can make a spark with nobody touching it.
Don't use acetone in a closed-up boat. Its vapor is about twice as heavy as air and runs downhill into the bilge, so ventilate from the low points to outside, not just across the cabin. OSHA's shipyard rule calls for enough ventilation to keep vapor under a tenth of its lower flammable limit.
Don't ventilate with a household box fan, shop vac or an extension cord plugged in below. OSHA's shipyard rule allows only explosion-proof lights near solvent work and no cable connections within 50 feet. The NTSB traced a 2019 barge explosion to static from an air mover that wasn't bonded, which lit leftover acetone vapor. Use a bonded air-driven or rated blower with every plug outside the boat.
Don't trust your nose. Gas-test the bilge before any hot or electrical work. Use a combustible-gas (LEL) meter and sample low. OSHA requires bilges to test below 10 percent of the lower flammable limit before shipyard hot work, and requires a Marine Chemist's certificate for hot work in or next to fuel tanks or spaces that held flammable liquid.
Don't bring more acetone below than you'll use in a few minutes, and never leave an open can at your feet. Pour a small amount into a closable container and cap the stock can. On Ultra Dash, the owners say, the open can is what burned the worker standing over it.
Don't carry acetone or two-part paint aboard as stores. Buy it when the job needs it and keep it off the boat or in a vented deck locker. The couple's own point: a fire like theirs needs someone to bring the solvent aboard.
Don't wad used rags into a trash bag or drop them in the bilge. OSHA's shipyard rule says rags soaked with these liquids go in a covered metal container. Take the can off the boat the same day.
Don't start without a charged fire extinguisher within reach and a clear way off. OSHA requires suitable extinguishers to be immediately available for solvent work. The owners say their boat was unlivable within about five minutes.
Don't treat a boat at the dock as being outdoors. Tell the marina or yard before solvent or hot work. Ultra Dash and the Leopard 45 were tied together, and the fire took both and the dock.
Don't skip gloves and eye protection, and don't keep going if you feel unwell. NIOSH lists eye, nose and throat irritation, headache and dizziness among the effects of breathing acetone. Stop, get into fresh air, and ventilate before going back.

The short version, before any solvent job

Ventilate from the low points to outside.
No sparks, electrical work or hot work while solvent is open.
Power off: battery switches, shore power, chargers and automatic pumps.
Use small amounts and keep the can closed.
Rags go in a covered metal can, off the boat.
Charged extinguisher within reach.
Gas-test the bilge before any hot work.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Sources

Background reading we checked while writing this page. The wording above is our own. The cause of the Ultra Dash fire is the owners' account in their video; no official report has been published.

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