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Preventing Explosions Aboard


Gasoline fumes are the major cause of boat explosions, and because they usually happen when someone is aboard, they often cause serious injuries. Here’s what you can do to make sure your boat is not a ticking time bomb.

The Incident

The port engine on the 1996 37-foot flybridge motor yacht just wasn’t right; the gas engine was running rough and losing power, so the owner contacted a local marine mechanic. While the boat was at her dock, the mechanic partially tore down the engine and found damage to the valves and pistons from normal wear and tear. The engine, he said, would have to be removed to repair it, so the owner prepared the boat. A few days later, the mechanic and an assistant arrived to move the boat, using the starboard engine. About ten minutes after the engine started, the boat exploded, throwing both men into the water. Amazingly, neither was seriously injured. The intense fire that followed destroyed another boat as well as much of the dock. During the subsequent investigation, the owner said he had shut off the fuel lines as he prepared for the engine to be removed; he had no idea what happened.

The fire analysis took more than the usual amount of time because most of the flybridge had collapsed into the engine room during the explosion. I inspected the wreckage with a fire investigator and found that the port bypass fuel shutoff was not completely closed, probably because over the years it became stiff from lack of use.


When the technician started the starboard engine, fuel ran as normal though the manifold to the starboard engine. Unfortunately, the owner had failed to completely close the port bypass fuel feed. Fuel dripped into the engine room, where the line had been disconnected from the carburetor. Gasoline fumes built up and were ignited.

The lesson to remember is that no matter how it happens, if gasoline gets into the bilge, it’s simply a spark away from an explosion. Oddly, not all explosions result in fire; many are so strong that they will blow out the flames. But the injury to passengers and crew can be devastating, even without a fire.

What You Can Do

Fiberglass boats last a long time, and the aging systems need continuous preventive maintenance. Fuel fill lines, feed lines, and vent lines are mostly in the engine room and are subjected to heat and vibration. On many boats they’re difficult to access for inspection. Hoses become hard and brittle over time, causing them to leak. Fittings rust, and many fuel shutoffs are not opened and closed during the season, so they stick; owners tend to leave them open all the time, even during winter storage. After a few years they don’t operate at all anymore. When you need to turn them off in an emergency, they might not work. (Question: Do you know where your shutoffs are?) As the above story illustrates, even an experienced owner can make a mistake that can cause an explosion. Fortunately, our experience has shown that if you pay attention to a few important things, you can avoid a catastrophe. For gasoline vapors to ignite, there must be a fuel leak somewhere, and something to ignite it. Remove both of these things, and you eliminate the potential for explosion. Proper refueling procedures are also critical to prevent a potential disaster.

Fuel Leaks

Unless you’ve been hiding under a rock, you probably know that during last several years, ethanol has been added to gasoline. This can degrade older fuel lines much faster then anticipated. Even newer hoses don’t have quite the same lifespan. Degraded fuel lines get brittle and will eventually leak — and a leaking fuel line is a disaster waiting to happen. If your hoses are more then ten years old (proper USCG-approved hoses are date-stamped when they were made), bend them, squeeze them, and see if they move or rotate on the fuel fittings. If so, they’re loose enough to leak. Sometimes fuel hoses are accidentally stepped on and damaged during routine engine maintenance. Run your hand along the hose or use a clean white rag and see if you smell gasoline (or worse, see it). If so, replace the hose using approved fuel line. While there are different types for different purposes on a boat, I recommend using only USCG-approved A1-15 hose. This hose has passed rigorous testing and can withstand a 2.5-minute burn test, which is designed to be enough time to put out a fire or abandon ship before the hose begins leaking. Most fuel-line manufacturers suggest that their fuel lines should replaced every 10 to 15 years even if there are no indications of leaks or damage. Proper fuel-lines are marked as shown, below.

Old hose gets brittle over time and tends to crack — and leak.
Use only USCG-approved fuel hose; the approval must be marked on the hose. Type A1-15 is the most durable.

Other places gasoline can leak are where hoses connect to other fittings. Fuel-fill spuds, fuel-tank lines and gas-tank gaskets, as well as carburetor and fuel-pump fittings can leak. Use the clean dry-rag method for these areas too.

Preventing Ignition

Gas fumes by themselves are relatively harmless. But the slightest spark can ignite the fumes with great power, enough to blow the deck off of a large boat or throw crew in the water. The other side of preventing explosions is to have no way to ignite gas fumes that may have built up. Any starters, alternators, or pumps — or any other electrical equipment — in your engine room or generator compartment must state that they are “Ignition Protected.” Ignition protection is a standard that makes a product, such as a starter or alternator, safe to be installed in an environment that could become explosive. It means it won’t spark, which is all that gas fumes need to ignite. Don’t listen to the kid at the auto parts store who says auto and marine parts are all the same — they’re not. It costs more to make marine ignition-protected parts, but they may just save your life. If you have any reservations about whether something is ignition protected, replace it.

How do you know if something is ignition-protected and safe for gas engine spaces? It must be marked like this.

Some pumps that you might consider safe because they’re installed on your boat are not necessarily ignition protected. Electric raw water pumps, for example, as well as some pumps used for pumping the bilge may not be ignition-protected. A previous owner could have installed a non-ignition-protected pump that could spark on startup. So check that all electrical parts that go on a gasoline engine (or in a gasoline engine space) have a label that says “Ignition Protected.” Note: even power tools used in a gasoline engine space can cause a spark sufficient to cause an explosion! Don’t take chances.

Engine Room Blowers And Their Hoses

A purpose-built marine-engine room blower is ignition-protected and therefore safe, but this can still be a problem area. Have you ever inspected your blower duct hose going into the bilge? These deteriorate over time. Eventually they’ll look like Swiss cheese, which means they’re no longer capable of removing dangerous fumes from the bilge. Replace them if they have any cuts or breaks. Blower hoses should extend to the lower 1/3 of the engine room. (Gasoline vapors are heavier than air and will sink to the lower part of the bilge.) Sufficiently running the blower (about four to five minutes) before starting the engine should evacuate any fumes that might have developed. Note that a bilge blower will not rid the compartment of spilled fuel, which will continue to emit vapors. As an added safety feature, a marine vapor detector should be installed in the engine room. These have indicators, generally on the helm station, that will alert you of any vapors. The detectors have a limited lifespan; usually after five years the manufacturer says they must be replaced.

Fueling Your Boat

When fueling, the electrical system on the vessel must be shut down. (Turn off battery switches.) All engines, including the generator, must be off. Close all hatches to prevent fumes from coming below. Remove all passengers. Make sure that the fuel-fill nozzle is in the fuel fill and not a rod holder or freshwater fill. Yep, that has happened many times. Before starting the engines, open hatches, and turn on the blower. After running it for four to five minutes, use your nose, and smell the bilge. If you smell gasoline, get off the boat and alert the dockmaster. If you find spilled gas in the bilge, call 911, and let the professionals deal with it. One more thing: Know where your fuel shutoffs are, and verify that they operate properly. Label them if necessary — some people even mark the location inside the boat so the crew or passengers easily see it.

Propane (LPG)

Many vessels today use propane cooking appliances. As builders have taken great pains over the years to make proper installations, explosions from them are fortunately rare. But especially on older boats, they still need regular inspections. Make sure that propane tanks are mounted in a compartment with a vent at the bottom to allow any propane that might leak from the tanks to go overboard. Shutoffs in the compartment can seize, and my experience with these tanks and valves is that they are rarely opened and closed; many are simply left open for convenience. Exercise them regularly, and close them when not in use. Consider this for a moment: I once saw a speaker installed with bare wires in the same compartment as the tanks. Don’t make this kind of dangerous mistake — nothing that could possibly make a spark can share space with a propane tank. Also, there should always be an electrical shutoff in the galley near the stove that closes a solenoid on the tank. You don’t want to have to clamber out of the galley to shut off the fuel supply in case there is a problem.

One of my pet peeves is that the hose or piping from the tank to the stove is sometimes not accessible and often hidden behind cabinets. The hose behind the stove also usually has limited access — you may have to dig a little to properly check the hoses. Inspect as much hose as possible, looking for cracks. You can use a leak-detection fluid (available at home-improvement stores or online) or a soapy solution along the hose, looking for bubbling while the valves are open. Following the ABYC’s procedure, you can then check the entire system for leaks: With the appliance valves off, open the cylinder supply valve. Close the cylinder supply valve. Observe the pressure gauge reading. The pressure indicated should remain constant for not less than three minutes. If any leakage is indicated by a drop in pressure, check the entire system with a leak detection fluid or detergent solution to locate the leak.

If you feel uncomfortable inspecting your gas or propane fuel lines or locating your shutoffs, ask your marine mechanic to examine the lines and show you where the shutoffs are; you’ll feel a lot safer on the water.

I Have Nothing To Worry About. I Have A Diesel Engine

Really? Well, the explosive nature of diesel is certainly not like gasoline, but during a recent claim I handled, the starter on a small sailboat diesel engine hung up when the engine was started and became red hot. Just above the starter was a non-approved automotive fuel-feed line to the engine. It melted and sprayed diesel fuel onto the hot starter, causing a fire. While it wasn’t an explosion, it could have been devastating. Fortunately, the owner had a fixed fire extinguishing system in the engine compartment that put out the fire. Over the years, I’ve seen gasoline cans for the dinghy’s outboard and small propane canisters stored in a diesel engine room. Remember, parts in a diesel engine room don’t have to be ignition-protected as they need to be in a gasoline engine room (and probably aren’t), so nothing explosive (gasoline, propane, flammable solvents, etc.) should be kept in a diesel engine room.

Kenneth Weinbrecht

Contributor, BoatUS Magazine