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Dangerous Gases Aboard

Here are four of the most common dangerous gases and fumes that most of us encounter on our boats. It's important to know how to handle them and when to sound the alarm.

Carbon monoxide alarm

Gasoline Fumes

Highly volatile and a leading cause of marine related explosions and fires, gasoline fumes, which are heavier than air, can easily accumulate in a vessel's bilge due to improper refueling or fuel system leaks. There, it's only a spark away from causing a fire or explosion.

Sources: While fuel system leaks due to damaged, degraded, or improperly installed components are an obvious source, gasoline vapor explosions can also be traced to things such as improper fueling techniques and lack of adequate ventilation. Mistakes during routine maintenance or repair of engines, generators, or fuel systems can also lead to an explosion.

Detection: Your nose is a marvel of nature, so put it to good use. Always sniff your engine compartment and bilge areas (particularly around fuel tanks) after refueling or any time prior to starting your engine. As good as your nose is, it can't be everywhere all of the time, so consider installing a gasoline vapor or fume detector. Fumes are heavier than air and will settle in the lowest areas of the bilge or compartment. As such, fume detector sensors (which are not waterproof) should be mounted as low as possible, but above the normal slosh range of bilge water.

Little Known Fact: Liquid gasoline doesn't burn; it's the vapors emanating from it that form an explosive mixture when combined with oxygen in the proper ratios. Fire is typically viewed as an inevitable result of any gasoline vapor explosion, but that's not always the case. Some explosions are so powerful they simply blow out any flames, resulting in catastrophic damage with little or no fire damage.

Prevention: Routine inspections and maintenance of your engine and fuel system are the keys to preventing fuel leaks. Typical problems I find while surveying include deteriorated fuel lines, loose or corroded hose clamps, cracked fuel system components, corroded fuel tanks, and fuel line damage due to lack of chafe protection. Out of sight components (such as fuel tank fill hoses) should be routinely accessed and inspected. I've found hose clamps reduced to a rusty paste and fill hoses so cracked and deteriorated they can be pulled apart by hand.

Rubber fuel lines and hoses have a finite lifespan, with the generally accepted norm being to replace those over 10 years of age, regardless of appearance. I routinely find original fuel lines and hoses on boats constructed in the '70s and '80s, which is scary.

All electrical equipment installed in engine or generator compartments (starters, alternators, pumps, battery chargers, water heaters, and so on) must be labeled as being Ignition Protected. This means they're nonsparking and safe to use in a potentially explosive environment.

The other side of explosion prevention is following established safety procedures when refueling, conducting engine or fuel system repairs, and the like. These include operating engine compartment blowers for at least 4 minutes and checking the engine compartment bilge for gasoline vapors every time before starting engines or generators.

Other precautions for refueling include turning off the engine, not smoking, and securing power to all ­electric motors, pumps, and so on, by turning off the main battery switch. A gasoline vapor-rich atmosphere can make engine starting difficult. So if your engine doesn't start don't simply keep cranking, investigate!

Gas Explosions On The Rise

Our BoatUS Marine Insurance claims files reveal an alarming statistic: In the past several years, the number of reported explosions due to leaking gas has increased by 30%.

Spilled gas boat fire

Photo: Getty Images/31moonlight31

In one case, the insured reported that, while preparing breakfast in the galley, there was a sudden loud "boom" and the cabin erupted in flames. Both people aboard suffered severe burns requiring hospitalization. It was later determined that the stove had a fault that allowed propane to leak into the cabin. Propane is heavier than air, so it collected in the boat's bilge, building to such a degree that when the boater lit the stove, the gas ignited. Had the boat been equipped with a gas detector, the alarm would have been raised well before there was sufficient gas in the boat to cause an explosion.

In another instance, a BoatUS insured had just filled his tanks with gasoline. When he turned on the ignition, all three people in the boat's cockpit, including himself, suddenly found themselves in the water from an explosion so strong that it blew them off the boat. The owner had forgotten to run the bilge blower for the necessary 5 minutes to dissipate built-up gas vapors after fill-up.

These reports are typical. In almost every instance, the explosive event was due to either faulty equipment or operator error and could have been avoided through routine maintenance and boater education. To guard against this risk, consider the following steps:

  • Have your boat inspected by your local U.S. Coast Guard Auxiliary; they'll perform a vessel safety check at no charge.
  • For a more comprehensive test, hire a suitably qualified marine surveyor to ensure that fuel and gas systems are safe and in compliance with recognized standards such as those published by the American Boat & Yacht Council and the National Fire Protection Agency.

— Mark Corke

Carbon Monoxide

Carbon monoxide (CO) is a potentially lethal gas produced when burning any carbon-based fuel (e.g., gasoline, wood, propane). CO is colorless, odorless, and tasteless, and mixes evenly with air, meaning it readily travels throughout a boat's interior spaces. CO enters the body through the lungs and is readily absorbed into the bloodstream, where it displaces oxygen levels in the body and can lead to carbon monoxide poisoning. Common signs of CO poisoning include headaches, dizziness, weakness, drowsiness, and nausea — symptoms that can easily be attributed to seasickness, alcohol consumption, or too much sun exposure rather than CO poisoning.

Sources: While the most common source of CO is engine exhaust, it can also be produced by any open flame device, such as a gas stove, heater, or grill. During surveys, I commonly encounter Darwin Award candidates for stupid decisions — people using camping stoves and heaters aboard, despite these explosive devices being clearly labeled for outdoor use only.

Engine and generator exhaust system leaks are a prime concern, however, even properly operating systems can introduce CO into the vessel under certain conditions. Examples include fumes being drawn belowdecks when a vessel is operated with aft hatches or portholes open (called the station wagon effect), or being anchored or docked near vessels operating engines or generators. Other factors such as inadequate or obstructed ventilation systems can also contribute to CO buildup.

CO2 detector

A CO detector should never be wired through the ignition circuit. All detectors should be permanently energized when people are aboard the boat.

Detection: As carbon monoxide is colorless, tasteless, and odorless, the only reliable way to detect it is by installing a CO detector. Marine grade CO detectors are recommended for all boats with enclosed accommodation compartments as per ABYC Standard A-24 and NFPA 302. ABYC also requires that CO detectors be tested to UL 2034 standards. If you utilize a generator to power air conditioners and other appliances while underway or at anchor, I'd recommend a CO alarm system designed to shut off the generator once CO is detected.

Neither ABYC (American Boat & Yacht Council) nor NFPA (National Fire Protection Association) states specifically where CO detectors should be located on your vessel, other than to say they must be located to monitor the atmosphere in the main cabin and each sleeping area. Unlike liquefied petroleum gas (LPG) or gasoline vapors (which are heavier than air) CO has roughly the same weight as oxygen, meaning detector placement is not limited to high or low areas of the cabin. Mounting them at roughly eye level makes it easier to monitor detector meters or warning lights. CO detectors also have a limited lifespan. Detectors or sensors (if part of a system) will typically need to be replaced every five years, however this may vary between units and should be verified with the manufacturer.

Little Known Fact: Although death can occur quickly in a CO-rich environment, exposure to smaller amounts can also be lethal. The effects of CO are cumulative and can build up gradually in a person's bloodstream for hours or even days before reaching critical levels.

Prevention: Ensure adequate ventilation exists when using open-flame devices (e.g., stoves), and never use stoves for comfort heating. Regularly inspect engine and generator exhaust systems for corrosion, raw water-cooling leaks, deteriorated hoses, loose hose clamps, and so on. Inspect exhaust system hoses along their entire length, paying attention to hose clamps, blockages, and chafe. Machinery spaces should also be sealed and vapor proof from the vessel's interior.

Liquid Propane Gas

There are a number of reasons why LPG is such a popular choice for cooking and heating on boats. It's efficient, relatively cheap, and widely available. It can also be extremely dangerous. LPG vapor is heavier than air and tends to "flow" like water, seeking the lowest possible point. As a boat's hull is essentially a watertight envelope, escaping LPG can be trapped in bilges or other low areas, where they can rapidly accumulate to explosive concentrations.

Propane mistakes

A well-meaning owner has made some "improvements" to the gas locker and in so doing has breached the gas-tight integrity of the installation, allowing any leaking gas to flow down into the bilge.

Sources: Installation standards and recommendations for LPG systems (such as those provided by ABYC) are comprehensive and, when heeded, go a long way toward preventing an "uncontrolled combustion" event. Assuming proper installation and barring physical damage to LPG system components, the most common source of leaks will be loose fittings and couplings. Other possible sources include improper tank stowage (which can introduce gas into the vessel interior should a leak occur), deteriorated flexible hose, and damage to LPG lines due to lack of chafe protection.

Physical damage to system components can also occur when LPG tank stowage lockers are used for general stowage. LPG tank lockers must be dedicated, meaning no other items (e.g., tools, lines, shackles) can be stored inside.

Detection: In addition to your nose, an LPG detector or "sniffer" is an excellent means of detecting leaks. Some sniffers consist of a single unit containing the sensor and alarm circuitry, however, most are sold as a "control package" that includes a monitor/control panel, remote sensor (one or more, depending on the unit), and a remote-controlled solenoid on/off valve to be installed at the tank.

Sniffers are designed to constantly monitor the air for LPG and should be configured to do so, even with no one onboard. This ensures those returning to the boat are alerted to a leak prior to boarding (and turning on a potential spark-inducing piece of equipment).

Those who leave their vessel unattended for long periods should look for sniffer units that provide the option to trigger an external alarm (such as a loud horn or marine strobe) to alert marina personnel or passersby that a problem exists.

As LPG vapor is heavier than air, sensors should be located beside and below appliances (under stoves for example) and at other low areas of the cabin or bilge where fumes are likely to collect. Sensors are not waterproof, so when mounted in the bilge they should be located as low as possible without becoming submerged or subject to bilge splash.

Little Known Fact: LPG suppliers are required by law to add an odorant to make gas leaks more noticeable. Just because you can't smell it doesn't mean it isn't there, however, as gas can remain after the odorant has dissipated.

Prevention: Check your LPG system weekly for leaks by conducting a leak down test. With the stove or appliance valves off and the solenoid (if so equipped) in the on position, open the manual cylinder supply valve to charge the system, then close and note the pressure gauge reading. The pressure should remain constant for not less than 3 minutes. A drop in pressure indicates a potential leak, meaning the system must be checked with leak-detection fluid or a solution of non-ammonia soapy water (no flames please) to locate the leak and correct.

Although LPG sensors and alarm systems should be professionally tested annually, they can also be tested using a butane lighter. Simply hold the lighter next to the sensor and press the trigger without lighting.

Portable Gasoline Generators

Cheap & Convenient, Yes. But Also Possibly Lethal

It's easy to see why the tempting call of easy AC power is difficult to resist, particularly when it can be had in five minutes and for one-eighth the cost of a permanently installed marine generator. But, there are numerous problems associated with using a portable gasoline generator onboard. As a marine surveyor I see many MacGyver-isms that boat owners have rigged up in efforts to address these issues (the ones they are aware of anyway). Others simply have no clue about the dangers to which a portable generator exposes them.

Portable gas generator 

One of the biggest problems with using a portable generator is the very real chance of carbon monoxide poisoning. No matter where you locate the generator during use — bow, swim platform, cockpit, flying bridge, even the forward cabin (yep, I've seen it) — it's a certainty that CO will at some point be introduced into the vessel's interior.

After realizing there's really no way to safely use a portable generator on board, some boaters think to circumvent the problem by attempting to permanently install them. There's a long list of requirements for permanently installed gasoline engines on boats. Most carry the weight of federal law and can be considered what I term "tombstone" regulations, a reference to the fact they were adopted due to previous loss of life.

These include strict installation requirements with regard to fuel systems, electrical systems, and ventilation systems — none of which can be easily met when trying to permanently install a portable generator. Additional challenges include the requirement for the generator to be ignition-protected (they're typically not), the problems associated with safely venting a hot or "dry" exhaust overboard, and of course the potential for disaster when refilling the gas tank of a hot generator in an enclosed compartment.

Hydrogen Sulfide

Hydrogen sulfide (H2S) is a colorless, toxic gas that is also flammable and highly corrosive. Symptoms of H2S exposure include skin and eye irritation, headaches, loss of balance, nausea, delirium, tremors, and convulsions. Inhalation of high concentrations of H2S can lead to rapid unconsciousness and death.

Sources: H2S gas occurs naturally during the breakdown of organic matter. It can be found in swamps, sewers, volcanic gases, and even water wells. In 2016, three Indonesian fishermen died after inhaling H2S emitting from a pile of rotten fish stored inside the hold of a fishing trawler. (The fish were being stored there prior to processing at a nearby fertilizer plant.)

The U.S. Coast Guard recently issued a safety alert regarding the formation of H2S gas within the waste oil storage tanks found aboard commercial vessels. The cause was determined to be the use of biodegradable cleaners to clean oily bilges and the chemical reactions involved in breaking down oil in an anaerobic (low oxygen) environment. For most recreational boats, the primary source of H2S gas is from overcharging lead acid batteries.

Detection: There are a number of portable H2S gas detectors available online, with many priced under $100. Your nose is also a valuable detector. Keep in mind that by the time you can smell it, hydrogen sulfide has already reached a level that's harmful to humans.

Little Known Fact: H2S gas smells like rotten eggs, and while very noticeable at first, it can quickly deaden the sense of smell (often in a few breaths), leaving potential victims unaware of its presence.

Prevention: As mentioned, the primary source of H2S (for the purpose of this article) is the overcharging of lead acid batteries. To avoid this, match your battery charger to the type and size of battery (or bank) you're charging. Set charging levels to the lowest, effective setting, and monitor battery voltage and temperature while charging. Adequate ventilation is also crucial.

If you smell or detect H2S, turn off the charger, ventilate the compartment, and stay outside until the gas is dispersed.

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Frank Lanier

Contributing Editor, BoatUS Magazine

Capt. Frank Lanier is a SAMS Accredited Marine Surveyor with more than 40 years of experience in the marine and diving industries. He’s also an author, public speaker, and multiple award-winning journalist whose articles on boat maintenance, repair, and seamanship appear regularly in numerous marine publications worldwide. He can be reached via his YouTube channel “Everything Boats with Capt. Frank Lanier” and website